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Cobalt phosphino-thiolate complexes: applications towards electrocatalytic small molecule conversion to synthetic fuels

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Cobalt Phosphino-Thiolate Complexes: Applications Towards Electrocatalytic Small Molecule
Conversion to Synthetic Fuels

By

Jeremy A. Intrator

A Dissertation to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

December 2022

Copyright 2022 Jeremy A. Intrator
ii

“ The secret of freedom lies in educating people, whereas the secret of tyranny is in keeping them ignorant.”
- Maximilian Robespierre

iii

For Boma and Apu

iv

Acknowledgements
I want to first acknowledge those that have stuck with me since, well, from the beginning.
To my parents, I know I can be independent and self-interested at times, so I want to thank you for
your patience and understanding. It most likely was very difficult to deal with my immediate
disappearance since basically after high school, off endeavoring with my ever-evolving pursuit of
knowledge and scholastic integrity. I have been on my own so long that it is hard to remember
when I was so dependent on you, but when I was a wee babe it probably was unknowable how
everything would turn out. And I know it has been difficult for you since I’ve been gone. And I
want you to know that it means more than I can encapsulate in mere sentences the ways you helped
me get to a point in my life where I have achieved that which I have set out to pursue. That is
something that I cannot give gratitude enough and is something I wish to share with my children
as well. And, similarly to my siblings, who support me every step of the way. I know it’s not easy
to be so far apart, but I hope I can still impact your lives positively going forward. I don’t intend
on being estranged in this distant land and expect me more involved going forward.
To Stephanie, you have been a light in my life since the moment you have entered it. I
cannot imagine where I would be today without your help and support. You have been there every
step and words cannot describe how you have transformed the way I see the world. You have taken
on so much to support what I endeavor going forward, and that is something I cannot thank you
enough in words. Just know that these words are a mere semblance of the feelings of gratitude I
have for you.
I want to thank Prof. Smaranda Marinescu for providing me the support and knowledge to
attain this prestigious station. You pushed me to become knowledgeable in areas that I was not
well-versed and taught me the skills and values to become a proficient and successful scientist and
v

researcher. To my lab mates, both past and present, your experience and knowledge has been
indispensable to my progress. To my mentor Andrew, thank you for dealing with my dumb
questions. Coming in as a relative neophyte must have been difficult to deal with so I appreciate
your patience and help whenever you gave it to me. Same goes out to other past members of the
group Eric, Nick, Ashley, Geo, and Keying. Your patience and support meant the world and I wish
you all the luck in your future endeavors. And Jeff, coming in clutch with your chemistry
knowledge and experience was a boon in more trying times. To the current members of the group,
David, Adam, and Jeremiah, it’s been a fun jaunt over the last few years. It’s been real hanging
out with you in the office, shooting jokes and memes. You are all smart and quick learners and I
expect you guys will continue the great chemistry coming out of group going forward. To new
members of the group, starting out is always the most difficult, so stick to it and you’ll be amazed
the distance you go.
To many others that have accompanied on this path, I want to acknowledge you as well.
To my fellow compatriots of my year, Kevin, Anuj, Justin, and many others, starting out together
in graduate school made transitioning to this level so much easier. And it was nice partying our
first year too. To John and the others at CNI, thank you for the opportunity to take over as RA for
the XPS. I appreciate your patience and apt advice in both my academic and career endeavors. To
Aviv, thanks for being my go-to bro on the west coast. I didn’t know anyone except you moving
here. I will always remember and cherish being roommates and our many adventures together. To
my friends out of school, thank you for dealing with my weird scheduling and introverted nature.
Just know that I cherished all my moments together.

vi

Table of Contents
Epigraph .........................................................................................................................................ii
Dedication .....................................................................................................................................iii
Acknowledgements........................................................................................................................iv
List of Tables.................................................................................................................................x
List of Figures................................................................................................................................xii
List of Schemes..........................................................................................................................xxiii
Abbreviations……......................................................................................................................xxiv
Abstract………...........................................................................................................................xxvi
Chapter 1: General Introduction…………………………………………………………………..1
1.1 Global Energy Overview: Outlook and Challenges ……………….………………….1
1.2 Synthetic Fuels………………………………………………………….……………..2
1.3 Renewable Chemical Feedstocks…………...…………………………………………5
1.4 Hydrogen Evolution Reaction (HER)…………………………………………………5
1.5 CO2 Reduction Reaction (CO2RR)……………………………………………………6
1.6 Catalyst Design………………………………………………………………………..8
1.7 References……………………………………………………………………..…..…..9
Chapter 2: Electronically-coupled Redox Centers in Trimetallic Cobalt Complexes…………...12
2.1 Introduction…………………………………………………………………………..12
2.2 Results and Discussion…………...………………………………………………….16
2.2.1 Synthesis and Characterization…………………………………………….18
2.2.2 Cyclic Voltammetry and Electrochemical Analysis……………………….27
2.2.3 Visible Spectroscopy and Spectroelectrochemisty…………………..…….35
2.2.4 Computational Studies……………………………………………………..43
2.3 Conclusion…………………………………………………………………………...50
2.5 Experimental Methods……………………………………………………………….53
vii

2.4.1 General……………………………………………………………………..53
2..2 NMR Spectroscopy………………………………………….……………....53
2.4.3 Elemental Analysis………………………………………………………....54
2.4.4 UV/Vis Spectroscopy……………………………………………………....54
2.4.5 Single Crystal X-ray Diffraction…………………………………………...54
2.4.6 X-ray Photoelectron Spectroscopy………………………………………....55
2.4.7 Cyclic and Differential Pulse Voltammetry (CV, DPV)…………………...55
2.4.8 Spectroelectrochemisty…………………………………………………….55
2.4.9 Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT)...56
2.4.10 Synthesis of [Co3(triphos)3(THT)][BF4]3 (1
3+
)…………………………...57
2.4.11 Synthesis of [Co3(triphos)3(BHT)][BF4]3 (2
3+
)…………………………...58
2.4.12 Synthesis of [Co(triphos)(BDT)][BF4] (3
+
) ………………………………58
2.4.13 Coordinates for Optimized Geometry……………………………………..59
2.5 References……………………………………………………………………………67
Chapter 3: Electrocatalytic CO2 Reduction to Formate by a Cobalt
Phosphino-Thiolate Complex………..……………………………………………….72
3.1 Introduction…………………………………………………………………………..72
3.2 Results and Discussion………...……………………………………………….…….80
3.2.1 Cyclic Voltammetry under Inert Conditions …………………..……..…....80
3.2.2 Chemical Reduction Studies……………………………………........….….82
3.2.3 Cyclic Voltammetry under Catalytic Conditions …………………..….......90
3.2.4 Controlled Potential Electrolysis and Product Analysis…...………….…….97
3.2.5 Catalytic Benchmarking and Comparisons…….……………….….……...105
3.2.6 Mechanistic Discussion and Computational Studies…………….……......108
3.3 Conclusion………………………………………………………………….…..…...115
3.4 Experimental Methods……………………………………………………………...116
3.4.1 General……………………………………………………………………116
3.4.2 Single-crystal X-ray Diffraction ……………………...……………….…116
3.4.3 NMR Spectroscopy…………………………………………...…………..116
viii

3.4.4 Variable Temperature NMR Spectroscopy………………….…….……...117
3.4.5 X-ray Photoelectron Spectroscopy ………………………..……………...117
3.4.6 Cyclic Voltammetry (CV)…………………………………………….......117
3.4.7 Controlled Potential Electrolysis…………………………………...……..118
3.4.8 Gas Chromatography……………………………………….……………..119
3.4.9 Formate Detection and Quantification……………………………..…......119
3.4.10 Density Functional Theory (DFT)………………………….....................120
3.4.11 Synthesis of [18-crown-6(K)][Co(triphos)(bdt)] ..……………………....120
3.4.12 Coordinates of DFT-computed structures ………………………………121
3.5 References…………………………………………………………………………..149
Chapter 4: Impact of Ligand Functionalization and Metal Center on the Activity and
Selectivity of a Phosphino-Thiolate Complex Towards CO2 Reduction to Formate…………....159
4.1 Introduction…………………………………………………………………………159
4.2 Results and Discussion………………………………………………….…………..161
4.2.1Synthesis and Characterization of [Co(trihpos)(bdtCl2)]
+

and [Fe(trihpos)(bdt)]
0
……….……………………………………………161
4.2.2 Cyclic Voltammetry under Inert Conditions……………………………...164
4.2.3 Cyclic Voltammetry under Catalytic Conditions…………..……………..170
4.2.4 Controlled Potential Electrolysis and Product Analysis……………...…..175
4.3 Conclusion…………………………………………………………………………..177
4.4 Experimental Method……………………………………………………………….178
4.4.1 General……………………………………………………………………178
4.4.2 Single-Crystal X-ray Diffraction…………………………….……………179
4.4.3 NMR Spectroscopy……………………………………………………….179
4.4.4 Cyclic Voltammetry (CV)………………………………………………...179
4.4.8 Synthesis of [Co(triphos)(bdtCl2)][BF4]·CH3CN…………………………180
4.5 References…………………………………………………………………………..181
Chapter 5: Highly Conjugated Multimetallic Cobalt Phosphino-Thiolate Complexes
towards Electrocatalytic CO2 Reduction……..……………………………..………183
5.1 Introduction……………………………………………………………….…...……183
ix

5.2 Results and Discussion……………………………………………………….……..185
5.2.1 Cyclic Voltammetry in MeCN under Inert Conditions…………..….……185
5.2.2 Cyclic Voltammetry in MeCN under Catalytic Conditions……………....189
5.2.4 Cyclic Voltammetry in DMF under Inert Conditions …………….……...194
5.2.5 Cyclic Voltammetry in DMF under Catalytic Conditions……………..…196
5.2.6 Controlled Potential Electrolysis and Product Analysis……………….….199
5.3 Conclusion……………………………………………………………….………….209
5.4 Experimental Method…………………………………………………….………....210
5.4.1 General……………………………………………………………….…...210
5.4.2 X-ray Photoelectron Spectroscopy………………….………………….…211
5.4.3 Cyclic Voltammetry (CV)…………………………………………….…..211
5.4.4 Controlled Potential Electrolysis…………………………………...……..211
2.4.5 Gas Chromatography……………………………………….………….….212
5.4.6 Formate Detection and Quantification……………………………...…......212
5.4.7 Synthesis of [Co3(triphos)3(THT)]………………………………………..213
4.5 References…………………………………………………………………………..213
Chapter 6: Computational Study on the Effects of Ligand Functionalization and Metal
Identity on the Activity of CO2 Reduction Catalysts………..……...………….……216
6.1 Introduction…………………………………………………………………...….…216
6.2 Results and Discussion……………………………………………………………...219
6.2.1 Computational studies on amine functionalized ReBpy complexes ……..219
6.2.2 Computational studies on the metal identity of a macrocyclic CO2
Reduction catalyst …………………………………...………………………....221
6.3 Conclusion………………………………………………………………………….228
6.4 Experimental Method……………………………………………………………....229
5.4.1 Density Functional Theory (DFT)………... ……….……………..……...229
5.4.2 Coordinates of DFT-computed structures …………………..……………230
6.5 References…………………………………………………………………………..240
Bibliography……………………………………………………………………………………243
x

List of Tables
Table 2.1 Average selected bond lengths (Å) for complex 1
3+
…………………………………...25
Table 2.2 Average selected bond angles (°) for complex 1
3+
……………………………………..26
Table 2.3 The comproportionality constants determined based on the Co
III/II
redox couples
present in complexes 1 and 2 (in MeCN) and those reported for Co
3
Cp
*
3
THT
and Co
3
Cp
*
3
BHT…………………………………………………………………………...……30
Table 2.4. Calculated ΔE1/2 of the Co
III/II
redox couples for complexes 1 and 2 in DMF,
MeCN, and DCM, where ΔE

refers to the difference in the reduction potential of the first and
second redox event (ΔE
a
) and of the second and third redox event (ΔE
b
) for the
respective species………………………………………………………………………..……….32
Table 2.5 λmax
and molar absorptivity values for 1
3+
, 2
3+
, and 3
+
in the visible range…………….36
Table 2.6. IVCT characteristic values for complex 2 in the near-IR range……………………….42
Table 2.7 DFT calculated orbital energies of frontier orbitals of complexes 1
3+
, 2
3+
, and 3
+
…….47
Table 2.8. TD-DFT calculated excited states for complex 3
+
and their associated excitation
energy, wavelength, and oscillator strength…………………………………………………....…48
Table 2.9. Calculated relative contribution of transitions to excited state 4 of complex 3
+
……..48
Table 2.10 Calculated relative contribution of transitions to excited state 2 of complex 3
+
……..49
Table 2.11 Calculated relative contribution of transitions to excited state 1 of complex 3
+
…….50
Table 3.1 Reported overpotentials and relative formate selectivity of active electrocatalysts
towards CO2RR. Table divided between formate selective (>85% FE, top section)
electrocatalysts and non-selective (≤85%, bottom section) electrocatalysts………….…………75
Table 3.2 Average selected bond lengths (Å) for [Co(triphos)(bdt)]
x
complexes, where
x = 1,0, -1………………………………………………………………….……………………..83
Table 3.3 Summary of the controlled potential electrolysis results and the conditions
used for the electrolysis of [Co(triphos)(bdt)]
+
in the presence of CO2 and
a proton source. Electrolyses were performed with 0.45 mM of [Co(triphos)(bdt)]
+

in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2……….…...100
Table 4.1 Average selected bond lengths (Å) for complexes [Co(triphos)(bdt)]
+

and [Co(triphos)(bdtCl
2
)]
+
…………………………………………………………….……...163
Table 4.2 Potentials of observed redox events both [Co(triphos)(bdt)]
+
and [Co(triphos)(bdtCl
2
)]
+
. All potentials are referenced versus Fc
+/0
………………………..165
Table 4.3 Summary of the controlled potential electrolysis results and the conditions
used for the electrolysis of [Co(triphos)(bdtX)]
+
(where X = H, Cl2) and in
the presence of CO2 and a proton source. Electrolyses were performed with
0.45 mM of [Co(triphos)(bdtX
2
)]
+
in a CH3CN solution containing 0.1 M [nBu4N][PF6]
under an atmosphere of CO2……………….……………………………………………..……..176

xi

Table 5.1 Potentials of the observed redox events for [Co(triphos)(THT)]
+
in MeCN and
DMF under an atmosphere of N2. All potentials are referenced versusFc
+/0
.………….……….188
Table 5.2 Summary of the controlled potential electrolysis results and the conditions used
for the electrolysis of [Co
3
(triphos)
3
(bdt)]
3+
in the presence of CO2 and a proton
source. Electrolyses were performed with 0.45 mM of [Co
3
(triphos)
3
(bdt)]
3+
in a CH3CN or
DMF solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2………………….195

Table 6.1 Comparison of selected experimental and calculated bond distances of
functionalized ReBpy complexes …………………….………………………………………..219
Table 6.2 Comparison of selected experimental and calculated bond distances………………223

xii

List of Figures
Figure 1.1 Global energy consumption by source (Left) and a comparison of
global average temperature and global atmospheric CO 2 concentrations (Right)………………...1
Figure 1.2 Relative share of energy sources over the last 10 years..…………………….……….2
Figure 1.3 Schematic representing generation of synthetic fuels through renewable
electricity sources. ………………………………………..……………………………………….3
Figure 1.4 Volumetric and gravimetric capacities of fuels and batteries…………………………3
Figure 1.5 Application of synthetic fuels as a means of energy transportation and export
to areas with low RCN capabilities………………………………………………………………...4
Figure 1.6 Model electrolytic cell for water splitting……………………………………………..6
Figure 1.7 CO2RR products and the associated standard reduction potential vs NHE in
aqueous conditions. List not exhaustive. ………………………………………………..………...7
Figure 1.8 Active sites of [NiFe] dehydrogenase (left), [NiFe] CO dehydrogenase (middle),
and formate dehydrogenase (right)……………………………………………………...………...9
Figure 2.1. Chemdraw illustrations of the dithiolene-based 2D MOFs containing
trinucleating ligands, such as triphenylene-2,3,6,7,10,11-hexathiolate (THT) and
benzene hexathiolate (BHT), and the proposed homogenization strategy of the
cobalt-dithiolene molecular units……………………………………………………..………….13
Figure 2.2. Examples of reported multimetallic complexes as a means of building-block
isolation of the associated MOF…………………………………………………………….……14
Figure 2.3. Chemdraw illustration of the synthesized trimetallic complexes 1
3+
and 2
3+
,
and the monometallic analogue 3
+
studied here…………………………………………………..15
Figure 2.4 500 MHz
1
H NMR spectrum of 1
3+
in dimethylsulfoxide-d6………………………..17
Figure 2.5 202 MHz
31
P-{
1
H} NMR (left) and 470 MHz
19
F-{
1
H} NMR (right) spectrum
of 1
3+
in dimethylsulfoxide-d6…………………….…………………………………………...…18
Figure 2.6 500 MHz
1
H NMR spectrum of 2
3+
in dimethylsulfoxide-d6…………………………19
Figure 2.7 202 MHz
31
P-{
1
H} NMR (left) and 470 MHz
19
F-{
1
H} NMR (right) spectrum
of 2
3+
in dimethylsulfoxide-d6………………………………………………………………...….20
Figure 2.8 500 MHz
1
H NMR spectrum of 3
+
in dimethylsulfoxide-d6…………………….…...20
Figure 2.9 202 MHz
31
P-{
1
H} NMR (left) and 470 MHz
19
F-{
1
H} NMR (right) spectrum
of 2
3+
in dimethylsulfoxide-d6………………………………………………………………….....21
Figure 2.10 XPS survey scan of complex a) 1
3+
, b) 2
3+
, c) 3
+
…………………………………...21
Figure 2.11. High-resolution X-ray photoelectron spectroscopy spectra of the a) Co 2p region
; b) F 1s region; c) B 1s and P 1s region; d) S 2p region; e) P 2p region for complex 1
3+
…..…..22

xiii

Figure 2.12. High-resolution X-ray photoelectron spectroscopy spectra of the a) Co 2p region;
b) F 1s region; c) B 1s and P 1s region; d) S 2p region; e) P 2p region for complex 2
3+
……….....22
Figure 2.13. High-resolution X-ray photoelectron spectroscopy spectra of the a) Co 2p region;
b) F 1s region; c) B 1s and P 1s region; d) S 2p region; e) P 2p region for complex 3
+
………….23
Figure 2.14. Top-down view of the solid-state structure of complex 1
3+
. Aryl and aliphatic
protons, counterions, and solvent molecules are omitted for clarity……………………………...24
Figure 2.15. Top-down view of the solid state structure of 2
3+
. Aryl and aliphatic
protons, counterions, and solvent molecules are omitted for clarity……………………………...24
Figure 2.16 a) CVs of complexes 1–3 (0.5 mM) and b) DPVs of 1–3 (0.5 mM) in MeCN
solutions containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s…….27
Figure 2.17 Cyclic voltammograms of 0.5 mM of a) 1 b) 2 c) 3 in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 10
to 1000 mV/s……………………………………………………………………………………..28
Figure 2.18 Plot of log of current density vs log of the scan rate for a) the first redox
couple of 1 b-d) each of the three redox couples of 2 e) and the redox couple
of 3 in MeCN………………………………………………………………………………….….29
Figure 2.19. Chemdraw illustrations of multi-valent states of complexes 1 and 2
accessible through electrochemical studies………………………………………………...…….30
Figure 2.20. CVs and DPVs of 1 (0.5 mM) (a,c) and 2 (0.5 mM) (b,d) in a solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2 in DMF (red), MeCN
(green), and DCM (blue). Scan rate is 100 mV/s……………………………………..…………..32
Figure 2.21 Cyclic voltammograms of 0.5 mM of 1 in a a) DCM and b) DMF solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 50
to 1000 mV/s……………………………………………………………………………..………34
Figure 2.22. Plot of log of current density vs log of the scan rate for each of the three redox
couple of 1 in a-c) DCM and d) the first redox couple in DMF……………………….…………34
Figure 2.23 Cyclic voltammograms of 0.5 mM of 2 in a a) DCM and b) DMF solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 50
to 1000 mV/s………………………………………………………………………………...…...35
Figure 2.24 Plot of log of current density vs log of the scan rate for each of the three redox
couple of 2 in a-c) DCM and d-f) DMF……………………………………………………….…35
Figure 2.25 Visible absorbance spectra of complexes 1
3+
, 2
3+
, and 3
+
in acetonitrile…………….37
Figure 2.26 UV-vis spectrum of a) 0.025 mM of 1
3+
, b) 0.025 mM of 2
3+
, c) 0.1 mM of
3
+
in acetonitrile………………………………………………………………………...………..37
Figure 2.27 Vis-NIR spectral changes of 1 in a DCM solution of 0.25M [nBu4N][PF6]
under an atmosphere of N2 as potential is cathodically shifted by 25 mV increments…………..38
Figure 2.28 Vis-NIR spectral changes of 2 in a DCM solution of 0.25 M [nBu4N][PF6]
xiv

under an atmosphere of N2 as potential is cathodically shifted by 25 mV increments……..…….39
Figure 2.29 Vis-NIR spectra of the electrochemical reduction of 2
3+
in a DCM solution of
0.25 M [nBu4N][PF6] under an atmosphere of N2: a) 2
3+
to 2
2+
b) 2
2+
c) 2
2+
to 2
+
d) 2
+

e) 2
+
to 2
0
. Solution electrolyzed cathodically by 25 mV increments……………………………40
Figure 2.30 Molecular orbital scheme of complexes 1
3+
, 2
3+
, and 3
+
(left), with the
corresponding orbital character of HOMO and LUMO (right)…………………………………..43
Figure 2.31 DFT calculated frontier orbitals for complex 1
3+
(6-31G*/PBE level of theory)……44
Figure 2.32 DFT calculated frontier orbitals for complex 2
3+
(6-31G*/PBE level of theory)……44
Figure 2.33 DFT calculated frontier orbitals for complex 3
+
(6-31G*/PBE level of theory)……..45
Figure 2.34 DFT calculated HOMO orbitals for complexes 1
3+
, 2
3+
, and 3
+
(6-31G*/PBE
level of theory)…………………………………………………………………...………………45
Figure 2.35 DFT calculated LUMO orbitals for complexes 1
3+
, 2
3+
, and 3
+
(6-31G*/PBE
level of theory)…………………………………………………………………………...………46
Figure 2.36 Origin of orbitals that highly contribute toward transition of excited state 4 in
complex 3
+
……………………………………………………………………………….………49
Figure 2.37 Origin of orbitals that highly contribute toward transition of excited state 2 of
complex 3
+
………………………………………………………………………………….……50
Figure 2.38 Origin of orbitals that highly contribute toward transition of excited state 1 of
complex 3
+
………………………………………………………………………………….……51
Figure 3.1 Chemdraw illustration of the synthesized cobalt triphosphine-thiolate
complex ([Co(triphos)(bdt)]
+
)…………………………………………………………..………79
Figure 3.2 CVs of 0.5 mM of [Co(triphos)(bdt)] in a MeCN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s…………………………80
Figure 3.3 Cyclic voltammograms of 0.5 mM of [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates is 0.1 V/s……..81
Figure 3.4 Cyclic voltammograms of 0.5 mM of [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from
0.1 to 1 mV/s……………………………………………………………………………………..82
Figure 3.5 Solid-state structure of [Co(triphos)(bdt)]
-
. Aryl and aliphatic protons,
counterions, and solvent molecules are omitted for clarity……………………………………….84
Figure 3.6 Variable temperature overlay of: a) 600 MHz
31
P-{
1
H} NMR
spectra of [Co(triphos)(bdt)]
-
in acetonitrile-d3; 600 MHz
1
H NMR spectra of
[Co(triphos)(bdt)]
-
in b) aromatic and c) aliphatic region in acetonitrile-d3.
Temperature varied between 26 and -35 °C……………………………………………………...84
Figure 3.7 600 MHz
31
P-{
1
H} NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at 26 °C.85
xv

Figure 3.8 600 MHz
31
P-{
1
H} NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3
at -35 °C……………………………………………………………………………..…………...85
Figure 3.9 600 MHz
1
H NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at 26 °C……...86
Figure 3.10 600 MHz
1
H NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at -35 °C….87
Figure 3.11 Overlay of 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in
acetonitrile-d3. Temperature varied between 26 and -35 °C…………………………………..…87
Figure 3.12 Overlay of 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in
acetonitrile-d3. Temperature varied between 26 and -35 °C……………………………………..88
Figure 3.12 Overlay of 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in
acetonitrile-d3. Temperature varied between 26 and -35 °C…………………………………...…88
Figure 3.13 CVs of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in
0.2 the presence of 0.3 M H2O (blue) or 0.3 M TFE (green). Scan rate is 100 mV………….……..90
Figure 3.14 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE.
Scan rate is 100 mV/s………………………………………………………………………..…...91
Figure 3.15 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdt)]
+
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under N2 (black) and in the presence of
0.3 M TFE under an atmosphere of N2 (green) and CO2 (blue). Scan rate is 100 mV/s. ………..92
Figure 3.16 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under N2 with increasing concentrations of TFE.
Scan rate is 100 mV/s………………………………………………………………………….....92
Figure 3.17 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] and 0.5 M H2O under an atmosphere of CO2.
Scan rates vary from 0.1, 0.5, and 1 mV/s……………………………………………………….93
Figure 3.18 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O.
Scan rate is 100 mV/s………………………………………………………………………….....94
Figure 3.19 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O.
Scan rate is 500 mV/s………………………………………………………………………..…...95
Figure 3.20 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of D2O.
Scan rate is 500 mV/s………………………………………………………………………..…...95
Figure 3.21 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under N2 (black) and in the presence of 0.3 M H2O
under an atmosphere of N2 (green) and CO2 (blue). Scan rate is 100 mV/s……………………….96
Figure 3.22 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN
xvi

solution containing 0.1 M [nBu4N][PF6] under N2 with increasing concentrations of H2O.
Scan rate is 100 mV/s…………………………………………………………………………….96
Figure 3.23 Comparison of the controlled potential electrolysis results – Faradaic
efficiencies (FE%) and Turnover numbers (TONs) – in the presence of: a) 0.3 M TFE,
0.3 M H2O, or no exogenous proton source (N/A) at -2.15 V vs Fc/Fc
+
, b) 0.3 M H2O at
potentials of -2.15 or -2.60 V vs Fc/Fc
+
, and c) 0.3 or 0.6 M H2O at -2.15 V vs Fc/Fc
+
.
All electrolyses were performed with 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2……………………….....98
Figure 3.24 Controlled potential electrolysis traces measured under 1 atm of CO 2. In all
cases a solution of 0.45 mM [Co(triphos)(bdt)]
+
in acetonitrile with 0.1 M
[nBu4N][PF6] supporting electrolyte was held at a potential of either -2.15 V or -2.60 V
vs Fc/Fc
+
for 2 hours in the presence of no added proton source (grey), 0.3 M TFE (red),
0.3 M H2O (green), or 0.3 M H2O (blue and purple). A control experiment was also
conducted (black) where electrolysis was performed at -2.15 V vs Fc/Fc
+
for 2 hours
in the presence of 0.3 M H2O in the absence of catalyst. Inset displays current in first
few minutes of the electrolysis experiments…………………………………………………….99
Figure 3.25 Controlled potential electrolysis traces measured under 1 atm of CO 2 in an
acetonitrile solution of 0.45 mM [Co(triphos)(bdt)]
+
with 0.1 M [nBu4N][PF6]
supporting electrolyte, which was held at a potential of -2.15 V vs Fc/Fc
+
for 8 hour in
the presence of 0.3 M H2O. Electrolysis was paused at the 4 hr mark to adjust for potential
drift of the Ag/Ag
+
pseudo reference electrode………………………..………………………...102
Figure 3.26 Cyclic voltammograms of a bare glassy carbon electrode (black – labeled
“bare GCE”) and a washed post-electrolysis glassy carbon electrode (red – labeled
“wash test”) in an acetonitrile solution containing 0.1 M [nBu4N][PF6] and 0.3 M H2O
under an atmosphere of CO2. The post-electrolysis glassy carbon electrode (wash test)
investigated here was generated upon performing an electrolysis with a glassy carbon
electrode for 8 hours at -2.15 V under CO2 and in a CH3CN solution with
0.45 M of [Co(triphos)(bdt)]
+
, 0.3 M H2O, and 0.1 M [nBu4N][PF6], and subsequently
washed with clean CH3CN under anaerobic conditions to prevent O 2 exposure
of the electrode……………………………………………………………………………….....103
Figure 3.27 Comparison of the controlled potential electrolysis trace of a washed
post-electrolysis glassy carbon electrode in an acetonitrile solution containing 0.1 M
[nBu4N][PF6] (black – labeled “rinse test”) to that of the CPE trace of
0.45 M of [Co(triphos)(bdt)]
+
in MeCN with 0.1 M [nBu4N][PF6] (red). Both electrolysis
in this figure were performed at a potential of -2.15 V vs Fc/Fc
+
for 2 hours in the presence
of 0.3 M H2O and 1 atm of CO2. The post-electrolysis glassy carbon electrode
(rinse test) investigated here was generated upon performing an electrolysis with a glassy
carbon electrode for 8 hours at -2.15 V under CO2 and in a CH3CN solution with
0.45 M of [Co(triphos)(bdt)]
+
, 0.3 M H2O, and 0.1 M [nBu4N][PF6], and subsequently
washed with clean CH3CN under anaerobic conditions to prevent O2 exposure
of the electrode………………………………………………………………………….………104
Figure 3.28 High-resolution X-ray photoelectron spectroscopy spectra of 1) Co 2p and 2)
S 2p region of a a) washed post-electrolysis glassy carbon electrode, b) electrode immersed in
xvii

0.45 M acetonitrile solution of [Co(triphos)(bdt)]
+
without applying any potential, and c)
bare electrode. Peak at ~169 eV in the S 2p region is associated with the Si 2p plasmon
loss structure from residual Si associated with Si-based polishing powder. The
post-electrolysis glassy carbon electrode investigated here was generated upon
performing an electrolysis with a glassy carbon electrode for 8 hours at -2.15 V under
CO2 and in a CH3CN solution with 0.45 M of [Co(triphos)(bdt)]
+
, 0.3 M H2O, and
0.1 M [nBu4N][PF6], and subsequently washed with clean CH 3CN……………………………105
Figure 3.29 Calculated relative energies of potential IV-CO
2
isomers at the 6-31+G*
/B3LYP level of theory………………………………………..………………………………...109
Figure 3.30 Calculated optimized structure of IV-CO
2
at the 6-31G*/B3LYP
level of theory. Phenyl substituents removed for clarity…………………………………...……110
Figure 3.31 Calculated relative energies of potential IV-H isomers at the 6-31+G*
/B3LYP level of theory………………………………………………………………….……...111
Figure 3.32 Calculated optimized structure of IV-H at the 6-31G*/B3LYP level of theory.
Phenyl substituents removed for clarity………………………………………………………...111
Figure 3.33 Calculated relative energies of potential [Co-SH]
0
isomers at the 6-31+G*
/B3LYP level of theory……………………………………………………………………..…..112
Figure 3.34 Calculated relative energies of IV-H and [Co-SH]
0
at the 6-31+G*/B3LYP
level of theory…………………………………………………………………………………...112
Figure 3.35 Calculated optimized structure of V at the 6-31G*/B3LYP level of theory.
Phenyl substituents removed for clarity………………………………………………………...113
Figure 3.36 Calculated optimized structure of VI at the 6-31G*/B3LYP level of theory.
Phenyl substituents removed for clarity………………………………………………………...115
Figure 4.1 Chemdraw illustration of the synthesized cobalt triphosphine-thiolate
complex ([Co(triphos)(bdt)]
+
), [Co(triphos)(bdtCl
2
)]
+
, [Fe(triphos)(bdt)]
+
…….………….160
Figure 4.2 500 MHz
1
H NMR spectrum of [Co(triphos)(bdtCl
2
)]
+
in DMSO-d6…………….161
Figure 4.3 202 MHz
31
P-{
1
H} NMR spectrum of [Co(triphos)(bdtCl
2
)]
+
in acetonitrile-d6….162
Figure 4.4 Solid-state structure of [Co(triphos)(bdtCl
2
)]
-
. Aryl and aliphatic protons,
counterions, and solvent molecules are omitted for clarity……………………………………...162
Figure 4.5 Visible absorbance spectra of complexes [Co(triphos)(bdt)]
+

and [Co(triphos)(bdtCl
2
)]
+
in acetonitrile…………………………………………..………….164
Figure 4.6 CVs of 0.5 mM of [Co(triphos)(bdtCl
2
)] in a MeCN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s………………..………166
Figure 4.7 Normalized CVs of [Co(triphos)(bdt)] and [Co(triphos)(bdtCl
2
)] in a MeCN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s……………166
Figure 4.8 Cyclic voltammograms of 0.5 mM of [Co(triphos)(bdtCl
2
)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from
xviii

a) 0.05 to 0.2 mV/s and b) 0.05 and 1V/s………………………………………………………..167
Figure 4.9 Plot of log of current density vs log of the scan rate for both
the [Co(triphos)(bdtCl
2
)]
+/0
redox couple in MeCN………………………………..…………168
Figure 4.10 CVs of 0.5 mM of [Fe(triphos)(bdt)]
0
in a MeCN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s……………………..…..169
Figure 4.11 Cyclic voltammograms of 0.5 mM of [Fe(triphos)(bdt)]
0
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 0.1
to 1 mV/s………………………………………………………………………………………..169
Figure 4.12 Plot of log of current density vs log of the scan rate for both the
[Fe(triphos)(bdt)]
+/0
and the [Fe(triphos)(bdt)]
0/-
redox couple in MeCN……………………170
Figure 4.13 CVs of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the
0.2 presence of 0.3 M H2O (blue). Scan rate is 100 mV……………………………………...…..171
Figure 4.14 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdtCl
2
)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2. Scan rates vary from
0.05 to 1 mV/s……………………………………………………………………………….….171
Figure 4.15 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdtCl
2
)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O.
Scan rate is 100 mV/s………………………………………………………………………..….172
Figure 4.16 CVs of 0.45 mM of [Co(triphos)(bdtCl
2
)]
+
(black) and [Co(triphos)(bdt)]
+

(green) in a CH3CN solution containing 0.1 M [nBu4N][PF6] CO2 and in the presence of
0.3 M H2O. Scan rate is 100 mV………………………………………………………………...173
Figure 4.17 CVs of 0.45 mM of [Fe(triphos)(bdt)]
0
in a CH3CN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the
0.2 presence of 0.3 0.3 M TFE (green)……………………………………………………….…174
Figure 4.18 Cyclic voltammograms of 0.45 mM [Fe(triphos)(bdt)]
0
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE.
Scan rate is 100 mV/s………………………………………………………………………..….175
Figure 4.19 Cyclic voltammograms of 0.45 mM of [Fe(triphos)(bdt)]
0
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] and 0.9 M H2O under an atmosphere of CO2.
Scan rates vary from 0.1, 0.5, and 1 mV/s……………………………………………………..175
Figure 4.20 Controlled potential electrolysis traces measured under 1 atm of CO 2 in an
acetonitrile solution of 0.45 mM [Co(triphos)(bdtCl
2
)]
+
with 0.1 M [nBu4N][PF6]
supporting electrolyte, which was held at a potential of -2.15 V vs Fc/Fc
+
for 2 hour in the
presence of 0.3 M H2O. ……………………………………………………………………..…..177
Figure 5.1 Modulation of ligand scaffold of a cobalt pyridyldiimine complex and its
effect on operational overpotential and activity towards the CO 2RR…………………………..184
xix

Figure 5.2 Chemdraw illustration of the synthesized cobalt triphosphine-thiolate
complex ([Co(triphos)(bdt)]
+
) and analogous trimetallic complex ([Co
3
(triphos)
3
(tht)]
3+
)….185
Figure 5.3 CVs of 0.5 mM of [Co
3
(triphos)
3
(tht)]
3+
in a MeCN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 500 mV/s………………………..187
Figure 5.4 CVs of 0.5 mM of [Co
3
(triphos)
3
(tht)]
3+
in a MeCN solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s…………………………187
Figure 5.5 Cyclic voltammograms of 0.5 mM of [Co
3
(triphos)
3
(tht)]
3+
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2.
Scan rates vary from 0.1 and 0.75V/s……………………………………………………..…….188
Figure 5.6 CVs of 0.45 mM of [Co
3
(triphos)
3
(tht)]
3+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red),
and under CO2 in the presence of 0.3 M H2O (blue) or 0.3 M TFE (green).
Scan rate is 100 mV…………………………………………………………………………..…189
Figure 5.7 CVs of 0.45 mM of [Co
3
(triphos)
3
(tht)]
+3
(CoTHT, blue) and
[Co(triphos)(bdt)]
+
(CoBDT, purple) in a CH3CN solution containing 0.1 M
[nBu4N][PF6] CO2 and in the presence of a)0.3M TFE b)0.3 M H2O.
Scan rate is 100 mV……………………………………………………………………….…….190
Figure 5.8 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing
concentrations of TFE. Scan rate is 100 mV/s………………………………………………….191
Figure 5.9 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in
a CH3CN solution containing 0.1 M [nBu4N][PF6] under CO2 (solid trace) and
N2 (dashed trace) with 0M TFE (black), 0.1 M TFE (red) and 0.3M TFE (blue).
Scan rate is 100 mV/s…………………………………………………………………...………191
Figure 5.10 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing
concentrations of TFE. Scan rate is 100 mV/s……………………………………….………….192
Figure 5.11 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in
a CH3CN solution containing 0.1 M [nBu4N][PF6] under CO2 (solid trace) and
N2 (dashed trace) with 2 M TFE (green) and 4M TFE (purple).
Scan rate is 100 mV/s……………………………………………………………………….…..192
Figure 5.12 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing
concentrations of H2O. Scan rate is 100 mV/s………………………………………………….193
Figure 5.13 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under CO2 (solid trace) and
N2 (dashed trace) with 0M TFE (black), 0.1 M TFE (red) and 0.3M TFE (green).
Scan rate is 100 mV/s…………………………………………………………………...………194
Figure 5.14 CVs of 0.45 mM of [Co
3
(triphos)
3
(tht)]
3+
in a MeCN solution
xx

containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 500 mV/s……………..195
Figure 5.15 Cyclic voltammograms of 0.45 mM of [Co
3
(triphos)
3
(tht)]
3+
in
a CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2.
Scan rates vary from 0.01 and 1V/s……………………………………………………………..195
Figure 5.16 Sequential cyclic voltammograms of 0.45 mM of [Co
3
(triphos)
3
(tht)]
3+

in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2.
Scan rates is 200 mV/s. Electrode was not polished between scans…………………….………196
Figure 5.17 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in
a DMF solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing
concentrations of TFE. Scan rate is 100 mV/s. Working electrode was
not polished between scans……………………………………………………………………..197
Figure 5.18 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(tht)]
+3
in a
DMF solution containing 0.1 M [nBu4N][PF6] under CO2 with increasing
concentrations of TFE. Scan rate is 100 mV/s. Working electrode was
polished between scans……………………………………………………………………..…..198
Figure 5.19 CVs of 0.45 mM of [Co
3
(triphos)
3
(tht)]
3+
in a) MeCN and b) DMF
solution containing 0.1 M [nBu4N][PF6] under an atmosphere CO2 in the presence
of 0.3 M H2O. Scan rate is 100 mV…………………………………………………………..…198
Figure 5.20 Comparison of the controlled potential electrolysis results –
Faradaic efficiencies (FE%) in the presence of 0.3 M TFE, 0.03 M TFE,
or no exogenous proton source (N/A) at a) -2.10 V or b) -2.60 V vs Fc/Fc
+
.
All electrolyses were performed with 0.45 mM of [Co
3
(triphos)
3
(bdt)]
3+
in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2…………….200
Figure 5.21 Comparison of the controlled potential electrolysis results –
Turnover numbers (TONs) in the presence of 0.3 M TFE, 0.03 M TFE, or no
exogenous proton source (N/A) at a) -2.10 V or b) -2.60 V vs Fc/Fc
+
.
All electrolyses were performed with 0.45 mM of [Co
3
(triphos)
3
(bdt)]
3+
in a
CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2………………...201
Figure 5.22 Controlled potential electrolysis traces of 0.45 mM
[Co
3
(triphos)
3
(bdt)]
3+
in acetonitrile with 0.1 M [nBu4N][PF6] measured under
1 atm of CO2 in the presence of 0.03M TFE at 2.60 V vs Fc/Fc
+
for 1.5 hours…………..……..202
Figure 5.23 Controlled potential electrolysis traces measured under 1 atm of CO 2.
In all cases a solution of 0.45 mM [Co
3
(triphos)
3
(bdt)]
3+
in acetonitrile with 0.1 M
[nBu4N][PF6] supporting electrolyte was held at a potential of either -2.15 V or -2.60 V
vs Fc/Fc
+
for 30 minutes in the presence of no added proton source (red, yellow),
0.03 M TFE (blue, green), 0.3 M TFE (violet, magenta)………………………………………...202
Figure 5.24 Cyclic voltammograms of a bare glassy carbon electrode (black –
labeled “bare GCE”) and a washed post-electrolysis glassy carbon electrode (red –
labeled “wash test”) in an acetonitrile solution containing 0.1 M [nBu4N][PF6] and
0.3 M TFE under an atmosphere of CO2. The post-electrolysis glassy carbon electrode
xxi

(wash test) investigated here was generated upon performing an electrolysis with a
glassy carbon electrode for 30 minutes at -2.60 V under CO2 and in a CH3CN solution
with 0.45 M of [Co
3
(triphos)
3
(bdt)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6], and
subsequently washed with clean CH3CN under anaerobic conditions to prevent O 2
exposure of the electrode…………………………………………………………………….….204
Figure 5.25 Cyclic voltammograms of a bare glassy carbon electrode (black –
labeled “bare GCE”) and a washed post-electrolysis glassy carbon electrode (red –
labeled “wash test scan x”, where x = First-Fifth) in an acetonitrile solution
containing 0.1 M [nBu4N][PF6] and 0.3 M TFE under an atmosphere of CO 2. The
post-electrolysis glassy carbon electrode (wash test) investigated here was
generated upon performing an electrolysis with a glassy carbon electrode for
30 minutes at -2.60V under CO2 and in a CH3CN solution with 0.45 M of
[Co
3
(triphos)
3
(bdt)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6], and subsequently
washed with clean CH3CN under anaerobic conditions to prevent O 2 exposure
of the electrode. ………………………………………………………………………………...205
Figure 5.26 High-resolution X-ray photoelectron spectroscopy spectra of Co 2p
region of a washed post-electrolysis glassy carbon electrode. The post-electrolysis
glassy carbon electrode investigated here was generated upon performing an electrolysis
with a glassy carbon electrode for 30 minutes at -2.60 V under CO2 and in a CH3CN
solution with 0.45 M of [Co
3
(triphos)
3
(bdt)]
3+
, 0.3 M TFE, and 0.1 M
[nBu4N][PF6], and subsequently washed with clean CH 3CN……………………………..…….206
Figure 5.27 UV-vis spectrum of a MeCN solution of [Co
3
(triphos)
3
(bdt)]
3+

(As-Prepared CoTHT) and a sample of the electrolyte solution from the working
compartment after CPE (Post-Electrolysis). The post-electrolysis solution investigated
here was generated upon performing an electrolysis with a glassy carbon electrode for
30 minutes at -2.60V under CO2 and in a CH3CN solution with 0.45 M of
[Co
3
(triphos)
3
(bdt)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6]…………………………………..206
Figure 5.28 Controlled potential electrolysis traces measured under 1 atm of CO 2.
In all cases a solution of 0.45 mM [Co
3
(triphos)
3
(bdt)]
3+
in dimethylformamide
with 0.1 M [nBu4N][PF6] supporting electrolyte was held at a potential of -2.60 V vs
Fc/Fc
+
for 2 hours in the presence of 0.3M TFE……………………...………………………….208
Figure 5.29 Cyclic voltammograms of a bare glassy carbon electrode (black –
labeled “bare GCE”) and a washed post-electrolysis glassy carbon electrode (blue –
labeled “wash test”) in an acetonitrile solution containing 0.1 M [nBu4N][PF6] and
0.3 M TFE under an atmosphere of CO2. The post-electrolysis glassy carbon electrode
(wash test) investigated here was generated upon performing an electrolysis with a
glassy carbon electrode for 2 hours at -2.60 V under CO2 and in a DMF solution with
0.45 M of [Co
3
(triphos)
3
(bdt)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6], and subsequently
washed with clean CH3CN under anaerobic conditions to prevent O 2 exposure
of the electrode……………………………………………………………………………….....209
Figure 6.1 Chemdraw illustration of the studied rhenium bipyridine amended
with NH2 substituents on bipyridine backbone…………………………………………………217
xxii

Figure 6.2 Chemdraw illustration of the studied aminopyridine metal complex (ML
1
)……….218
Figure 6.3 Molecular orbital scheme of complexes 4,4′-NH2-Re, 5,5′-NH2-Re,
6,6′-NH2-Re (left), with the corresponding orbital images of HOMO and
LUMO (right). Calculations were performed using the M06 functional with the
6-311G* basis set for H, C, N, and O atoms and the LANL2DZ effective core potential
and basis set for Cl and Re atoms……………………………………………………………….220
Figure 6.4 Calculated relative energy of spin states for [FeL
1
]
+2
at the 6-31+G*/B3LYP
level of theory………………………………………………………………………………......221
Figure 6.5 a) Computed and b)experimental UV/Vis spectra for [FeL
1
]
2+
…………………….222
Figure 6.6 a) Computed and b)experimental UV/Vis spectra for [NiL
1
]
2+
…………………….223
Figure 6.7 Molecular orbital scheme of complexes [FeL
1
]
0
(left), with the corresponding
orbital character of HOMO, HOMO
-1
, and LUMO (right) at the 6-31+G*/B3LYP
level of theory…………………………………………………………………………………...224
Figure 6.8 Molecular orbital scheme of complexes [NiL
1
]
0
(left), with the corresponding
orbital character of HOMO, HOMO
-1
, and LUMO (right) at the 6-31+G*/B3LYP l
evel of theory……………………………………………………………………………………225
Figure 6.9 Optimized structure of [NiL
1
]
+2
(left) and [NiL
1
]
0
. Solvent molecules
removed for clarity……………………………………………………………………………...225
Figure 6.10 Molecular orbital scheme of complexes [CoL
1
]
0
(left), with the corresponding
orbital character of HOMO, HOMO
-1
, and LUMO (right) at the 6-31+G*/B3LYP
level of theory…………………………………………………………………………………...226
Figure 6.11 High-resolution X-ray photoelectron spectroscopy spectra of Ni 2p and of a
washed post-electrolysis glassy carbon electrode and bare electrode. The post-electrolysis
glassy carbon electrode investigated here was generated upon performing an electrolysis
with a glassy carbon electrode for 2 hours at -2.90 V under CO2 and in a DMF solution
with 1.3 M TFE and 0.1 M [nBu4N][PF6], and subsequently washed with clean DMF………….227
Figure 6.12 Molecular orbital scheme of complexes [FeL
1
]
0
, [CoL
1
]
0
, and [NiL
1
]
0

at the 6-31+G*/B3LYP level of theory………………………………………………………….228

xxiii

List of Schemes
Scheme 2.1. Synthetic procedure for complexes 1
3+
and 2
3+
………………………….…………16
Scheme 3.1 Uses of formate/formic acid towards energy storage and conversion……………….70
Scheme 3.2 Reported pathways of electrocatalytic CO 2RR to formate………………………….74
Scheme 3.3 Synthetic procedure for the reduction of [Co(triphos)(bdt)]
0
to
[Co(triphos)(bdt)]
-
………………………………………………………………………………83
Scheme 3.4 Proposed mechanism for electrocatalytic CO 2RR to HCOO
-

Employing [Co(triphos)(bdt)]
+
…………………..……………...……………………………..108

xxiv

Abbreviations
Abbreviation Meaning

MV Multi-valent
IVCT Inter-Valence Charge Transfer
MOF Metal-Organic Framework
DFT Density Functional Theory
TD-DFT Time Dependent Density Functional Theory
CPE Controlled-Potential Electrolysis
CV Cyclic Voltammetry
ECEC Electronic-Chemical-Electronic-Chemical Steps
CO
2
RR CO2 Reduction Reaction
HER Hydrogen Evolution Reaction
OER Oxygen Reduction Reaction
NMR Nuclear Magnetic Resonance Spectroscopy
UV-Vis Ultraviolet-Visible
VT Variable Temperature
TON Turnover Number

FE% Faradaic Efficiency
RCN Renewable Carbon-Neutral
XPS X-ray Photoelectron Spectroscopy
HOMO Highest Occupied Molecular Orbital
LUMO Lowest Occupied Molecular Orbital
SOMO Singly Occupied Molecular Orbital
xxv

KIE Kinetic Isotope Effect
MeCN,CH
3
CN Acetonitrile
DMSO Dimethyl sulfoxide
TFE 2,2,2-Trifluoroethanol
THF Tetrahydrofuran
DCM Dichloromethane
DMF N,N-Dimethylformamide

xxvi

Abstract

With increases global population projected, an ever-increasing energy supply will be
needed to satiate coupled energy demand. To combat this issue, generating energy from renewable
carbon-neutral (RCN) sources has been proposed to compensate for an every-increasing demand
for energy while avoiding long term ecological issues associated with anthropogenic combustion
of fossil fuels. Implementation of renewable energy sources is limited, due to their intermittent
nature, in addition to major energy-consuming sectors such as transportation heavy reliance on
non-renewable liquid-based fuels. Synthetic fuels are novel energy storage platform where
electricity derived from renewable energy sources is used to electrochemically convert abundant
small molecules to value-added products, in which energy is stored in the form of chemical bonds.
These products can then be employed as a means of energy storage during peak renewable energy
supply and employed towards the liquid-dependent transportation sector, while also providing a
framework for large scale energy commodization and provide a renewable source for chemical
feedstocks.
To effectively implement this strategy at the required scales necessary for sustainable
applications, a key research goal is to develop Earth-abundant electrocatalysts that drive these
conversion efficiently at high rates under mild conditions. In biological settings, evolution has
provided highly efficient catalytic systems that can efficiently produce the targeted products under
mild conditions while employing earth-abundant components. As a result, research into the
incorporation of common structural motifs found in enzymatic active sites into synthetic catalytic
systems demands attention as means to produce similarly active Earth-abundant catalytic systems.

xxvii

In this dissertation, several electrocatalytic systems towards electrochemical H 2 production
and CO2 reduction were developed and/or further studied. Chapter 1 presents a general overview
of global energy demand and supply, energy storage through synthetically derived fuels,
fundamentals of both hydrogen evolution and CO 2 reduction, and catalysts design principles
incorporating design elements from enzymatically-derived systems. In Chapter 2, building blocks
of a series of cobalt-dithiolene derived metal organic frameworks active towards electrocatalytic
hydrogen evolution are isolated as homogenous multimetallic complexes using a phosphine
capping unit. These complexes are further studied using electrochemical means to further
investigate the electronic coupling between redox active sites present in the analogous materials.
In Chapter 3, a cobalt phosphino-thiolate complex is investigated as a selective electrocatalyst
towards CO2 reduction to formate. Employing thiolate-based moieties similarly observed in
enzymatic active sites, this complex exhibits robust stability, with formate selectivities as high as
91%. Chapter 4 explores functionalization of the core cobalt phosphino-thiolate complex through
modification of the ligand scaffold and variation of the metal center identity as a means of tuning
electrocatalytic activity and selectivity. Chapter 5 investigates utilizing a highly-conjugated
multimetallic cobalt phosphino-thiolate complex towards CO2RR. Lastly, Chapter 6 employs
computational techniques to further study the effects of ligand functionalization and metal identity
on established CO2 reduction electrocatalysts.

Chapter 1: General Information
1.1 The Global Energy Overview: Outlook and Challenges
Global populations are projected to increase to 9.7 billion by the year 2050 and 11.2 billion
by the year 2100.
1
With this increase global population, an ever increasing energy supply will be
needed to satiate coupled energy demand, with demand estimated to hit 26 TW by 2040.
2
As of
2020, 88% of our energy budget relies on non-renewable fossil fuel-based sources including
petroleum, natural gas, and coal. Of the 12% of energy sources sourced from renewable sources,
39% originate from the burning of biomass (biofuels, wood, etc).
3
These energy sources can be
problematic due to combustion products primarily acting as strong greenhouse gases in the
atmosphere, which has been associated with increases in global average temperatures of ~1 °C
since the pre-industrial era, and other issues including rising ocean acidity and acid rain.
4–6

Figure 1.1 Global energy consumption by source (Left) and a comparison of global average
temperature and global atmospheric CO2 concentrations (Right). Reprinted with permission from
Ref 3 and 4.
To combat this issue, generating energy from renewable carbon-neutral (RCN) sources has
been proposed to compensate for an every-increasing demand for energy while avoiding long term
ecological issues associated with anthropogenic combustion of fossil fuels. This shift towards
RCNs can be illustrated in an increase in grid-level capacity from these sources over the last 20
years, though issues with full grid-wide conversion has been limiting.
7

2

Figure 1.2 Relative share of energy sources over the last 10 years. Reprinted with permission from
Ref 7.
One notable limiting factor of most renewable energy sources, especially in the case of solar and
wind sourced energy, is its intermittent nature. Specifically, while daiy insolation often peaks
during midday, this often does not align with peak grid-level demand, centered when activity is
highest in the home (mornings and evenings).
8
Additionally, wind generation can be difficult to
predict and peak wind energy supply often occurs during overnight hours when energy demand is
quite low.
9
To combat these issues, the storage of renewable energy during peak supply has been
proposed to mitigate this spatio-temporal demand mismatch.
1.2 Synthetic Fuels
Synthetic fuels are novel energy storage platform compared to current energy storage
technologies. In this strategy, using electricity derived from RCN energy sources, abundant small
molecules, such as H2O, CO2, and N2, are electrochemically converted to value-added products,
such as H2, CO, HCOO
-
, NH3, etc., where energy is stored in the form of chemical bonds.
10–12

3

Figure 1.3 Schematic representing generation of synthetic fuels through renewable electricity
sources. Reprinted with permission from Ref 10.
This method of electro-synthetically derived fuels has several benefits over contemporary
storage technologies. In terms of overall efficiency, chemical fuels display superior gravimetric
and volumetric capacities compared to popular alternatives such as lithium-ion batteries.
13

Figure 1.4 Volumetric and gravimetric capacities of fuels and batteries. Reprinted with
permission from Ref 13.
This issue in scalability will likely limit battery applications in grid-level storage compared to
synthetic fuels. Alternatively, synthetic fuels provide other benefits in RCN energy sources
4

viability in the transport sector. The transportation sector is the major consumer of energy, with
34% of the energy produced in California going to this sector in 2020.
14
Particularly, a majority of
RCN sources produce electricity, which is ill-suited to transportation industries that are primarily
dependent on liquid fuel.
11
To bridge this gap, synthetic fuels derived from these RCN sources can
be directly implemented in conventional internal-combustion engines or newly developed fuel
cells to help convert this relatively slow-adopting sector to become more renewable energy reliant.
Moreover, the long charging times, short driving range, and reduced cargo and passenger space
due to volumetric capacity of batteries make this sector more suitable to synthetic fuel
applications.
15
Synthetic fuel can be also considered a medium for energy transportation itself. In
areas of net surplus of RCN energy generation, production of synthetic fuels can be used as a
means of energy export to regions with low RCN capabilities.

Figure 1.5 Application of synthetic fuels as a means of energy transportation and export to areas
with low RCN capabilities Source: The Commonwealth of Australia.
In Australia, the Austalian Renewable Energy Agency has proposed a mechanism of energy
export through the conversion of RCNs to natural gas and ammonia and its export to other East
Asian countries such as Korea and Japan.
16
This method of surplus export can be help supply
regions with either little RCN sources availability or significant industrial development to
5

transition local energy consumption to renewable sources. As a result, fundamental research into
catalytic systems that can drive these reactions efficiently can help with implementation of this
technology in real-life scenarios.
1.3 Renewable Chemical Feedstocks
Another particular application of electrochemical conversion of abundant small molecules
to value added products is the renewable generation of industrially relevant chemical feedstocks.
With a wide range of chemical applications due to the scope of hydrogenation chemistry, H 2 sees
use in ammonia production, Fisher-Tropsch chemistry, and as a reagent in food and medicinal
applications.
17
Moreove, C1 building blocks are essential to the synthesis of industrially important
oragnic compounds that intergral to current day economies, with examples including CO, which
can be used in industrial relevant Fischer-Tropsch and carbonylation conversions, and methanol
which can be employed in methanol to gasoline and olefin processes.
18
Modern production of H2
and various C1 building blocks are produced primarily through non-renewable sources, indluding
H2 and CO (Syn Gas) production through steam reforming of fossil fuel-drived methane, and
which then can be employed to produce methanol using copper and zinc oxide catalysts.
19,20
Due
to inherent reliance on non-renewable resources to generate these essential chemical feedstocks,
developing new methods of synthsizing these building blocks from naturally abundant small
molecules through electrochemical means will be beneficial in employing renewable energy
sources to these conversions in the future.
1.4 Hydorgen Evolution Reaction (HER)
H2 generation electrochemically can be considered one half reaction in the overall
electrochemical water splitting process (Equation 3):
6

4𝐻 +
+ 4𝑒 −
→ 2𝐻 2
(1)
2𝐻 2
𝑂 → 4𝐻 +
+ 𝑂 2
+ 4𝑒 −
(2)
𝑂𝑣𝑒𝑟𝑎𝑙𝑙 : 2𝐻 2
𝑂 → 2𝐻 2
+ 𝑂 2
𝐸 0
= 1.23 𝑉 (3)
where Equation 1 represent the hydrogen evolution reaction (HER), and Equation 2 represent the
oxygen evolution reaction (OER). Unlike previously discussed methods that rely on non-
renewable fossil fuels, water electrolysis utilizes abundant H 2O and only discharges gaseous
oxygen as a byproduct, another abundant compound in our environment.

Figure 1.6 Model electrolytic cell for water splitting. Reprinted with permission from Ref 21.
In a model electrolytic cell (Figure 1.6), external power supply applied to the cell, where upon
OER is performed at the anode. Protons then migrate through a polymer electrolyte member where
they are subsequently combined with electrons to evolve H 2.
21
Water electrolysis requires an
operating potential of at least 0 V vs SHE as a thermodynamic driving force for the conversion,
but often catalyst must be used to limit potential kinetic barriers and allow operating potentials to
approach those of the thermodynamic barrier. This additional driving force, what is defined as the
overpotential, is the difference between the operating potential of the reaction that meets a certain
rate or selectivity versus the potential at which the reaction can be driven according to purely
7

thermodynamic principles. For HER, platinum is often used as the industry standard catalyst as it
can drive significant catalytic currents close to the thermodynamic barrier with minimal
overpotentials.
22
However, platinum is among a class of non-Earth abundant noble metals that are
not sufficiently abundant to facilitate large scale applications. As a result, developing catalytic
systems employing Earth-abundant components that can effectively compare to the activity and
efficiency of platinum is of upmost importance if H 2 generation is to be implemented at scales as
an effective synthetic fuel and feedstock alternative.
1.5 The CO
2
Reduction Reaction (CO
2
RR)
The CO2 reduction reaction can yield to various products depending on electrolytic
conditions. The direct 1e
-
reduction of CO2 generates the bent CO2 radical anion. Due to the large
degree of reorganization energy from the linear CO 2 molecule to the bent radical anion, this
transformation is quite thermodynamically unfavorable, with a standard reduction potential of
-1.90V (Equation 1).
18

𝐶 𝑂 2
+ 𝑒 −
→ 𝐶 𝑂 2
.−
𝐸 0
= −1.90 𝑉 ( 𝟏 )
𝐶 𝑂 2
+ 2𝐻 +
+ 2𝑒 −
→ 𝐶𝑂 + 𝐻 2
𝑂 𝐸 0
= −0.53 𝑉 ( 𝟐 )
𝐶 𝑂 2
+ 2𝐻 +
+ 2𝑒 −
→ 𝐻𝐶 𝑂 2
𝐻 𝐸 0
= −0.61 𝑉 ( 𝟑 )
𝐶 𝑂 2
+ 4𝐻 +
+ 4𝑒 −
→ 𝐻𝐶𝐻𝑂 + 𝐻 2
𝑂 𝐸 0
= −0.48 𝑉 (4)
𝐶 𝑂 2
+ 6𝐻 +
+ 6𝑒 −
→ 𝐶 𝐻 3
𝑂𝐻 + 𝐻 2
𝑂 𝐸 0
= −0.38 𝑉 ( 𝟓 )
𝐶 𝑂 2
+ 8𝐻 +
+ 8𝑒 −
→ 𝐶 𝐻 4
+ 2𝐻 2
𝑂 𝐸 0
= −0.24 𝑉 ( 𝟔 )
Figure 1.7 CO2RR products and the associated standard reduction potential vs NHE in aqueous
conditions. List not exhaustive.
As a result, coupling reduction of CO2 with the addition of protons can lead to more mild reduction
potentials and can produce a variety of potential reduction products (Equations 2-6).
Accompanying these multielectron proton coupled reactions is the significant kinetic barriers
8

associated with the multitude of bond formation and breaking steps which can lead to sluggish
reaction rates, necessitating significant overpotentials to drive these conversions at reasonable
rates.
18
Moreover, in addition to the variability in the CO 2RR product formation, due to the
presence of protons in solution and the relatively large negative potentials needed to drive these
reaction, competitive HER is a common competitive faradaic process that can limit the selectivity
of these processes. In order to implement the CO 2RR towards grid-scale synthetic fuel generation,
it is necessary to develop catalysts that can carry out these transformations with high selectivities
and low overpotentials without compromising reaction rates. Current heterogeneous catalysts for
the CO2RR are often limited due to their reliance on precious metals, such as Ag and Au, toxic
metals, such as Hg and Pb, and metals that display low selectivity, such as with Cu, and ill-defined
surface mechanisms inhibitive towards rational iterative improvement.
23–25
On the other hand,
homogeneous catalysts can provide modular platforms that can be synthetically tuned to increase
rates and selectivities. As a result, fundamental research of homogeneous catalytic systems can
provide insights into new chemical strategies to further optimize and increase the efficiencies of
future catalytic systems.
1.6 Catalyst Design
As mentioned previously, it is paramount to develop efficient catalytic systems towards
both the HER and CO2RR without relying on non-Earth abundant and toxic metal-based catalytic
systems. While the current state of artificial CO 2RR and HER catalysts is limited, evolution has
provided highly efficient catalytic systems for these reactions in biological settings. Enzymes such
as hydrogenase can reversibly catalyze the HER and the hydrogen oxidation reaction, while CO
dehydrogenase and Formate dehydrogenase can selectively and reversibly convert CO 2 to CO or
formate, respectively, near the thermodynamic potential.
25–29
Notably, the active sites of these
9

enzymes feature relatively abundant first row transition metals. One common motif is the extensive
presence of thiolate moieties in the form of cysteine residues in the active sites of both hydrogenase
and CO dehydrogenase and the molybdopterin ligand cofactor within Formate dehydrogenase.

Figure 1.8 Active sites of [NiFe] dehydrogenase (left), [NiFe] CO dehydrogenase (middle), and
Formate dehydrogenase (right).
25–29

Research suggests that highly donating nature of these ligands increase the nucleophilicity of the
bound metal centers and act as redox mediators that can stabilize low oxidation states on the active
metal sites, increasing the activity of the enzymes toward their select chemical conversions.
25

While some research has explored the reactivity of these enzymes directly as electroactive
catalysts,
30–32
research into the incorporation of these motifs into synthetic catalytic systems
demands attention as means to produce highly active and selective catalytic systems that employ
Earth-abundant transition metals similarly found in enzymatic settings.

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high-every-year-why-isn’t-every-year-hotter-last.
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coasts/ocean-acidification.
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Solar Energy Conversion, University of Southern California, 2018.
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Carbon Dioxide - Methods, Mechanisms, and Catalysts. Chem. Rev. 2018, 118 (9), 4631–
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in Graphene-Based Electrocatalysts for Hydrogen Evolution Reactions. RSC Adv. 2017, 7
(43), 27033–27039. https://doi.org/10.1039/C7RA04115B.
(20) De Luna, P.; Hahn, C.; Higgins, D.; Jaffer, S. A.; Jaramillo, T. F.; Sargent, E. H. What
Would It Take for Renewably Powered Electrosynthesis to Displace Petrochemical
Processes? Science. 2019, 364 (6438). https://doi.org/10.1126/science.aav3506.
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production-electrolysis.
11

(22) Morales-guio, C. G.; Stern, L.; Hu, X.; Stern, L. Chem Soc Rev Nanostructured
Hydrotreating Catalysts for Electrochemical Hydrogen Evolution. 2014, 6555–6569.
https://doi.org/10.1039/c3cs60468c.
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Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendor, I.
Progress and Perspectives of Electrochemical CO 2 Reduction on Copper in Aqueous
Electrolyte. 2019. https://doi.org/10.1021/acs.chemrev.8b00705.
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Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem.
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J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.;
Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.; Thauer, R.
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Function Relationships of Anaerobic Gas-Processing Metalloenzymes. Nature 2009, 460
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0728(01)00502-2.

12

Chapter 2: Electronically-coupled redox centers in trimetallic cobalt complexes
2.1 Introduction
Metal-organic frameworks (MOFs) represent a class of crystalline coordination polymers
that display high porosities and surface areas, while also providing access to sorption and catalytic
characteristics not displayed in the traditional organic-based polymers.
1,2
Two-dimensional metal-
organic frameworks (2-D MOFs) are a class of MOFs that display structures analogous to that of
graphene, with two-dimensional polymeric sheets.
3,4
Recent studies into this class of modified 2-
D conjugated coordination polymers have reported high charge mobilities, conductivities, and
porosities.
3,4
Importantly, various physical characteristics such as porosity, band gap, conductivity,
and magnetic ordering are highly tailorable by altering the metal center, organic ligand, or specific
synthetic conditions.
4
This tunability allows for the development of materials with applications in
electronics as transistors,
5
capacitors,
6
spintronic devices,
7
and electrocatalysts.
8
Conductive 2-D
materials have displayed notable activity towards the oxygen evolution reaction (OER), oxygen
reduction reaction (ORR), and the hydrogen evolution reaction (HER).
8,9
One particular class of
2D MOFs active towards electrocatalytic HER utilizes the cobalt metal centers ligated to
trinucleating triphenylene or benzene hexathiolate ligands (Figure 2.1). These cobalt-dithiolene
based 2D MOFs display high catalyst loading with exceptional, robust electrocatalytic activity
even in highly acidic conditions, while also demonstrating band-like metallic conductivity and
superconductivity.
10–13
Due to the heterogeneous nature of MOFs, the fundamental
characterization of the metal centers and conjugated organic ligand is a limiting factor for
controlling the properties of these materials. As a result, isolating analogous molecular units of
MOFs for studies in solution may elucidate physical characteristics that are not directly observable
using heterogeneous characterization techniques.
13

Figure 2.1. Chemdraw illustrations of the dithiolene-based 2D MOFs containing trinucleating
ligands, such as triphenylene-2,3,6,7,10,11-hexathiolate (THT) and benzene hexathiolate (BHT),
and the proposed homogenization strategy of the cobalt-dithiolene molecular units.
The isolation of molecular building blocks of 2-D MOFs for study can be achieved by the
addition of capping ligands, which hinder polymerization, resulting in the formation of
multimetallic complexes (Figure 2.1). Due to the highly coupled nature of the redox-active
moieties, many studies have been conducted on this class of multimetallic complexes employing
various ligand architectures, metals, and capping ligands to investigate the electronic properties of
these complexes.
14

Figure 2.2. Examples of reported multimetallic complexes as a means of building-block isolation
of the associated MOF.
14–18

A trimetallic complex incorporating a trinucleating hexahydroxytriphenylene (HHTP) ligand
coordinating three ruthenium bis(bipyridine) centers was reported to display strong coupling
between the redox-active dioxolene ligands, resulting in stabilization of intermediate redox
states.
18
Analogous copper and nickel-coordinated HHTP complexes were also investigated and
shown to display similar electronic coupling between the redox-active sites, suggesting that this
electronic communication mechanism occurs in the associated 2-D MOF material as well.
14,15

Attempts to study the copper-coordinated hexaiminotriphenylene congener through
electrochemical studies were hindered by the instability of this complex to disproportionation
under reductive conditions.
15
Lastly, tri-nucleating complexes employing the dithiolene-based
triphenylene-2,3,6,7,10,11-hexathiolate (THT)
16
and benzene hexathiolate (BHT)
17
ligands
(Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, respectively, for the cobalt-based derivatives) with
pentamethylcyclopentadienyl (Cp
*
) as the capping ligand, have previously been reported
incorporating group 9 and 10 metals. These complexes notably exhibit metal-based redox-active
15

centers and display strong electronic communication between the metal centers, evidenced by
electrochemical and spectroscopic studies, with metallic coupling strongly affected by the choice
of dithiolene bridging ligand and the metal center.

Figure 2.3. Chemdraw illustration of the synthesized trimetallic complexes 1
3+
and 2
3+
, and the
monometallic analogue 3
+
studied here.
Herein, we report two new trinuclear cobalt complexes incorporating dithiolene ligands,
such as triphenylene-2,3,6,7,10,11-hexathiolate (THT) (1
3+
), and benzene hexathiolate (BHT)
(2
3+
), with 1,1,1,-tris(diphenylphosphinomethyl) ethane (triphos) employed as the capping ligand
(Figure 2.3). Inspired by the results from similar THT
16
and BHT
17
complexes mentioned above,
as well as our previous studies detailing BHT- and THT-based 2-D MOFs
10–13,19,20
, we have
investigated the influence of the coordination environment and the electrochemical medium on the
electronic coupling between the metal centers. Consequently, the complexes described herein can
serve as molecular models to provide complementary insight into the electronic properties of the
materials with THT and BHT ligands. X-ray crystallography, UV-Vis-NIR spectroscopy, and
electrochemical studies were carried out to investigate the electronic coupling of the redox active
metal centers in complexes 1 and 2, and these results are supported through density-functional
16

theory (DFT) calculations. We further compare these results to previously-reported
Co
3
Cp
*
3
THT
16
and Co
3
Cp
*
3
BHT
17
systems to highlight the influence of the capping ligand on
the electronic structure of these model complexes. A mononuclear cobalt complex
[Co(triphos)(BDT)][BF
4
] (3
+
), where BDT = 1,2-benzenedithiolate, was synthesized and studied
to highlight the electronic effects resulting from the multimetallic nature of complexes 1 and 2
(Figure 2), and to probe the effect of capping ligand relative to the Cp
*
- analogue (CoCp
*
BDT).
16

2.2 Results and Discussion
2.2.1 Synthesis and Characterization

Scheme 2.1. Synthetic procedure for complexes 1
3+
and 2
3+
.
17

Complex 1
3+
was synthesized via a two-step one-pot process (Scheme 2.1). First, 1,1,1-
tris(diphenylphosphinomethyl)ethane (triphos) was added to cobalt(II) tetrafluoroborate in
acetonitrile following published literature procedure
21
to form the
cobalt(triphos)(tetrafluoroborate) complex. Subsequent addition of the THT ligand to the reaction
mixture in the presence of triethylamine led to the formation of a red precipitate. After overnight
stirring, the mixture was exposed to ambient atmospheric conditions, whereupon any apparent red
precipitate dissolved to generate a blue solution. After removal of the volatiles and washing with
THF, complex 1
3+
was isolated as a blue powder. The
1
H nuclear magnetic resonance (NMR)
spectrum of 1
3+
in DMSO-d6 displays sharp features, indicative of a diamagnetic species (Figure
2.4).

Figure 2.4 500 MHz
1
H NMR spectrum of 1
3+
in dimethylsulfoxide-d6.
18

Two broad aliphatic singlets are observed at δ 2.98 and 2.00 ppm in a 6:3 ratio, corresponding to
the methylene and methyl moieties on the triphos ligand, respectively. Four aromatic signals
appear at δ 9.93 (s), 7.29 (t), 7.18 (m), and 7.10 (t) ppm in a 2:6:12:12 ratio, corresponding to the
protons of the triphenylene core (δ 9.93 ppm), and the aryl substituents of the triphos ligand (δ
7.29-7.10 ppm range).

Figure 2.5 202 MHz
31
P-{
1
H} NMR (left) and 470 MHz
19
F-{
1
H} NMR (right) spectrum of 1
3+

in dimethylsulfoxide-d6.
The
19
F-{
1
H} NMR spectrum of 1
3+
in DMSO-d6 displays one fluorine environment at δ -148.0
ppm, corresponding to the BF4
-
anion (Figure 2.5). The presence of the BF4
-
counteranion, along
with the observation of a diamagnetic species by
1
H NMR spectroscopy, suggests that complex
1
3+
contains Co (III) centers. The
31
P-{
1
H} NMR spectrum of 1
3+
displays a single broad peak at
δ 33.8 ppm, corresponding to the triphos ligand (Figure 2.5).
The synthetic procedure for complex 2
3+
was similar to that of complex 1
3+
. BHT was
added to the in situ generated cobalt(II)(triphos) (tetrafluoroborate) complex in the presence of
triethylamine to directly form a green-blue solution. After overnight stirring, the mixture was
exposed to ambient atmospheric conditions, where the solution color changed from green to blue.
After removal of the volatiles and subsequent washings, complex 2
3+
was isolated as a blue
19

powder. The
1
H NMR spectrum of 2
3+
in DMSO-d6 also displays sharp features, indicative of a
diamagnetic species (Figure 2.6).

Figure 2.6 500 MHz
1
H NMR spectrum of 2
3+
in dimethylsulfoxide-d6.
Two broad aliphatic singlets are observed at δ 2.97 and 2.01 ppm in a 6:3 ratio, assigned to the
triphos methylene (CH2) and methyl (CH3) moieties, respectively. Three aromatic signals appear
at δ 7.20 (t), 7.15 (m), and 6.88 (t) ppm, in a 6:12:12 ratio, representing the protons of the aryl
substituents of the triphos ligand. The
19
F-{
1
H} NMR spectrum of 2
3+
in DMSO-d6 displays one
fluorine environment at δ -148.0 ppm, corresponding to the BF4
-
anion (Figure 2.7). Similar to
complex 1
3+
, the presence of the BF4
-
counteranion, along with the observation of a diamagnetic
species by
1
H NMR spectroscopy, suggests that complex 2
3+
contains Co (III) centers.

20

Figure 2.7 202 MHz
31
P-{
1
H} NMR (left) and 470 MHz
19
F-{
1
H} NMR (right) spectrum of 2
3+

in dimethylsulfoxide-d6.
The
31
P-{
1
H} NMR spectrum of 2
3+
displays one broad peak at δ 31.4 ppm, corresponding to the
triphos ligand (Figure 2.7). Synthesis of complex 3
+
was adapted from the literature,
22

incorporating tetrafluoroborate as the counter-anion instead of the reported hexafluorophosphate
to allow for accurate comparisons of the complexes synthesized (Figures 2.8-2.9).

Figure 2.8 500 MHz
1
H NMR spectrum of 3
+
in dimethylsulfoxide-d6.
21

Figure 2.9 202 MHz
31
P-{
1
H} NMR (left) and 470 MHz
19
F-{
1
H} NMR (right) spectrum of 2
3+

in dimethylsulfoxide-d6.
The
19
F-{
1
H} NMR spectrum of 3
+
in DMSO-d6 displays one fluorine environment at δ -148.0
ppm, indicating the successful incorporation of the BF4
-
anion (Figure 2.9).
X-ray photoelectron spectroscopy (XPS) surveys on crystals of complexes 1
3+
, 2
3+
, and 3
+

confirm the presence of Co, S, P, C, F, and B (Figures 2.10).

Figure 2.10 XPS survey scan of complex a) 1
3+
, b) 2
3+
, c) 3
+
.
22

Figure 2.11. High-resolution X-ray photoelectron spectroscopy spectra of the a) Co 2p region; b)
F 1s region; c) B 1s and P 1s region; d) S 2p region; e) P 2p region for complex 1
3+
.

Figure 2.12. High-resolution X-ray photoelectron spectroscopy spectra of the a) Co 2p region; b)
F 1s region; c) B 1s and P 1s region; d) S 2p region; e) P 2p region for complex 2
3+
.
23

Figure 2.13. High-resolution X-ray photoelectron spectroscopy spectra of the a) Co 2p region; b)
F 1s region; c) B 1s and P 1s region; d) S 2p region; e) P 2p region for complex 3
+
.
The high-resolution scans of the Co 2p region for complexes 1
3+
and 2
3+
display similar features
at binding energies of 794.7 eV and 779.8 eV for complex 1
3+
and 794.9 eV and 779.9 eV for
complex 2
3+
, corresponding to the Co 2p1/2 and 2p3/2 levels, respectively (Figures 2.11a-2.12a).
Analogous scans of the Co 2p region for complex 3
+
display features at binding energies of 795.2
eV and 780.2 eV, indicating a small shift in the 2p1/2 and 2p3/2 levels toward higher binding energies
(Figures 2.13a). The Co 2p regions for complexes 1
3+
, 2
3+
, and 3
+
display no satellite features at
786 eV, suggesting that the cobalt centers for each complex are in the formal +3 oxidation states
(i.e., Co
III
). High resolution scans of S 2p and P 2p regions of complexes 1
3+
, 2
3+
, and 3
+
show
features at binding energies of 163.9 eV and 162.7 eV, corresponding to the S 2p 1/2 and S 2p3/2
levels, respectively, and 132.4–132.5 eV and 131.5–131.6 eV, corresponding to the P 2p1/2 and P
2p3/2, respectively (Figures 2.11d,e-2.13d,e). Lastly, high resolution scans of the F 1s and B 1s
24

regions display characteristic peaks at 685.3–684.9 eV and 193.8 eV, respectively, reflecting the
presence of the BF4
-
anion (Figures 2.11b, c-2.13b,c).

Figure 2.14. Top-down view of the solid-state structure of complex 1
3+
. Aryl and aliphatic
protons, counterions, and solvent molecules are omitted for clarity.

Figure 2.15. Top-down view of the solid-state structure of 2
3+
. Aryl and aliphatic protons,
counterions, and solvent molecules are omitted for clarity.
X-ray quality crystals of complexes 1
3+
and 2
3+
were grown via vapor diffusion of diethyl
ether into acetonitrile solutions containing complexes 1
3+
and 2
3+
, and solid-state crystal structures
were obtained for both complexes (Figures 2.14, 2.15). Due to low quality diffraction data for
complex 2
3+
, bond lengths and angles will only be discussed for complex 1
3+
. Notably, to our
knowledge, this is the first solid state structure reported for a triphenylene-based Co-dithiolene
trimetallic complex. Single crystal X-ray analysis of 1
3+
indicates a species containing three cobalt
25

metal centers, each coordinated to a central THT ligand and capped with a single triphos ligand.
Additionally, BF4
-
anions were detected in the lattice as outer-sphere counterions. Three BF4
-

counteranions are present for each molecular unit in the lattice, suggesting that complex 1
3+
has
an overall charge of +3 and that each Co center has a +3 formal oxidation state. The angular
structural parameter, τ, can inform on the coordination environment about the metal center and is
defined as follows.
𝜏 =
𝛽 −𝛼 60
(1)
where β and α are the two largest angles of the coordination center, when β > α. When τ
approximates 0 this corresponds to the metal complex displaying a square pyramidal geometry,
whereas when τ approximated 1, this corresponds to a trigonal bipyramidal geometry. The τ
parameter was calculated to be < 0.1 for each metal center (Equation 2.1), suggesting that each
cobalt center adopts a distorted square pyramidal geometry.
Table 2.1 Average selected bond lengths (Å) for complex 1
3+
.
Bond Bond Length (Å)
Co–S 2.159(2)
Co–P
apical
2.173(5)
Co–P
basal
2.241(4)
C–S 1.736(5)

26

Table 2.2 Average selected bond angles (°) for complex 1
3+
.
Bond Bond Angle (Å)
S–Co–S 89.78(6)
S–Co–P
apical
107.02(77)
S–Co–P
basal
87.14(7)
P
apical
–Co–P
basal
92.71(4)

Average selected bond lengths and angles can be found in Tables 2.1 and 2.2. We can identify two
asymmetric Co–P bonding environments within the structure; one environment is within the basal
plane of the square pyramidal coordination sphere (labeled as Co–Pbasal), and the other is
perpendicular to the square pyramidal plane (labeled as Co–Papical). Comparing the same Co–P
bonding environments reported in the solid state structure for complex 3
+
(Co–Papical: 2.183(2) Å,
Co–Pbasal: 2.232(2) Å),
23
we find that complex 1
3+
displays slightly shorter Co–Papical bond lengths
(by ~0.010 Å) and slightly longer Co–Pbasal bond lengths (by ~0.009 Å). The average Co–S bond
for complex 1
3+
is measured to be 2.159(2) Å, which is slightly shorter than the Co–S bond
reported for complex 3
+
(2.169 (2) Å).
23
Complex 1
3+
exhibits an average bond length of 1.736(5)
Å for the C–S bond, which is in good agreement with reported dithiolene ligands in their reduced
thiolate state, further supporting the +3 oxidation assignment of the metal centers
24
. Significant
bowing is apparent in the triphenylene backbone, with an angle of 22.6(7)° between the plane of
the central six carbon atoms of the triphenylene core and that of the asymmetric cobalt-dithiolene
moiety (Figure S16). This is ascribed to a crystal packing effect, similar to the bowing of internal
ligands observed in analogous complexes.
16

27

2.2.2 Cyclic Voltammetry and Electrochemical Analysis

Figure 2.16 a) CVs of complexes 1–3 (0.5 mM) and b) DPVs of 1–3 (0.5 mM) in MeCN solutions
containing 0.1 M [nBu4N] [PF6] under an atmosphere of N2. Scan rate is 100 mV/s.
Cyclic voltammetry (CV) studies of complexes 1
3+
, 2
3+
, and 3
+
(0.5 mM) under N2
atmosphere were carried out using a glassy carbon working electrode in acetonitrile (MeCN)
solutions with 0.1 M tetrabutylammonium hexafluorophosphate ([nBu4N] [PF6]) as the supporting
electrolyte. All potentials are referenced versus Fc
+/0
and all CVs were first scanned cathodically
and subsequently returned anodically. CV studies of complex 1 display a reversible redox feature
centered at -0.69 V, which can be deconvoluted using differential pulse voltammetry (DPV) to
separate three reversible one-electron events with standard potentials (E
0
1/2) at -0.63 V, -0.71 V,
and -0.80 V (Figure 2.16). Analogous metal dithiolene complexes have shown similarly
convoluted redox features, assigned to the sequential reduction of electronically-coupled redox-
active metal centers.
16,25
Similar electrochemical behavior is observed for complex 1, and is
therefore attributed to electronic coupling between the cobalt centers in this trimetallic system.
CV studies of complex 2 display three reversible one-electron features with standard
potentials (E
0
1/2) at -0.65 V, -0.93 V, and -1.27 V, with a greater degree of separation relative to
28

those observed in complex 1. These three separate reversible one-electron events can be similarly
associated with electronic coupling between the cobalt centers, comparable in nature to what is
observed for complex 1. The shift of each sequential E
0
1/2 for complex 2 can be attributed to the
increased stability of the mixed-valent species, due to stronger electronic coupling of the redox
active sites through the central benzene ligand compared to the bridging triphenylene ligand of
complex 1.
16
CV studies of complex 3 exhibit a single reversible redox feature at -0.74 V, which
was previously assigned to the Co
III/II
couple (Figure 2.16a).
22
As only one reduction feature was
observed for complex 3 within the potential range studied, the three redox features of 1 and 2 can
also be attributed to sequential Co
III/II
couples for each metal center. Randles-Sevcik plots for
complexes 1–3 (Figures 2.17) in acetonitrile yield slopes of approximately 0.5 (Figures 2.18), as
expected for freely diffusing molecular species in solution.

Figure 2.17 Cyclic voltammograms of 0.5 mM of a) 1 b) 2 c) 3 in a CH3CN solution containing
0.1 M [nBu4N] [PF6] under an atmosphere of N2. Scan rates vary from 10 to 1000 mV/s.
29

Figure 2.18 Plot of log of current density vs log of the scan rate for a) the first redox couple of 1
b-d) each of the three redox couples of 2 e) and the redox couple of 3 in MeCN.
As the metal centers in complexes 1 and 2 are sequentially reduced, mixed valence (MV)
states are formed. A comproportionality constant (K c) for the formation of the MV state relative
to disproportionation can be described using the following equation:
𝐾 𝑐 =
[𝑿 𝒏 ]
2
[𝑋 𝑛 +1
][𝑋 𝑛 −1
]
= exp {[𝐸 1
2
0
( 𝑋 𝑛 +1
/𝑋 𝑛 )− 𝐸 1
2
0
( 𝑋 𝑛 /𝑋 𝑛 −1
) ] 𝐹 /𝑅𝑇 } = exp {∆𝐸 1
2
0
𝐹 /𝑅𝑇 } (2)
where n is the charge, [X
n
] is the concentration of the MV species with charge n, [X
n+1
] and [X
n-1
]

are the concentrations of the isovalent species, and E
0
(X
n+1
/X
n
) and E
0
(X
n
/X
n-1
) are the standard
potentials of the respective species.
16,26,27
In the case of complexes 1 and 2 there are three separate
redox couples consisting of E
0
(X
2+/3+
), E
0
(X
1+/2+
), and E
0
(X
+/0
), generating two possible MV states,
X
2+
for the [Co
II
Co
III
Co
III
] species and X
+
for the [Co
II
Co
II
Co
III
] species, resulting in two separate
ΔE1/2 and Kc values for each state (Figure 2.19). The Log Kc values of both states of complex 1
and 2, labeled as X
2+
for ΔE1/2[(X
3+
/X
2+
) - (X
2+
/X
1+
)] and X
+
for ΔE1/2[(X
2+
/X
+
) - (X
+
/X
0
)], are
presented in Table 2.3.
30

Figure 2.19. Chemdraw illustrations of multi-valent states of complexes 1 and 2 accessible
through electrochemical studies.
Table 2.3 The comproportionality constants determined based on the Co
III/II
redox couples present
in complexes 1 and 2 (in MeCN) and those reported for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT.
Complex

E
0
1/2
(X
3+
/X
2+
)
(V vs
Fc/Fc
+
)
E
0
1/2
(X
2+
/X
+
)
(V vs
Fc/Fc
+
)
E
0
1/2
(X
+
/X
0
)
(V vs
Fc/Fc
+
)
Log
K
c
(X
2+
)
Log
K
c
(X
+
)
Ref
1 -0.63 -0.71 -0.80 1.4 1.5 This work
2 -0.65 -0.93 -1.27 4.7 5.8 This work
Co
3
Cp
*
3
THT
-1.31 -1.38 -1.46 1.2 1.4 1
Co
3
Cp
*
3
BHT
-1.35 -1.58 -1.92 3.9 5.8 2

31

The free energy of comproportionation of the MV species relies on various thermodynamic factors
including contributions from inductive, electrostatic, and magnetic effects in addition to
resonance-based stabilization effects.
27
While the degree of comproportionation cannot directly
indicate the magnitude of the resonance-stabilized coupling in MV species, it can provide a
diagnostic handle in comparing relative changes in electronic coupling in structurally similar
systems.
28,29
The Kc values measured for complex 2 is several orders of magnitude larger than that
of complex 1 (> 10
4
), which is likely a result of the stronger coupling of the metal centers through
the bridging benzene ligand of complex 2. The measured Kc values for complexes 1 and 2 are
comparable to those reported for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, since the electronic coupling
is primarily through the conjugated THT and BHT ligands, respectively (Table 2.3). Notably, the
sequential Co
III/II
couples for complexes 1 and 2 are shifted to more positive potentials (by > 70
mV) compared to those reported for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, likely due to the neutral
capping triphos ligand in contrast to the anionic Cp
*
ligand. Additional factors such as solvation
effects can contribute significantly to the stability of mixed valence states, and ΔE1/2 values are
expected to shift in conjunction with solvent polarity. CV studies of complexes 1
3+
and 2
3+
were
performed in dichloromethane (DCM) and N,N-dimethylformamide (DMF) to determine the effect
of solvent polarity on stabilization of the mixed-valency of both complexes (Figures 2.20). The
ΔE1/2 values observed in each solvent for the Co
III/II
couples of complexes 1 and 2 can be found in
Table 2.4.

32

Figure 2.20. CVs and DPVs of 1 (0.5 mM) (a, c) and 2 (0.5 mM) (b,d) in a solution containing
0.1 M [nBu4N][PF6] under an atmosphere of N2 in DMF (red), MeCN (green), and DCM (blue).
Scan rate is 100 mV/s.
Table 2.4. Calculated ΔE1/2 of the Co
III/II
redox couples for complexes 1 and 2 in DMF, MeCN,
and DCM, where ΔE

refers to the difference in the reduction potential of the first and second redox
event (ΔE
a
) and of the second and third redox event (ΔE
b
) for the respective species.
Solvent Dielectric
Constant
30

ΔE
a
(X
3+/2+
-
X
2+/+
) for 1
(mV)
ΔE
b
(X
2+/+
-
X
+/0
) for 1
(mV)
ΔE
a
(X
3+/2+
-
X
2+/+
) for 2
(mV)
ΔE
b
(X
2+/+
-
X
+/0
) for 2
(mV)
DMF 38.25 80 101 270 420
MeCN 36.65 80 100 260 360
DCM 8.93 139 126 340 500

As the solvent polarity decreases from MeCN to DCM, a marked increase is observed in
the ΔE1/2s for complexes 1 and 2. Previous studies on the behavior of MV systems have
33

demonstrated that low-polarity, low-donor strength solvents (such as DCM) increase MV
stability.
31,32
The capacity of the solvent to electronically shield the charged metal centers on
complexes 1 and 2 is directly associated with its dielectric constant. As the metal centers of the
isovalent states have a higher intrinsic electrostatic repulsion compared to the MV state (the
difference in charge in Equation 2.2 is always 1, electrostatically favoring the MV state), solvents
of high dielectric constants can shift the equilibrium towards the isovalent species relative to the
MV species due to greater stabilization of this coulombic repulsive force by dielectric
shielding.
33,34
Additionally, solvents with relatively high intrinsic Lewis basicity, such as MeCN,
can localize charge on the cationic metal centers via inductive effects, reducing delocalization as
a stabilizing feature in MV species.
32,35
Moreover, solvents of low polarity may also favor the
solvation of the partially reduced MV states of complexes 1 and 2, as the cationic charge is
sequentially reduced, thus favoring the less polar solvents such as DCM.
31
It should be noted that
utilization of solvents of low polarity have also been reported to increase ion-pairing effect due to
a decrease in solvation of both the MV species and electrolyte in solution. This effective increase
in ion-analyte interaction has been reported to accentuate the effect of the electrolyte ion identity
as the primary means of affecting ∆E1/2.
31
Notably, this trend is not observed in the CV studies of
complexes 1 and 2 in the stronger donating solvent DMF. Comparison of the ΔE1/2 values of
complexes 1 and 2 in MeCN and DMF demonstrates the opposite trend, where an increase in the
ΔE1/2 is observed in the more donating DMF, albeit a weaker shift compared to the one observed
upon switching from MeCN to DCM. Randles-Sevcik analysis was performed on complexes 1 and
2 in both DMF and DCM, confirming that the trimetallic complexes are freely-diffusing at the
electrode’s double-layer in these solvents as well (Figures 2.21-2.24).
34

Figure 2.21 Cyclic voltammograms of 0.5 mM of 1 in a a) DCM and b) DMF solution
containing 0.1 M [nBu4N] [PF6] under an atmosphere of N2. Scan rates vary from 50 to 1000
mV/s.

Figure 2.22. Plot of log of current density vs log of the scan rate for each of the three redox couple
of 1 in a-c) DCM and d) the first redox couple in DMF.

35

Figure 2.23 Cyclic voltammograms of 0.5 mM of 2 in a a) DCM and b) DMF solution
containing 0.1 M [nBu4N] [PF6] under an atmosphere of N2. Scan rates vary from 50 to 1000
mV/s.

Figure 2.24 Plot of log of current density vs log of the scan rate for each of the three redox
couple of 2 in a-c) DCM and d-f) DMF.
2.2.3 Visible Spectroscopy and Spectroelectrochemistry
To further probe the electronic structure of these complexes, UV-Vis absorption
spectroscopy studies were performed on complexes 1
3+
, 2
3+
, and 3
+
in acetonitrile. Absorbance
spectra of these species show two peaks in the visible range (Figures 2.25, 2.26). The wavelengths
of maximum absorption (λmax) and molar absorptivities (ε) are displayed in Table 2.5. Complexes
1
3+
and 2
3+
display prominent transitions (λ1) at 624 nm and 618 nm, respectively, consistent with
36

charge-transfer transitions. Additionally, weaker transitions at longer wavelengths (λ2) were
detected at 738 nm for complex 1
3+
, and 756 nm for complex 2
3+
. These absorptions were not
observed for Co
3
Cp
*
3
THT or Co
3
Cp
*
3
BHT, suggesting substantial contribution from the triphos
capping ligand. A slight blue shift in λ1 was observed for complex 1
3+
relative to complex 2
3+
(by
6 nm), as well as a red shift in λ2 (by 18 nm). The UV-vis spectrum of complex 3
+
displays a
prominent transition at 540 nm (λ1), and a weaker absorption at 720 nm (λ2).
23
The absorption
peaks for complex 3
+
are blue-shifted compared to those of complex 1
3+
(by 84 nm for λ1 and 18
nm for λ2) and complex 2
3+
(by 78 nm for λ1 and 36 nm for λ2).
Table 2.5 λmax
and molar absorptivity values for 1
3+
, 2
3+
, and 3
+
in the visible range.
Complex Transition λ
max
(nm) ε (10
-4
M
-1
cm
-1
)
1
3+
λ
1 624 3.49(0.04)
λ
2
738 3.17(0.04)
2
3+
λ
1
618 1.83(0.04)
λ
2
756 1.88(0.04)
3
1+
λ
1
540 0.73(0.02)
λ
2
720 0.204(0.002)

37

Figure 2.25 Visible absorbance spectra of complexes 1
3+
, 2
3+
, and 3
+
in acetonitrile.

Figure 2.26 UV-vis spectrum of a) 0.025 mM of 1
3+
, b) 0.025 mM of 2
3+
, c) 0.1 mM of 3
+
in
acetonitrile.
38

To further investigate the degree of electronic coupling in complexes 1 and 2, MV states
of complexes 1 and 2 were electrochemically generated and studied using Vis-NIR spectroscopy.
All measurement were performed under N2 atmosphere and were carried out in a sealed OTTLE
(optically transparent thin-layer electrochemistry) cell using a platinum working electrode in
dichloromethane (DCM) solutions with 0.25 M (nBu4N) (PF6) as supporting electrolyte.
Electrolytic reduction was performed via stepwise 25 mV increments vs Ag/AgCl and the extant
of the reduction and formation of MV states were monitored using Vis-NIR spectra. Spectra of
electrolyzed solution of complex 1
3+
display a decrease in the intensity of the isovalent parent
absorbances at 624 and 618 nm and emergence of a new NIR transitions, ascribed to intervalence
charge transfer (IVCT) transitions in the generated MV state (Figure 2.27).

Figure 2.27 Vis-NIR spectral changes of 1 in a DCM solution of 0.25M [nBu4N] [PF6] under an
atmosphere of N2 as potential is cathodically shifted by 25 mV increments.
These transitions are subsequently suppressed as complex 1 is further reduced to the isovalent
neutral oxidation state. Due to the low degree of separation of the E
0
1/2 of complex 1, sole in-situ
formation of each MV state of complex 1 was not accessible, leading to mixtures of MV and
isovalent likely observed in the Vis-NIR spectra. In contrast, spectra of electrolyzed solutions of
39

complex 2
3+
display the formation of three distinct species in solution, with clean isobestic points
indicating facile conversion of each species to subsequent MV states (Figures 2.28,2.29). Facile
formation of MV species of complex 2, such as 2
2+
and 2
+
(Figures 2.29a, c), is indicated by the
emergence of absorbances in the near-IR, ascribed to intervalence charge transfer (IVCT)
transitions in the generated MV state. Spectra of MV states 2
2+
and 2
+
were further deconvoluted
and fit with gaussian peak forms and the characteristic peak energies (νmax), molar absorptivity
(ε
IVCT
), and peak width at half-heights (Δν1/2) of the IVCT bands can be found in Table 2.6.

Figure S2.28 Vis-NIR spectral changes of 2 in a DCM solution of 0.25 M [nBu4N] [PF6] under an
atmosphere of N2 as potential is cathodically shifted by 25 mV increments.
40

Figure 2.29 Vis-NIR spectra of the electrochemical reduction of 2
3+
in a DCM solution of 0.25 M
[nBu4N] [PF6] under an atmosphere of N2: a) 2
3+
to 2
2+
b) 2
2+
c) 2
2+
to 2
+
d) 2
+
e) 2
+
to 2
0
. Solution
electrolyzed cathodically by 25 mV increments.
As 2
3+
is electrochemically reduced to 2
2+
, we observe a suppression of the isovalent parent
transitions at 618 and 756 nm, and the emergence of two new absorption bands at 802 and 1186
nm (ν1), with the latter attributed to an IVCT (Figures 2.29a, b). As the potential is stepped further
cathodically, the absorption bands in the visible region are further reduced in intensity, with two
new NIR transitions developing at 1092 (ν2) and 1291nm (ν3), indicating the conversion of 2
2+
to
2
+
(Figures 8c, d). Lastly, as 2
+
is reduced to the 2
0
, we observe a general decrease in intensity of
absorption bands in both the visible and NIR spectrum, suggesting a conversion from the mixed -
valent 2
+
to the isovalent 2
0
(Figures 2.29e). The degree of electronic coupling in complex 2 was
41

further assessed using the electronic parameters of the IVCT bands obtained from spectra of 2
2+

and 2
+
. The extent of electronic coupling in MV species has been classically distinguished between
Class II (localized charge, weak to moderate coupling), borderline Class II-III (localized-
delocalized charge, moderate coupling), and Class III (delocalized charge, strong coupling).
36–38

Using the classification method developed by Brunschwig, Creutz, and Sutin, the magnitude of
divergence of the theoretical IVCT bandwidth from that of the experimental band width can inform
on the degree of electronic coupling in the system (Equation 2.3):
38

Γ = 1 − ( Δ𝑣 1
2
/Δ𝑣 1
2
0
) (3)
Δ𝑣 1
2
0
= [16𝑅𝑇𝑙𝑛 2( 𝑣 𝑚𝑎𝑥 ) ]
1/2
(4)
𝐻 𝑎𝑏
=
2.06𝐸 −2
(𝑣 𝑚𝑎𝑥
𝜀 𝐼𝑉𝐶𝑇 𝛥 𝑣 1
2
)
1/2
𝑟 𝑎𝑏
(5)
where Δν
0
1/2 is the theoretical IVCT bandwidth, which can be calculated according to Equation
2.4, where R is the ideal gas constant, T is the temperature in K, and νmax is the energy at the max
peak height of the IVCT band. The measured Γ values can be found in Table 2.6. Magnitude of
the Γ values fall well below 0.1 indicating our system can be classified as a Class II localized
system, in agreement with previous designation to the Co
3
Cp
*
3
BHT.

42

Table 2.6. IVCT characteristic values for complex 2 in the near-IR range.
Complex Transition ν
max
(cm
1
)
[nm]
εIVCT
(10
3
M
-1
cm
-1
)
Δν
1/2
(cm

1
)
Γ H
ab

(10
3

cm
1
)
2
2+
ν
1 8430
[1186]
2.4 20440 -3.6

1.7
2
+
ν
2 9158
[1092]
5.1 45040 -8.8 4.0
ν
3
7749
[1291]
0.81 42505 -9 1.4
Based on this assignment, the proper equation can be employed to directly measure the electronic
coupling parameter (Hab) for the detected IVCT transitions (Equation 2.5), where rab is the through-
space geometrical distance between redox active sites in Å.
39,40
These values are located in Table
4. An intermetallic distance of 7.4 Å was chosen to be the distance between redox active sites
(metal centers in this case) (rab) for the reported values for Co
3
Cp
*
3
BHT and will be used in this
study to allow for accurate comparison between both complexes. Additionally, this value is
analogous to the cobalt-cobalt distance in the calculated geometrically optimized structure (7.467
Å, vide infra). It should be noted that the choice of rab is often a point of significant error in reported
Hab values due to the inherent ambiguity in the effective charge transfer distance, and its congruity
with formally assigned redox sites.
41
Utilizing the IVCT bands of complexes 2
2+
and 2
+
to measure
Hab values yields results that are an order of magnitude larger than those reported for
Co
3
Cp
*
3
BHT.
17
This difference in strength of electronic coupling in complex 2 compared to
Co
3
Cp
*
3
BHT is striking upon comparison with the previous similarities of comproportionation
43

values for both these complexes. Spectroscopic analysis of the MV states can be more diagnostic
of the true nature of electronic coupling in MV species, and as a result we conclude that complex
2 displays stronger electronic communication than Co
3
Cp
*
3
BHT. As the free energy of
comproportionation is influenced by many different thermodynamic components such as
inductive, magnetic exchange, and electrostatic contributions, other stabilizing/destabilizing
components outside of electronic coupling may be generating similar comproportionation values
for complexes 2 and Co
3
Cp
*
3
BHT.
27

2.2.4 Computational Studies

Figure 2.30 Molecular orbital scheme of complexes 1
3+
, 2
3+
, and 3
+
(left), with the corresponding
orbital character of HOMO and LUMO (right).
A series of unrestricted density functional theory (DFT) calculations at 6-31G*/PBE level
of theory were performed on complexes 1
3+
, 2
3+
, and 3
+
to supplement the experimental results
(Figure 2.30). For calculations performed on complexes 1
3+
, 2
3+
, and 3
+
, the aryl substituents of
the triphos capping ligand were modeled as methyl groups to decrease computing complexity and
44

time. Based on these calculations, the highest occupied molecular orbitals (HOMOs) for all three
complexes are primarily localized on the dithiolene ligand and exhibit Co(dxy)-S(pz) π* anti-
bonding character (Figures 2.30-34).

Figure 2.31 DFT calculated frontier orbitals for complex 1
3+
(6-31G*/PBE level of theory).

Figure 2.32 DFT calculated frontier orbitals for complex 2
3+
(6-31G*/PBE level of theory).

45

Figure 2.33 DFT calculated frontier orbitals for complex 3
+
(6-31G*/PBE level of theory).

Figure 2.34 DFT calculated HOMO orbitals for complexes 1
3+
, 2
3+
, and 3
+
(6-31G*/PBE level of
theory).
46

Figure 2.35 DFT calculated LUMO orbitals for complexes 1
3+
, 2
3+
, and 3
+
(6-31G*/PBE level of
theory).
The lowest unoccupied molecular orbitals (LUMOs) of complexes 1
3+
, 2
3+
, and 3
+
yield orbitals
that are metal-center dominant, with Co(d z
2
)-P(pz) σ* and Co(d z
2
)-S(pz) π* antibonding character
and considerable orbital contribution from the triphos ligand (Figures 2.30-33, 2.35). Contribution
of the dithiolene ligand to the LUMO follows a trend of 3
+
> 2
3+
> 1
3+
(Figures 2.30-33, 2.35),
suggesting stronger electronic exchange between metal sites in complex 2, compared to that in
complex 1. This is also supported by the electrochemical studies discussed above. Compared to
the frontier orbitals calculated for Co
3
Cp
*
3
THT, Co
3
Cp
*
3
BHT, and CoCp
*
BDT, we do not
observe a destabilization of the HOMOs of complexes 1
3+
and 2
3+
relative to that of complex 3
+
.
Rather, an inverted relationship is observed in which the HOMO of complex 3
+
is destabilized
relative to those of complexes 1
3+
and 2
3+
(destabilized by 0.1006 and 0.2422 eV compared to the
HOMO of complex 1
3+
and 2
3+
, respectively, Figure 2.30 and Table 2.7). The LUMO of complex
3
+
is also significantly destabilized compared to the LUMOs of complexes 1
3+
and 2
3+
(destabilized
by 0.3646 and 0.4599 eV compared to the LUMOs of complex 1
3+
and 2
3+
, respectively). This
difference may be due to the cationic nature of these complexes; the greater π-conjugation of
47

complexes 1
3+
and 2
3+
leads to an overall stabilization of the charged system, in contrast to the
neutral Co
3
Cp
*
3
THT, Co
3
Cp
*
3
BHT, and CoCp
*
BDT complexes.
16

Table 2.7 DFT calculated orbital energies of frontier orbitals of complexes 1
3+
, 2
3+
, and 3
+
.

Orbitals
Calculated Orbital Energy (eV)
Complex 1
3+
Complex 2
3+
Complex 3
+

LUMO+2 -4.14702 -4.16334 -1.31159
LUMO+1 -4.14974 -4.17151 -2.75107
LUMO -4.23137 -4.32661 -3.86674
HOMO -5.445 -5.5865 -5.34432
HOMO-1 -5.47221 -5.6246 -5.55385
HOMO-2 -5.49942 -5.80147 -6.15794

Furthermore, comparison of the frontier orbitals of complexes 1
3+
and 2
3+
indicates a contrasting
trend to that reported for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, where the frontier orbitals of complex
2
3+
are slightly stabilized relative to those of complex 1
3+
. This relationship can be rationalized as
a stronger stabilization of the frontier orbitals of complex 2
3+
due to the greater degree of
conjugated coupling between metal centers through the benzene backbone of complex 2
3+

compared to the weaker coupling through the larger triphenylene backbone in complex 1
3+
.
11

Time-dependent DFT (TD-DFT) calculations were carried out for complex 3 as a model
system to establish a qualitative understanding for the observed electronic transitions of these
complexes. The aryl substituents of the triphos ligand were explicitly included in these calculations
to better clarify the role of the capping ligand in the observed transitions. Based on these
calculations, the observed λ1 transition for complex 3
+
at 540 nm can be principally attributed to a
HOMO-LUMO transition, suggesting a singlet LMCT transition (Tables 2.8-9, Figure 2.36).
48

Table 2.8. TD-DFT calculated excited states for complex 3
+
and their associated excitation energy,
wavelength, and oscillator strength.
Excited State Excitation Energy
(eV)
Wavelength (nm) Oscillator Strength
1 1.5474 801.2149 0.0063
2 1.8375 674.7211 0.0167
3 2.4501 506.0202 0.0252
4 2.6771 463.1131 0.0792
5 2.7158 456.5137 0.0382
6 2.8492 435.1397 0.0145
7 2.9278 423.4579 0.0110
8 3.0549 405.8398 0.0065
9 3.1439 394.351 0.0060
10 3.1702 391.0794 0.0018

Table 2.9. Calculated relative contribution of transitions to excited state 4 of complex 3
+
.
Transition % Contribution
HOMO-15 -> LUMO 10.96015
HOMO-11-> LUMO 13.18357
HOMO-3 -> LUMO 17.95751
HOMO-1 -> LUMO 13.73247
HOMO -> LUMO 44.1663

49

Figure 2.36 Origin of orbitals that highly contribute toward transition of excited state 4 in complex
3
+
.
The weaker absorption band detected at 720 nm is calculated to involve contributions from
HOMO-1, HOMO-7, and HOMO-12 to the LUMO in addition to a significant contribution from
a HOMO-LUMO transition (Tables 2.8,2.10-11, Figures 2.37-38).
Table 2.10 Calculated relative contribution of transitions to excited state 2 of complex 3
+
.
Transitions % Contributions
HOMO-20 -> LUMO 6.515144
HOMO-12 -> LUMO 21.60247
HOMO-11 -> LUMO 6.860259
HOMO-7 -> LUMO 17.32073
HOMO-3 -> LUMO 7.461315
HOMO-2 -> LUMO 7.220255
HOMO-1 -> LUMO 9.656316
HOMO -> LUMO 23.36351

50

Table 2.11 Calculated relative contribution of transitions to excited state 1 of complex 3
+
.
Transitions % Contributions
HOMO-19->LUMO 5.84626
HOMO-7->LUMO 8.469689
HOMO-5->LUMO 7.233084
HOMO-2->LUMO 10.46369
HOMO-1->LUMO 31.40727
HOMO->LUMO 36.5800

Figure 2.37 Origin of orbitals that highly contribute toward transition of excited state 2 of complex
3
+
.

51

Figure 2.38 Origin of orbitals that highly contribute toward transition of excited state 1 of complex
3
+
.
Due to significant involvement of the triphos capping ligand to these HOMO orbitals (Figures
2.37-38), as well as contribution by the dithiolene ligand, a singlet LMCT transition is also
consistent with this assignment. We attribute the transitions observed for complexes 1
3+
and 2
3+
to
LMCT-type transitions based on these predictions, with the considerable broadening of the
absorption bands resulting from the increased orbital degeneracies in these species (Figure 2.30).
Significant contribution of the triphos ligand to the absorption at 720 nm for complex 3
+
also
suggests why these transitions are not present in the visible spectra of Co
3
Cp
*
3
THT,
Co
3
Cp
*
3
BHT, and Co
3
Cp
*
3
BDT.
11,20
Based on these calculations, we can determine the origin of
the relative shifts displayed in the visible spectra of complexes 1
3+
, 2
3+
, and 3
+
. As previously
stated, a strong blue shift is observed in the absorption maxima for complex 3
+
, compared to
complexes 1
3+
and 2
3+
. As the λ1 transition at 624 nm is predicted to involve substantial
contribution from a HOMO-LUMO transition, the shift in this peak can be rationalized by the
destabilization of the LUMO of complex 3
+
relative to that of complexes 1
3+
and 2
3+
. Similarly,
the slight blue shift in the λ1 transition of complex 2
3+
compared to that of complex 1
3+
can be
rationalized as an increase in the HOMO-LUMO/(HOMO-1)-LUMO gap of complex 2
3+

compared to that of complex 1
3+
.

52

2.3 Conclusions
In summary, we report a series of trinuclear cobalt complexes incorporating triphenylene-
2,3,6,7,10,11-hexathiolate (THT) (1
3+
), and benzene hexathiolate (BHT) (2
3+
) to provide insight
into the electronic properties of the analogous cobalt-dithiolene 2-D MOFs. We have investigated
the electronic coupling between multiple metal centers in the trimetallic systems as a function of
the coordination sphere and the electrochemical medium, and have compared these results to those
of the previously-reported Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT complexes to clarify the role of the
capping ligand.
16,17
The solid state crystal structure of complex 1
3+
reveals three five-coordinate
cobalt centers that are bound to the triphos and the THT ligands in a distorted square pyramidal
geometry. Cyclic voltammetry (CV) studies of complexes 1 and 2 display three reversible redox
events (assigned to the Co
III/II
couple), resulting in the formation of mixed valence states following
sequential reductions of electronically-coupled redox-active metal centers. The
comproportionality constants (Log Kc) based on the Co
III/II
redox couples were determined to be
1.4 and 1.5 for complex 1, and 4.7 and 5.8 for complex 2. These results are similar to those reported
for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, suggesting minimal influence of the capping ligand on the
stability of the MV states of complexes 1 and 2. Electrochemical studies in solvents of different
polarities were conducted, which demonstrate that the ΔE1/2 of the Co
III/II
couple shifts as a function
of solvent polarity, indicating a negative correlation between polarity of the electrochemical
medium and the stability of the species. UV-Vis absorption spectroscopy studies indicate a red
shift in the absorption bands of complexes 1
3+
and 2
3+
compared to the monometallic congener 3
+
.
These transitions are not observed for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, suggesting they are
attributed to contributions from the triphos ligand. Spectroscopic analysis of electrochemically
generated MV states of complexes 1 and 2 yield Vis-NIR spectra that display IVCT bands in the
53

NIR. Analysis of the IVCT transitions for complex 2 indicate a class II localized MV species, with
Hab values significantly larger than what is reported for Co
3
Cp
*
3
BHT, suggesting stronger
electronic communication in complex 2. Density functional theory (DFT) calculations predict a
significant deviation in the relative energies of the frontier orbitals of complexes 1
3+
, 2
3+
, and 3
+

that contrasts those calculated for Co
3
Cp
*
3
THT and Co
3
Cp
*
3
BHT, suggesting that the capping
ligand contributes significantly to the overall electronic structure of the system.
2.4 Experimental Methods
2.4.1 General
All manipulations of air and moisture sensitive materials were conducted under a nitrogen
atmosphere in a Vacuum Atmospheres drybox or on a dual manifold Schlenk line. The glassware
was oven-dried prior to use. All solvents were degassed with nitrogen and passed through activated
alumina columns and stored over 4Å Linde-type molecular sieves. All the chemical reagents were
purchased from commercial vendors and used without further purification. Ligands triphenylene-
2,3,6,7,10,11-hexathiol (THT) and benzenehexathiolate (BHT) were prepared according to the
reported procedures.
16,42

2.4.2 NMR Spectroscopy
Proton NMR spectra were acquired at room temperature using Varian (Mercury 400 2-Channel,
VNMRS-500 2-Channel, VNMRS-600 3-Channel, and 400-MR 2-Channel) spectrometers and
referenced to the residual
1
H resonances of the deuterated solvent (
1
H: DMSO, δ 2.50) and are
reported as parts per million (ppm) relative to tetramethylsilane.
31
P resonances are reported as
parts per million relative to 85% H3PO4, which is set as 0 ppm.
19
F resonances are reported as parts
per million relative to fluorobenzene, which is set as –112.7 ppm.
43

54

2.4.3 Elemental analyses
Elemental analyses were performed by Complete Analysis Laboratories, Inc., Parsippany, New
Jersey or Robertson Microlit Laboratories, New Jersey.

2.4.4 UV-Vis NIR spectroscopy
Spectra were obtained using a Lambda 950 UV/Vis/NIR Spectrophotometer. Samples were
analyzed in transmittance mode with a 1 cm quartz cuvette, and the spectrum measured for a blank
acetonitrile sample was subtracted as background.

2.4.5 Single-crystal X-ray Diffraction
Diffraction data were collected on a Bruker SMART APEX DUO 3-circle platform diffractometer,
equipped with an APEX II CCD, using Mo Kα radiation (TRIUMPH curved-crystal
monochromator) from a fine-focus tube. The diffractometer was equipped with an Oxford
Cryosystems Cryostream 700 apparatus for low-temperature data collection. The frames were
integrated using the SAINT algorithm to give the hkl files corrected for Lp/decay. The absorption
correction was performed using the SADABS program. The structures were solved by intrinsic
phasing and refined on F2 using the Bruker SHELXTL Software Package and ShelXle. All non-
hydrogen atoms were refined anisotropically.

55

2.4.6 X-ray Photoelectron Spectroscopy
XPS data were collected using a Kratos AXIS Ultra instrument. The monochromatic X-ray source
was the Al K α line at 1486.6 eV. Low-resolution survey spectra were acquired between binding
energies of 1–1200 eV. Higher-resolution detailed scans, with a resolution of ~0.1 eV, were
collected on individual XPS lines of interest at a pass energy of 20. The sample chamber was
maintained at < 2 × 10
–8
torr. The XPS data were analyzed using the CasaXPS software.

2.4.7 Cyclic and Differential Pulse Voltammetry (CV, DPV)
CV and DPV electrochemistry experiments were carried out using a Pine potentiostat. The
experiments were performed in a single compartment electrochemical cell under a nitrogen
atmosphere using a 3 mm diameter glassy carbon electrode as the working electrode, a platinum
wire as auxiliary electrode and a silver wire as the reference electrode. All experiments in this
paper were referenced relative to ferrocene (Fc) with the Fe
3+/2+
couple at 0.0 V, and all CVs were
first scanned cathodically and subsequently returned anodically. All electrochemical experiments
were performed in acetonitrile (MeCN), dichloromethane (DCM), or dimethylformamide (DMF)
with 0.5 mM analyte concentration and 0.1 M tetrabutylammonium hexafluorophosphate as the
supporting electrolyte. Ohmic drop was compensated using the positive feedback compensation
implemented in the instrument. All electrochemical experiments were performed with internal
resistance compensation using the current interrupt (RUCI) method in AfterMath.

2.4.8 Spectroelectrochemistry
Vis-NIR spectroelectrochemistry was carried out using an OTTLE cells equipped with a Pt
minigrid auxiliary electrode, an Ag microwire pseudoreference electrode, a Pt minigrid working
56

electrode and CaF2 windows.
44
Spectra were obtained using a Lambda 950 UV/Vis/NIR
Spectrophotometer. A dichloromethane (DCM) solution of 0.25 M (TBA)(PF6) was used to better
solvate the produced reduced species and to avoid excess migration of reduced species to the
auxiliary electrode. Potentials were applied using a PalmSens EMStat3+. Electrolysis was
performed at 25 mV increments and monitored via Vis-NIR. Starting potentials were chosen where
no change in absorbance was observed for the initial isovalent species, and end points were
achieved where no change in absorbance was observed. Equilibrium was attained via electrolysis
for several minutes and verified using Vis-NIR. Reversibility was verified by reversing the
electrolysis and ensuring that the spectrum of the starting material could be regenerated. The molar
extinction coefficients for the MV species were obtained from the initial concentration of the
parent isovalent complex, absorbance of the MV species, and pathlength of the OTTLE cell.

2.4.9 Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT)
All calculations were run using the Q-CHEM program package.
45
Geometry optimizations were
run with unrestricted DFT calculations at the PBE level of theory using a 6-31G* basis set.
46–49

Solvation effects were considered using conductor-like polarizable continuum model (C-PCM).
50

TD-DFT calculations were performed using B3LYP level of theory using a 6-31G* basis set. Ten
excited states were considered and calculated for both singlet and triplet transitions.
51
Solvation
effects were considered for all optimization and single point energy calculation employed
dichloromethane (8.93)
30
as the model solvent, while acetonitrile (36.65)
30
was used for TD-DFT
calculations as this was the solvent used for experimental UV-Vis spectroscopy experimentation.
No substantial differences in the geometric values were observed upon comparing geometry
57

optimization calculations in vacuum or with C-PCM solvent considerations. Atomic van der Waal
radii used in C-PCM calculation were considered employing universal force field (UFF) values.
52

2.4.10 Synthesis of [Co
3
(triphos)
3
(THT)] [BF
4
]
3
(1
3+
)
Solids [Co (CH3CN)6][BF4]2 (155 mg, 0.323 mmol), and 1,1,1-
tris(diphenylphosphinomethyl)ethane (triphos) (194 mg, 0.310 mmol), along with 25 mL of
acetonitrile were added to a 250 mL Schlenk flask under N2. The reaction was stirred at room
temperature for 1 hour, during which the mixture turned a lime green color. After one hour,
triphenylene-2,3,6,7,10,11-hexathiol (THT) (45 mg, 0.107 mmol) and triethylamine (300 μL, 2.15
mmol) were added to the reaction mixture. After stirring at room temperature for two hours, a rust
red precipitate formed. After an additional 12 hours of stirring, the reaction vessel was exposed to
room atmosphere with ample mixing and bubbling using compressed atmospheric air, and the red
precipitate dissolved, forming a blue solution. The mixture was first vacuum filtered, and the
solvent of the filtrate was subsequently removed under vacuum, yielding a blue powder. The blue
powder was washed with ~700 mL of cold THF and recrystallized by vapor diffusion of diethyl
ether into an acetonitrile solution, to generate dark blue crystals in 80% yield.
1
H NMR (500 MHz,
DMSO-d6): δ 9.93 (s, 2H, C18H6S6), 7.29 (t, 6H, P(C6H5)2), 7.18 (m, 12H, P(C6H5)2)), 7.10 (t, 12H,
P(C6H5)2), 2.98 (s, 6H, PCH2), 2.00 (s, 3H, CH3).
31
P-{
1
H} NMR (202 MHz, DMSO-d6): δ 33.8
(br s).
19
F-{
1
H} NMR (470 MHz, DMSO-d6): δ -148.0 (m). Elem. Anal. Calcd for
C141H123Co3P9S6B3F12: C 62.13; H 4.55. Found: C 60.86; H 4.88.

58

2.4.11 Synthesis of [Co
3
(triphos)
3
(BHT)] [BF
4
]
3
(2
3+
)
Solids [Co (CH3CN)6][BF4]2 (343 mg, 0.718 mmol), and 1,1,1-
tris(diphenylphosphinomethyl)ethane (triphos) (625 mg, 0.643 mmol), along with 20 mL of
acetonitrile were added to a 250 mL Schlenk flask under N2. The reaction was stirred at room
temperature for 1 hour, during which the mixture turned a lime green color. After one hour,
benzenehexathiol (BHT) (271 mg, 0.248 mmol) and triethylamine (300 μL, 2.15 mmol) were
added to the reaction mixture. After stirring at room temperature for two hours, the solution turned
a blue-green color. After 12 hours of stirring, the reaction vessel was exposed to room atmosphere
with ample mixing and bubbling using compressed atmospheric air, and the solution slowly turned
more blue in color. The mixture was first vacuum filtered, and the solvent of the filtrate was
subsequently removed under vacuum, yielding a blue powder. The blue powder was washed with
~700 mL of cold THF and recrystallized by vapor diffusion of diethyl ether into an acetonitrile
solution, to generate dark blue crystals in 76% yield.
1
H NMR (500 MHz, DMSO-d6): 7.20 (t, 6H,
P(C6H5)2), 7.15 (m, 12H, P(C6H5)2), 6.88 (t, 12H, P(C6H5)2), 2.97 (s, 6H, PCH2), 2.01 (s, 3H, CH3).
31
P-{
1
H} NMR (202 MHz, DMSO-d6): δ 31.4 (br s).
19
F-{
1
H} NMR (470 MHz, DMSO-d6): δ -
148.0 (m). Elem. Anal. Calcd for C129H117Co3P9S6B3F12: C 60.16; H 4.58. Found: C 59.73; H 4.74.

2.4.12 Synthesis of [Co(triphos)(BDT)] [BF
4
] (3
+
)
The complex was synthesized using the reported procedure and was adapted to accommodate
tetrafluoroborate as the counterion herein.
22
To a 250 mL flask under N2, 411 mg (0.5 mmol) of
the synthesized cobalt[1,1,1-tris(diphenylphosphinomethyl)ethane][benzene-1,2-dithiolate]
(complex Co(triphos)(BDT), where triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane, and
59

BDT = benzene-1,2-dithiolate), 136 mg (0.5 mmol) of ferrocenium tetrafluoroborate (FcBF4), and
50 mL of dichloromethane were added, yielding a violet solution. The reaction was stirred for 2 hr
at room temperature. The mixture was transferred to a 1 L flask and stirred vigorously with ~ 700
mL of hexanes, yielding a purple precipitate, which was isolated by vacuum filtration and washed
with copious amounts of hexanes. The product was recrystallized by vapor diffusion of diethyl
ether into an acetonitrile solution, to generate violet crystals in 87% yield.
1
H NMR (500 MHz,
DMSO-d6) δ 8.29 (m, 2H, C6H4S2), δ 7.55 (m, 2H, C6H4S2), 7.26-7.05 (m, 30H, P(C6H5)2), 2.95
(s, 6H, PCH2), 1.98 (s, 3H, CH3).
31
P-{
1
H} NMR (202 MHz, DMSO-d6) δ 33.9 (s).
19
F-{
1
H} NMR
(470 MHz, DMSO-d6): δ -148.0 (m). Elem. Anal. Calcd for C47H43CoP3S2BF4: C 61.99; H 4.76.
Found: C 61.68; H 4.71.

2.4.13 Coordinates for Optimized Geometry (Charge and spin specified by first two digits
respectively for each structure. Atomic coordinates listed in an order of the X, Y , then Z coordinate
values, respectively.)
1
3+

3 1
C 0.2597921 0.0391097 -4.1329811
C -0.5016903 -1.1384727 -3.9316776
C -0.8244486 -1.5370481 -2.6275679
H -1.4018387 -2.4567702 -2.5089695
C -0.4386488 -0.7803799 -1.5049202
C -0.8100236 -1.1931224 -0.1433198
C -1.5883763 -2.3449758 0.0874133
H -1.9274247 -2.9642470 -0.7461153
C -1.9702149 -2.7299501 1.3801392
C -1.5585033 -1.9475452 2.4870639
C -0.7709862 -0.8089685 2.2753721
H -0.4812270 -0.2244485 3.1515368
C 0.4270954 0.8016321 0.7768355
60

C 0.8958779 1.5609532 1.8654649
H 0.7035925 1.2418810 2.8922530
C 1.6321520 2.7386931 1.6800883
C 1.8928455 3.1968757 0.3649816
C 1.4491714 2.4423149 -0.7291925
H 1.6735319 2.8203363 -1.7291817
C 0.7407138 1.2368507 -0.5581817
C 0.3237223 0.4244441 -1.7099430
C 0.6676056 0.7931342 -3.0246547
H 1.2647094 1.6878527 -3.2142582
C -5.3937852 -6.6305965 4.9764258
H -6.2725564 -6.1546360 5.4477489
H -5.7350765 -7.6139328 4.6051888
C -4.2784606 -6.8318421 6.0388312
C -4.1029132 -5.5491021 6.8919605
H -3.2195436 -5.6587587 7.5468237
H -4.9741213 -5.4236209 7.5613835
C -2.9319600 -7.2491635 5.3839410
C 4.4379137 7.4640275 4.6275599
H 3.6755275 7.6491632 5.4057558
H 5.4200760 7.5704743 5.1247578
C 4.3109484 8.5217404 3.5005785
C 5.2543715 8.1626174 2.3177322
H 6.2534348 7.8988111 2.7069028
H 5.3958053 9.0496904 1.6739841
C 2.8351619 8.6557337 3.0252216
H 2.7879350 9.2923923 2.1234078
H 2.2500708 9.1789757 3.8039151
C 4.7512501 9.8867351 4.0712708
H 4.6140844 10.6872734 3.3241126
H 5.8161018 9.8648698 4.3605369
H 4.1581247 10.1456795 4.9652422
Co -0.2475039 -0.9792273 -7.0838921
Co -3.1968068 -4.2702109 3.8658762
Co 3.2134242 5.4310216 2.1831376
P -1.8265287 -1.9898774 -8.2319560
P 1.2221021 -2.2813972 -7.9235603
P -4.8825259 -5.6414333 3.4828595
P -3.9235511 -3.9657461 5.9268114
P -1.9816146 -5.9044053 4.5112158
P 4.2922187 5.6885423 4.0840204
P 4.6398968 6.7958102 1.2145571
P 1.9321092 7.0662559 2.6637059
S 0.6445469 0.5188321 -5.7760116
S -1.0230257 -2.0478578 -5.3348829
S -2.9576031 -4.1471876 1.6879810
61

S -2.0716957 -2.4237980 4.0925721
S 2.2507631 3.6663457 3.0317612
S 2.7713407 4.7013077 0.1665734
C -4.7213810 -7.9746541 6.9782602
H -5.7198052 -7.7678494 7.4003785
H -4.7721255 -8.9326979 6.4328159
H -4.0116522 -8.0920451 7.8149896
P 0.0355382 0.4315999 -8.7392169
C 0.4196964 -0.3773891 -10.3693131
H -0.0565253 0.2377064 -11.1549795
H 1.5096106 -0.3138939 -10.5401450
C -1.5353897 -1.9923743 -10.0718456
H -1.9381660 -2.9492583 -10.4505946
H -2.1403391 -1.1947407 -10.5375834
C 0.8725189 -2.7951861 -9.6809371
H 0.4431212 -3.8121456 -9.6692114
H 1.8523198 -2.8781985 -10.1875505
C -0.0485882 -1.8498451 -10.5055528
C 0.0618735 -2.2592150 -11.9895393
H 1.0833029 -2.0810594 -12.3672508
H -0.1720310 -3.3302316 -12.1176816
H -0.6406230 -1.6776263 -12.6110188
H -2.2616351 -7.6579309 6.1628271
H -3.1059871 -8.0636564 4.6590229
C 1.5069077 -3.8558036 -7.0115029
H 1.8869184 -3.6052236 -6.0087160
H 0.5713546 -4.4216592 -6.8961405
H 2.2521311 -4.4718696 -7.5436458
C 2.9277569 -1.5858304 -8.0017254
H 3.6239857 -2.3654597 -8.3561088
H 2.9772409 -0.7309478 -8.6914852
H 3.2337454 -1.2497597 -6.9984907
C -1.4377874 1.4900348 -9.0927449
H -2.2649467 0.9064161 -9.5214884
H -1.7785504 1.9618758 -8.1568368
H -1.1611981 2.2807642 -9.8110858
C 1.3463717 1.7242633 -8.6020498
H 1.4418955 2.2402297 -9.5718196
H 1.0643991 2.4566235 -7.8301932
H 2.3163670 1.2870223 -8.3238076
C -2.9735232 -2.8735498 7.0723152
H -1.9109463 -3.1532817 7.1240391
H -3.0392635 -1.8325156 6.7213052
H -3.4161624 -2.9430462 8.0801069
C -5.5901934 -3.1744515 6.0201053
H -6.3730123 -3.8364830 5.6243148
62

H -5.8278970 -2.9352728 7.0706748
H -5.5774549 -2.2431820 5.4310105
C -4.6763700 -6.9320108 2.1783610
H -5.6542700 -7.4103567 2.0003952
H -4.3333406 -6.4630574 1.2426245
H -3.9593810 -7.7136846 2.4717552
C -0.3819649 -0.4078845 0.9839833
C -6.4013165 -4.7976443 2.8628777
H -7.2506623 -5.5014696 2.8424950
H -6.6665436 -3.9204507 3.4707176
H -6.1896277 -4.4518016 1.8382175
C -1.0523985 -6.7827544 3.1863648
H -0.5254459 -7.6539268 3.6115111
H -1.7361475 -7.1128377 2.3911568
H -0.3163216 -6.0908722 2.7479536
C 0.7089779 7.5093963 1.3598751
H -0.0014217 6.6757226 1.2427689
H 0.1595255 8.4181810 1.6594852
H 1.2095307 7.6764905 0.3957218
C 4.0707950 7.7105388 -0.2860795
H 3.5809744 7.0150240 -0.9855100
H 3.3709504 8.5237418 -0.0399032
H 4.9509541 8.1533981 -0.7824701
C 6.1420690 5.9599322 0.5416906
H 5.8315773 5.3823400 -0.3436168
H 6.8953541 6.7090811 0.2446011
H 6.5812400 5.2641307 1.2706846
C 6.0354452 5.0805255 4.0292214
H 6.0483491 4.0611227 3.6116233
H 6.6653557 5.7320730 3.4068350
H 6.4495109 5.0599434 5.0511821
C 3.6709768 4.8326916 5.5963170
H 4.2498530 5.1775995 6.4696934
H 2.6027468 5.0338685 5.7684139
H 3.8036751 3.7460894 5.4813043
C -0.6450309 -5.4494728 5.6930854
H -0.0209499 -6.3319266 5.9171678
H -0.0213575 -4.6592933 5.2473613
H -1.0737082 -5.0715102 6.6322583
C -2.1464142 -3.7742571 -7.8789641
H -2.3318236 -3.9234734 -6.8040735
H -3.0362952 -4.0972812 -8.4448509
H -1.2933396 -4.3974998 -8.1853894
C 0.8430484 6.8390025 4.1340259
H 1.4343702 6.6950523 5.0498052
H 0.2075162 7.7324170 4.2621480
63

H 0.2048800 5.9543895 3.9833843
C -3.5016779 -1.2680199 -7.9602915
H -4.2226926 -1.6842440 -8.6840027
H -3.8152852 -1.5311409 -6.9370067
H -3.4890308 -0.1717350 -8.0420466

2
3+

3 1
C 0.4574964 6.1309865 8.0202568
C 3.9532810 5.1836927 8.1615271
C 0.7459574 4.1816345 2.6527928
C 3.5145485 3.5401842 2.9214639
C 2.1714947 7.2980351 5.8728927
H 1.5468883 8.1375388 5.5165305
H 2.6639645 7.6472412 6.7971570
C 4.3902378 6.1192827 5.4236669
H 4.9927184 6.7517878 6.1021742
H 5.0748497 5.7776603 4.6268589
C 2.6248801 6.2459817 3.5716324
H 3.3366396 6.2906611 2.7263008
H 1.7216473 6.7931120 3.2492061
C 3.2478739 6.9626522 4.8024546
C 3.8553824 8.2960670 4.3136998
H 3.0991687 8.9049905 3.7887349
H 4.6915748 8.1126004 3.6170524
H 4.2385518 8.8853905 5.1643750
C 1.1675865 0.8226340 6.0008794
C -0.1095446 1.4169549 5.9781930
Co 1.8319972 3.9042540 5.8270890
S 2.5691808 1.8656303 6.0075725
S -0.2112294 3.1604958 5.9547096
P 1.0030581 5.9018520 6.2734294
P 3.8335283 4.6351977 6.4014251
P 2.1693074 4.4506929 3.7910319
C -1.2850551 0.5972442 5.9670867
C -1.1592998 -0.8046411 5.9452000
S -2.8904137 1.2859590 5.9912997
S -2.6163460 -1.7655140 5.8955437
C 0.1374159 -1.4104133 5.9709850
C 1.2915541 -0.6047078 6.0155930
S 0.3356482 -3.1452660 5.9619713
S 2.8474495 -1.3962717 6.0848398
C -5.3521989 -2.8806893 7.9281148
C -6.2703364 0.5746075 8.4348612
C -5.6227597 2.7303175 6.7064655
C -7.3748328 -1.8290929 6.0195942
64

H -7.8127803 -2.7643455 5.6271453
H -7.8415851 -1.6592232 7.0064563
C -7.5081971 0.7070094 5.7798699
H -8.3040119 0.8579923 6.5325910
H -7.6143878 1.5276387 5.0476310
C -6.8755158 -0.7278911 3.7562515
H -7.3521362 -0.0948267 2.9853063
H -6.8849372 -1.7594199 3.3628681
C -7.7156268 -0.6535667 5.0652531
C -9.2060276 -0.7691480 4.6799183
H -9.3901019 -1.6821705 4.0878447
H -9.5261865 0.1000112 4.0798086
H -9.8378905 -0.8131936 5.5835505
Co -4.2881198 -0.3802017 5.8488133
P -5.5541245 -2.1438818 6.2480661
P -5.8782950 0.9036406 6.6591007
P -5.0994241 -0.1682692 3.8843342
C 4.7935308 -3.5630267 8.3910823
C 2.2406248 -6.1040459 8.1803197
C 0.3226473 -6.2870750 6.1142235
C 3.5184393 -2.7018185 2.8311245
C 1.6069984 -4.8206109 2.9190908
C 5.1963044 -5.5546847 6.2192289
H 6.2734015 -5.4436805 5.9991175
H 5.1294321 -6.1768028 7.1293283
C 3.1246520 -6.8537676 5.4940122
H 3.2839024 -7.7146847 6.1688908
H 2.5679444 -7.2363601 4.6207416
C 4.3285919 -5.3459662 3.8084336
H 4.1235147 -5.9583574 2.9103641
H 5.2784322 -4.8180377 3.6129634
C 4.4952384 -6.2864111 5.0384943
C 5.3878087 -7.4716546 4.6121281
H 6.3462420 -7.1122789 4.1988022
H 4.8873921 -8.0800658 3.8390570
H 5.6085491 -8.1246752 5.4741029
Co 2.4691135 -3.5387536 5.9260073
P 4.5406470 -3.8520166 6.5877169
P 2.0294844 -5.6491367 6.4029639
P 2.9670815 -4.0827280 3.9212087
H -4.3596394 -3.3585435 7.9604544
H -6.1301122 -3.6413107 8.1101837
H -5.3903956 -2.1171526 8.7175751
H -6.7249806 -0.4166187 8.5693457
H -6.9742997 1.3379998 8.8076671
H -5.3426864 0.6234646 9.0283651
65

C -0.5193681 6.3923973 5.3533337
H 1.2160276 5.7756347 8.7320769
H -0.4543299 5.5290432 8.1634795
H 0.2293648 7.1912627 8.2203914
H 3.3329952 6.0727590 8.3435452
H 5.0015550 5.4282138 8.4025060
H 3.6121365 4.3707018 8.8225961
H 4.6416022 -2.4869882 8.5733577
H 5.8162729 -3.8469002 8.6906252
H 4.0646804 -4.1199126 8.9976056
H 3.2972024 -6.0867734 8.4828768
H 1.8472793 -7.1214825 8.3476017
H 1.6752864 -5.3948418 8.8062588
C -5.1381071 1.5245235 3.1567065
H -5.2954556 3.1260831 5.7339756
H -4.8489292 2.9647944 7.4536086
H -6.5677991 3.2187227 6.9970035
H -0.2974805 6.5328998 4.2854807
H -0.9015567 7.3439041 5.7601504
H -1.2968169 5.6196661 5.4582758
H -5.5673521 1.4686366 2.1416856
H -4.1171356 1.9317194 3.0962982
H -5.7573572 2.2041541 3.7604739
C -5.2633073 -3.6288875 5.1887780
H -0.0423759 -6.0683367 5.0995627
H -0.3613918 -5.8109151 6.8340096
H 0.3132252 -7.3780830 6.2769281
H -5.5038639 -3.4186466 4.1363267
H -5.9037826 -4.4550458 5.5405900
H -4.2091868 -3.9402062 5.2556001
C -4.1982532 -1.1096380 2.5804655
H 1.9803711 -5.0342803 1.9026726
H 0.7635009 -4.1156014 2.8548251
H 1.2525057 -5.7597248 3.3673395
H -4.1304738 -2.1761310 2.8411257
H -3.1764592 -0.7051618 2.5048765
H -4.7076937 -0.9959654 1.6079711
C 5.8312931 -2.7484546 5.8625052
C 5.2691052 3.4802092 6.3033897
H 5.9891464 -2.9647956 4.7950469
H 6.7845850 -2.9013491 6.3950870
H 5.5270804 -1.6954579 5.9667813
H 5.3371385 2.9943552 5.3185531
H 5.1523636 2.6967344 7.0678975
H 6.1991731 4.0395499 6.4982839
H 1.0065288 4.5249014 1.6369211
66

H -0.1465224 4.7181675 3.0062371
H 0.5157344 3.1048257 2.6242231
H 3.5627916 3.8731274 1.8705957
H 3.3054161 2.4598237 2.9566531
H 4.4896729 3.7258027 3.3935579
H 3.7833617 -3.0929167 1.8333832
H 4.3835495 -2.1777520 3.2634761
H 2.6936719 -1.9784728 2.7276101

3
+

1 1
C 0.8898390 0.6991335 -3.4168502
C 0.8313210 0.7931190 -4.8260612
H -0.1265425 0.6665727 -5.3398135
C 1.9983909 1.0514035 -5.5400919
H 1.9585157 1.1301583 -6.6306418
C 3.2338574 1.2096868 -4.8697374
H 4.1421391 1.4091357 -5.4460875
C 3.3063211 1.1114910 -3.4828148
H 4.2594449 1.2323299 -2.9588805
C 2.1274760 0.8594041 -2.7442896
C -0.2364741 -3.0228221 -1.1522465
C 1.8275857 -2.4906106 0.7475353
C -0.9613825 -2.2473125 1.5813880
H -1.8918893 -2.6588831 1.1493548
H -0.4951714 -3.0696769 2.1562116
C -1.3145324 -1.0840769 2.5545155
C -1.9338740 -1.7021723 3.8268915
H -2.2844276 -0.9167279 4.5189644
H -1.1955536 -2.3225837 4.3642426
H -2.7957004 -2.3442957 3.5741939
C -2.3633015 -0.1266762 1.9232893
H -2.4633436 0.7780985 2.5482743
H -3.3542716 -0.6168728 1.9236276
C -0.0430926 -0.2973645 2.9647390
H 0.7584008 -1.0049280 3.2439469
H -0.2563385 0.3099525 3.8647900
C 2.4291579 0.9766328 2.1249139
C 0.0740797 2.5294889 2.1848626
C -2.7320274 2.0557747 -0.0660458
C -3.1936714 -0.6512567 -0.8035443
Co 0.1208152 0.2335287 -0.3779167
P 0.1995724 -1.8236503 0.1783604
P -1.9995656 0.3781635 0.1629326
P 0.6453166 0.8471992 1.6577063
67

S -0.5333252 0.3732201 -2.4523154
S 2.1430711 0.7372516 -0.9993972
H 0.5071686 -2.9384216 -1.9607336
H -1.2225188 -2.7852840 -1.5768612
H -0.2344697 -4.0554408 -0.7621157
H 2.5815208 -2.3418364 -0.0416862
H 1.7371018 -3.5704170 0.9596192
H 2.1737468 -1.9823369 1.6599573
H 2.8729885 1.8463109 1.6157769
H 2.9983821 0.0856529 1.8201007
H 2.5174121 1.1121429 3.2162957
H 0.3831505 3.2653666 1.4241573
H 0.5470065 2.7919182 3.1466032
H -1.0176762 2.5847347 2.3077460
H -2.7783627 2.2405765 -1.1516724
H -2.0956375 2.8329787 0.3802531
H -3.7472945 2.1127948 0.3618993
H -3.0704999 -0.4481665 -1.8789597
H -4.2273903 -0.4022335 -0.5089357
H -3.0256275 -1.7251546 -0.6300301

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72

Chapter 3: Electrocatalytic CO
2
Reduction to Formate by a Cobalt Phosphino-Thiolate
Complex
3.1 Introduction
Due to the inherent unsustainable nature of fossil fuels, the development of renewable
energy alternatives, which can sustain the rising global energy demands without detrimental
environmental impacts, is paramount.
1,2
Unfortunately, large scale application of these sources
such as solar and wind, has been deficient due to their intermittent nature, with peak supply often
asynchronous with matching demand.
3
To combat this issue, the storage of renewable energy into
chemical bonds during peak supply has been proposed to mitigate this spatio-temporal demand
mismatch.
4–9
In one strategy, naturally abundant small molecules, such as H 2O and CO2, are
electrochemically converted into value added products, such as H 2 in the hydrogen evolution
reaction (HER) and C1 products in the CO2 reduction reaction (CO2RR).
7,10–16
Additionally, the
CO2RR provides a renewable means to produce CO as a C 1 feedstock for essential industrial
processes that produce methanol and diesel that would otherwise rely on fossil fuel sourced
substrates.
17–19
Current heterogeneous catalysts for the CO 2RR are often limited due to their low
selectivity, with their ill-defined surface mechanisms inhibitive towards rational iterative
improvement.
7,20
On the other hand, homogeneous catalysts can provide modular platforms that
can be synthetically tuned to increase rates and selectivities.
7,21,22
As a result, fundamental research
of homogeneous catalytic systems can provide insights into new chemical strategies to further
optimize and increase the efficiencies of future catalytic systems.

Scheme 3.1 Uses of formate/formic acid towards energy storage and conversion.
73

One possible route towards small molecule-based energy storage is the reduction of CO2
to formate/formic acid (Equation 1), due to its unique opportunities towards energy storage and
conversion. While other CO2RR products potentially need additional chemical steps to yield
substrates amenable to energy extraction, such as in the case of CO, formate/formic acid can be
directly and effectively employed in fuel cells.
23,24
Additionally, formic acid can be effectively
utilized as a easily transportable and non-toxic “liquid H2 carrier” which can release H2 via
oxidation back to CO2.
24–26
One strategy of reducing CO2 to formate is through hydrogenation with
gaseous H2 (Equation 2).
11,27
Drawbacks of this approach are the necessary high pressure and
temperatures needed to drive these reactions.
11,27
As a result, decoupling the addition of protons
and electrons through electrochemical methods has been a proposed solution to circumvent this
issue (Equation 1).
Amongst homogeneous electrocatalysts active in the CO 2RR, production of formate is
rare
7,27,28
, with purely selective formate producing electrocatalysts with FE > 85% and based on
Eearth-abundant elements even more scarce due to the competitive HER (Table 2.1).
24,29
Some
electrocatalysts of note that are highly selective towards formate production include iridium pincer
complexes that display faradaic efficiency (FE) of 85-97%.
30–32
Platinum phosphine complexes
were also reported to electrocatalyze the reversible conversion between CO2 and HCO2
-
with high
selectivity and low overpotential.
33–36
Dinuclear rhodium complexes were also reported as active
electrocatalysts for CO2 reduction to formate with FEs up to 93%.
37

Catalysts based on non-precious elements, such as the Fe-carbonyl clusters containing an
interstitial N atom have been reported to perform the CO 2RR selectively with FE as high as 96%
in aqueous media.
38
Notably, upon exchanging the nitrogen atom for carbon in the Fe-carbonyl
clusters, the selectively shifts toward the HER, indicating the hydricity of the cluster can be tuned
74

and it drastically affects reactivity.
39
A similarly selective cobalt-pentadienyl complex
incorporating a disphosphine ligand amended with two pendant amines was reported to selectively
produce formate with 90% FE and at high turnover frequency (TOF > 1000 s
-1
).
40
Moreover, ligand
functionalization has also been reported to alter CO 2RR product selectivity, as in the case of a
series of manganese carbonyl bipyridine and phenanthroline complexes where functionalization
of the ligand framework with tertiary amines within the secondary coordination sphere shifted
selectivity from CO production towards HCO2
-
production, with selectivity towards HCO2
-

reported as large as 89% FE.
41
Lastly, an iron tetradentate phosphine complex was reported to
display exceptionally high formate selectivity as large as 97% FE, and was shown to form
methanol in the presence of an amine cocatalyst.
42

Notably, these catalysts have been identified to produce formate via a hydride transfer
mechanism where CO2 inserts into the metal-hydride bond (Scheme 3.2a). It should be noted that
while this is the more commonly reported mechanism, a separate mechanism involving protonation
of the carbon atom of the metal-CO2 adduct is a possible pathway (Scheme 3.2b), albeit less
commonly reported.
43–45
Though these previous reported catalysts provide a useful knowledge in
understanding CO2RR formate selectivity, the scarcity of these reports in the literature necessitates
additional research into new catalytic platforms that can provide additional knowledge in
controlling the selectivity of these systems in the CO 2RR.

Scheme 3.2 Reported pathways of electrocatalytic CO 2RR to formate.

75

Table 3.1 Reported overpotentials and relative formate selectivity of active electrocatalysts
towards CO2RR. Table divided between formate selective (>85% FE, top section) electrocatalysts
and non-selective (≤85%, bottom section) electrocatalysts.

Catalytic System
Calculated
Overpotential vs
Fc/Fc
+
(mV)
a

Reported
Overpotential
vs Fc/Fc
+

(mV)
a

Formate
FE%

Reference
[CpCo(P
R
2N
R‘
2)]
2+

700-800

500-700

95-99

40

[Fe4N(CO)12]
-

230-440

230-440

95

38

fac-Mn(N∧N)(CO)
3
Br
370 370 89
41

[Fe(PP
3
)](BF
4
)
2

240 N/R 97
42

[Pt(dmpe)2]
2+

600 90 >90
33

76

[Ir(POCOP)]
+

650-690 N/R 85-93
32

(PN
H
P)IrH3
330 N/R 97
31

[Rh
2
L
2
(Phen)
2
](BF
4
)
2

640 N/R 93
37

Fe(
Me
crebpy)Cl
830 N/R 71-85
46

Me
P3CoCl
700 N/R 58
47

[Co(bpy)(pynt)2]
+

200 110-280 57-64
48

[(bdt)Mo(O)S2CuCN]
2-

570

800

74

49

77

[Ni(qpdt)2]
-

300

300

70

50

[FeN5Cl2]
+

310 310 75-80
44

a
Overpotential determined where electrolysis was performed and produced the highest formate
selectivity. N/R – not reported.

While the current state of artificial CO 2RR and HER catalysts is limited, evolution has
provided highly efficient catalytic systems for these reactions in biological settings. Enzymes such
as hydrogenase can reversibly catalyze the HER and the hydrogen oxidation reaction, while CO
dehydrogenase and Formate dehydrogenase can selectively and reversibly convert CO 2 to CO or
formate, respectively, near the thermodynamic potential.
11,18,51–53
While some research has
explored the reactivity of these enzymes directly as electroactive catalysts,
53–55
synthetic chemists
have studied metal complexes with common structural motifs located in the active sites of these
enzymes for insights into their catalytic performance. One such common motif is the extensive
presence of thiolate moieties, which have been subsequently incorporated into reported catalysts
for both the HER and CO2RR.
7,11,18,56,57
In one set of catalysts, thiolates and their heavier chalcogen
congeners were employed as ligands in cobalt bis(dithiolene) and bis(diselenolene) complexes.
58,59

Both catalytic systems were reported to exhibit high activity towards the HER with catalytic
turnovers (TONs), and TOFs as high as 9,000 and 3,400 h
-1
, respectively. These systems suggest
that chalcogenide-based ligands are an activating ancillary ligand in biologically inspired systems,
due to its propensity to act as a proton relay.
58,60–62
Based on the success of these homogeneous
78

species, the metal dithiolene motifs were incorporated into a heterogeneous metal-organic
frameworks and polymers, displaying exceptionally high activity and stability under aqueous
acidic electrocatalytic conditions.
63,64

Catalytic systems featuring metal thiolates moieties have also been studied for activity
towards the CO2RR. Cobalt pyridyl thiolates incorporating diphosphine ancillary ligands have
exemplified significant activity and selectivity towards electrocatalytic CO production with FE
>92%, and low overpotentials accessible due to the proton shuttling of the activating ligand.
65,66
A
similar pyridyl cobalt thiolate complex incorporating bipyridine ligands displayed markedly low
overpotentials, modest TOFs, selectivity towards formate production as high as 64%, but suffer
from catalyst deactivation due to CO poisoning.
48
A cobalt complex incorporating the non-
innocent phosphinobenzenethiolate ligand has been reported to produce variable CO:H 2 ratios as
a function of acid pKa with faradaic efficiencies >99%.
67
A structurally-derived formate
dehydrogenase-based catalyst has also been synthesized, comprising of a Ni bis(dithiolene) metal
center with the dithiolene ligands structurally similar to the molybdopterin motif found in the
active center of the enzyme.
50,68
This catalysts was notably selective towards formate production,
though a prior irreversible reduction of the ligand is necessary to produce the active catalyst.
50,68

Similarly, a biologically inspired bimetallic oxo Mo-Cu benzenedithiolate complex derived from
the active site of Mo-Cu CO dehydrogenase was reported to display formate selectivity of 74%,
with experimental data indicating that oxo transfer to CO 2 to form carbonate is necessary before
the active catalyst could be generated.
49
Though these prior studies have demonstrated positive
results of the use of metal complexes with metal-thiolate motif towards small molecule reduction,
there still exists a scarcity of catalytic reports on sulfur-based metal complexes towards these
catalytic processes, necessitating the continual study in this area.
79

Motivated by the success of aforementioned cobalt-based thiolate complexes towards the
electrocatalytic reduction of small molecules, herein we report the reactivity of a cobalt based
catalyst ([Co(triphos)(bdt)]
+
) incorporating 1,1,1,-tris(diphenylphosphinomethyl) ethane
(triphos) and 1,2-benezenedithiolate as the ancillary ligands towards the electrocatalytic CO 2RR
(Figure 3.1). A multidentate phosphine donor ligand was chosen as an ancillary ligand due to its
extensive use in catalysts active towards the chemical
11,47
and electrochemical
7,28,29,47
CO2RR.
Moreover, triphos was selected as the phosphine ligand of choice based on reports of Co-triphos
complexes active towards the HER
69
and towards the CO2 hydrogenation to methanol
70
, in addition
to a similarly constructed Fe(triphos)(bdt) complex reported as an active HER electrocatalyst.
71

Cyclic voltametric studies were performed to characterize the electrochemical behavior of
[Co(triphos)(bdt)]
+
under reducing conditions, and the reactivity of the complex in the presence
of CO2 and Brønsted acids was investigated. Controlled potential electrolysis studies were
employed to explore the selectivity of the catalyst under various conditions and the reaction
intermediates were synthesized and characterized via NMR spectroscopy to determine possible
mechanistic pathways. Lastly, density functional theory (DFT) computational methods were
utilized to help elucidate potential mechanistic pathways.

Figure 3.1 Chemdraw illustration of the synthesized cobalt triphosphine-thiolate complex
([Co(triphos)(bdt)]
+
).
80

3.2 Results and Discussion
3.2.1 Cyclic Voltammetry under Inert Conditions
Complex [Co(triphos)(bdt)]
+
was synthesized according to a reported literature
procedure.
72,73
Cyclic voltammograms (CVs) of [Co(triphos)(bdt)]
+
(0.45 mM) were obtained
under an N2 atmosphere using a glassy carbon electrode (GCE) in acetonitrile (MeCN) solutions
with 0.1 M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) as the supporting
electrolyte. All potentials are referenced versus Fc
+/0
and all CVs were first scanned cathodically
and subsequently returned anodically. CVs of [Co(triphos)(bdt)]
+
reveal a reversible redox couple
at -0.74 V. This reversible feature is attributed to a formal Co
III/II
process based on previous reports
and is assigned the [Co(triphos)(bdt)]
+/0
couple.
72–74
Upon scanning further cathodically, CVs
exhibit an irreversible feature at -2.08 V, and an quasi-reversible couple at -2.39 V, and are
assigned to the [Co(triphos)(bdt)]
0/-
and [Co(triphos)(bdt)]
-/-2
couples, respectively (Figure 3.2).

Figure 3.2 CVs of 0.5 mM of [Co(triphos)(bdt)] in a MeCN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s.
81

Figure 3.3 Cyclic voltammograms of 0.5 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates is 0.1 V/s.
Scanning anodically from this potential, two irreversible redox events are observed at -1.51
V and -1.13 V. These oxidation features do not appear in the CVs if the potential is reversed before
reaching [Co(triphos)(bdt)]
0/-
, suggesting that these oxidative events originate from the reduction
of [Co(triphos)(bdt)]
0
(Figure 3.3). The irreversible nature of the redox feature of the
[Co(triphos)(bdt)]
0/-
is likely due to a chemical/structural change to the complex in conjunction
with the associated reduction of the complex (vide infra). CVs of variable scan rates performed on
the [Co(triphos)(bdt)]
0/-
feature (Figure 3.4) do not display any change in the reversibility of the
feature, though some changes were observed in the return oxidation features. This may be a result
of the chemical step occurring too rapid to be observed compared to the CV timescale.
82

Figure 3.4 Cyclic voltammograms of 0.5 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 0.1 to 1 mV/s.
3.2.2 Chemical Reduction Studies
To understand the electrochemical behavior observed at the [Co(triphos)(bdt)]
0/-
couple,
[Co(triphos)(bdt)]
0
was chemically reduced using excess KC8 and in the presence of 18-crown-6
(Scheme 3.3). X-ray quality crystals were grown via vapor diffusion of n-pentane into
tetrahydrofuran solutions containing the reduced complex. Solid state structure of this species
indicates a four-coordinate cobalt complex with the triphos ligand acting as a bidentate ligand in
this structure, with one of the phosphine linkers disassociated from the metal center (Figure 3.5).
The angular structural parameter, τ, was calculated as follows:
𝜏 =
360° −( 𝛽 +𝛼 )
360° −2cos
−1
( −
1
3
)
(3)
where β and α are the two largest angles of the coordination center, when β > α. When τ
approximates 0 this corresponds to the metal complex displaying a square pyramidal geometry,
whereas when τ approximated 1, this corresponds to a trigonal bipyramidal geometry. The τ
parameter was calculated to be ~0.3 (equation 3.3), suggesting that the metal center adopts a
83

distorted square planar geometry, with a torsion angle of 30.91°. Additionally, a single potassium
cation chelated by 18-crown-6 is present as a counterion for each molecular unit in the lattice,
suggesting this complex can be identified as [Co(triphos)(bdt)]
-
.

Scheme 3.3 Synthetic procedure for the reduction of [Co(triphos)(bdt)]
0
to [Co(triphos)(bdt)]
-
.

Figure 3.5 Solid-state structure of [Co(triphos)(bdt)]
-
. Aryl and aliphatic protons, counterions,
and solvent molecules are omitted for clarity.

Table 3.2 Average selected bond lengths (Å) for [Co(triphos)(bdt)]
x
complexes, where x = 1,0, -
1.

Bond Length (Å)
Bond [Co(triphos)(bdt)]
+
[Co(triphos)(bdt)]
0
[Co(triphos)(bdt)]
-

Co–S 2.169(2) 2.223(1) 2.186(1)
Co–P
apical
2.183(2) 2.300(1) -
Co–P
basal
2.232(2) 2.212(1) 2.107(1)
C–S 1.734(4) 1.750(4) 1.750(3)
Reference
73

75
this work

84

Average bond lengths for [Co(triphos)(bdt)]
-
can be found in Table 3.2 in addition to those
reported for [Co(triphos)(bdt)]
+
and [Co(triphos)(bdt)]
0
.
73,75
Although an elongation of the Co–
S bond from 2.169(2) to 2.223(1) Å is reported upon reduction of [Co(triphos)(bdt)]
+
to
[Co(triphos)(bdt)]
0
, a subsequent contraction of the Co–S bond to 2.186(1) Å is observed upon
further reduction to [Co(triphos)(bdt)]
-
. On the other hand, consecutive contractions of the basal
Co–P bond (labeled as Co–Pbasal) are observed from 2.232(2) Å in [Co(triphos)(bdt)]
+
to 2.212(1)
Å in [Co(triphos)(bdt)]
0
, and then to 2.107(1) Å in [Co(triphos)(bdt)]
-
. Additionally, while the
C–S bond length was observed to slightly elongate from 1.734(4) to 1.750(4) Å upon reduction
from [Co(triphos)(bdt)]
+
to [Co(triphos)(bdt)]
0
, no change to the bond length is observed upon
generating [Co(triphos)(bdt)]
-
(C–S bond of 1.750(3) Å in [Co(triphos)(bdt)]
-
), indicating the
innocent nature of the dithiolene ligand at the [Co(triphos)(bdt)]
0/-
couple.

Figure 3.6 Variable temperature overlay of: a) 600 MHz
31
P-{
1
H} NMR spectra of
[Co(triphos)(bdt)]
-
in acetonitrile-d3; 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in b)
aromatic and c) aliphatic region in acetonitrile-d3. Temperature varied between 26 and -35 °C.
To determine if the solution-state structure of [Co(triphos)(bdt)]
-
is comparable to that of
its solid-state structure,
1
H and
31
P-{
1
H} nuclear magnetic resonance (NMR) spectrum of
[Co(triphos)(bdt)]
-
were acquired at varying temperatures (Figures 3.6-13). At 26 °C, the
31
P-
{
1
H} NMR spectrum of [Co(triphos)(bdt)]
-
in MeCN-d6 displays two broad peaks at δ 46.67 and
-30.96 ppm (Figure 3.6a and 3.7). At -35 °C (Figures 3.6a and 3.8), two sharp peaks are observed
85

at δ 41.75 and -28.20 ppm in a 2:1 ratio and are attributed to both the bound and un-bound
phosphines, respectively.

Figure 3.7 600 MHz
31
P-{
1
H} NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at 26 °C.

Figure 3.8 600 MHz
31
P-{
1
H} NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at -35 °C.
The
1
H NMR spectrum of [Co(triphos)(bdt)]
-
in MeCN-d6 at 26 °C (Figures 3.6b, c, and
3.9) displays two broad aliphatic singlets at δ 2.23 (s) and 0.41 (s) ppm in a 6:3 ratio corresponding
86

to the methylene and methyl moieties on the triphos ligand, respectively. Three aromatic signals
appear at δ 6.43 (m), 7.21 (m), and 7.24 (m) ppm attributed to the aromatic protons on both the
dithiolene and triphos ligand, in addition to a broad peak at δ 7.75 pm.

Figure 3.9 600 MHz
1
H NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at 26 °C.
At -35 °C, the
1
H NMR spectrum of [Co(triphos)(bdt)]
-
in MeCN-d6 (Figures 3.6b, c, and
3.10) displays three new features in the aliphatic region, including two doublets at δ 2.26 and 2.19
ppm and a singlet at δ 2.04 ppm in a ratio of 2:2:2 attributed to the individual methylene linkers
on both the ligated phosphine and the unbound phosphines, respectively, and three new aromatic
singlets at δ 8.10, 7.68, and 7.26 ppm, attributed to the aromatic protons of the unbound phosphine
of the triphos ligand.
87

Figure 3.10 600 MHz
1
H NMR spectrum of [Co(triphos)(bdt)]
-
in acetonitrile-d3 at -35 °C.

Figure 3.11 Overlay of 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in acetonitrile-d3.
Temperature varied between 26 and -35 °C.
88

Figure 3.12 Overlay of 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in acetonitrile-d3.
Temperature varied between 26 and -35 °C.

Figure 3.12 Overlay of 600 MHz
1
H NMR spectra of [Co(triphos)(bdt)]
-
in acetonitrile-d3.
Temperature varied between 26 and -35 °C.
The reversibility of the observed temperature-dependent solution-state changes was investigated
by remeasuring the
1
H and
31
P NMR spectra of [Co(triphos)(bdt)]
-
at room temperature, and the
observed spectra are identical to the ones observed previously. The presence of peak coalescence
and broadening in the proton and phosphorus resonances of [Co(triphos)(bdt)]
-
at room
89

temperatures is indicative of a fast exchange process on the NMR timescale. As the temperature is
decreased the exchange can be limited. The
31
P NMR spectrum at -35 °C is indicative of two
different phosphine environments in the 2:1 ratio, corresponding to the bound and deligated
phosphines, respectively. The
1
H NMR spectrum of [Co(triphos)(bdt)]
-
at low temperatures also
suggests that one of the phosphines in the triphos ligand is dissociated from the metal center. These
results indicate that the low temperature solution-state structure is in agreement to the solid-state
crystal structure. The
1
H and
31
P-{
1
H} NMR spectra of [Co(triphos)(bdt)]
-
at room temperature
suggest that the deligated phosphine is interchanging with the bound phosphine moieties in a fast
exchange process. Based on the variable temperature (VT) NMR data obtained of the bound
methylene linkers [Co(triphos)(bdt)]
-
, the exchange rate constant (kc) and the free energy of
activation (ΔG
‡
) at coalescence can be calculated as follows:
𝑘 𝑐 =
𝜋 Δ𝑣 √2
(4)
Δ𝐺 ‡
= 𝑎 𝑇 𝑐 [10.319+ log ( 𝑇 𝑐 /𝑘 𝑐 ) ] (5)

where Δv is the maximum peak separation of the associated resonances displaying exchange in
Hz, Tc is the temperature at coalescence in K, and a is 4.575 × 10
-3
kcal mol
-1
K
-1
s
-1
. As these
equations are only valid if the exchanging nuclei are not coupled, only the Δv (58.4 Hz at 238.15
K) and the Tc (277.4 K) of the exchanging bound methylene linkers of the triphos ligand were
considered. Based on equations 3.4 and 3.5, the kc and the free energy of activation ΔG
‡
were
determined to be 129 s
-1
and 13.5(5) kcal/mol at the coalescence temperature of 4° C, respectively.
Consequently, VT studies indicate similar low temperature solution state and solid-state crystal
structures where one phosphine moiety is deligated from the metal center, whereas at room
90

temperature a fast exchange is observed. This chemical step is likely the origin of the irreversibility
of the [Co(triphos)(bdt)]
0/-
couple.
3.2.3 Cyclic Voltammetry under Catalytic Conditions
The catalytic behavior of [Co(triphos)(bdt)]
+
was studied using CV experiments in the
presence of CO2 and with variable proton sources. CVs of [Co(triphos)(bdt)]
+
under CO2 display
enhanced current densities at potential corresponding to the irreversible [Co(triphos)(bdt)]
0/-
couple (Figure 3.13).

Figure 3.13 CVs of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the presence of 0.3
M H2O (blue) or 0.3 M TFE (green). Scan rate is 100 mV.
An anodic shift is observed at the onset of the [Co(triphos)(bdt)]
0/-
couple under CO2, indicative
of an association of CO2 to the metal center upon reduction, suggesting an EC mechanism.
76–78

Upon scanning anodically, the oxidative features at -1.51 V and -1.13 V are not observed under
CO2, suggesting that the faradaic process associated with the observed current response is catalytic
in nature and is consuming the electrons that would otherwise be available for oxidation at these
oxidation features. Addition of 0.3 M of a Brønstred acid, such as 2,2,2-trifluoroethanol (TFE),
91

under a CO2 atmosphere leads to the formation of a characteristic catalytic plateau, with a 5-fold
increase in the current density (Figure 3.13). Increasing the concentration of TFE yields an increase
in the catalytic current density and an anodic shift in the catalytic onset potential (Figure 3.14).
Upon reaching 0.7 M TFE, a new catalytic feature appears at -2.54 V, which progressively
increases in current density upon further titration of TFE (Figure 3.14). Addition of 0.3 M TFE
under N2 display only mild increases in the current response at the [Co(triphos)(bdt)]
0/-
couple
(Figures 3.15 and 3.16), which represents a 2.5-fold decrease in the current density displayed under
CO2. This result indicates that the current responses observed under CO 2 in the presence of TFE
are likely not largely contributed by direct TFE/proton reduction, similar to what is observed for
other active CO2RR catalysts.
79–82

Figure 3.14 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE. Scan rate is 100
mV/s.

92

Figure 3.15 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under N2 (black) and in the presence of 0.3 M TFE under an
atmosphere of N2 (green) and CO2 (blue). Scan rate is 100 mV/s.

Figure 3.16 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under N2 with increasing concentrations of TFE. Scan rate is 100
mV/s.
Addition of 0.3 M H2O as a proton source under a CO2 atmosphere yields a catalytic
response that displays a trace-crossing event upon scanning anodically (Figure 3.13). This
phenomenon has been previously attributed to the formation and subsequent reduction of a newly
93

formed species with a standard reduction potential more positive than that of
[Co(triphos)(bdt)]
0/-
.
83–85
Notably, this trace-crossing event is not observed in the CVs performed
with scan rates of 0.5 and 1 V/s (Figure 3.17),

suggesting that at fast scan rates the rate of formation
of this species is too sluggish to be observed on the CV timescale.
41
Titrations of H2O beyond the
0.3 M concentration yields CV traces that decrease in catalytic current, which is in contrary to
what is expected for CO2RR dependent on proton concentration, indicating a separate chemical
step is occurring in conjunction with the CO2RR (Figure 3.18).

Figure 3.17 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] and 0.5 M H2O under an atmosphere of CO2. Scan rates vary from
0.1, 0.5, and 1 mV/s.

94

Figure 3.18 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O. Scan rate is 100
mV/s.
Titration of H2O at lower concentrations (20-100 μM) yield CV traces that increase in current,
suggesting this additional chemical step is disfavored at low acid concentrations (Figure 3.19).
Titration of D2O were performed under a CO2 atmosphere at similarly low concentrations to
determine a H/D kinetic isotope effect (KIE) (Figure 3.20). Calculation of KIE values was derived
from previous reports and adapted for this study (Equation S4).
40

𝐾𝐼𝐸 = 𝑘 𝐻 2
𝑂 /𝑘 𝐷 2
𝑂 = (
𝑠𝑙𝑜𝑝 𝑒 𝐻 2
𝑂 𝑠𝑙𝑜𝑝 𝑒 𝐷 2
𝑂 )
2
(6)
𝑘 ∝ (
𝑖 𝑐𝑎𝑡 𝑖 𝑝 )
2
(7)

95

Figure 3.19 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O. Scan rate is 500
mV/s.

Figure 3.20 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of D2O. Scan rate is 500
mV/s.
As the observed reaction rate constant (k) is proportional to the square of icat/ip (Equation 3.6), KIE
values can be obtained by squaring the ratio of the slope of icat/ip as a function of [H2O] or [D2O],
respectively (Equation 3.7). In this study, icat is the peak current observed under catalytic
conditions at the catalytic couple – the [Co(triphos)(bdt)]
0/-
couple in this case, in the presence of
substrate and ip is the peak current at a reversible couple – the [Co(triphos)(bdt)]
+/0
couple in this
case, giving rise to a KIE of 5.9(8), indicating a hydride-based mechanism .

96

Figure 3.21 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under N2 (black) and in the presence of 0.3 M H2O under an
atmosphere of N2 (green) and CO2 (blue). Scan rate is 100 mV/s.

Figure 3.22 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdt)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under N2 with increasing concentrations of H2O. Scan rate is 100
mV/s.
Further addition of both H2O or D2O past 0.1 M leads to no subsequent increase in current,
indicative of saturation kinetics. Addition of 0.3 M H2O under N2 displays only a minor increase
in current at the [Co(triphos)(bdt)]
0/-
couple (Figure 3.21). The onset of the [Co(triphos)(bdt)]
0/-
97

couple shifts anodically as [H2O] increases, indicating association of a substrate upon initial
reduction (EC mechanism), which in this case suggests the formation of a metal-hydride (Figure
3.22). Additionally, the oxidative features at -1.51 V and -1.13 V are still present at all [H2O] under
N2, suggesting that the current response is not catalytic, and no faradaic process is present to
consume the electrons provided at the [Co(triphos)(bdt)]
0/-
couple which can be subsequently
oxidized. This result implies that [Co(triphos)(bdt)]
+
exhibits very little activity towards HER
with H2O as a proton source, and the current response exhibited under CO2 can exclude HER as a
dominant competitive faradaic process.
3.2.4 Controlled Potential Electrolysis
To identify and quantify the products generated at the observed catalytic features,
controlled potential electrolysis (CPE) experiments were performed in acetonitrile for 2 hours
under 1 atm of CO2 with either TFE or H2O as the proton source. CPEs were performed with both
TFE and H2O either at -2.15 V or at -2.60 V vs Fc
+/0
and with various acid concentrations to
determine if the product selectivity and total turnover number changes as a function of these
variables. At the end of the CPE experiment, gaseous products were sampled from the head space
of the electrolysis cell, and quantification was determined by gas chromatography (GC) analysis.
Products in the liquid phase were detected and quantified using
1
H NMR spectroscopy. Results of
these experiments are shown in Figure 3.23, 3.24, and Table 3.3. Turnover numbers (TONs) and
faradaic efficiencies (FE%) were determined from the CPE studies, based on established
equations:

𝐹𝑎𝑟𝑎𝑑𝑎𝑖𝑐 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 𝑡 ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒 𝑑 𝑎𝑐𝑐𝑜𝑟𝑑𝑖𝑛𝑔 𝑡𝑜 𝑐 ℎ𝑎𝑟𝑔𝑒 𝑝𝑎𝑠𝑠𝑒𝑑 𝑥 100 (8)

98

𝑇𝑢𝑟𝑛𝑜𝑣𝑒𝑟 𝑁𝑢𝑚𝑏𝑒𝑟𝑠 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (9)

Electrolysis of [Co(triphos)(bdt)]
+
in the presence of 0.3 M TFE at -2.15 V (Entry 1) yields
formate as the primary CO2 reduction product with a faradaic efficiency (FE) of 59%. Gaseous
products such as H2 and CO were detected at FE of 6% and 2%, respectively, yielding a combined
FE% of 67%.

Figure 3.23 Controlled potential electrolysis traces measured under 1 atm of CO 2. In all cases a
solution of 0.45 mM [Co(triphos)(bdt)]
+
in acetonitrile with 0.1 M [nBu4N][PF6] supporting
electrolyte was held at a potential of either -2.15 V or -2.60 V vs Fc/Fc
+
for 2 hours in the presence
of no added proton source (grey), 0.3 M TFE (red), 0.6 M H 2O (green), or 0.3 M H2O (blue and
purple). A control experiment was also conducted (black) where electrolysis was performed at -
2.15 V vs Fc/Fc
+
for 2 hours in the presence of 0.3 M H2O in the absence of catalyst. Inset displays
current in first few minutes of the electrolysis experiments.

99

Figure 3.24 Comparison of the controlled potential electrolysis results – Faradaic efficiencies
(FE%) and Turnover numbers (TONs) – in the presence of: a) 0.3 M TFE, 0.3 M H2O, or no
exogenous proton source (N/A) at -2.15 V vs Fc/Fc
+
, b) 0.3 M H2O at potentials of -2.15 or -2.60
V vs Fc/Fc
+
, and c) 0.3 or 0.6 M H2O at -2.15 V vs Fc/Fc
+
. All electrolyses were performed with
0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.

100

Table 3.3 Summary of the controlled potential electrolysis results and the conditions used for the
electrolysis of [Co(triphos)(bdt)]
+
in the presence of CO2 and a proton source. Electrolyses were
performed with 0.45 mM of [Co(triphos)(bdt)]
+
in a CH3CN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of CO2.
Entry Acid
Time
(hours)
[Acid]
(M)
Potential
(V) vs
Fc/Fc
+

Charge
(C) (±4)
H2 CO HCOO
-

Total
FE%
(±6)
FE%
(±3)
TON
(±0.3)
FE%
(±2)
TON
(±0.10)
FE%
(±3)
TON
(±1.5)
1 TFE
2
0.3 -2.15 11 6 0.2 2 0.04 59 1.8 67
2

H 2O
0.3 -2.15 20 10 0.6 4 0.25 84 4.9 98
3 0.3 -2.60 38 4 0.5 1 0.06 83 9.2 88
4 0.6 -2.15 16 7 0.3 1 0.04 80 3.7 88
5 - - -2.15 1 Trace Trace Trace Trace 3 0.01 3
6 H 2O 8 0.3 -2.15 93 10 2.6 1 0.39 91 24.3 >99

Upon employing H2O as a proton source (entry 2), CPE results display a significant shift
in selectivity towards formation of formate at 84%, with a near unity total FE% and increase in
total TON of CO2RR products. This high selectivity towards the CO 2RR at the expense of HER is
corroborated by substantially low current densities observed in the CVs of [Co(triphos)(bdt)]
+
in
the presence of H2O under N2 (Figure 3.22) compared to CO2 (Figures 3.13, 3.18, and 3.21). An
initial increase in the absolute value of the catalytic current is observed in the CPEs employing
H2O as a proton source (Figure 3.23), which is contrary to the initial decrease in current expected
in electrolysis experiments due to the initial rapid consumption of substrate in the electrode’s
double layer. This preliminary induction period may indicate the formation of a more active form
of the [Co(triphos)(bdt)] catalyst, which may also be associated with trace-crossing events
observed in the CVs experiments under CO 2 and in the presence of H2O. After this initial increase
in the absolute value of the catalytic current, stable currents are observed throughout the rest of
the CPE (Figure 3.23). Performing CPE at a larger overpotential (-2.60 V) in the presence of 0.3
M H2O (entry 3) yields higher HCOO
-
TONs, 9.2 compared to 4.9, at the expense of a loss of unity
101

total FE% due to lower quantities of side products such as H 2 (4%, 0.5 TON) and CO (1%, 0.4
TON) detected. Moreover, performing electrolysis with 0.6 M H 2O at -2.15 V (entry 4) results in
a reduction in the HCOO
-
TONs compared to Entry 2 from 4.9 to 3.7, along with a similar loss in
FE% from 84 to 80, in addition to similar loss of total faradaic efficiency (from 98% to 88%).
Notably, charge consumed throughout electrolysis is inversely proportional to acid concentration
for 0.3 vs 0.6 M H2O conditions (entry 2 vs 4). This decrease in catalytic performance upon
increasing the water concentration is in agreement with a decrease in the current density observed
in the CVs obtained in the presence of [Co(triphos)(bdt)]
+
and CO2 upon titration of H2O (Figure
3.18). Lastly, performing CPE at -2.15 V without an added exogenous Brønsted acid (entry 5)
yields only trace gaseous products and only minor amounts of formate detected with a TON of
0.01 and a FE% of 3%, indicating the presence of a proton donor is necessary for significant
product formation.
Due to high selectivity towards formate production in the presence of 0.3 M H 2O at -2.15
V, additional control, long term stability, and degradation experiments were performed under these
conditions. Control experiments in the absence of catalyst yield only trace amounts of CO 2RR
products (Figure 3.23), indicating the presence of [Co(triphos)(bdt)]
+
is necessary to generate the
products discussed above. Performing CPE experiments for 8 hours displays good current stability
and exhibits a notable increase in selectivity towards formation of formate at 91%, with a total
formate TON of 24.3 (Figure 3.25, and Table 3.3 entry 6).
102

Figure 3.25 Controlled potential electrolysis traces measured under 1 atm of CO 2 in an acetonitrile
solution of 0.45 mM [Co(triphos)(bdt)]
+
with 0.1 M [nBu4N][PF6] supporting electrolyte, which
was held at a potential of -2.15 V vs Fc/Fc
+
for 8 hour in the presence of 0.3 M H2O. Electrolysis
was paused at the 4 hr mark to adjust for potential drift of the Ag/Ag
+
pseudo reference electrode.
To determine if a catalytically active phase is depositing on the electrode during electrolysis, the
working electrode was rinsed with clean CH 3CN post-electrolysis and placed back in the working
compartment with a CH3CN solution containing 0.1 M [nBu4N][PF6] and 0.3 M H2O under an
atmosphere of CO2. CVs of the electrodes post-CPE display negligible current density, with
currents comparable to those observed using a bare glassy carbon electrode (Figure 3.26).
Performing a 2 hr-electrolysis with the rinsed post-CPE electrode produces only trace products,
suggesting no catalytically active phase is deposited on the electrodes during CPE (Figure 3.27).
103

Figure 3.26 Cyclic voltammograms of a bare glassy carbon electrode (black – labeled “bare
GCE”) and a washed post-electrolysis glassy carbon electrode (red – labeled “wash test”) in an
acetonitrile solution containing 0.1 M [nBu4N][PF6] and 0.3 M H2O under an atmosphere of CO2.
The post-electrolysis glassy carbon electrode (wash test) investigated here was generated upon
performing an electrolysis with a glassy carbon electrode for 8 hours at -2.15 V under CO2 and in
a CH3CN solution with 0.45 M of [Co(triphos)(bdt)]
+
, 0.3 M H2O, and 0.1 M [nBu4N][PF6], and
subsequently washed with clean CH3CN under anaerobic conditions to prevent O 2 exposure of the
electrode.

104

Figure 3.27 Comparison of the controlled potential electrolysis trace of a washed post-electrolysis
glassy carbon electrode in an acetonitrile solution containing 0.1 M [nBu4N][PF6] (black – labeled
“rinse test”) to that of the CPE trace of 0.45 M of [Co(triphos)(bdt)]
+
in MeCN with 0.1 M
[nBu4N][PF6] (red). Both electrolysis in this figure were performed at a potential of -2.15 V vs
Fc/Fc
+
for 2 hours in the presence of 0.3 M H2O and 1 atm of CO2. The post-electrolysis glassy
carbon electrode (rinse test) investigated here was generated upon performing an electrolysis with
a glassy carbon electrode for 8 hours at -2.15 V under CO2 and in a CH3CN solution with 0.45 M
of [Co(triphos)(bdt)]
+
, 0.3 M H2O, and 0.1 M [nBu4N][PF6], and subsequently washed with clean
CH3CN under anaerobic conditions to prevent O 2 exposure of the electrode.
The rinsed electrode was additionally analyzed using X-ray photoelectron spectroscopy (XPS) to
determine if a cobalt-containing species is deposited on the working electrode during electrolysis.
XPS spectra of the working electrode indicates trace amounts of cobalt and sulfur on the electrode
surface (Figure 3.28). XPS spectra of a glassy carbon electrode immersed in a 0.45 mM CH 3CN
solution of [Co(triphos)(bdt)]
+
exhibits similar Co 2p and S 2p features with the ones displayed
in the spectra of the post-electrolysis electrode, suggesting that that these features originate from
physiosorbed material, and that chemical deposition of cobalt-containing materials on the
electrode surface during electrolysis is unlikely (Figure 3.28).

105

Figure 3.28 High-resolution X-ray photoelectron spectroscopy spectra of 1) Co 2p and 2) S 2p
region of a a) washed post-electrolysis glassy carbon electrode, b) electrode immersed in 0.45 M
acetonitrile solution of [Co(triphos)(bdt)]
+
without applying any potential, and c) bare electrode.
Peak at ~169 eV in the S 2p region is associated with the Si 2p plasmon loss structure from residual
Si associated with Si-based polishing powder. The post-electrolysis glassy carbon electrode
investigated here was generated upon performing an electrolysis with a glassy carbon electrode
for 8 hours at -2.15 V under CO2 and in a CH3CN solution with 0.45 M of [Co(triphos)(bdt)]
+
,
0.3 M H2O, and 0.1 M [nBu4N][PF6], and subsequently washed with clean CH 3CN.
3.2.5 Catalytic Benchmarking and Comparisons
To evaluate the electrocatalytic activity of [Co(triphos)(bdt)]
+
, the CO2RR selectivity
towards formate and over-potential are compared relative to the values reported for other
molecular catalysts. Turnover frequency and the use of Tafel plot are avoided in this discussion
due to the inherent coupled chemical steps corresponding to [Co(triphos)(bdt)]
+
upon using H2O
as a proton source, as illustrated by CV and CPE plots, in addition to the lack of an “S” shaped
curve marking a purely kinetic regime, which makes these values not representative of the CO 2RR
106

catalytic kinetics. In that light, other factors such as relative overpotential and selectivity towards
formate are used to help compare the activity of [Co(triphos)(bdt)]
+
relative to other reported
catalysts. The overpotential for the CO2RR to formate ( η ) is determined by taking the difference
of the standard reduction potential of CO 2/HCOOH relative to the applied overpotential and is
considered at the applied CPE potential of -2.15 V vs Fc/Fc
+
where high selectivity towards
formate production was displayed. Calculation of the standard reduction of potential of CO 2 to
HCOOH was adapted from previous reports and adapted to this study (Equation 3.10).
40,86

𝐸 𝐶 𝐻 3
𝐶𝑁
0
(
𝐶 𝑂 2
𝐻𝐶𝑂𝑂𝐻 ) = 𝐸 𝑎𝑞
0
(
𝐶 𝑂 2
𝐻𝐶𝑂𝑂𝐻 ) −
𝑅𝑇𝑙𝑛 10
𝐹 𝑝 𝐾 𝑎 ( 𝐶 𝐻 3
𝐶𝑁 )
( 𝐻 2
𝐶 𝑂 3
)−
𝑅𝑇
2𝐹 ln (
𝐾 ℎ,𝐶 𝑂 2
,𝑎𝑞 /𝑔 𝐾 ℎ,𝐶 𝑂 2
,𝐶 𝐻 3
𝐶𝑁 /𝑔 )−
2∆𝐺 𝑡 ,𝐻 +
,𝐶 𝐻 3
𝐶𝑁 /𝑎𝑞
0
−∆𝐺 𝑡 ,𝐻𝐶𝑂𝑂𝐻 ,𝐶 𝐻 3
𝐶𝑁 /𝑎𝑞
0
2𝐹 (10)

𝐸 𝑎𝑞
0
(
𝐶 𝑂 2
𝐻𝐶𝑂𝑂𝐻 ) = −0.197 𝑉 𝑣𝑠 𝑁𝐻𝐸
∆𝐺 𝑡 ,𝐻 +
,𝐶 𝐻 3
𝐶𝑁 /𝑎𝑞
0
= −0.48 𝑒𝑉
∆𝐺 𝑡 ,𝐻𝐶𝑂𝑂𝐻 ,𝐶 𝐻 3
𝐶𝑁 /𝑎𝑞
0
= −0.063 𝑒𝑉
𝐾 ℎ,𝐶 𝑂 2
,𝑎𝑞 /𝑔 = 29
𝐾 ℎ,𝐶 𝑂 2
,𝐶 𝐻 3
𝐶𝑁 /𝑔 = 3.6

When H2O is added to the system, carbonic acid is employed as the proton source as it is the
strongest acid in solution as a result of hydration of CO 2 in the presence of H2O (pKa = 17.03).
Potential was referenced vs Fc/Fc
+
couple with EAg/AgCl = 0.210 V vs NHE and EFc/Fc
+
= 0.51 V vs
Ag/AgCl and further adjusted with the inter-liquid junction potential (EL/CH3CN = 0.099 V).
107

Considering this, the standard reduction potential of CO 2/HCOO
-
in acetonitrile is calculated to be
-1.40 V vs Fc/Fc
+
. Using this method, an overpotential of 750 mV was determined and is compared
with other reported electrocatalysts for electrocatalytic CO 2RR with their associated overpotentials
and relative selectivities (Table 3.1). Selective electrocatalytic conversion of CO 2 to formate >
85% FE are quite rare amongst reported electrocatalysts active in the CO 2RR, making
[Co(triphos)(bdt)]
+
an effective electrocatalyst towards selective conversion of CO 2 to formate
over other side reactions such as HER. Amongst formate selective catalysts employing Earth-
abundant metals, [Co(triphos)(bdt)]
+
displays comparably high selectivities similar to those
reported for other catalytic systems such as fac-Mn(N∧N)(CO)3Br
41
, Fe4N(CO)12
38
, and
[Fe(PP3)](BF4)
42
, though at moderately higher operating overpotential. Interestingly, while
[Co(triphos)(bdt)]
+
exhibits significant similarities to [CpCo(P
R
2N
R‘
2)I]
40
in both structure,
selectivity, and effective overpotential, [Co(triphos)(bdt)]
+
displays longer term electrolytic stability
(~8hr) compared to [CpCo(P
R
2N
R‘
2)I] (~1hr).
40
Additionally, [Co(triphos)(bdt)]
+
exhibits
superior formate selectivity compared to reported catalytic systems featuring metal thiolates
moieties, which have not been reported to exceed formate selectivity >74% FE and have not been
reported to employ water as the exogenous proton source.
48–50

108

3.3.6 Mechanistic Discussion and Computational Studies

Scheme 3.4 Proposed mechanism for electrocatalytic CO 2RR to HCOO
-
employing
[Co(triphos)(bdt)]
+
.
Based on experimental data, a proposed mechanism for the electrochemical conversion of
CO2 to HCOO
-
utilizing [Co(triphos)(bdt)]
+
can be found in Scheme 3.4. To supplement
experimental results, DFT calculation were employed to help elucidate potential intermediates and
pathways that were not accessible via experimental methods. As formate was found to be the
primary CO2RR product, mechanistic discussion will be limited to this product. Based on obtained
electrochemical data, we only observe enhanced currents in the presence of CO 2 and a proton
source at the [Co(triphos)(bdt)]
0/-
couple, suggesting generation of [Co(triphos)(bdt)]
-
(III) is
necessary before any catalytic activity is observed. Moreover, based on chemical reduction
109

experiments, we can determine an additional chemical step in the form of apical phosphine
deligation occurs concurrently upon reduction of [Co(triphos)(bdt)]
0
(II). CVs of
[Co(triphos)(bdt)]
+
display an anodic shift at the [Co(triphos)(bdt)]
0/-
couple under an
atmosphere of CO2 and in the presence of a proton source under N 2, indicating the favorable
binding of both CO2 and H
+
. As a result, DFT calculations were performed to model both Co-H
(IV-H) or Co-CO2 (IV-CO
2
) adducts to study their role in the reduction of CO 2 to formate.
Modelling of IV-CO
2
yields a favored structure with CO2 bound apically in a position trans
to the methyl moiety of the triphos ligand within a square pyramidal metal coordination
environment favored by 4.8 kcal/mol compared to its associated isomer (Figures 3.29 and 3.30

Figure 3.29 Calculated relative energies of potential IV-CO
2
isomers at the 6-31+G*/B3LYP
level of theory.
110

Figure 3.30 Calculated optimized structure of IV-CO
2
at the 6-31G*/B3LYP level of theory.
Phenyl substituents removed for clarity.

Formation of IV-CO
2
has a free energy change of -3.6 kcal/mol. This considerably low free energy
change can be rationalized due to the large degree of ligand reorientation from that of a distorted
square planar structure of III to that of a planar geometric orientation in IV-CO
2
upon adduct
formation (Figure 3.30). These results, in addition to CO being detected only as a negligible
product, indicates that the formation of the CO 2 bound adduct is not the predominant mechanism,
and the metal hydride pathway will be considered in this discussion continuing forward.
A large KIE value of 5.9(8) supports a hydride-based mechanism, with a rate limiting step
(RLS) most likely involving a Co-H formation or transfer step, as reported in related
electrocatalysts with similarly large KIE values.
40,48
Modelling of IV-H yields a similar result to
IV-CO
2
, with the optimized structure displaying a hydride adduct in a position trans to the methyl
moiety of the triphos ligand, albeit with a smaller relative difference in energy of 1.7 kcal/mol,
indicating that both of these isomers could be present in solution (Figure 3.31). Notably, the
calculated structure of IV-H retains the distorted square planar structure of both phosphine and
thiolate ligands in II, with the proton in the apical position (Figure 3.32).

111

Figure 3.31 Calculated relative energies of potential IV-H isomers at the 6-31+G*/B3LYP level
of theory.

Figure 3.32 Calculated optimized structure of IV-H at the 6-31G*/B3LYP level of theory.
Phenyl substituents removed for clarity.
Protonation of the dithiolene was also explored due to previous reports of protonation as an initial
step before turnover on similarly constructed complexes.
62,67,87
Due to both thiolate moieties being
symmetrically inequivalent, four possible thiol permutations were calculated (Figure 3.33).
Comparing the calculated IV-H structure to the lowest energy Co(S-H) state indicates the metal
hydride is the thermodynamically favored product over the thiolate protonation product by 21.4
kcal/mol (Figure 3.34).
112

Figure 3.33 Calculated relative energies of potential [Co-SH]
0
isomers at the 6-31+G*/B3LYP
level of theory.

Figure 3.34 Calculated relative energies of IV-H and [Co-SH]
0
at the 6-31+G*/B3LYP level of
theory.

Based on the optimized structure of IV-H and the calculated energy of a solvated free hydride, the
hydricity (ΔGH-) of IV-H was calculated compiutationally according to the following equation:
113

𝐺 𝑀𝐻
− 𝐺 𝑀 − 𝐺 𝐻 −
= Δ𝐺 𝐻 −
(11)

where GMH is the free energy of the computed metal hydride structure, G M is the free energy of the
computed complex by itself (without the hydride), and G H- is the computed free energy of the free
hydride. Using Equation 3.11, the hydricity of IV-H was calculated to be 58.7 kcal/mol. This ΔGH-
value is larger compared to that reported for the hydricity of formate in acetonitrile (44 kcal/mol)
suggesting that a formal hydride transfer from IV-H to CO2 is not thermodynamically favored.
33,88

Similar reports on cobalt hydrides suggest an additional formal reduction of the generated Co(III)-
H to Co(II)-H is necessary before a hydride transfer to the substrate can be thermodynamically
driven.
40,48,69
As a result, the 1e
-
reduction of IV-H was considered and produces complex V. The
optimized structure of V displays a distorted square pyramidal structure (τ = 0.62) with the hydride
ligand in the axial position (Figure 3.35).

Figure 3.35 Calculated optimized structure of V at the 6-31G*/B3LYP level of theory. Phenyl
substituents removed for clarity.

Calculating the hydricity of V yields a ΔGH- of 37.8 kcal/mol, indicating that reduction of IV-H to
V is necessary to produce a Co-H hydridic enough to convert CO2 to HCOO
-
. Moreover, the
reduction potential of IV-H to V was calculated according to the following equation:
41

114

𝐸 𝑋 𝑎 /𝑋 𝑎 +𝑛 0
= 𝐸 𝐹𝑐 /𝐹 𝑐 +
0
−
∆𝐺 𝑋 𝑎 /𝑋 𝑎 +𝑛 𝑛𝐹
(12)
where E
0
X
a
/X
a+n
is the standard reduction potential of X
a
/X
a+n
couple vs the ferrocene/ferrocenium
couple in acetonitrile, ΔGX
a
/X
a+n
is the change in the Gibbs free energy of the X
a
/X
a+n
couple, F is
23.0605 kcal mol
-1
V
-1
, n is the number electrons consumed in the conversion of X
a
to X
a+n
, and
E
0
Fc/Fc
+
is the absolute standard reduction potential of ferrocene/ferrocenium couple in acetonitrile
(-4.804 V).
89,90
Using Equation 3.12, the potential at which IV-H is reduced to V was calculated
to occur at -1.61 V vs Fc/Fc
+
, well anodic of potentials where onset of catalysis is observed,
indicating a strong electrochemical driving force for the conversion to V upon protonation of III.
With this in mind, the trace-crossing behavior observed in CVs under catalytic conditions can be
rationalized as follows. Protonation of III to form IV-H

is proposed to be rate limiting, in
agreement with large KIE values discussed above. Since formation of IV-H

from III is rate
limiting, it is possible that III can diffuse away from the electrode before reacting with proton
donors in the bulk solution to form IV-H. Species IV-H can then diffuse back towards the
electrode where it is rapidly reduced at the electrode to produce V, yielding the observed trace-
crossing at low scan rates. CVs at high scan rates do not display this trace-crossing behavior as the
rate of formation of IV-H is too sluggish to be observed on the CV timescale (Figure 3.17).

115

Figure 3.36 Calculated optimized structure of VI at the 6-31G*/B3LYP level of theory. Phenyl
substituents removed for clarity.
Intermediate V is proposed to react with CO2 and lead to the formation of the formato-complex
VI (Figure 3.36), followed by deligation of formate and religation of the apical phospine linker to
regenerate II in a stepwise or concerted manner. Intermediate V can also directly reduce exogenous
proton donors in solution to form H2 as is observed in the electrolysis experiments.

3.3 Conclusions.
This report focuses on the investigation of the electrocatalytic activity of
[Co(triphos)(bdt)]
+
towards the CO2RR. In the presence of an exogenous proton source such as
H2O, selective electrochemical conversion of CO 2 to HCOO
-
is observed with faradic yields as
high as 91% at an overpotential of 750 mV. The catalyst displays robust stability, with 8 hour CPE
experiment displaying negligible reduction in current and no evidence of deposition on the
electrode during electrolysis. Chemical reduction studies of [Co(triphos)(bdt)]
+
indicate that
deligation of the apical phosphine likely occurs before catalysis. A mechanism is proposed to occur
through a hydride transfer pathway, and DFT calculation indicate an additional reduction of the
[Co(triphos)(bdt)(H)]
0
to [Co(triphos)(bdt)(H)]
-
is necessary for turnover, suggesting an overall
ECEC mechanism. Ultimately, this study provides additional experimental evidence towards the
beneficial role sulfur-based moieties play in molecular metal complexes as a method to increase
116

their selectivity as electrocatalysts towards CO 2RR. Further studies are underway to improve
aspects of this catalyst, such as the relatively large overpotential, through functionalization of the
ancillary ligands.
3.4 Experimental Methods
3.4.1 General
All manipulations of air and moisture sensitive materials were conducted under a nitrogen
atmosphere in a Vacuum Atmospheres drybox or on a dual manifold Schlenk line. The glassware
was oven-dried prior to use. All solvents were degassed with nitrogen and passed through activated
purchased from commercial vendors and used without further purification.
3.4.2 Single-crystal X-ray Diffraction
An opaque black plate specimen was mounted for the X-ray crystallographic analysis, with
approximate dimensions of 0.58 × 0.383 × 0.209 mm
3
. The X-ray intensity data were measured on
a XtaLAB Synergy, Dualflex, HyPix system equipped with a micro-focus sealed tube (Cu Kα λ =
1.54184 Å), a goniometer (4-axis kappa with telescopic detector sled), and a detector (HPC HyPx-
6000HE 77.5 × 80.3 mm
2
). Data was collected on CrysAlisPro 1.171.41.122a (Rigaku OD, 2021)
and a total of 2584 frames were collected and integrated. The SHELXT 2014/5 Software Package
was used to determine the structure solution with direct methods. The SHELXL Software Package
was used for refinement by full-matric least-squares on F2. OLEX2-1.5 program was used for both
structure solution and refinement.

3.4.3 NMR Spectroscopy
Proton NMR spectra were acquired at room temperature using Varian (Mercury 400 2-Channel,
VNMRS-500 2-Channel, VNMRS-600 3-Channel, and 400-MR 2-Channel) spectrometers and
referenced to the residual
1
H resonances of the deuterated solvent and are reported as parts per
117

million (ppm) relative to tetramethylsilane (
1
H: MeCN-d3, δ 1.94 ppm).
31
P resonances are reported
as parts per million relative to an external sample of 85% H3PO5, which is set as 0 ppm.

3.4.4 Variable Temperature NMR Spectroscopy
Variable temperature (VT)
1
H and
31
P-{
1
H} NMR spectra were acquired using Varian VNMRS-
600 3-channel and referenced to the residual
1
H resonances of the deuterated solvent and are
reported as parts per million (ppm) relative to tetramethylsilane (
1
H: MeCN-d3, δ 1.94 ppm).
31
P
resonances are reported as parts per million relative to an external sample of 85% H3PO4, which
is set as 0 ppm. The authentic probe temperature for each set experiment was verified using an
external methanol standard. Reversibility was verified by ensuring that the spectrum of the starting
material could be regenerated upon returning to room temperature.

3.4.5 X-ray Photoelectron Spectroscopy
XPS data were collected using a Kratos AXIS Ultra instrument. The monochromatic X-ray source
was the Al K α line at 1486.6 eV. High-resolution detailed scans, with a resolution of ~0.1 eV,
were collected on individual XPS lines of interest at a pass energy of 20. The sample chamber was
maintained at < 2 × 10
–8
Torr. The XPS data were analyzed using the CasaXPS software.

3.4.6 Cyclic Voltammetry (CV)
Electrochemistry experiments were carried out using a Pine potentiostat. The experiments were
performed in a single compartment electrochemical cell under a nitrogen atmosphere using a 3
mm diameter glassy carbon electrode as the working electrode and a platinum wire as auxiliary
electrode. The reference electrode was a Ag wire in a 0.1 M electrolyte solution in MeCN and was
separated from the rest of the solution by a Vycor tip. Ohmic drop was compensated using the
positive feedback compensation implemented in the instrument. All experiments in this paper were
referenced relative to ferrocene (Fc) with the Fe
3+/2+
couple at 0.0 V. All electrochemical
118

experiments were performed in acetonitrile (MeCN) with 0.45 mM analyte concentration and 0.1
M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. All
electrochemical experiments were performed with iR compensation using the current interrupt
(RUCI) method in AfterMath.

3.4.7 Controlled Potential Electrolysis.
Controlled potential electrolysis (CPE) measurements to determine Faradaic efficiency were
conducted in a sealed two-chambered H cell with two chambers separated by a fine porosity glass
frit. The first chamber held the working and reference electrodes in 40 mL of electrolyte solution
(0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in MeCN and the second chamber
held the auxiliary electrode in 21 mL of electrolyte solution. The reference electrode was a Ag
wire in a 0.1 M TBAPF6 electrolyte solution in MeCN and was separated from the rest of the
solution by a Vycor tip. CPE experiments were performed in 0.1 M TBAPF6 solution in MeCN
with 0.45 mM of analyte. Glassy carbon plate electrodes (6 cm × 1 cm × 0.3 cm; Tokai Carbon
USA) were used as the working and auxiliary electrodes. The working compartment was sparged
with N2 or CO2 for 15 minutes before the experiment. Wash tests were performed by removing the
post electrolysis solution of the working compartment from the H-cell via syringe under a positive
pressure of CO2 and rinsing the chamber of the electrolysis cell three times with acetonitrile. The
cell was maintained under 1 atm of CO 2, and the electrode was not removed from the cell during
these washings to prevent O2-exposure. No visible material is present on the electrode following
CPE, and negligible current densities are observed in the wash test relative to any of the preparative
CPE studies. As such, these studies strongly suggest that the observed reactivity is due to the
solubilized catalyst, as no gaseous or liquid products are observed following the wash procedure.

119

3.4.8 Gas Chromatography
Gaseous products were quantified using a Shimadzu GC-2010-Plus instrument equipped with a
BID detector and a Restek ShinCarbon ST Micropacked column. In a typical experiment, 2 mL of
gas were withdrawn from the headspace of the electrochemical cell with a gas-tight syringe and
injected into the instrument. Calibration plots were prepared with multiple injections of syngas
standards purchased from Praxair, Inc.

3.4.9 Formate Detection and Quantification
Detection: Initial detection of formate was performed according to literature precedent.
91

Following CPE studies, a 5 mL aliquot of each electrolysis solution was collected and extracted
with 2 mL D2O. The aqueous portion was acidified with one drop HCl and tested for formate by
1
H NMR spectroscopy.
Quantification: Quantification of formate was performed according to the following procedure.
A NaOH solution (0.25 mL of 0.075 mM) was added to 5 mL aliquot of the electrolysis solution,
and the resulting solution was subsequently reduced under pressure and mild heating until dry.
Two 1 mL aliquots of D2O were added sequentially to the dry solid, and the resulting mixture was
mixed vigorously, and sonicated for 5 minutes, before being filtered through a glass frit. One mL
from the filtered solution was transferred to an NMR sample tube. A capillary containing a solution
of DMF in D2O was added to the NMR sample tube. A
1
H NMR spectrum (128 scans with a 10 s
relaxation time) was taken for each sample in a Varian 400-MR 2-channel instrument. Integration
ratios were then taken of the formate proton (δ = 8.47 ppm) and the DMF formyl proton (δ = 7.96
ppm). The formate concentration of the electrolysis solution was then determined using the
calibration plot described below.
Calibration: Standard formate solutions were prepared with concentrations of 0.1 M, 0.01 M,
0.001 M, and 0.0001 M. A
1
H NMR spectrum was taken for each solution with the aforementioned
120

DMF/D2O capillary, and a calibration curve was generated consisting of the integration ratios of
formate to DMF against the known formate concentration.

3.4.10 Density Functional Theory (DFT)
All calculations were run using the Q-CHEM program package.
92
Geometry optimizations were
run with unrestricted DFT calculations at the B3LYP level of theory using a 6-31G* basis set.
93–
98
Solvation effects were considered using the SMD implicit solvent model, with acetonitrile as
the modeled solvent.
99
Single point energy calculations were run on each computed structure with
a larger 6-31+G* basis set for all atoms and energies from these calculations were used throughout.
Vibrational frequency calculations were run from each computed structure to confirm the lack of
imaginary frequencies in geometry optimized structures.
3.4.11 Synthesis of [18-crown-6(K)][Co(triphos)(bdt)]
In a glove box, 47 mg (0.057 mmol) of the synthesized cobalt[1,1,1-
tris(diphenylphosphinomethyl)ethane][benzene-1,2-dithiolate] (complex Co(triphos)(bdt),
where triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane, and bdt = benzene-1,2-dithiolate),
was added to a 20 mL scintillation vial. The vial was then charged with 12 mL of THF and allowed
to stir at room temperature until all the powder dissolved. KC8 (23 mg, 0.17 mmol) was added to
the vial, whereupon the reaction mixture slowly turned from an amber color to a dark yellow color.
The reaction mixture was then stirred for 1 hr at room temperature, after which the resulting
solution was filtered. 18-crown-6 (15 mg, 0.057 mmol) was added to the filtered reaction mixture
and the solution was allowed to stir overnight at room temperature. The volatiles were removed
under vacuum and black crystals (60% yield) were generated upon recrystallization of the resulting
solid by vapor diffusion of n-pentane into a THF solution.
1
H NMR (500 MHz, CH3CN-d3, -30
°C): δ 8.10 (s, 2H, meta-P(C6H5)2), δ 7.68 (s, 4H, ortho-P(C6H5)2), 7.26 (s, 4H, para-P(C6H5)2),
121

7.22 (t, 8H, ortho/meta-P(C6H5)4), 7.18 (t, 8H, ortho/meta-P(C6H5)4), 7.17 (t, 4H, para-P(C6H5)4),
7.14 (m, 2H, C6H4S2), 6.39 (m, 2H, C6H4S2), 3.51 (s, 24H, 18-crown-6), 2.26 (d, 2H, PCH2), 2.19
(d, 2H, PCH2), 2.04 (s, 2H, PCH2), 0.23 (s, 3H, CH3).
31
P-{
1
H} NMR (202 MHz, CH3CN-d3, -30
°C): δ 41.8 (br s), 28.2 (br s).

3.4.12 Coordinates of DFT-computed structures (Charge and spin specified by first two digits
respectively for each structure. Atomic coordinates listed in an order of the X, Y, then Z
coordinate values, respectively.)

I
1 1
C 0.9271373 0.5690094 -3.4899568
C 0.8956738 0.5533566 -4.8981657
H -0.0320388 0.3203339 -5.4139346
C 2.0485177 0.8349538 -5.6196298
H 2.0202560 0.8224377 -6.7057662
C 3.2507216 1.1289457 -4.9521479
H 4.1503847 1.3427040 -5.5226254
C 3.2947751 1.1424326 -3.5641180
H 4.2226995 1.3653294 -3.0440665
C 2.1294600 0.8676964 -2.8225170
C 0.0529539 -3.1889772 -1.1144424
C 0.7972340 -3.0998086 -2.3025253
H 1.4604535 -2.2587584 -2.4760380
C 0.6928787 -4.0913492 -3.2771709
H 1.2725944 -4.0058417 -4.1920312
C -0.1536172 -5.1855252 -3.0794255
H -0.2373259 -5.9554768 -3.8414356
C -0.8880786 -5.2864694 -1.8975816
H -1.5441297 -6.1364740 -1.7312392
C -0.7844482 -4.2977785 -0.9153598
H -1.3614354 -4.4053468 -0.0033889
C 1.9132129 -2.5022376 0.8969213
C 3.0940227 -2.2827416 0.1683068
H 3.0766060 -1.6803278 -0.7326984
C 4.3051821 -2.8268554 0.5943277
H 5.2064806 -2.6414538 0.0165730
122

C 4.3594131 -3.6030817 1.7540806
H 5.3033435 -4.0256731 2.0869424
C 3.1914208 -3.8336643 2.4812472
H 3.2178243 -4.4373294 3.3842466
C 1.9752244 -3.2932642 2.0554647
H 1.0892966 -3.4979919 2.6450181
C -1.0054758 -2.2433478 1.4686005
H -1.9176210 -2.4755546 0.9112254
H -0.7385390 -3.1527919 2.0117575
C -1.3224595 -1.0975852 2.4796056
C -1.9476197 -1.7578943 3.7267070
H -2.3087587 -0.9970815 4.4281729
H -1.2111534 -2.3816237 4.2459927
H -2.7950987 -2.3923119 3.4428659
C -2.3747654 -0.1097450 1.9130139
H -2.3896688 0.7746657 2.5548277
H -3.3694479 -0.5627182 1.9594265
C -0.0559267 -0.3390091 2.9524767
H 0.7143788 -1.0669683 3.2088336
H -0.2871001 0.2023442 3.8734806
C 2.4680838 1.0086848 2.2391917
C 3.1396589 0.0273238 2.9842622
H 2.6403916 -0.8791256 3.2996926
C 4.4801189 0.1966413 3.3396097
H 4.9775341 -0.5781512 3.9163706
C 5.1719938 1.3463411 2.9597868
H 6.2145483 1.4752703 3.2373112
C 4.5119327 2.3331598 2.2244309
H 5.0357014 3.2373758 1.9269492
C 3.1735661 2.1684486 1.8696152
H 2.6818854 2.9554132 1.3093103
C 0.0645275 2.5472879 2.1399099
C -0.3224511 2.9168313 3.4379579
H -0.3294494 2.1935069 4.2465231
C -0.6924740 4.2340057 3.7136880
H -0.9932007 4.5048138 4.7220270
C -0.6687624 5.1993020 2.7043784
H -0.9533987 6.2245370 2.9244259
C -0.2759175 4.8428282 1.4133406
H -0.2553395 5.5855971 0.6210883
C 0.0845641 3.5241989 1.1313055
H 0.3834961 3.2570366 0.1222900
C -2.7357462 2.1604939 0.0385565
C -3.4540191 2.7654362 1.0824585
H -3.6310738 2.2443938 2.0160390
C -3.9689791 4.0552241 0.9326791
123

H -4.5206176 4.5076347 1.7520847
C -3.7793564 4.7562453 -0.2590576
H -4.1785898 5.7604795 -0.3719025
C -3.0779482 4.1568614 -1.3075807
H -2.9300337 4.6904060 -2.2425696
C -2.5609797 2.8696466 -1.1618955
H -2.0184578 2.4177486 -1.9857178
C -3.4115449 -0.4957183 -0.7904052
C -4.7508907 -0.0840912 -0.6623768
H -5.0044312 0.7812973 -0.0586749
C -5.7728979 -0.7780510 -1.3091668
H -6.8004648 -0.4428424 -1.1976734
C -5.4776166 -1.8935304 -2.0972443
H -6.2744156 -2.4307602 -2.6043336
C -4.1539332 -2.3110897 -2.2285318
H -3.9080410 -3.1780060 -2.8356394
C -3.1280626 -1.6175417 -1.5805905
H -2.1112210 -1.9624655 -1.7083534
Co 0.1494902 0.2241868 -0.4121856
P 0.3033836 -1.9127789 0.1956578
P -2.1027929 0.4261865 0.1492976
P 0.6896023 0.8573601 1.7422141
S -0.5026128 0.2216546 -2.5312798
S 2.1315247 0.9020261 -1.0657709
II
0 2
C 1.1337604 0.7253148 -3.5697088
C 1.1591142 0.7210596 -4.9756575
H 0.2467141 0.4944799 -5.5230482
C 2.3363344 0.9993851 -5.6700544
H 2.3381321 0.9877116 -6.7573218
C 3.5103895 1.2878386 -4.9634500
H 4.4335620 1.5028300 -5.4961770
C 3.4953756 1.2973511 -3.5685352
H 4.4058258 1.5213553 -3.0167108
C 2.3147055 1.0183409 -2.8583798
C -0.0883310 -3.3390687 -1.0347713
C 0.5034163 -3.2402883 -2.3051665
H 1.1106312 -2.3745395 -2.5503883
C 0.3062118 -4.2380138 -3.2618208
H 0.7713914 -4.1448398 -4.2397655
C -0.4919998 -5.3458491 -2.9664999
H -0.6508094 -6.1197731 -3.7127618
C -1.0843810 -5.4537356 -1.7060214
H -1.7035286 -6.3142362 -1.4654508
124

C -0.8803664 -4.4611045 -0.7441584
H -1.3434587 -4.5724501 0.2313460
C 1.8506145 -2.6907077 0.8825563
C 3.0106086 -2.5033505 0.1101444
H 2.9550354 -1.9282941 -0.8098067
C 4.2340693 -3.0384579 0.5151276
H 5.1187922 -2.8812352 -0.0965248
C 4.3224962 -3.7656612 1.7050175
H 5.2752961 -4.1789113 2.0251701
C 3.1771923 -3.9585887 2.4801833
H 3.2337377 -4.5254184 3.4060500
C 1.9487531 -3.4310961 2.0701716
H 1.0759278 -3.6051795 2.6911541
C -1.0289760 -2.2334980 1.5049010
H -1.9561858 -2.4573369 0.9669307
H -0.8055037 -3.1204040 2.1052485
C -1.2958610 -1.0231750 2.4594632
C -1.9456972 -1.5978578 3.7376891
H -2.2769687 -0.7922416 4.4036640
H -1.2341002 -2.2239992 4.2890726
H -2.8170606 -2.2140510 3.4857824
C -2.3095358 -0.0052506 1.8660498
H -2.2822925 0.8928251 2.4907407
H -3.3218435 -0.4147457 1.9369470
C 0.0067757 -0.2989800 2.8976050
H 0.7776595 -1.0469651 3.0917040
H -0.1694943 0.2115118 3.8487807
C 2.4863993 1.0973108 2.2049288
C 3.2115484 0.0432905 2.7811384
H 2.7500007 -0.9198054 2.9596622
C 4.5535977 0.2083316 3.1332981
H 5.0914838 -0.6231877 3.5811325
C 5.1976685 1.4263821 2.9136226
H 6.2412357 1.5525781 3.1894400
C 4.4879577 2.4818716 2.3369624
H 4.9750149 3.4370744 2.1592143
C 3.1473794 2.3190167 1.9867010
H 2.6159662 3.1535001 1.5428943
C 0.0737643 2.5964153 2.2006415
C -0.2244299 2.9301011 3.5316608
H -0.1325255 2.1938763 4.3239345
C -0.6358080 4.2219891 3.8647215
H -0.8685192 4.4603176 4.8992049
C -0.7422261 5.2044726 2.8769572
H -1.0608186 6.2098388 3.1391823
C -0.4346649 4.8886270 1.5527194
125

H -0.5118776 5.6447351 0.7762096
C -0.0340412 3.5932698 1.2174538
H 0.1987086 3.3572492 0.1832807
C -2.7891157 2.1953781 -0.0329649
C -3.5752587 2.7616783 0.9828293
H -3.7527864 2.2288120 1.9105807
C -4.1458093 4.0267839 0.8188176
H -4.7476531 4.4498668 1.6187787
C -3.9464105 4.7412516 -0.3635549
H -4.3894472 5.7256335 -0.4892663
C -3.1763087 4.1807451 -1.3858984
H -3.0188368 4.7265678 -2.3126813
C -2.6029294 2.9190975 -1.2235436
H -2.0011301 2.4946968 -2.0226163
C -3.3110595 -0.5023097 -0.8021617
C -4.6754147 -0.2668003 -0.5500860
H -4.9784934 0.5242890 0.1295760
C -5.6604242 -1.0355617 -1.1690181
H -6.7080227 -0.8350454 -0.9601152
C -5.3018084 -2.0571512 -2.0536176
H -6.0690134 -2.6564813 -2.5367255
C -3.9534624 -2.2994438 -2.3120492
H -3.6588449 -3.0925358 -2.9942372
C -2.9655890 -1.5263996 -1.6927745
H -1.9265975 -1.7248159 -1.9186239
Co 0.2098242 0.3657364 -0.4595590
P 0.2741741 -1.9985770 0.1905501
P -2.0279672 0.5101957 0.0910159
P 0.7102662 0.9342099 1.6889817
S -0.3607646 0.3773685 -2.6845784
S 2.2906612 1.0577411 -1.0859867
III
-1 1
Co 0.9175760 0.7084739 -1.9214445
P 0.0260450 -1.0879864 -1.1925298
S 0.2812027 0.2178843 -4.0349676
P 0.9874577 1.7084127 -0.0321927
S 2.4643160 2.1060792 -2.7646213
P -1.7871428 -2.1642575 3.4093357
C 1.3263757 1.1741871 -5.0972992
C 1.0528829 -2.5494602 -1.7307997
C 0.6881851 -3.3851431 -2.7975926
C 2.2832927 2.0451560 -4.5255267
C 2.9540699 2.7656570 -6.7582861
C 0.6089829 0.8177083 1.5730832
126

C -0.1688058 3.1756810 0.0760911
C 3.0831177 2.8359852 -5.3713926
C 4.9178227 2.0998563 1.2115575
C 3.6578674 1.6026319 0.8764942
C 2.0132946 1.8944440 -7.3252736
C 1.5371882 -4.4055595 -3.2356448
C 1.2104836 1.1086518 -6.4984637
C -1.8529462 0.2933177 1.2042023
C -1.2243259 5.0061485 1.2939891
C 2.3069732 -2.7565114 -1.1288469
C -0.0360797 -1.4344713 0.6422708
C 2.7718744 -4.6096701 -2.6168583
C -0.9281481 3.5301043 -1.0485078
C 3.1543327 -3.7780855 -1.5605680
C -0.3264916 3.9379582 1.2474208
C 2.6161680 2.4633597 0.4870850
C 5.1710900 3.4734578 1.1471098
C -1.8288637 4.5983720 -1.0069112
C -0.4767015 -0.2843974 1.5752560
C -1.9823445 5.3375045 0.1669857
C 2.8895177 3.8372260 0.4151443
C 4.1536418 4.3383968 0.7423668
C -0.5045416 -0.8297499 3.0355414
C -1.0126463 -2.9657179 4.8966001
C 0.2479335 -3.5748221 4.7464168
C 0.1804822 -4.3895868 7.0262339
C 0.8417289 -4.2698484 5.8004095
C -1.0795694 -3.8101132 7.1822291
C -1.6713914 -3.1068356 6.1286261
C -1.7036845 -1.6159552 -1.6751709
C -2.2143194 -2.8889631 -1.3627749
H -1.5784528 -3.6303991 -0.8862290
C -3.5346800 -3.2286473 -1.6622877
H -3.9061751 -4.2199306 -1.4140219
C -4.3765124 -2.2976357 -2.2782362
H -5.4056505 -2.5608272 -2.5091536
C -3.8866621 -1.0291187 -2.5913850
H -4.5325447 -0.2958532 -3.0680664
C -2.5624668 -0.6939733 -2.2906757
H -2.1891375 0.2953030 -2.5376873
C -3.1981746 -1.2066958 4.1316418
C -3.0492715 -0.1730961 5.0731918
H -2.0600549 0.0980288 5.4318541
C -4.1625534 0.5085766 5.5669932
H -4.0284690 1.3055323 6.2941169
C -5.4464578 0.1663766 5.1304136
127

H -6.3126486 0.6984478 5.5150912
C -5.6106022 -0.8607822 4.1998717
H -6.6051000 -1.1332387 3.8559284
C -4.4935979 -1.5405286 3.7051275
H -4.6300307 -2.3359243 2.9761115
H -0.2649311 -3.2414134 -3.2955267
H 3.5821731 3.3855958 -7.3941085
H 0.3593339 1.5693833 2.3294951
H 1.5516757 0.3612444 1.8924986
H 3.8127779 3.5112735 -4.9286009
H 5.7034192 1.4132262 1.5175510
H 3.4921487 0.5283838 0.9100211
H 1.9056183 1.8297048 -8.4057459
H 1.2300801 -5.0405018 -4.0632329
H 0.4777166 0.4345745 -6.9378200
H -1.8438659 0.7648309 0.2176466
H -2.1587337 1.0554771 1.9307524
H -2.6237764 -0.4852877 1.1874732
H -1.3310444 5.5805586 2.2109860
H 2.6365036 -2.1097108 -0.3197249
H 0.9714046 -1.7432130 0.9410292
H -0.6865556 -2.3026251 0.7918747
H 3.4318860 -5.4036948 -2.9564128
H -0.8108775 2.9566769 -1.9641415
H 4.1160106 -3.9203390 -1.0736539
H 0.2609791 3.7091779 2.1323478
H 6.1530244 3.8626081 1.4038811
H -2.4092577 4.8487181 -1.8914964
H -2.6832562 6.1675106 0.2051100
H 2.1151627 4.5286771 0.0992382
H 4.3395301 5.4079186 0.6786896
H -0.6387272 0.0051172 3.7337115
H 0.4717338 -1.2727579 3.2634965
H 0.7747778 -3.5106511 3.7968406
H 0.6411638 -4.9327952 7.8471234
H 1.8202186 -4.7228032 5.6606892
H -1.6077916 -3.8994331 8.1284537
H -2.6513134 -2.6636476 6.2777261
IV-CO2
’

-1 1
Co 0.2420993 1.0816993 -2.1582949
P -0.1769032 -0.9482689 -1.2464392
S 0.8866541 0.0314736 -4.0717435
P 0.5429382 1.9585409 -0.0792458
S 0.9652392 3.0261248 -3.0373872
128

P -1.6923910 -2.0939478 3.4334352
C 1.2311913 1.3407676 -5.2068955
C 1.1965530 -2.1541366 -1.5833853
C 1.0482316 -3.1947690 -2.5131369
C 1.2866291 2.6738533 -4.7420938
C 1.8867746 3.4263670 -6.9819347
C -0.5919688 1.4390240 1.3051045
C 0.4949178 3.8001038 0.1615827
C 1.6203266 3.7028840 -5.6414011
C 3.8721942 0.8105839 2.1751227
C 2.5597843 1.1187346 1.8057171
C 1.8232012 2.1057347 -7.4458355
C 2.1039440 -4.0691064 -2.7825866
C 1.4976951 1.0767428 -6.5633661
C -2.5469416 -0.1381347 0.9723393
C 1.4938115 5.9034179 0.8724208
C 2.4390512 -2.0002251 -0.9455900
C -0.1833855 -1.0267755 0.6207469
C 3.3282457 -3.9173811 -2.1295293
C -0.6793497 4.4993052 -0.1724081
C 3.4922257 -2.8765375 -1.2123530
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130

O -2.0305403 1.5914116 -3.5316699
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IV-CO2
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131

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133

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IV-H
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137

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C -3.9586709 -0.9024519 -2.5836793
H -4.6018845 -0.1564856 -3.0430170
C -2.6388168 -0.5722729 -2.2593772
H -2.2785181 0.4321798 -2.4623021
C -3.1371245 -1.2183677 4.2067245
C -2.9716301 -0.1979399 5.1598622
H -1.9759168 0.0717531 5.5012619
C -4.0766424 0.4711721 5.6880219
H -3.9301658 1.2577475 6.4239628
C -5.3683271 0.1292975 5.2747227
H -6.2281619 0.6514146 5.6863473
C -5.5486947 -0.8848495 4.3330396
H -6.5493968 -1.1569687 4.0073661
138

C -4.4400698 -1.5521844 3.8037813
H -4.5892755 -2.3379545 3.0668829
H -0.3671882 -3.0323317 -3.4008270
H 4.2457673 2.5550015 -7.3321144
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H 4.3006967 2.9411692 -4.8927010
H 5.6945687 1.6216869 1.3221932
H 3.5072856 0.6810662 0.6868100
H 2.4273213 1.1863496 -8.3533226
H 1.1514843 -4.7266182 -4.3368681
H 0.6357141 0.2473562 -6.9044138
H -1.8778231 0.8115760 0.2831832
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H -2.6345218 -0.4469449 1.2614602
H -1.7329453 5.4541012 2.1924927
H 2.5721199 -2.0798345 -0.3890221
H 0.9448999 -1.7135640 0.9127058
H -0.7215128 -2.2610308 0.8013379
H 3.3792599 -5.1303414 -3.2983291
H -0.6853016 3.1051360 -2.0463880
H 4.0680234 -3.7927982 -1.3131578
H -0.1145502 3.6066042 2.1912708
H 6.0039085 4.0901804 1.4624963
H -2.3034186 4.9768379 -2.0472732
H -2.8430084 6.1579410 0.0792638
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H 4.0884758 5.5979035 0.9595269
H -0.5979604 0.0175994 3.7630509
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H -2.5188964 -2.6883672 6.3278509
H 0.0969093 -0.7764328 -4.4400414
IV- S H ’ ’
Co 0.8663018 0.7497978 -1.9349955
P -0.0289510 -1.0729499 -1.1972426
S 0.4384049 0.1093964 -4.0625635
P 0.9107850 1.7422906 -0.0185991
S 2.1801634 2.3625299 -2.7961596
P -1.7443715 -2.1890186 3.4259172
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C 0.9842947 -2.5289228 -1.7492983
139

C 0.5740139 -3.4153841 -2.7551839
C 2.3690901 2.0352541 -4.5541903
C 3.3044837 2.5452724 -6.7167105
C 0.6122378 0.8145937 1.5715639
C -0.2751381 3.1699909 0.0959882
C 3.2293457 2.7983668 -5.3475896
C 4.9018520 2.1669221 0.9337651
C 3.6272115 1.6588567 0.6843077
C 2.4958521 1.5475015 -7.2766205
C 1.4140540 -4.4471007 -3.1838166
C 1.6268268 0.8055863 -6.4807255
C -1.8611908 0.3057779 1.2551277
C -1.3458376 4.9851253 1.3172984
C 2.2686292 -2.6891746 -1.1996153
C -0.0689283 -1.4234707 0.6324499
C 2.6810484 -4.6055068 -2.6198415
C -1.0570410 3.4984698 -1.0219859
C 3.1071854 -3.7196646 -1.6263548
C -0.4285444 3.9343993 1.2660623
C 2.5479732 2.5177482 0.4059314
C 5.1285148 3.5462989 0.8912645
C -1.9767990 4.5494886 -0.9730483
C -0.4795668 -0.2819694 1.5881957
C -2.1253846 5.2932970 0.1984744
C 2.7920285 3.8979612 0.3510260
C 4.0719385 4.4074822 0.5944987
C -0.4745255 -0.8448527 3.0414805
C -0.9431166 -2.9840382 4.9020881
C 0.3097373 -3.6016856 4.7252038
C 0.2963369 -4.3927493 7.0140473
C 0.9267523 -4.2893573 5.7705391
C -0.9559517 -3.8044268 7.1967281
C -1.5709491 -3.1082100 6.1516519
C -1.7653622 -1.5011095 -1.6962831
C -2.3444526 -2.7442816 -1.3836641
H -1.7551977 -3.5145918 -0.8937356
C -3.6764606 -3.0136066 -1.7009384
H -4.1031146 -3.9824277 -1.4542907
C -4.4581454 -2.0409609 -2.3311639
H -5.4966384 -2.2492020 -2.5749151
C -3.8982256 -0.8013265 -2.6427639
H -4.4984444 -0.0368231 -3.1293251
C -2.5625483 -0.5347107 -2.3274421
H -2.1379185 0.4348480 -2.5706453
C -3.1572428 -1.2456803 4.1623049
C -3.0108770 -0.2014528 5.0923522
140

H -2.0214091 0.0876975 5.4357466
C -4.1275279 0.4682638 5.5949580
H -3.9955349 1.2737625 6.3129519
C -5.4119517 0.1031085 5.1791195
H -6.2807051 0.6259035 5.5706298
C -5.5734316 -0.9353313 4.2606964
H -6.5683021 -1.2258662 3.9330425
C -4.4532979 -1.6027579 3.7566768
H -4.5880454 -2.4069138 3.0370367
H -0.4042780 -3.3064030 -3.2108526
H 3.9803852 3.1232005 -7.3402865
H 0.3985573 1.5512711 2.3532976
H 1.5694988 0.3517705 1.8335789
H 3.8392981 3.5779223 -4.8981941
H 5.7191588 1.4855903 1.1558698
H 3.4741131 0.5826352 0.6988894
H 2.5430927 1.3483227 -8.3443667
H 1.0742834 -5.1263324 -3.9616555
H 1.0051099 0.0346000 -6.9282724
H -1.8715775 0.8032689 0.2813052
H -2.1540806 1.0491597 2.0055320
H -2.6311987 -0.4731002 1.2324934
H -1.4513509 5.5636460 2.2315229
H 2.6250722 -2.0011305 -0.4372211
H 0.9389197 -1.7515285 0.9078259
H -0.7286053 -2.2854625 0.7769575
H 3.3332645 -5.4080585 -2.9540080
H -0.9492909 2.9225654 -1.9379802
H 4.0941749 -3.8277297 -1.1838916
H 0.1753490 3.7194375 2.1432951
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H -2.5758071 4.7822550 -1.8496188
H -2.8412519 6.1099811 0.2410688
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H -1.4600704 -3.8815430 8.1570212
H -2.5442674 -2.6579552 6.3209443
H 3.4474150 2.0959203 -2.4075924
IV- S H ’ ’ ’
0 1
141

Co 0.9480736 0.6683936 -1.9124598
P 0.0045968 -1.1130502 -1.1597214
S 0.4363249 0.0868629 -4.0511338
P 0.9934599 1.7049409 -0.0177668
S 2.5060683 2.0535259 -2.7470374
P -1.7769518 -2.1482834 3.4496619
C 1.4171750 1.1424154 -5.1335905
C 1.0415357 -2.5467234 -1.7293756
C 0.6926424 -3.3429000 -2.8313057
C 2.3373947 2.0002420 -4.5120121
C 3.0010689 2.7565556 -6.7315398
C 0.6237355 0.8087131 1.5803796
C -0.1783559 3.1478187 0.0247465
C 3.1315104 2.8126853 -5.3463382
C 4.9315778 2.0853823 1.1546278
C 3.6726241 1.5896225 0.8150193
C 2.0801266 1.8863897 -7.3291157
C 1.5575712 -4.3353839 -3.3006563
C 1.2898556 1.0675270 -6.5227490
C -1.8431681 0.3036943 1.2219134
C -1.2895509 4.9777449 1.1865007
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C -0.0492151 -1.4462164 0.6697108
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C -0.8825982 3.4959603 -1.1377582
C 3.1572552 -3.7503197 -1.5924320
C -0.3900933 3.9102764 1.1867627
C 2.6194871 2.4591272 0.4803243
C 5.1670762 3.4634284 1.1528222
C -1.7846136 4.5632613 -1.1404384
C -0.4723078 -0.2835081 1.5949687
C -1.9926384 5.3047685 0.0234340
C 2.8721524 3.8379301 0.4700322
C 4.1352710 4.3359801 0.8049094
C -0.4955952 -0.8170507 3.0591661
C -0.9893438 -2.9360089 4.9372008
C 0.2662010 -3.5529462 4.7774861
C 0.2278226 -4.3340761 7.0695098
C 0.8721403 -4.2351583 5.8328692
C -1.0274937 -3.7470684 7.2350374
C -1.6314567 -3.0563568 6.1799356
C -1.7194211 -1.5943344 -1.6671649
C -2.2357001 -2.8783484 -1.4149189
H -1.6079471 -3.6389670 -0.9584017
C -3.5521614 -3.1992528 -1.7473339
H -3.9299534 -4.1984629 -1.5463216
142

C -4.3823251 -2.2397035 -2.3347737
H -5.4081302 -2.4897974 -2.5922963
C -3.8880341 -0.9588575 -2.5822866
H -4.5270203 -0.2021550 -3.0298346
C -2.5678055 -0.6404144 -2.2488669
H -2.2068633 0.3696186 -2.4253121
C -3.1853855 -1.1871427 4.1711822
C -3.0340951 -0.1407798 5.0980799
H -2.0437813 0.1394058 5.4464361
C -4.1469605 0.5417345 5.5916613
H -4.0112350 1.3485102 6.3075282
C -5.4324712 0.1877191 5.1694918
H -6.2982527 0.7204484 5.5541195
C -5.5988327 -0.8522806 4.2537227
H -6.5946057 -1.1341582 3.9213052
C -4.4823788 -1.5327461 3.7588904
H -4.6206380 -2.3383627 3.0415359
H -0.2609648 -3.1966101 -3.3276576
H 3.6236187 3.3955529 -7.3529187
H 0.3905972 1.5615791 2.3406745
H 1.5690464 0.3471324 1.8846874
H 3.8494029 3.4926752 -4.8952973
H 5.7294118 1.3949465 1.4162573
H 3.5149707 0.5137873 0.8028096
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H 1.2636280 -4.9431959 -4.1526677
H 0.5754098 0.3806842 -6.9690217
H -1.8328050 0.7603391 0.2282518
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H -0.7128616 -2.3041283 0.8205603
H 3.4629648 -5.3178863 -3.0462196
H -0.7225674 2.9240485 -2.0474541
H 4.1177146 -3.8988946 -1.1055874
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H 6.1479746 3.8512835 1.4148895
H -2.3227435 4.8112417 -2.0516340
H -2.6947459 6.1344336 0.0259693
H 2.0859343 4.5346981 0.1987808
H 4.3086427 5.4091350 0.7918822
H -0.6248355 0.0248357 3.7494695
H 0.4811181 -1.2591925 3.2865790
H 0.7796402 -3.5045735 3.8196040
143

H 0.6978761 -4.8673033 7.8916112
H 1.8465716 -4.6944922 5.6859376
H -1.5425085 -3.8207134 8.1898065
H -2.6073619 -2.6068782 6.3362739
H -0.7629632 0.6800272 -4.2580671
IV-SH
0 1
Co 0.9404643 0.6070799 -1.9150306
P -0.0053884 -1.1756927 -1.1634848
S 0.2504228 0.2611249 -4.0328611
P 0.9877161 1.6590730 -0.0289839
S 2.7014855 1.6011063 -2.7428289
P -1.7797353 -2.1618782 3.4613163
C 1.3180966 1.2591546 -5.0450318
C 1.0368754 -2.6076655 -1.7227121
C 0.6805066 -3.4093153 -2.8182059
C 2.3944376 1.9596194 -4.4776830
C 2.9773336 2.9393681 -6.6004513
C 0.6249922 0.7837056 1.5776762
C -0.1899931 3.0958784 -0.0411973
C 3.2284574 2.7834043 -5.2378672
C 4.9327603 2.1652458 1.0791768
C 3.6786692 1.6284733 0.7885426
C 1.9165867 2.2395279 -7.1922348
C 1.5353717 -4.4148042 -3.2770393
C 1.1036036 1.4048504 -6.4296603
C -1.8408905 0.2732067 1.2150688
C -1.3522486 4.9374459 1.0467213
C 2.2845453 -2.8282112 -1.1126420
C -0.0499336 -1.4844136 0.6749617
C 2.7649512 -4.6339255 -2.6528400
C -0.8180071 3.4461262 -1.2462300
C 3.1369693 -3.8348391 -1.5686078
C -0.4645315 3.8616966 1.1045960
C 2.6027759 2.4596063 0.4268831
C 5.1439002 3.5448286 0.9923485
C -1.7064225 4.5231601 -1.3066783
C -0.4701146 -0.3124465 1.5924820
C -1.9776644 5.2692475 -0.1587880
C 2.8312371 3.8407945 0.3295845
C 4.0923379 4.3780091 0.6103555
C -0.4952543 -0.8366073 3.0603795
C -0.9964513 -2.9349451 4.9586933
C 0.2592092 -3.5541934 4.8089713
C 0.2131218 -4.3126620 7.1084601
144

C 0.8614046 -4.2262324 5.8729727
C -1.0423733 -3.7233259 7.2642590
C -1.6425051 -3.0425840 6.2005518
C -1.7275628 -1.6759331 -1.6542395
C -2.2367297 -2.9525076 -1.3545107
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C -3.5539426 -3.2892346 -1.6676529
H -3.9269653 -4.2825428 -1.4312841
C -4.3908583 -2.3524364 -2.2816835
H -5.4176653 -2.6143541 -2.5233532
C -3.9013182 -1.0802785 -2.5790478
H -4.5448058 -0.3432153 -3.0524449
C -2.5798257 -0.7458384 -2.2666661
H -2.2113328 0.2483757 -2.4993206
C -3.1896980 -1.1926476 4.1692194
C -3.0415602 -0.1346142 5.0832291
H -2.0524539 0.1509230 5.4305913
C -4.1562877 0.5532719 5.5651226
H -4.0230465 1.3692028 6.2710089
C -5.4404257 0.1931366 5.1439726
H -6.3075555 0.7302285 5.5193716
C -5.6036503 -0.8585836 4.2410932
H -6.5983007 -1.1453278 3.9094847
C -4.4854028 -1.5443595 3.7579173
H -4.6212071 -2.3588288 3.0501874
H -0.2692853 -3.2530734 -3.3188325
H 3.6067226 3.5938643 -7.1967175
H 0.3883100 1.5377893 2.3359154
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H 4.0603097 3.3028268 -4.7682781
H 5.7467464 1.5049767 1.3671754
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H 4.0941235 -3.9906829 -1.0774857
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145

H 6.1220858 3.9633897 1.2135417
H -2.1844367 4.7751958 -2.2496812
H -2.6696215 6.1063244 -0.2011630
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H 0.6802055 -4.8381358 7.9372236
H 1.8359949 -4.6875988 5.7336448
H -1.5604969 -3.7873841 8.2180368
H -2.6186092 -2.5908924 6.3492448
H 2.9396971 2.8698000 -2.3287190

V
-1 2
Co 1.3554720 0.4394604 -1.8239469
P 0.1251150 -1.3097946 -1.1910931
S 0.2216091 1.3143533 -3.6475493
P 0.9625603 1.7770068 -0.0187751
S 3.2926791 0.6371909 -3.0085543
P -1.6514598 -2.1976086 3.4483487
C 1.4918587 1.7425482 -4.8079093
C 0.9664064 -2.8552093 -1.7813082
C 0.6790214 -3.3205513 -3.0778701
C 2.8489093 1.4395749 -4.5323970
C 3.4952661 2.4027334 -6.6835574
C 0.7108010 0.7804043 1.5488451
C -0.3360334 3.1028746 0.1448340
C 3.8296975 1.7758013 -5.4815765
C 4.7832250 2.8296646 1.2395020
C 3.5993810 2.1221985 1.0183254
C 2.1560027 2.7048739 -6.9550560
C 1.3639777 -4.4075328 -3.6217651
C 1.1702848 2.3752171 -6.0237233
C -1.7328969 0.1892117 1.1486082
C -1.5142089 4.8040886 1.4332996
C 1.9727142 -3.5093534 -1.0521627
C 0.0852562 -1.5523078 0.6601103
C 2.3560814 -5.0553616 -2.8806213
C -1.1794372 3.3786628 -0.9412943
C 2.6560009 -4.6012706 -1.5950857
C -0.5131313 3.8374630 1.3315947
C 2.4895275 2.7395640 0.4156618
C 4.8900449 4.1670033 0.8509643
146

C -2.1828736 4.3485430 -0.8416511
C -0.3536248 -0.3559777 1.5495755
C -2.3558504 5.0599536 0.3459024
C 2.6158381 4.0829669 0.0206136
C 3.8015360 4.7888825 0.2353356
C -0.3850005 -0.8606200 3.0264847
C -0.8939671 -2.8884615 4.9980378
C 0.3873297 -3.4666274 4.9165041
C 0.2786833 -4.1521719 7.2367786
C 0.9708031 -4.0819464 6.0243663
C -1.0015896 -3.6034506 7.3257100
C -1.5832170 -2.9793616 6.2180373
C -1.6332613 -1.6921924 -1.6703472
C -2.2351108 -2.9141141 -1.3182439
H -1.6605902 -3.6715448 -0.7912074
C -3.5661934 -3.1780731 -1.6421528
H -4.0121827 -4.1284637 -1.3596176
C -4.3229429 -2.2233231 -2.3288560
H -5.3602467 -2.4273639 -2.5815356
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H -4.3152424 -0.2627123 -3.2283880
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H -1.9561907 0.1962686 -2.6577762
C -3.0999804 -1.2286438 4.0782701
C -2.9977335 -0.1397451 4.9615832
H -2.0248574 0.1672969 5.3353951
C -4.1370894 0.5515445 5.3763649
H -4.0397050 1.3925105 6.0583607
C -5.4002643 0.1636323 4.9179695
H -6.2863636 0.7041590 5.2404829
C -5.5181139 -0.9199530 4.0461336
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C -4.3751709 -1.6091201 3.6303274
H -4.4752218 -2.4476394 2.9450159
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H 4.2745011 2.6546944 -7.3992758
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H 1.1210977 -4.7481492 -4.6252631
H 0.1289056 2.6103968 -6.2345172
H -1.7162609 0.6082367 0.1390241
H -2.0478840 0.9851777 1.8330195
147

H -2.4952943 -0.5970244 1.1662034
H -1.6367661 5.3590990 2.3601831
H 2.2367524 -3.1769960 -0.0534425
H 1.1015728 -1.8292560 0.9561996
H -0.5558614 -2.4134901 0.8771162
H 2.8905152 -5.9028813 -3.3016788
H -1.0429917 2.8381784 -1.8737676
H 3.4254038 -5.0961905 -1.0075604
H 0.1382119 3.6625109 2.1839338
H 5.8120040 4.7165788 1.0218944
H -2.8269100 4.5459026 -1.6951520
H -3.1365500 5.8122578 0.4251631
H 1.7814250 4.5889643 -0.4566522
H 3.8712138 5.8278918 -0.0774290
H -0.5320576 -0.0073453 3.6984729
H 0.5955457 -1.2840575 3.2724537
H 0.9388469 -3.4412090 3.9791749
H 0.7312993 -4.6336719 8.0996327
H 1.9656540 -4.5118580 5.9373395
H -1.5536051 -3.6552989 8.2610723
H -2.5791718 -2.5577410 6.3140469
H 2.2582495 -0.2201657 -0.7973206
VI
-1 2
Co 1.8321525 1.4071391 0.2388507
P 0.8730201 0.1608310 -1.3802683
S 3.1503152 2.3725803 -1.3810759
P 0.2469800 0.9120462 1.7724586
S 2.6107967 3.0063934 1.7022370
P -2.4303698 -3.6463009 -0.8369047
C 3.9963284 3.7144553 -0.5851711
C 2.0859014 -0.5959756 -2.5579976
C 2.2459308 -0.1111260 -3.8656296
C 3.7557604 3.9954909 0.7777474
C 5.3390440 5.8486708 0.6696648
C -0.5055242 -0.7885213 1.6404746
C -1.1882848 2.0689499 1.9162293
C 4.4337295 5.0664188 1.3873736
C 2.2984795 -0.3132962 5.1436395
C 1.7951913 -0.2100273 3.8461009
C 3.1964894 -0.6697848 -4.7238700
C 4.9101695 4.5111878 -1.2979926
C -2.2642495 -0.3789479 -0.1595561
C -3.3866414 2.6624032 2.7783072
C 2.9183827 -1.6435572 -2.1258268
148

C 0.0027423 -1.3787644 -0.7940112
C 4.0051098 -1.7216793 -4.2905131
C -1.1658509 3.2806051 1.2098330
C 3.8632410 -2.2041579 -2.9869031
C -2.3119561 1.7753576 2.7094310
C 0.8785341 0.8044253 3.5137123
C 1.9075601 0.5981275 6.1287592
C -2.2415931 4.1704226 1.2789735
C -1.0976184 -1.2706192 0.2895548
C -3.3556764 3.8619013 2.0603731
C 0.5032304 1.7218616 4.5063817
C 1.0133146 1.6186337 5.8036876
C -1.6111239 -2.7139509 0.5861363
C -2.2408831 -5.3946037 -0.2358450
C -0.9413712 -5.8976224 -0.0339120
C -0.7347791 -7.2301117 0.3240929
C -3.3219822 -6.2800909 -0.1000842
C -0.3308407 0.9911241 -2.5103410
C -1.0469725 0.2775161 -3.4871335
H -0.8836502 -0.7891659 -3.6114517
C -1.9659653 0.9281283 -4.3105720
H -2.5134120 0.3627471 -5.0603170
C -2.1812426 2.3031983 -4.1734220
H -2.8993091 2.8081660 -4.8142801
C -1.4689171 3.0237593 -3.2138935
H -1.6266471 4.0933947 -3.1034806
C -0.5486039 2.3704389 -2.3892843
H 0.0062708 2.9353825 -1.6470822
C -4.2354908 -3.3022758 -0.5956771
C -4.8894273 -3.3522057 0.6479314
H -4.3387696 -3.6347511 1.5410182
C -6.2484545 -3.0519519 0.7503196
H -6.7389771 -3.0932229 1.7195741
C -6.9783841 -2.6997167 -0.3897617
H -8.0361633 -2.4636711 -0.3074650
C -6.3442459 -2.6535777 -1.6323247
H -6.9050822 -2.3830224 -2.5232224
C -4.9824600 -2.9530748 -1.7320469
H -4.4930687 -2.9073373 -2.7021263
H 1.6305308 0.7065843 -4.2250349
H -1.2713200 -0.8805277 2.4187662
H 0.3090170 -1.4653657 1.9197489
H 4.2464268 5.2809442 2.4375130
H 3.0019824 -1.1067481 5.3836916
H 2.1367760 -0.8997142 3.0815847
H 3.3016434 -0.2792649 -5.7329659
149

H 5.0958805 4.2924248 -2.3474991
H -1.9432110 0.6548021 -0.3041300
H -3.0641821 -0.3792441 0.5894654
H -2.6861236 -0.7270672 -1.1088402
H -4.2480991 2.4171642 3.3941184
H 2.8475128 -2.0089045 -1.1079743
H 0.8025096 -2.0174995 -0.4061505
H -0.4055643 -1.8851015 -1.6752904
H 4.7416588 -2.1585608 -4.9597529
H -0.3000178 3.5330907 0.6055648
H 4.4921391 -3.0177381 -2.6341630
H -2.3514609 0.8543949 3.2838740
H 2.3011320 0.5155886 7.1386084
H -2.2046683 5.1023837 0.7210235
H -4.1945155 4.5509217 2.1136861
H -0.1903311 2.5230982 4.2756593
H 0.7076603 2.3389939 6.5584208
H -2.2970168 -2.6831145 1.4402119
H -0.7584403 -3.3285576 0.8958564
H -0.0786021 -5.2472493 -0.1597794
H 0.2778269 -7.5923103 0.4836967
H -4.3357154 -5.9292309 -0.2649677
C 5.5789141 5.5692140 -0.6810921
C -1.8218185 -8.0969063 0.4702019
C -3.1140721 -7.6172165 0.2508848
H 5.8546886 6.6698753 1.1621645
H 6.2831820 6.1707635 -1.2510701
H -1.6610231 -9.1359040 0.7455237
H -3.9681189 -8.2820346 0.3548694
H 4.6675664 0.4150106 0.8482586
C 4.0729294 -0.5026225 1.0775140
O 2.8169920 -0.4051544 0.8399114
O 4.6696963 -1.4920240 1.5243170

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159

Chapter 4: Impact of Ligand Functionalization and Metal Center on the Activity and
Selectivity of a Phosphino-Thiolate Complex Towards CO
2
Reduction to Formate
4.1 Introduction
Amongst homogeneous electrocatalysts active in the CO 2RR, catalysts that are selective
formate producing electrocatalysts with faradaic efficiencies (FE) > 85%, operate at low
overpotentials, and are based on Earth-abundant elements are quite rare.
1,2
Recently, we reported
a catalyst that employs relatively Earth-abundant components and that can selectively catalyze the
conversion of CO2 into formate electrocatalytically at FE% as high as 91% (Chapter 3). Drawbacks
of this catalytic system is its relatively large overpotential and its utilization of a cobalt metal
center. Large overpotentials can lead to reduced efficiency in electrochemical conversions as the
system is inherently held at a potential either more negative or positive than what is necessary
thermodynamically, leading to a lower energy efficiency compared to systems that can operate
closer to the thermodynamic limit. Moreover, while cobalt is an element with a larger crustal
abundance than 2
nd
and 3
rd
row transition metals, it is several orders of magnitude less abundant
compared to other first row transition metals congeners such as Fe and Mn.
3
Moreover, due to the
burgeoning Li-ion battery market and related interest in Co-based cathode material, future cobalt
supply-chain infrastructure will be severely stressed and lead to inflated prices and reduced
supply.
4
Overall, employing more abundant metals such as Fe can lead to catalytic systems that
can be more scalable due to their lower overall cost.
Motivated by the success of aforementioned cobalt-based phosphino-thiolate complexes
[Co(triphos)(bdt)]
+
towards the CO2RR, and as a means to reduce overpotential and employ more
earth abundant components, a series of complexes in the form of an analogous iron-based complex
[Fe(triphos)(bdt)]
+
and an analogous cobalt complex employing a 3,6-dichloro-1,2-
benzenedithiolate ligand [Co(triphos)(bdtCl
2
)]
+
(Figure 4.1) were synthesized. Previous
160

computational studies on these sets of complexes indicate significant contribution of the
benzenedithiolate (bdt) ligand towards frontier orbitals of [Co(triphos)(bdt)]
+
primarily localized
on the metal center (Chapter 2).
5
Amending the ancillary benzenedithiolate ligand with electron-
withdrawing groups will be used to reduce the Lewis basicity of the bdt ligand and reduce electron
density on the metal center. This will potentially shift the [Co(triphos)(bdtX)]
0/-
(X = H, Cl) redox
event associated with the onset of catalysis to more anodic potentials and lower operational
overpotentials. A similarly constructed complex employing an iron metal center will also be
synthesized and assessed as an electrocatalyst towards the CO 2RR. Cyclic voltametric studies were
performed to characterize the electrochemical behavior of [Fe(triphos)(bdt)]
+
and
[Co(triphos)(bdtCl
2
)]
+
under reducing conditions, and the reactivity of the complex in the
presence of CO2 and Brønsted acids was investigated. Controlled potential electrolysis studies
were employed to explore the selectivity of the catalyst under ideal catalytic conditions determined
for [Co(triphos)(bdt)]
+
.

Figure 4.1 Chemdraw illustration of the synthesized cobalt triphosphine-thiolate complex
([Co(triphos)(bdt)]
+
), [Co(triphos)(bdtCl
2
)]
+
, [Fe(triphos)(bdt)]
+
.

161

4.2 Results and Discussion
4.2.1 Synthesis and Characterization of [Co(triphos)(bdtCl
2
)]
+
and of [Fe(triphos)(bdt)]
0

Complex [Fe(triphos)(bdt)]
0
was synthesized according to a reported literature
procedure.
6
A similar synthetic procedure was employed for [Co(triphos)(bdtCl
2
)]
+
, however 3,6-
dichloro-1,2-benzenedithiol was used instead of 1,2-benzenedithiol.
5,7,8
The
1
H NMR spectrum of
[Co(triphos)(bdtCl
2
)]
+
in DMSO-d6 displays two broad aliphatic singlets at δ 3.03 and 2.01 ppm,
corresponding to the methylene and methyl moieties on the triphos ligand, and 3 aromatic signals
appear at δ 7.72 (s), 7.27 (br t), and 7.06 (br t) ppm corresponding to the protons of the dithiolate
ligand and the para- and overlapped ortho- and meta- protons of the aryl substituents of the triphos
ligand, respectively, in a 6:3:2:6:24 ratio (Figure 4.2). The
31
P-{
1
H} NMR spectrum of
[Co(triphos)(bdtCl
2
)]
+
in MeCN-d3 displays one broad peak at δ 32.5 ppm, corresponding to the
triphos ligand (Figure 4.3).

Figure 4.2 500 MHz
1
H NMR spectrum of [Co(triphos)(bdtCl
2
)]
+
in DMSO-d6.
162

Figure 4.3 202 MHz
31
P-{
1
H} NMR spectrum of [Co(triphos)(bdtCl
2
)]
+
in acetonitrile-d3.
X-ray quality crystals of [Co(triphos)(bdtCl
2
)]
+
were grown via layering of pentane onto
a dichloromethane solution containing [Co(triphos)(bdtCl
2
)]
+
. Solid state structure of
[Co(triphos)(bdtCl
2
)]
+
indicates the successful ligation of the chlorine functionalized bdt ligand
to the metal center without significant change to coordination about the metal center compared to
[Co(triphos)(bdt)]
+
(Figure 4.4). Average bond lengths for [Co(triphos)(bdtCl
2
)]
+
can be found
in Table 3.1 in addition to those reported for [Co(triphos)(bdt)]
+
.

Figure 4.4 Solid-state structure of [Co(triphos)(bdtCl
2
)]
+
. Aryl and aliphatic protons,
counterions, and solvent molecules are omitted for clarity.
163

Table 4.1 Average selected bond lengths (Å) for complexes [Co(triphos)(bdt)]
+
and
[Co(triphos)(bdtCl
2
)]
+
.

Bond [Co(triphos)(bdt)]
+
[Co(triphos)(bdtCl
2
)]
+

Co–S 2.161(3) 2.169(2)
Co–P
apical
2.183(2) 2.183(2)
Co–P
basal
2.249(5) 2.232(2)
C–S 1.731(6) 1.731(5)
Reference
8
This Work

The solid-state structure of [Co(triphos)(bdtCl
2
)]
+
displays a slightly longer Co–S bond (2.169(2)
Å) compared to the one in the structure of [Co(triphos)(bdt)]
+
(2.161(3) Å), although no
difference in bond length is observed in the C–S bond. Concurrently, a shorter bond lengths of the
Co–P bonds in the basal plane of the coordination environment is observed between
[Co(triphos)(bdt)]
+
(2.249(5) Å) and [Co(triphos)(bdtCl
2
)]
+
(2.232(2) Å), with no shift in the
apical Co-P bond length. These bond length shifts indicate significant changes in the electronic
structure of the complex.

164

Figure 4.5 Visible absorbance spectra of complexes [Co(triphos)(bdt)]
+
and
[Co(triphos)(bdtCl
2
)]
+
in acetonitrile.
To further probe the electronic structure of these systems, UV/Vis spectroscopy studies
were performed on [Co(triphos)(bdtCl
2
)]
+
. Absorbance spectra of [Co(triphos)(bdtCl
2
)]
+

displays similar transitions as the ones observed in the spectra of [Co(triphos)(bdt)]
+
, with a 6 nm
blue shift of the transition at 540 nm for [Co(triphos)(bdt)]
+
to 534 nm for
[Co(triphos)(bdtCl
2
)]
+
. No discernable shift in the absorption band at the longer wavelength
(~720 nm) was observed. Previous computational studies of [Co(triphos)(bdt)]
+
have suggested
that the absorption band at 540 nm can be principally attributed to a HOMO–LUMO transition.
Assuming a similar HOMO-LUMO transition can be attributed to the band at 536 nm in the spectra
of [Co(triphos)(bdtCl
2
)]
+
, we surmise functionalization of the bdt with an electron withdrawing
group such as Cl significantly alters the relative energy of the frontier orbitals in
[Co(triphos)(bdtCl
2
)]
+
relative to those in [Co(triphos)(bdt)]
+
.
4.2.2 Cyclic Voltammetry under Inert Conditions
Cyclic voltammograms (CVs) of [Fe(triphos)(bdt)] and

[Co(triphos)(bdtCl
2
)]
+
(0.45
mM) were obtained under an N2 atmosphere using a glassy carbon electrode (GCE) in acetonitrile
165

(MeCN) solutions with 0.1 M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) as the
supporting electrolyte. All potentials are referenced versus Fc
+/0
and all CVs were first scanned
cathodically and subsequently returned anodically. Potentials of redox events observed in the CVs
for both [Co(triphos)(bdt)]
+
and [Co(triphos)(bdtCl
2
)]
+
are located in Table 4.2. CVs of
[Co(triphos)(bdtCl
2
)]
+
reveal a reversible redox couple at -0.61 V (Figure 4.6). This reversible
feature is attributed to a formal Co
III/II
process based on previous reports on [Co(triphos)(bdtCl
2
)]

and is assigned the [Co(triphos)(bdtCl
2
)]
+/0
couple. This reduction event displays a 140 mV
anodic shift compared to the one observed for [Co(triphos)(bdt)]
+/0
(Figures 4.6 and 4.7).
5,7,8

Table 4.2 Potentials of the observed redox events for [Co(triphos)(bdt)]
+
and
[Co(triphos)(bdtCl
2
)]
+
. All potentials are referenced versus Fc
+/0
.
E
0
vs Fc/Fc
+

Couple (X = H, Cl) [Co(triphos)(bdt)]
+
[Co(triphos)(bdtCl
2
)]
+

[Co(triphos)(bdtX
2
)]
+/0
-0.75 -0.61
[Co(triphos)(bdtX
2
)]
0/-
-2.08 -2.00
[Co(triphos)(bdtX
2
)]
-/-2
-2.39 -2.41
Ox 1 -1.51 -1.36
Ox 2 -1.13 -0.99

166

Figure 4.6 CVs of 0.5 mM of [Co(triphos)(bdtCl
2
)] in a MeCN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s.

Figure 4.7 Normalized CVs of [Co(triphos)(bdt)]
+
and [Co(triphos)(bdtCl
2
)]
+
in a MeCN
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s.
Upon scanning further cathodically, CVs exhibit an irreversible feature at -2.00 V, and an
quasi-reversible couple at -2.41 V, and are assigned to the [Co(triphos)(bdt)]
0/-
and
[Co(triphos)(bdt)]
-/-2
couples and are shifted by 80 mV cathodically and 20 mV anodically
compared to the non-chlorinated derivative, respectively (Figures 4.6 and 4.7). On the return sweep
(scanning anodically from -2.6 V), two irreversible redox events are observed at -1.36 V and -0.99
167

V, similarly shifted to more positive potentials compared to those observed upon electrochemically
oxidizing [Co(triphos)(bdt)]
-
analogue (Figure 4.7). These oxidation features do not appear in the
CVs if the potential is reversed before reaching [Co(triphos)(bdt)]
0/-
(Figure 3.3), suggesting that
these oxidative events originate from the reduction of [Co(triphos)(bdt)]
0
and are most likely
similarly the case for [Co(triphos)(bdtCl
2
)]
0
as well. The irreversible nature of the redox feature
of the [Co(triphos)(bdtCl
2
)]
0/-
is likely due to a chemical/structural change to the complex upon
reduction of [Co(triphos)(bdtCl
2
)]
0
,

similarly observed in the reduction of [Co(triphos)(bdt)]
0
(Chapter 3). CVs of variable scan rates performed on the [Co(triphos)(bdtCl
2
)]
0/-
feature (Figure
4.8) display some change in the reversibility of this event at low scan rates (50 mV/s) and in the
return oxidation features at high scan rates (≥ 200 mV/s), which is not observed in similar CVs of
[Co(triphos)(bdt)]
+
, indicating a shift in the kinetics of phosphine deligation is associated with
functionalization of the bdt ligand. Randles-Sevcik plots for Co(triphos)(bdtCl
2
)]
+
(Figures 4.8)
in acetonitrile yield slopes of approximately 0.5 (Figures 4.9), as expected for freely diffusing
molecular species in solution.

Figure 4.8 Cyclic voltammograms of 0.5 mM of [Co(triphos)(bdtCl
2
)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from a) 0.05 to 0.2
V/s and b) 0.05 and 1 V/s.

168

Figure 4.9 Plot of log of current density vs log of the scan rate for cathodic and anodic scan of the
[Co(triphos)(bdtCl
2
)]
+/0
redox couple in MeCN measured at -0.63 V and -0.56V vs Fc/Fc
+

respectively.
CVs of [Fe(triphos)(bdt)]
0
reveal a two reversible redox couple at -0.32 V and -1.99 V
and are attributed to the [Fe(triphos)(bdt)]
+/0
and the [Fe(triphos)(bdt)]
0/-
couple, respectively
(Figure 4.10). Randles-Sevcik plots for [Fe(triphos)(bdt)]
0
(Figures 4.11) in acetonitrile yield
slopes of approximately 0.5 (Figure 4.12), as expected for freely diffusing molecular species in
solution. Notably, [Fe(triphos)(bdt)]
0/-
couple is fully reversible as opposed to what is observed
for [Co(triphos)(bdt)]
0/-
couple, indicating no deligation of the apical phosphine occurs upon
reduction to [Fe(triphos)(bdt)]
-
. This is likely the case as [Fe(triphos)(bdt)]
-
is isoelectronic with
[Co(triphos)(bdt)]
0
, which is five-coordinate as well. Further electrochemical characterization is
reported in the literature.
6

169

Figure 4.10 CVs of 0.5 mM of [Fe(triphos)(bdt)]
0
in a MeCN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s.

Figure 4.11 Cyclic voltammograms of 0.5 mM of [Fe(triphos)(bdt)]
0
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 0.1 to 1 V/s.
170

Figure 4.12 Plot of log of current density vs log of the scan rate for both the
[Fe(triphos)(bdt)]
+/0
,measured at

-0.35 and -0.29 V vs Fc/Fc
+
respectively and the
[Fe(triphos)(bdt)]
0/-
redox couples, measured at -2.02, and -1.94 V vs Fc/Fc
+
respectively, in
MeCN.
4.2.3 Cyclic Voltammetry under Catalytic Conditions
The catalytic behavior of [Co(triphos)(bdtCl
2
)]
+
and [Fe(triphos)(bdt)]
0
was studied
using CV experiments in the presence of CO 2 and with variable proton sources. CVs of
[Co(triphos)(bdtCl
2
)]
+
under CO2 display enhanced current densities at potential corresponding
to the irreversible [Co(triphos)(bdtCl
2
)]
0/-
couple (Figure 4.13). An anodic shift is observed at the
onset of the [Co(triphos)(bdtCl
2
)]
0/-
couple under CO2, indicative of an association of CO2 to the
metal center upon reduction, suggesting an EC mechanism.
9–11
Upon scanning anodically, the
oxidative features at -1.36 V and -0.99 V are not present under CO2, and a new feature at -1.44 V
is observed instead, suggesting that the faradaic process associated with the observed current
response is catalytic in nature and is producing a new species which is oxidized at this potential.

171

Figure 4.13 CVs of 0.45 mM of [Co(triphos)(bdtCl
2
)]
+
in a CH3CN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the presence of 0.3
M H2O (blue). Scan rate is 100 mV/s.

Figure 4.14 Cyclic voltammograms of 0.45 mM of [Co(triphos)(bdtCl
2
)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2. Scan rates vary from 0.05 to 1 V/s.
172

Figure 4.15 Cyclic voltammograms of 0.45 mM [Co(triphos)(bdtCl
2
)]
+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O. Scan rate is 100
mV/s.
Performing CVs at multiple scan rates induce a shift in this redox event, shifting the feature
catholically (Figure 4.14). Addition of 0.3 M H 2O as a proton source under a CO2 atmosphere
leads to the formation of a characteristic catalytic plateau, with a 2-fold increase in the current
density (Figure 4.13). Increasing the concentration of H2O yields an increase in the catalytic
current density and an anodic shift in the catalytic onset potential (Figure 4.15). Upon addition of
0.5 M H2O, a CV trace yields a catalytic response that displays a trace-crossing event upon
scanning anodically (Figure 4.15). This phenomenon has been previously attributed to the
formation and subsequent reduction of a newly formed species with a standard reduction potential
more positive than that of [Co(triphos)(bdtCl
2
)]
0/-
and is similarly reported for
[Co(triphos)(bdt)]
+
to describe the formation of the [CoH(triphos)(bdt)]
0
species (Figure 3.13,
Chapter 3).
12–14
Comparatively, CV traces of [Co(triphos)(bdtCl
2
)]
+

under a CO2 atmosphere in
the presence of 0.3 M H2O yield catalytic plateaus with current densities 1.75 fold less than those
observed under similar conditions with [Co(triphos)(bdt)]
+
, with onset of catalysis shifted
positively by 24 mV, respectively (Figure 4.16).
173

Figure 4.16 CVs of 0.45 mM of [Co(triphos)(bdtCl
2
)]
+
(black) and [Co(triphos)(bdt)]
+
(green)
in a CH3CN solution containing 0.1 M [nBu4N][PF6] CO2 and in the presence of 0.3 M H2O. Scan
rate is 100 mV.
This corroborates our initial hypothesis that amending electron-withdrawing groups on the
bdt ligand will lower electron density on the metal center, and shift onset of catalysis anodically
and lower the operational overpotential. On the other hand, reduction in electron density on the
metal center can also reduce nucleophilicity and potentially reduce catalytic activity, which may
be illustrated by lower observed catalytic current density. This scaling relationship between
activity and operational overpotential has been observed in several other electrocatalytic systems
such as iron carbonyl clusters and iron tetraphenyl porphyrin complexes towards the CO2RR and
d
8
nickel complexes towards HER.
1,15,16

174

Figure 4.17 CVs of 0.45 mM of [Fe(triphos)(bdt)]
0
in a CH3CN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the presence of 0.3
0.3 M TFE (green).
CVs of [Fe(triphos)(bdt)]
0
under CO2 display no change in the current density at the
potential corresponding to the reversible [Fe(triphos)(bdt)]
0/-
couple (Figure 4.17). Addition of
0.3 M of a Brønstred acid, such as 2,2,2-trifluoroethanol (TFE), under a CO2 atmosphere leads to
the formation of a characteristic catalytic plateau, with a 0.5-fold increase in the current density
(Figure 4.17). Increasing the concentration of TFE yields an increase in the catalytic current
density and an anodic shift in the catalytic onset potential (Figure 4.18). Upon performing CVs
under CO2 atmosphere and in the presence of 0.9 M TFE at scan rates greater than 100 mV/s an
oxidation feature is observed upon scanning anodically from the catalytic plateau, suggesting that
at fast scan rates the rate of catalysis is too sluggish to be observed on the CV timescale and the
return oxidation associated with the [Fe(triphos)(bdt)]
0/-
can be observed (Figure 4.19). This
reaction rate is likely a function of proton donor concentration as well, as a return oxidation feature
can also be observed at [TFE] < 0.5 M (Figure 4.18).
175

Figure 4.18 Cyclic voltammograms of 0.45 mM [Fe(triphos)(bdt)]
0
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE. Scan rate is 100
mV/s.

Figure 4.19 Cyclic voltammograms of 0.45 mM of [Fe(triphos)(bdt)]
0
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] and 0.9 M TFE under an atmosphere of CO2. Scan rates vary from
0.1, 0.5, and 1 V/s.
4.2.4 Controlled Potential Electrolysis and Product Analysis of [Co(triphos)(bdtCl
2
)]
+

To identify and quantify the products generated at the observed catalytic features,
controlled potential electrolysis (CPE) experiments were performed in acetonitrile for 2 hours
176

under 1 atm of CO2 with H2O as the proton source at -2.15 V vs Fc
+/0
. The activity and selectivity
of [Co(triphos)(bdtCl
2
)]
+
was directly compared to the ones observed under the ideal conditions
for formate production with [Co(triphos)(bdt)]
+
. At the end of the CPE experiment, gaseous
products were sampled from the head space of the electrolysis cell, and quantification was
determined by gas chromatography (GC) analysis. Products in the liquid phase were detected and
quantified using
1
H NMR spectroscopy. Results of these experiments are shown in Figure 4.20
and Table 4.3. Turnover numbers (TONs) and faradaic efficiencies (FE%) were determined from
the CPE studies, based on established equations:

𝐹𝑎 𝑟 𝑎𝑑𝑎𝑖𝑐 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 𝑡 ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 𝑎𝑐𝑐𝑜𝑟𝑑𝑖𝑛𝑔 𝑡𝑜 𝑐 ℎ𝑎𝑟𝑔𝑒 𝑝𝑎𝑠𝑠𝑒𝑑 𝑥 100 (1)

𝑇𝑢𝑟𝑛𝑜𝑣𝑒𝑟 𝑁𝑢𝑚𝑏𝑒𝑟𝑠 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (2)

Electrolysis of [Co(triphos)(bdtCl
2
)]
+
in the presence of 0.3 M TFE at -2.15 V yields formate as
the primary CO2 reduction product with a faradaic efficiency (FE) of 24%. Gaseous products such
as H2 and CO were detected at FE of 1% and 0.4%, respectively, yielding a combined FE% of
25%.

Table 4.3 Summary of the controlled potential electrolysis results of [Co(triphos)(bdtX)]
+
(where
X = H, Cl2). Electrolyses were performed with 0.45 mM of [Co(triphos)(bdtX
2
)]
+
in a CH3CN
solution containing 0.1 M [nBu4N][PF6] and 0.3M H2O under an atmosphere of CO2 at -2.15V vs
Fc/Fc
+
.
Catalyst
Charge
(C) (±4)
H2 CO HCOO
-

Total
FE%
(±7)
FE% H2
(±3)
TON H2
(±0.3)
FE%CO
(±2)
TONCO
(±0.1)
FE% HCOO-
(±3)
TON HCOO -
(±1.5)
[Co
Cl2
]
+

4.8 1 0.02 0.4 0.004 24 0.5 25
[Co]
+
20 10 0.6 4 0.25 84 4.9 98
a
catalyst denoted [Co
Cl2
]
+
corresponds to [Co(triphos)(bdtCl 2)]
+
and [Co]
+
corresponds to [Co(triphos)(bdt)]
+

177

This is a significant deviation from the relatively selective formation of formate (84%)
when [Co(triphos)(bdt)]
+
is used under similar conditions. Additionally, generally lower TONs
are observed in comparison to the values observed in the presence of [Co(triphos)(bdt)]
+
in
addition to a lack of unity in faradaic efficiency. This overall trend suggests that
[Co(triphos)(bdtCl
2
)]
+
is a less active and selective electrocatalyst than [Co(triphos)(bdt)]
+

under similar conditions. Moreover, the lack of unity total FE% indicates a parasitic faradic
process not associated with the CO2RR is occurring when [Co(triphos)(bdtCl
2
)]
+
is electrolyzed
under these conditions.

Figure 4.20 Controlled potential electrolysis traces measured under 1 atm of CO 2 in an acetonitrile
solution of 0.45 mM [Co(triphos)(bdtCl
2
)]
+
with 0.1 M [nBu4N][PF6] supporting electrolyte,
which was held at a potential of -2.15 V vs Fc/Fc
+
for 2 hour in the presence of 0.3 M H2O.
4.3 Conclusion
This report focuses on the investigation of the electrocatalytic activity of
[Co(triphosCl
2
)(bdt)]
+
and [Fe(triphos)(bdt)]
0
towards the CO2RR. Electrochemical analysis
of [Co(triphosCl
2
)(bdt)]
+
yields CVs that indicate significant positive shift of the
[Co(triphosCl
2
)(bdt)]
+/0
and the [Co(triphosCl
2
)(bdt)]
0/-
redox couples compared to those
178

observed for [Co(triphos)(bdt)]
+
. In the presence of CO2 and H2O, the onset of catalysis is
observed to occur at more cathodic potentials when [Co(triphos)(bdtCl
2
)]
+
is used in place of
[Co(triphos)(bdt)]
+
, though at the loss of significant catalytic current density (~1.75 fold less).
Electrolysis of [Co(triphos)(bdtCl
2
)]
+
in the presence of 0.3 M H2O at -2.15 V yields low formate
selectivity, activity, and a significant loss of the total faradaic efficiency compared to
[Co(triphos)(bdt)]
+
. [Fe(triphos)(bdt)]
+
displays a reversible redox couple at the
[Fe(triphos)(bdt)]
0/-
, and a catalytic feature is observed upon introducing an exogenous proton
source in the presence of CO2 and TFE.
For future work, further experimentation to determine the catalytic parameters for both
species should be undertaken. This entails determining the ideal catalytic conditions for
[Co(triphos)(bdtCl
2
)]
+
at more anodic operational overpotentials and various H 2O concentration.
Employing H2O as a proton source using [Fe(triphos)(bdt)] should be explored and electrolysis
under CO2 atmosphere should be performed to determine the activity of this complex towards the
CO2RR.
4.4 Experimental Methods
4.4.1 General
All manipulations of air and moisture sensitive materials were conducted under a nitrogen
atmosphere in a Vacuum Atmospheres drybox or on a dual manifold Schlenk line. The glassware
was oven-dried prior to use. All solvents were degassed with nitrogen and passed through activated
alumina columns and stored over 4Å Linde-type molecular sieves. All the chemical reagents were
purchased from commercial vendors and used without further purification

179

4.4.2 Single-crystal X-ray Diffraction
An opaque black plate specimen was mounted for the X-ray crystallographic analysis, with
approximate dimensions of 0.58 × 0.383 × 0.209 mm
3
. The X-ray intensity data were measured on
a XtaLAB Synergy, Dualflex, HyPix system equipped with a micro-focus sealed tube (Cu Kα λ =
1.54184 Å), a goniometer (4-axis kappa with telescopic detector sled), and a detector (HPC HyPx-
6000HE 77.5 × 80.3 mm
2
). Data was collected on CrysAlisPro 1.171.41.122a (Rigaku OD, 2021)
and a total of 2584 frames were collected and integrated. The SHELXT 2014/5 Software Package
was used to determine the structure solution with direct methods. The SHELXL Software Package
was used for refinement by full-matric least-squares on F2. OLEX2-1.5 program was used for both
structure solution and refinement.

4.3.3 NMR Spectroscopy
Proton NMR spectra were acquired at room temperature using Varian (Mercury 400 2-channel,
VNMRS-500 2-channel, VNMRS-600 3-channel, and 400-MR 2-channel) spectrometers and
referenced to the residual
1
H resonances of the deuterated solvent and are reported as parts per
million (ppm) relative to tetramethylsilane (
1
H: DMSO-d6, δ 2.50 ppm).
31
P resonances are
reported as parts per million relative to an external sample of 85% H3PO4, which is set as 0 ppm.

4.4.4 Cyclic Voltammetry (CV)
Electrochemistry experiments were carried out using a Pine potentiostat. The experiments were
performed in a single compartment electrochemical cell under a nitrogen atmosphere using a 3
mm diameter glassy carbon electrode as the working electrode and a platinum wire as auxiliary
electrode. The reference electrode was a Ag wire in a 0.1 M electrolyte solution in MeCN and was
separated from the rest of the solution by a Vycor tip. Ohmic drop was compensated using the
positive feedback compensation implemented in the instrument. All experiments in this paper were
referenced relative to ferrocene (Fc) with the Fe
3+/2+
couple at 0.0 V. All electrochemical
180

experiments were performed in acetonitrile (MeCN) with 0.45 mM analyte concentration and 0.1
M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) as the supporting electrolyte. All
electrochemical experiments were performed with internal resistance compensation using the
current interrupt (RUCI) method in AfterMath. Normalized current is calculated by taking the ratio
of the observed current density (i) and the peak current density displayed at the cathodic peak of
the [Co(triphos)(bdt)]
+/0
couple (ip).

4.4.8 Synthesis of [Co(triphos)(bdtCl
2
)][BF
4
]·CH
3
CN
A solution of CoCl2·H2O (238 mg, 1 mmol) in 25 ml of ethanol was transferred to a solution of
1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) (623 mg, 1 mmol) in 20 mL of acetone under
N2, and was stirred at room temperature for 1 hour, during which the mixture turned a wine color
(Solution A). Then, a solution of sodium tertbutoxide (197 mg, 2 mmol) dissolved in 15 mL of
methanol was prepared, to which 211 mg (1 mmol) of 3,6-dichloro-1,2-benzenedithiol (bdtCl 2)
was added and allowed to mix for 30 min yielding a yellow solution (Solution B). Solution B was
then added dropwise to Solution A, whereupon a red precipitate forms after 10 min. of mixing.
The reaction was then allowed to mix under N2 overnight. The solution is then reduced under
vacuum to a dry powder, after which it is transferred to a glove box and washed with 20 mL of
H2O ( 3), 20 mL of EtOH ( 3), and 20 mL of pentane ( 2), and then dried under vacuum. The
product was recrystallized by vapor diffusion of pentane into a dichloromethane solution. To a 20
mL vial under N2, 194 mg (0.21 mmol) of the synthesized product, 57 mg (0.21 mmol) of
ferrocenium tetrafluoroborate (FcBF4), and 20 mL of dichloromethane were added, yielding a
violet-red solution. The reaction mixture was stirred for 1 hr at room temperature. The mixture
was transferred to a 1 L flask and stirred vigorously with ~ 700 mL of hexanes, yielding a purple-
red precipitate, which was isolated by vacuum filtration and washed with copious amounts of
181

hexanes. The product was recrystallized by layering pentane on a dichloromethane solution of the
complex.
1
H NMR (500 MHz, DMSO-d6) δ 7.72 (m, 2H, C6H2Cl2S2), δ 7.26 (m, 6H, para-
P(C6H5)2), 7.06 (m, 24H, ortho/meta -P(C6H5)2), 3.03 (s, 6H, PCH2), 2.01 (s, 3H, CH3).
31
P-{
1
H}
NMR (202 MHz, DMSO-d6) δ 32.5(s).

Elem. Anal. Calcd for C47H41Cl2CoP3S2BF4·CH3CN: C
57.67; H 4.35;N 1.37. Found: C 57.56; H 4.12; N 1.22.
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183

Chapter 5: Highly Conjugated Multimetallic Cobalt Phosphino-Thiolate Complexes
towards Electrocatalytic CO
2
Reduction
5.1 Introduction
Current heterogeneous catalysts for the CO 2RR are often limited due to their low
selectivity, with their ill-defined surface mechanisms inhibitive towards rational iterative
improvement.
1,2
On the other hand, homogeneous catalysts can provide modular platforms that
can be synthetically tuned to increase rates and selectivities.
1,3,4
As a result, fundamental research
of homogeneous catalytic systems can provide insights into new chemical strategies to further
optimize and increase the efficiencies of future catalytic systems. One parameter that is often
focused on improving is the operational overpotential, which is the difference between the
operating potential of the reaction that meets a certain rate or selectivity versus the potential at
which the reaction can be driven according to purely thermodynamic principles.
One effective strategy towards catalyst design is employing common structural motifs
found biological systems active in the reaction of interest. On such set of systems are enzymes
such as CO dehydrogenases and formate dehydrogenase, which can selectively and reversibly
convert CO2 to CO or formate, respectively, near the thermodynamic potential. A common
structural motif found in these systems is the presence of redox-active moieties in the form of FeS
clusters in CO dehydrogenase and molybdopterin ligand in formate dehydrogenase, which act as
electron conduits and reservoirs that can stabilize reaction intermediates through delocalization of
the electrons throughout these conjugated moieties.
5
Similarly, this strategy has been employed in
various synthetically-derived molecular systems, where ligands are synthetically modified with
extended conjugated moieties, which has been found to stabilize reduced metal centers and store
excess electron equivalents, lowering effective overpotentials and increasing relative activity.
6–10

Another strategy to reduce overpotential is functionalizing ligand frameworks with cationic
184

species that can stabilize CO2 intermediates and reduced metal centers via through-space
Coulombic interactions.
6,11–14
In some cases both strategies are employed, as with a cobalt
pyridyldiimine complex, where the pyridine backbone was functionalized with phenyl, pyridyl,
and N-methylpyridinium groups. As each of these functionalities were introduced to the ligand
scaffold, an anodic shift in catalytic onset potential was detected with an associated increase in the
faradaic efficiency towards CO and the intrinsic activity by 4 orders of magnitude.
6

Figure 5.1 Modulation of ligand scaffold of a cobalt pyridyldiimine complex and its effect on the
operational overpotential and activity towards the CO 2RR. Reprinted with permission from
Ref 23. Copyright 2021 American Chemical Society.
Recently, we reported a cobalt phosphino-thiolate complex ([Co(triphos)(bdt)]
+
) that can
effectively convert CO2 to formate (as high as 91%) in the presence of H2O, displaying negligible
current degradation over 8 hours (Chapter 3). However, this catalytic system displays a large
overpotential (750 mV) relative to its similarly selective congeners (Table 3.1). As a result, to
lower the effective overpotential and to attempt to increase activity, we report the reactivity of
trimetallic cobalt complexes incorporating a trinucleating dithiolene ligands in the form of
triphenylene-2,3,6,7,10,11-hexathiolate (THT) with 1,1,1,-tris(diphenylphosphinomethyl) ethane
(triphos) employed as the capping ligand
([Co
3
(triphos)
3
(THT)]
3+
) (Figure 5.2). Based on our previous reports, this multimetallic system
185

displays extensive conjugation across the THT ligand with significant electronic communication
across the three cationic cobalt centers.
15
As a result, we hypothesize that these synergistic effects
can be an effective strategy to lower overpotential and increase the activity of the phosphino-
thiolate active site within the molecular structure of [Co
3
(triphos)
3
(THT)]
3+
. Cyclic voltametric
studies were performed to characterize the electrochemical behavior of [Co
3
(triphos)
3
(THT)]
3+
under reducing conditions, and the reactivity of the complex in the presence of CO 2 and Brønsted
acids was investigated. Controlled potential electrolysis studies were employed to explore the
selectivity, activity, and stability of the catalyst under various conditions.

Figure 5.2 Chemdraw illustration of the synthesized cobalt triphosphine-thiolate complex
([Co(triphos)(bdt)]
+
) and analogous trimetallic complex ([Co
3
(triphos)
3
(tht)]
3+
).

5.2 Results and Discussion
5.2.2 Cyclic Voltammetry in MeCN under Inert Conditions
Complex [Co
3
(triphos)
3
(THT)]
3+
was synthesized according to a reported literature procedure.
15

Cyclic voltammograms (CVs) of [Co
3
(triphos)
3
(THT)]
3+
(0.45 mM) were obtained under an N2
atmosphere using a glassy carbon electrode (GCE) in acetonitrile (MeCN) or dimethylformamide
(DMF) solutions with 0.1 M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) as the
186

supporting electrolyte. All potentials are referenced versus Fc
+/0
and all CVs were first scanned
cathodically and subsequently returned anodically. Potentials of redox events observed in the
CVs for [Co(triphos)(THT)]
3+
are located in Table 5.1. CVs of [Co
3
(triphos)
3
(THT)]
3+
in
MeCN have been previously reported by us to reveal a three-electron reversible redox couple
centered at -0.69 V, which has been assigned to the Co
III/II
redox event at each metal site
sequentially (Figure 5.3, Chapter 3).
15
Upon scanning further cathodically, CVs exhibit two
irreversible feature at -2.01 and -2.07 V, attributed to a similar convoluted three-electron redox
event described above and will be assigned to the [Co
3
(triphos)
3
(tht)]
0/3-
couple as a whole
(Figure 5.3). This amounts to a 70 mV shift in this first redox event compared to the
[Co(triphos)(bdt)]
0/-
couple (-2.08 V), suggesting the highly conjugated THT ligand and the
cationic cobalt centers are influencing the redox properties of [Co
3
(triphos)
3
(THT)]
3+
. Scanning
anodically from this potential, three irreversible redox events are observed at -1.86 V, -1.49 V,
and -1.31 V. These oxidation features do not appear in the CVs of the [Co
3
(triphos)
3
(THT)]
3+

complex if the potential is reversed before reaching the [Co
3
(triphos)
3
(THT)]
0/3-
couple,
suggesting that these oxidative events originate from the reduction of [Co
3
(triphos)
3
(THT)]
0
(Figure 5.4). The irreversible nature of the redox feature of the [Co
3
(triphos)
3
(THT)]
0/3-
couples
is likely due to a chemical/structural change to the complex upon reduction of
[Co
3
(triphos)
3
(THT)]
0
similarly observed in the reduction of [Co(triphos)(bdt)]
0
(Chapter 3).
CVs of variable scan rates of [Co
3
(triphos)
3
(THT)]
+3
(Figure 5.5) display no significant change
in the reversibility of the [Co
3
(triphos)
3
(THT)]
0/3-
couple.

187

Figure 5.3 CVs of 0.5 mM of [Co
3
(triphos)
3
(THT)]
3+
in a MeCN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate is 500 mV/s.

Figure 5.4 CVs of 0.5 mM of [Co
3
(triphos)
3
(THT)]
3+
in a MeCN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s.

188

Table 5.1 Potentials of the observed redox events for [Co(triphos)(THT)]
+
in MeCN and DMF
under an atmosphere of N2. All potentials are referenced versus Fc
+/0
.
E
0
vs Fc/Fc
+

of [Co
3
(triphos)
3
(THT)]
Couple
Solvent
MeCN DMF
[Co(triphos)(THT)]
+/0
-0.69 -0.72
[Co(triphos)(THT)]
0/-
-2.04 -2.21
[Co(triphos)(THT)]
-/-2
-2.39 -
Ox 1 -1.86 -1.59
Ox 2 -1.49 -
Ox 3 -1.31 -

Figure 5.5 Cyclic voltammograms of 0.5 mM of [Co
3
(triphos)
3
(THT)]
3+
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 0.1 and 0.75 V/s.

189

5.2.2 Cyclic Voltammetry in MeCN under Catalytic Conditions
The catalytic behavior of [Co
3
(triphos)
3
(THT)]
3+
was studied using CV experiments in
MeCN in the presence of CO2 and with variable proton sources. CVs of [Co
3
(triphos)
3
(THT)]
3+

under CO2 display enhanced current densities at the potential corresponding to the irreversible
[Co
3
(triphos)
3
(THT)]
0/3-
couple (Figure 5.6).

Figure 5.6 CVs of 0.45 mM of [Co
3
(triphos)
3
(THT)]
3+
in a CH3CN solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the presence of 0.3
M H2O (blue) or 0.3 M TFE (green). Scan rate is 100 mV.
Upon scanning anodically, the oxidative features at -1.86 V, -1.49 V, and -1.31 V are not observed
under CO2, suggesting that the faradaic process associated with the observed current response is
catalytic in nature and is consuming the electrons that would otherwise be available for oxidation
at these oxidation features. Addition of 0.3 M of a Brønstred acid, such as 2,2,2-trifluoroethanol
(TFE), under a CO2 atmosphere leads to the formation of a characteristic catalytic plateau, with a
2-fold increase in the current density (Figure 5.6). Comparatively, CV traces of
[Co
3
(triphos)
3
(THT)]
+3
under a CO2 atmosphere in the presence of 0.3 M TFE yield catalytic
190

plateaus with similar current densities than those observed in the presence of [Co(triphos)(bdt)]
+
,
in addition to a positively shifted onset of catalysis by 39 mV (Figure 5.7).

Figure 5.7 CVs of 0.45 mM of [Co
3
(triphos)
3
(THT)]
+3
(CoTHT, blue) and [Co(triphos)(bdt)]
+

(CoBDT, purple) in a CH3CN solution containing 0.1 M [nBu4N][PF6], 1 atm of CO2, and in the
presence of a) 0.3 M TFE; b) 0.3 M H2O. Scan rate is 100 mV/s.
Increasing the concentration of TFE yields an increase in the catalytic current density and
an anodic shift in the catalytic onset potential (Figure 5.8). Upon reaching 0.5 M TFE, a new
catalytic feature appears at -2.5 V, which progressively increases in current density and shifts
anodically upon further titration of TFE, similar to what is observed under similar conditions in
the CVs of [Co(triphos)(bdt)]
+
(Figure 5.8). Titrating 0.1 M and 0.3 M of TFE under N2 display
only mild increases in the current response at the [Co(triphos)(bdt)]
0/-
couple (Figure 5.9),
indicating that the current responses observed under CO 2 in the presence of TFE at these
concentrations are likely not largely contributed by direct TFE/proton reduction, similar to what
is observed for other active CO2RR catalysts.
16–19
Upon reaching 2 M TFE, a new shoulder feature
appears at -1.99 V, which progressively increases in current density and shifts cathodically upon
further titration of TFE (Figure 5.10). Titrating 2 M and 4 M of TFE under N2 display similar
catalytic shoulder at a similar current density, indicating this feature may originate from direct
TFE/proton reduction (Figures 5.11).
191

Figure 5.8 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE. Scan rate is 100
mV/s.

Figure 5.9 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 (solid trace) and N2 (dashed trace) with no exogenous
proton source (black), 0.1 M TFE (red) and 0.3 M TFE (blue). Scan rate is 100 mV/s.

192

Figure 5.10 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE. Scan rate is 100
mV/s.

Figure 5.11 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 (solid trace) and N2 (dashed trace) with 2 M TFE
(green) and 4 M TFE (purple). Scan rate is 100 mV/s.
Addition of 0.3 M H2O as a proton source under a CO2 atmosphere yields a similar catalytic
response, with an increase in the current density relative to the CVs performed under only CO2
(Figure 5.6). Comparatively, CV traces of [Co
3
(triphos)
3
(THT)]
+3
under a CO2 atmosphere in the
193

presence of 0.3 M H2O display catalytic plateaus with larger current densities and more anodic
onsets than those observed under similar conditions with [Co(triphos)(bdt)]
+
(Figure 5.7b).

Figure 5.12 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of H2O. Scan rate is 100
mV/s.
Increasing the concentration of H2O yields an increase in the catalytic current density and an
anodic shift in the catalytic onset potential (Figure 5.12). Upon reaching 0.5 M TFE, a new
catalytic feature appears at -2.66 V, which progressively increases in current density and shifts
anodically upon further titration of H2O (Figure 5.12), similar to the feature associated with the
TFE titrations in Figure 5.8. Notably, no trace crossing is observed in the catalytic feature under
CO2 and in the presence of H2O, in addition to increases in the current density as a function of
[H2O], which deviates from the electrochemical behavior observed for [Co(triphos)(bdt)]
+
under
similar conditions (Chapter 3, Figure 3.18).
194

Figure 5.13 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a CH3CN solution
containing 0.1 M [nBu4N][PF6] under CO2 (solid trace) and N2 (dashed trace) with 0 M TFE
(black), 0.1 M TFE (red) and 0.3 M TFE (green). Scan rate is 100 mV/s.
5.2.4 Cyclic Voltammetry in DMF under Inert Conditions
CVs of [Co(triphos)(THT)]
+
in DMF reveal a reversible redox feature at -0.71 V, which
has been assigned to Co
III/II
redox events at each metal site sequentially (Figure 5.14, Chapter 3).
15

Upon scanning further cathodically, CVs exhibit two irreversible features at -2.14 and -2.21 V,
attributed to the [Co
3
(triphos)
3
(THT)]
0/3-
couple, occurring more cathodic than observed CVs
performed in MeCN (Figure 5.14). On the return sweep (scanning anodically from -2.8 V), only
one irreversible redox event is observed at -1.58 V as opposed to the three features observed in the
CV performed in MeCN. Variable scan rates experiments of [Co
3
(triphos)
3
(THT)]
+3
(Figure 5.15)
in DMF display no significant change in the reversibility of the [Co
3
(triphos)
3
(THT)]
0/3-
couple,
though a anodic shift is observed of the return oxidation feature.
195

Figure 5.14 CVs of 0.45 mM of [Co
3
(triphos)
3
(THT)]
3+
in a DMF solution containing 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate is 500 mV/s.

Figure 5.15 Cyclic voltammograms of 0.45 mM of [Co
3
(triphos)
3
(THT)]
3+
in a DMF solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates vary from 0.01 to 1 V/s.

196

Figure 5.16 Sequential cyclic voltammograms of 0.45 mM of [Co
3
(triphos)
3
(THT)]
3+
in a DMF
solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rates is 200 mV/s.
Electrode was not polished between scans.
Performing a series of CVs without polishing between scans yields a shift and reduction of the
[Co
3
(triphos)
3
(THT)]
0/3-
couple to more negative potentials and the emergence of a new redox
event at -2.26 V (Figure 5.16). Polishing the electrode regenerates the original CV trace. This
behavior suggests deposition of a newly formed species to the electrode at the
[Co
3
(triphos)
3
(THT)]
0/3-
couple when CVs are performed in DMF with [Co
3
(triphos)
3
(THT)]
3+
.
5.2.5 Cyclic Voltammetry in DMF under Catalytic Conditions
The catalytic behavior of [Co
3
(triphos)
3
(THT)]
3+
was studied using CV experiments in
DMF and in the presence of CO2 and TFE. CV experiments were first performed with the working
electrode unpolished between the subsequent scans. CVs of [Co
3
(triphos)
3
(THT)]
3+
under CO2
display enhanced current densities at the potential corresponding to the irreversible
[Co
3
(triphos)
3
(THT)]
0/3-
couple (Figure 5.17). Upon scanning anodically, the oxidative feature at
-1.58 V is not observed under CO2, suggesting that the faradaic process associated with the
197

observed current response is catalytic in nature and is consuming the electrons that would
otherwise be available for oxidation at these oxidation features.

Figure 5.17 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a DMF solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE. Scan rate is 100
mV/s. Working electrode was not polished between scans.
Addition of 0.1 M of TFE under a CO2 atmosphere displays a catalytic feature with a two-fold
reduction in the current density and a cathodic shift in the catalytic onset relative to CVs performed
under CO2 without an exogenous proton source (Figure 5.17). Additionally, titrations of TFE
beyond the 0.1 M concentration yield no change in the current density, indicative of saturation
kinetics. Overall, these electrochemical characteristics is in contrary to what is expected for the
CO2RR dependent on proton concentration. Upon polishing the electrode between scans, the CVs
display a marked difference in the response as a function of [TFE] (Figure 5.18). Increasing the
concentration of TFE yields an increase in the catalytic current density and an anodic shift in the
catalytic onset potential. Upon reaching 0.3 M TFE, the current response saturates as a function of
[TFE].

198

Figure 5.18 Cyclic voltammograms of 0.45 mM [Co
3
(triphos)
3
(THT)]
+3
in a DMF solution
containing 0.1 M [nBu4N][PF6] under CO2 with increasing concentrations of TFE. Scan rate is 100
mV/s. Working electrode was polished between scans.

Figure 5.19 CVs of 0.45 mM of [Co
3
(triphos)
3
(THT)]
3+
in an MeCN (blue) or DMF (red)
solution containing 0.1 M [nBu4N][PF6] under an atmosphere CO2 in the presence of 0.3 M H2O.
Scan rate is 100 mV/s.

199

Overall, polishing between scans yields CVs that display more traditional electrochemical
characteristics of what is expected for the CO2RR dependent on proton concentration. These
experiments indicate that deposition of an electrocatalytic inactive phase on the working electrode
occurs during catalysis. CVs of [Co
3
(triphos)
3
(THT)]
3+
performed in DMF under CO2 in the
presence of 0.3 M TFE display a 1.6-fold increase catalytic current density compared to those
displayed in the CVs under similar conditions performed in MeCN (Figure 5.19). Alternatively,
CV traces under DMF display a 90 mV negative shift in the onset of catalysis compared to the
feature observed in MeCN (Figure 5.19)
5.2.6 Controlled Potential Electrolysis and Product Analysis
To identify and quantify the products generated at the observed catalytic features,
controlled potential electrolysis (CPE) experiments were performed in acetonitrile for 30 minutes
or in dimethylformamide for 120 minutes under 1 atm of CO 2 with TFE as the proton source. CPEs
were performed at -2.10 V or at -2.60 V vs Fc
+/0
and with various acid concentrations to determine
if the product selectivity and total turnover number changes as a function of these variables. At the
end of the CPE experiment, gaseous products were sampled from the head space of the electrolysis
cell, and quantification was determined by gas chromatography (GC) analysis. Products in the
liquid phase were detected and quantified using
1
H NMR spectroscopy. Results of these
experiments are shown in Figures 5.20, 5.21, 5.23 and Table 5.1. Turnover numbers (TONs) and
faradaic efficiencies (FE%) were determined from the CPE studies, based on established
equations:

𝐹𝑎𝑟𝑎𝑑𝑎𝑖𝑐 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 𝑡 ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 𝑎𝑐𝑐𝑜𝑟𝑑𝑖𝑛𝑔 𝑡𝑜 𝑐 ℎ𝑎𝑟𝑔𝑒 𝑝𝑎𝑠𝑠𝑒𝑑 𝑥 100 (8)

200

𝑇𝑢𝑟𝑛𝑜𝑣𝑒𝑟 𝑁𝑢𝑚𝑏𝑒𝑟𝑠 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (9)

Electrolysis of [Co
3
(triphos)
3
(THT)]
3+
was performed for 30 minutes in MeCN in the presence
of CO2 and 0.3 M TFE at -2.10 V (Entry 5) and yields formate as the primary CO2 reduction
product with a faradaic efficiency (FE) of 26%. Gaseous products such as H 2 and CO were detected
at FEs of 17% and 13%, respectively, yielding a combined FE% of 56%. CPE experiments
conducted past approximately 30 minutes led to the formation of a red precipitate and an associated
inflection point within the CPE trace, and subsequently were not performed past this point for all
CPEs (Figure 5.22). Due to the incongruity of the CPE lengths, direct comparison of TONs with
[Co(triphos)(bdt)]
+
will not be made.

Figure 5.20 Comparison of the controlled potential electrolysis results – Faradaic efficiencies
(FE%) in the presence of 0.3 M TFE, 0.03 M TFE, or no exogenous proton source (N/A) at a)
-2.10 V or b) -2.60 V vs Fc/Fc
+
. All electrolyses were performed with 0.45 mM of
[Co
3
(triphos)
3
(THT)]
3+
in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
201

Figure 5.21 Comparison of the controlled potentiMeCNal electrolysis results - Turnover numbers
(TONs) in the presence of 0.3 M TFE, 0.03 M TFE, or no exogenous proton source (N/A) at a) -
2.10 V or b) -2.60 V vs Fc/Fc
+
. All electrolyses were performed with 0.45 mM of
[Co
3
(triphos)
3
(THT)]
3+
in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an
atmosphere of CO2.
Table 5.2 Summary of the controlled potential electrolysis results and the conditions used for the
electrolysis of [Co
3
(triphos)
3
(THT)]
3+
in the presence of CO2 and a proton source. Electrolyses
were performed with 0.45 mM of [Co
3
(triphos)
3
(THT)]
3+
in a CH3CN or DMF solution
containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2.
Entry Solvent
Time
(min)
[Acid]
(M)
Potential
(V) vs
Fc/Fc
+

Charge
(C)
(±4)
H2 CO HCOO
-

Total
FE%
(±6)
FE%
(±3)
TON
(±0.3)
FE%
(±2)
TON
(±0.10)
FE%
(±3)
TON
(±1.5)
1
MeCN 30
0
-2.10 1 0 0 0 0 0 0 0
2 -2.60 5 2 0.04 5 0.05 8 0.1 15
3
0.03
-2.10 4 3 0.04 2 0.02 5 0.03 9
4 -2.60 5 2 0.04 1 0.02 4 0.04 7
5
0.3
-2.10 7 17 0.3 13 0.5 26 0.4 56
6 -2.60 5 17 0.5 34 1 16 0.4 67
7 DMF 120 0.3 -2.60 20 3 0.2 1 0.06 6 0.4 10

202

Figure 5.22 Controlled potential electrolysis traces of 0.45 mM [Co
3
(triphos)
3
(THT)]
3+
in
acetonitrile with 0.1 M [nBu4N][PF6] measured under 1 atm of CO2 in the presence of 0.03 M TFE
at -2.60 V vs Fc/Fc
+
for 1.5 hours.

Figure 5.23 Controlled potential electrolysis traces measured under 1 atm of CO 2. In all cases a
solution of 0.45 mM [Co
3
(triphos)
3
(THT)]
3+
in acetonitrile with 0.1 M [nBu4N][PF6] supporting
electrolyte was held at a potential of either -2.10 V or -2.60 V vs Fc/Fc
+
for 30 minutes in the
presence of no added proton source (red and yellow, respectively), 0.03 M TFE (blue and green,
respectively), and 0.3 M TFE (violet and magenta, respectively).

203

Comparing these results to those obtained with [Co(triphos)(bdt)]
+
, CPE results for
[Co
3
(triphos)
3
(bdt)]
3+
yield significantly lower selectivity towards formate production, while
producing larger amounts of side products such as CO and H 2. Performing CPE at a larger
overpotential (-2.60 V) in the presence of 0.3 M TFE (Entry 6) display a significant shift in
selectivity towards formation of CO with 34% FE (TONCO = 1), at the expense of lower quantities
of formate (FEHCO2– = 16%, TONHCO2– = 0.4), while production of H2 stays relatively stable (FE H2
= 17%, TONH2 = 0.5). CPE results at either low TFE concentrations (0.03 M) or without an added
exogenous Brønsted acid (Entries 1-4) display a mixture of products, with an overall low total
faradaic yields and catalytic TONs.
Across the CPE experiments, the total FE% of all detected products does not approach
100%, suggesting the presence of a parasitic secondary process stoichiometrically consuming
electrons throughout electrolysis experiments. To further investigate the stability of this catalytic
system, degradation experiments were performed under the conditions employed in the CPEs
discussed above. To determine if a catalytically active phase is depositing on the electrode during
electrolysis, the working electrode was rinsed with clean CH 3CN post-electrolysis and placed back
in the working compartment with a CH3CN solution containing 0.1 M [nBu4N][PF6] and 0.3 M
TFE under an atmosphere of CO2.
204

Figure 5.24 Cyclic voltammograms of a bare glassy carbon electrode (black – labeled “bare
GCE”) and a washed post-electrolysis glassy carbon electrode (red – labeled “wash test”) in an
acetonitrile solution containing 0.1 M [nBu4N][PF6] and 0.3 M TFE under an atmosphere of CO2.
The post-electrolysis glassy carbon electrode (wash test) investigated here was generated upon
performing an electrolysis with a glassy carbon electrode for 30 minutes at -2.60 V under CO2 and
in a CH3CN solution with 0.45 M of [Co
3
(triphos)
3
(THT)]
3+
, 0.3 M TFE, and 0.1 M
[nBu4N][PF6], and subsequently washed with clean CH 3CN under anaerobic conditions to prevent
O2 exposure of the electrode.
CVs of the electrodes post-CPE display redox features with the onset at ~ -2.0 V, with currents
larger than those observed using a bare glassy carbon electrode, suggesting an electroactive
deposited material is present on the electrode (Figure 5.24). Scanning through this feature
successively leads to a decrease in the observed current upon each subsequent scan, indicating that
this deposited material display low stability at these reducing potentials (Figure 5.25). The washed
electrode was additionally analyzed using X-ray photoelectron spectroscopy (XPS) to determine
if a cobalt-containing species is deposited on the working electrode during electrolysis. XPS
spectra of the working electrode indicates trace amounts of cobalt on the electrode surface (Figure
5.26).

205

Figure 5.25 Cyclic voltammograms of a bare glassy carbon electrode (black – labeled “bare
GCE”) and a washed post-electrolysis glassy carbon electrode (red – labeled “wash test scan x”,
where x = First-Fifth) in an acetonitrile solution containing 0.1 M [nBu4N][PF6] and 0.3 M TFE
under an atmosphere of CO2. The post-electrolysis glassy carbon electrode (wash test) investigated
here was generated upon performing an electrolysis with a glassy carbon electrode for 30 minutes
at -2.60 V under CO2 and in a CH3CN solution with 0.45 M of [Co
3
(triphos)
3
(THT)]
3+
, 0.3 M
TFE, and 0.1 M [nBu4N][PF6], and subsequently washed with clean CH 3CN under anaerobic
conditions to prevent O2 exposure of the electrode.

206

Figure 5.26 High-resolution X-ray photoelectron spectroscopy spectra of Co 2p region of a
washed post-electrolysis glassy carbon electrode. The post-electrolysis glassy carbon electrode
investigated here was generated upon performing an electrolysis with a glassy carbon electrode
for 30 minutes at -2.60 V under CO2 and in a CH3CN solution with 0.45 M of
[Co
3
(triphos)
3
(THT)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6], and subsequently washed with
clean CH3CN.

Figure 5.27 UV-vis spectrum of a MeCN solution of [Co
3
(triphos)
3
(THT)]
3+
(as-prepared
CoTHT) and a sample of the electrolyte solution from the working compartment after CPE (post-
electrolysis). The post-electrolysis solution investigated here was generated upon performing an
electrolysis with a glassy carbon electrode for 30 minutes at -2.60 V under CO2 and in a CH3CN
solution with 0.45 M of [Co
3
(triphos)
3
(THT)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6].
207

UV-vis spectra taken of an electrolyte solution after CPE display new transitions that are not
present in the visible spectrum of a MeCN solution of [Co
3
(triphos)
3
(THT)]
3+
, suggesting
degradation of [Co
3
(triphos)
3
(THT)]
3+
during electrolysis (Figure 5.27). This correlates with our
observations of deposition of an electroactive material on the working electrode surface during
electrolysis, precipitation of a red powder after 30 min of CPE in conjunction with an inflection in
the CPE trace, and the lack of unity FE%, indicating the presence of a parasitic parallel
decomposition event consuming electron equivalents and depositing cobalt containing material on
the electrode.
Electrolysis of [Co
3
(triphos)
3
(THT)]
3+
was performed for 2 hours in DMF in the presence
of 0.3 M TFE and CO2 at -2.10 V (Entry 7). CPE experiments conducted past approximately 30
minutes did not lead to similar formation of a red precipitate as observed in the CPE experiments
performed in MeCN, with no similarly observed inflection point observed within the CPE, and as
a result the CPEs were performed for 2 hours (Figure 5.28). CPE results display a mixture of
products, with overall low total faradaic yields similar to what is observed for 30 minutes CPEs
performed in MeCN with little to no exogenous proton source. Total TONs of products formed for
H2 and HCOO
-
are similar to those displayed for the 30 minutes CPEs performed in MeCN at
similar operational potentials and [TFE]. CVs of the electrodes post-CPE display lower currents
comparable to those observed using a bare glassy carbon electrode, suggesting the presence of a
passivating material deposited on the electrode during CPE experiments, similarly observed in the
CV experiments described above (Figure 5.29). As a result, formation and deposition of this
passivation material is most likely the origin for the lack of unity FE% in detected products.

208

Figure 5.28 Controlled potential electrolysis traces measured under 1 atm of CO 2. In all cases a
solution of 0.45 mM [Co
3
(triphos)
3
(THT)]
3+
in dimethylformamide with 0.1 M [nBu4N][PF6]
supporting electrolyte was held at a potential of -2.60 V vs Fc/Fc
+
for 2 hours in the presence of
0.3 M TFE.

209

Figure 5.29 Cyclic voltammograms of a bare glassy carbon electrode (black – labeled “bare
GCE”) and a washed post-electrolysis glassy carbon electrode (blue – labeled “wash test”) in an
acetonitrile solution containing 0.1 M [nBu4N][PF6] and 0.3 M TFE under an atmosphere of CO2.
The post-electrolysis glassy carbon electrode (wash test) investigated here was generated upon
performing an electrolysis with a glassy carbon electrode for 2 hours at -2.60 V under CO2 and in
a DMF solution with 0.45 M of [Co
3
(triphos)
3
(THT)]
3+
, 0.3 M TFE, and 0.1 M [nBu4N][PF6],
and subsequently washed with clean CH3CN under anaerobic conditions to prevent O 2 exposure
of the electrode.

5.3 Conclusion
This report focuses on the investigation of the electrocatalytic activity of
[Co
3
(triphos)
3
(THT)]
3+
towards the CO2RR. Electrochemical analysis of
[Co
3
(triphos)
3
(THT)]
3+
in MeCN indicate that in the presence of CO 2 and H2O, the onset of
catalysis is occurs at more cathodic potentials than that observed in the CVs of [Co(triphos)(bdt)]
+
under similar conditions. Electrolysis of [Co
3
(triphos)
3
(THT)]
3+
in the presence of 0.3 M TFE at
-2.10 V yields low formate selectivity, with a significant mixture of additional products, such as
CO and H2, while electrolysis at -2.60 V display larger selectivity towards CO formation, with
overall total faradaic efficiency across all different conditions significantly below unity.
Electrolysis in MeCN was limited to 30 min due to precipitation of a red material, making direct
210

activity comparison with [Co(triphos)(bdt)]
+
not feasible. Additionally, wash tests and UV-vis
spectroscopy studies of the post-CPE solution indicate significant catalyst decomposition during
electrolysis. Electrochemical analysis of [Co
3
(triphos)
3
(THT)]
3+
in DMF suggests the formation
of a passivating phase on the electrode under both N 2 and CO2. CPE performed in DMF do not
display similar precipitation events, though product analysis yields limited faradaic yields towards
HCOO
-
, CO, and H2. These results indicate that [Co
3
(triphos)
3
(THT)]
3+
displays limited stability
under these conditions at these reducing potential and can only be used as a catalysts towards
CO2RR in a limited capacity.
For future work, further experimentation on employing alternative proton sources should
be pursued. This entails utilizing H2O as a proton source in the CPE experimentation as this is the
ideal proton source for [Co(triphos)(bdt)]
+
. Moreover, investigation into other highly-conjugated
multimetallic phosphino-thiolate cobalt complexes, such as the previously reported
[Co
3
(triphos)
3
(BHT)]
3+
employing the trinucleating benzenehexathiolate ligand, should be
pursued as well.
5.4 Experimental Methods
5.4.1 General
All manipulations of air and moisture sensitive materials were conducted under a nitrogen
atmosphere in a Vacuum Atmospheres drybox or on a dual manifold Schlenk line. The glassware
was oven-dried prior to use. All solvents were degassed with nitrogen and passed through activated
alumina columns and stored over 4Å Linde-type molecular sieves. All the chemical reagents were
purchased from commercial vendors and used without further purification

211

5.4.2 X-ray Photoelectron Spectroscopy
XPS data were collected using a Kratos AXIS Ultra instrument. The monochromatic X-ray source
was the Al K α line at 1486.6 eV. High-resolution detailed scans, with a resolution of ~0.1 eV,
were collected on individual XPS lines of interest at a pass energy of 20. The sample chamber was
maintained at < 2 × 10
–8
Torr. The XPS data were analyzed using the CasaXPS software.

5.4.3 Cyclic Voltammetry (CV)
Electrochemistry experiments were carried out using a Pine potentiostat. The experiments were
performed in a single compartment electrochemical cell under a nitrogen atmosphere using a 3
mm diameter glassy carbon electrode as the working electrode and a platinum wire as auxiliary
electrode. The reference electrode was a Ag wire in a 0.1 M electrolyte solution in MeCN and was
separated from the rest of the solution by a Vycor tip. Ohmic drop was compensated using the
positive feedback compensation implemented in the instrument. All experiments in this paper were
referenced relative to ferrocene (Fc) with the Fe
3+/2+
couple at 0.0 V. All electrochemical
experiments were performed in acetonitrile (MeCN) with 0.45 mM analyte concentration and 0.1
M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) as the supporting electrolyte. All
electrochemical experiments were performed with iR compensation using the current interrupt
(RUCI) method in AfterMath.

5.4.4 Controlled Potential Electrolysis.
Controlled potential electrolysis (CPE) measurements to determine Faradaic efficiency were
conducted in a sealed two-chambered H cell with two chambers separated by a fine porosity glass
frit. The first chamber held the working and reference electrodes in 40 mL of electrolyte solution
(0.1 M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) in MeCN) and the second
chamber held the auxiliary electrode in 21 mL of electrolyte solution. The reference electrode was
a Ag wire in a 0.1 M [nBu4N][PF6] electrolyte solution in MeCN and was separated from the rest
212

of the solution by a Vycor tip. CPE experiments were performed in 0.1 M [nBu4N][PF6] solution
in MeCN with 0.45 mM of analyte. Glassy carbon plate electrodes (6 cm × 1 cm × 0.3 cm; Tokai
Carbon USA) were used as the working and auxiliary electrodes. The working compartment was
sparged with N2 or CO2 for 15 minutes before the experiment. Wash tests were performed by
removing the post electrolysis solution of the working compartment from the H-cell via syringe
under a positive pressure of CO2 and rinsing the chamber of the electrolysis cell three times with
acetonitrile. The cell was maintained under 1 atm of CO 2, and the electrode was not removed from
the cell during these washings to prevent O2-exposure.

5.4.5 Gas Chromatography
Gaseous products were quantified using a Shimadzu GC-2010-Plus instrument equipped with a
BID detector and a Restek ShinCarbon ST Micropacked column. In a typical experiment, 2 mL of
gas were withdrawn from the headspace of the electrochemical cell with a gas-tight syringe and
injected into the instrument. Calibration plots were prepared with multiple injections of syngas
standards purchased from Praxair, Inc.

5.4.6 Formate Detection and Quantification
Detection: Initial detection of formate was performed according to literature precedent.
20

Following CPE studies, a 5 mL aliquot of each electrolysis solution was collected and extracted
with 2 mL D2O. The aqueous portion was acidified with one drop of HCl and tested for formate
by
1
H NMR spectroscopy.
Quantification: Quantification of formate was performed according to the following procedure.
A NaOH solution (0.25 mL of 0.075 mM) was added to 5 mL aliquot of the electrolysis solution,
and the resulting solution was subsequently reduced under pressure and mild heating until dry.
Two 1 mL aliquots of D2O were added sequentially to the dry solid, and the resulting mixture was
213

mixed vigorously, and sonicated for 5 minutes, before being filtered through a glass frit. One mL
from the filtered solution was transferred to an NMR sample tube. A capillary containing a solution
of DMF in D2O was added to the NMR sample tube. A
1
H NMR spectrum (128 scans with a 10 s
relaxation time) was taken for each sample in a Varian 400-MR 2-Channel instrument. Integration
ratios were then taken of the formate proton (δ = 8.47 ppm) and the DMF formyl proton (δ = 7.96
ppm). The formate concentration of the electrolysis solution was then determined using the
calibration plot described below.
Calibration: Standard formate solutions were prepared with concentrations of 0.1 M, 0.01 M,
0.001 M, and 0.0001 M. A
1
H NMR spectrum was taken for each solution with the aforementioned
DMF/D2O capillary, and a calibration curve was generated consisting of the integration ratios of
formate to DMF against the known formate concentration.

5.4.7 Synthesis of [Co
3
(triphos)
3
(THT)]
See Section 2.4.10

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(16) Chapovetsky, A.; Welborn, M.; Luna, J. M.; Haiges, R.; Miller, T. F.; Marinescu, S. C.
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216

Chapter 6: Computational Study on the Effects of Ligand Functionalization and Metal
Identity on the Activity of CO
2
Reduction Catalysts
6.1 Introduction
Optimization of molecular systems established as efficient electrocatalysts towards the
CO2RR is an active area of research. One such system is the rhenium bipyridine (ReBpy) based
catalyst that has been reported to efficiently convert CO 2 to CO.
1,2
As a strategy to tune catalytic
efficiency, studies have investigated modifications to the bipyridine backbone as a means to
introduce secondary and primary-sphere hydrogen bonding effects which increase local proton
concentration and stabilize reaction intermediates.
3
A previous study from our group reported the
functionalization of primary, secondary, and tertiary amines at the 6,6’position of a ReBpy system,
where the primary amine variant was observed to outperform the secondary and tertiary analogous
and undergo hydrogen-bonding interactions.
4
Herein, we investigated the electronic and steric
effects of amending primary amine substituents to various position on the bipyridine backbone of
the ReBpy system and its implications towards electrocatalytic activity to the CO 2RR (Figure 6.1).
Experimental results indicate that while the 4,4'-NH
2
-Re yields higher faradaic efficiency towards
CO, 6,6'-NH
2
-Re displays a more positive onset of catalysis and larger TONs towards CO
indicating the presence of a pendant amines adjacent to the metal center is integral to increasing
activity. Herein, to supplement experimental results found in this study, density functional theory
(DFT) computational methods were employed to help explicate the effect of ligand
functionalization on the electronics and activity of this set of complexes.

217

Figure 6.1 Chemdraw illustration of the studied rhenium bipyridine amended with NH2
substituents on bipyridine backbone.
Another method of homogeneous catalyst modulation is varying the identity of metal site.
In this manner, direct control on the electronics of the system can be tuned using the metal center
identity, altering both selectivity and activity of these systems within similar ligand scaffold sets.
In one study, metal complexes utilizing a pentadentate macrocyclic N-donor ligand was
synthesized employing both cobalt and iron.
5
Selectivity of the system was highly dependent on
the identity of the metal center, with the Co-based complex primarily yielding CO at 82% faradaic
efficiency (FE%) under electrochemical conditions and a TOF of 21.9 h
-1
under photochemical
conditions. Employing iron as the active metal center, electrolysis results displayed formate as the
primary product at a faradaic efficiency of 75-80%. In a similar study, a series of metal
quaterpyridine complexes incorporating cobalt and iron displayed significant activity as a function
of metal identity.
6
The cobalt-based complex displayed electrochemical conversion of CO 2 to CO
at a faradic yield of 97% at an overpotential of 140 mV, while the iron analogue displayed
significantly lower activity (FE%CO = 48%) toward the CO2RR due to CO poisoning. As a result,
these examples underscore the influence the metal site identity has towards tuning the CO 2RR
catalysts.
In a previous study, our group has demonstrated the catalytic activity of a cobalt
aminopyridine macrocyclic complex towards the CO2RR.
7
Varying the number of pendant
secondary and tertiary amines in the ligand framework was found to directly impact the activity of
218

the catalytic system, with a direct correlation between number of secondary amines and catalytic
activity, indicating the importance of hydrogen-bonding to the system as a whole.
8
Recently, a
computational report indicated the influences of various functionalities of the Co-macrocyclic
catalytic system, such as the choice of the metal center on the electronics of the system and effects
towards minimizing thermodynamic barriers towards reduction of CO 2 to CO.
9
However, these
studies do not incorporate experimentally-derived data into their computational work, and lack
further analysis of the relative molecular orbital character and energetics of systems. In this current
report, we investigate a series of ML
1
complexes with iron or nickel metal centers to study the
influence of transition metal identity on the reactivity of this system (Figure 6.2). Electrochemical
studies of [FeL
1
]
+2
and [NiL
1
]
+2
display marked difference in activity towards CO 2 reduction
compared to the reported Co-based congener. As a result, DFT computational methods were
employed to help elucidate the effect of metal center identity on the electronics and activity of this
set of macrocyclic complexes.

Figure 6.2 Chemdraw illustration of the studied aminopyridine metal complexes (ML
1
).

219

6.2 Results and Discussion
6.2.1 Computational studies on amine functionalized ReBpy complexes
A series of unrestricted density functional theory (DFT) calculations were performed on
4,4'-NH
2
-Re, 5,5'-NH
2
-Re, 6,6'-NH
2
-Re to probe the electronic implication of amine position on
the bipyridine ligand. The M06 functional was used throughout this study, as it provides reduced
Hartree–Fock exchange contributions and includes empirical fitting for accuracy in organometallic
systems in similar Re-bpy based systems reported in the literature.
10
Similarly, for heavier
elements such as Cl and Re, the Hay–Wadt VDZ (n +1)effective core potentials and basis sets
(LANL2DZ) were used, while 6-311G* basis set was used for lighter elements such as H, C, N,
and O atoms, as reported for computational work on Re-Bpy complexes in the literature.
10

Optimized structures of this of these complexes are in relatively good agreement with the
structures obtained through X-ray crystallographic techniques (Table 6.1).
Table 6.1 Comparison of selected experimental and calculated bond distances of functionalized
ReBpy complexes.

4,4' 5,5' 6,6'
Bond
Experimental
(Å)
Calculated
(Å)
Experimental
(Å)
Calculated
(Å)
Experimental
(Å)
Calculated
(Å)
Re-C(eq) 1.927(5) 1.919 1.925(2) 1.920 1.917(2) 1.921
Re-C(ap) 1.895(4) 1.910 1.899(2) 1.91 1.919(2) 1.909
C-O(eq) 1.146(6) 1.161 1.151(2) 1.160 1.153(2) 1.159
Re-N 2.169(3) 2.207 2.173(1) 2.206 2.184(2) 2.227
(eq) correspond to the CO in the equatorial plane and (ap) correspond to the CO in the apical position
Based on these calculations, HOMOs of both 4,4'-NH
2
-Re and 6,6'-NH
2
-Re display significant
stabilization as compared to the HOMO of 5,5'-NH
2
-Re (by 11 and 35 eVs, respectively) (Figure
6.3). Moreover, both 4,4'-NH
2
-Re and 6,6'-NH
2
-Re exhibit significantly more HOMO orbital
contribution from both the metal center and equatorial CO ligands compared to the HOMOs of
220

5,5'-NH
2
-Re, while the HOMO of 4,4'-NH
2
-Re exhibits the lowest contribution from the
bipyridine ligand (Figure 6.3). Upon comparing the HOMO-LUMO gap of these complexes, the
following trend of 4,4'-NH
2
-Re > 6,6'-NH
2
-Re > 5,5'-NH
2
-Re was determined. To evaluate if
electron density on the metal center is dependent on amine positionality, Mulliken electron
population analysis was performed on each NH
2
-Re complex. Based on these calculations, the
metal center of 6,6'-NH
2
-Re (+0.748) displays the lowest partial positive charge compared to 5,5'-
NH
2
-Re (+0.830) and 4,4'-NH
2
-Re (+0.834), indicating a greater degree of electron density on the
metal center of 6,6'-NH
2
-Re compared to its related congeners. In addition to hydrogen bonding
interactions, this trend supports the more positive reduction potential of 6,6'-NH
2
-Re than those
of the 4,4'- and 5,5'- analogues which are very similar, substantiating that both sterics and
electronics play a role in the catalytic activity.

Figure 6.3 Molecular orbital scheme of complexes 4,4'-NH
2
-Re, 5,5'-NH
2
-Re, 6,6'-NH
2
-Re
(left), with the corresponding orbital images of HOMO and LUMO (right). Calculations were
performed using the M06 functional with the 6-311G* basis set for H, C, N, and O atoms and the
LANL2DZ effective core potential and basis set for Cl and Re atoms.

221

6.2.2 Computational studies on the metal identity of a macrocyclic CO
2
reduction catalyst
A series of unrestricted density functional theory (DFT) calculations was first performed
on [ML
1
]
+2
to ascertain the ground spin states of these complexes and determine the relative energy
gap between alternative excited spins states. As the structure of these macrocycle complexes have
been displayed to coordinate solvent molecules in the axial positions, explicit DMF molecules
were bound to the metal center for these studies as DMF is the primary electrochemical medium
of use in associated electrochemical studies. For computational studies of [FeL
1
]
+2
, an octahedral
coordination environment with two DMF molecules in the axial position was considered based on
the obtained crystal structures and the d
6
electron count of the Fe(II) metal center.

Calculations for
[FeL
1
]
+2
indicate an energetically preferred singlet spin state compared to a triplet state by 4.28
kcal/mol, in agreement with Evan’s methods obtained from NMR spectra of [FeL
1
]
+2
(Figure 6.4).

Figure 6.4 Calculated relative energy of spin states for [FeL
1
]
+2
at the 6-31+G*/B3LYP level of
theory.
Calculations for the [FeL
1
]
+2
indicate an energetically preferred singlet spin state compared to a
triplet state by 4.28 kcal/mol, in agreement with Evan’s methods obtained from the NMR spectra
of [FeL
1
]
+2
(Figure 6.4). Calculations of the quintet spin state of [FeL
1
]
+2
were attempted but did
not converge. Additionally, time-dependent DFT (TD-DFT) calculations of [FeL
1
]
+2
display
222

similar UV transitions as displayed in experimentally obtained UV-vis spectra of [FeL
1
]
+2
(Figure
6.5), though a bathochromic shift is observed in the computationally derived spectra.

Figure 6.5 a) Computed and b) experimental UV/Vis spectra for [FeL
1
]
2+
.
For computational studies of [NiL
1
]
+2
, a square pyramidal coordination environment with one
DMF molecule in the axial position was considered based on obtained crystal structures and the
d
8
electron count of the Ni(II) metal center.

Calculations for [NiL
1
]
2+
yield only the triplet spin
state, as the singlet state was unable to converge, in agreement with Evan’s methods obtained from
NMR spectra of [NiL
1
]
+2
. Additionally, TD-DFT calculations of [NiL
1
]
+2
display similar UV
transitions as displayed in experimentally obtained UV-vis spectra of [NiL
1
]
+2
(Figure 6.6), and a
similar bathochromic shift is observed in the computationally derived spectra. Optimized
structures of both [FeL
1
]
+2
and [NiL
1
]
+2
are in good agreement with the structures obtained
through X-ray crystallographic techniques (Table 6.2).
223

Figure 6.6 a) Computed and b) experimental UV/Vis spectra for [NiL
1
]
2+
.
Table 6.2 Comparison of selected experimental and calculated bond distances of [ML
1
].

[FeL
1
]
+2
[NiL
1
]
+2

Bond
Experimental
(Å)
Calculated
(Å)
Experimental
(Å)
Calculated
(Å)
M-N 1.996 2.029 2.068 2.069

To determine the viability of the iron and nickel analogues of the [ML
1
] complex towards
the CO2RR, the electronic structure of the doubly reduced [ML
1
]
0
was explored with series of
unrestricted DFT calculations. For computational studies of [FeL
1
]
0
, a square pyramidal
coordination environment with one DMF molecule in the axial position was considered based on
the d
8
electron count of the Fe(0) metal center.

Both singlet and triplet spin states were considered
for [FeL
1
]
0
, though only the singlet state was found to converge (Figure 6.7). Based on these
calculations, the highest occupied molecular orbital (HOMO) of [FeL
1
]
0
are primarily localized
on both the metal center and ligand, and exhibit Fe(d z
2
)-Npyridyl(pz) bonding character, forming a
pocket of electron density above the metal center. On the other hand, the HOMO-1 of [FeL
1
]
0

display orbitals that are metal-center dominant, with Fe dxz symmetry, and the lowest unoccupied
molecular orbital (LUMO) is primarily ligand dominant.
224

Figure 6.7 Molecular orbital scheme of complexes [FeL
1
]
0
(left), with the corresponding orbital
character of HOMO, HOMO-1, and LUMO (right) at the 6-31+G*/B3LYP level of theory.
For computational studies of [NiL
1
]
0
, a square planar coordination environment without DMF
coordination was considered based on the closed shell nature of the Ni(0) metal center (Figure
6.8). Based on these calculations, the optimized structure exhibits significant distortion within the
macrocyclic ligand (Figure 6.9). The HOMO of [NiL
1
]
0
are primarily localized on both the metal
center and the ligand, and exhibit Ni(d x
2
-y
2
)-Npyridyl(px/y) antibonding character. On the other hand,
the HOMO-1 of [NiL
1
]
0
display metal-center dominant, with Ni d z
2
symmetry, and a LUMO of
mixed metal-ligand character. To effectively compare the electronic structures of [FeL
1
]
0
and
[NiL
1
]
0
to [CoL
1
]
0
, unrestricted DFT calculations were used to generate the electronic structure of
[CoL
1
]
0
.
225

Figure 6.8 Molecular orbital scheme of complexes [NiL
1
]
0
(left), with the corresponding orbital
character of HOMO, HOMO-1, and LUMO (right) at the 6-31+G*/B3LYP level of theory.

Figure 6.9 Optimized structure of [NiL
1
]
+2
(left) and [NiL
1
]
0
(right). Solvent molecules removed
for clarity.
226

Figure 6.10 Molecular orbital scheme of complexes [CoL
1
]
0
(left), with the corresponding orbital
character of HOMO, HOMO-1, and LUMO (right) at the 6-31+G*/B3LYP level of theory.
Based on previous computational work performed on [CoL
1
]
0
,
8
a previously optimized doublet
structure was employed in single-point energy calculations to generate molecular orbital diagram
of [CoL
1
]
0
(Figure 6.10). Both singly occupied molecular orbitals (SOMO and SOMO-1) of
[CoL
1
]
0
are localized on both the metal center and ligand, and exhibit Ni(d xz/yz)-Npyridyl(pz) bonding
character, while the LUMO is of mixed metal-ligand character.
The apparent variations in the calculated molecular orbital energy and the character
between the transition metal variant of [ML
1
]
0
provide indications toward the differences in
activity of these complexes towards CO 2 reduction. Electrolysis of both [FeL
1
]
+2
and [NiL
1
]
+2

under a CO2 atmosphere in the presence of a proton source yield no detected reduction products.
Moreover, post-electrolysis analysis of the working electrode suggests decomposition and
deposition of [NiL
1
]
+2
during electrolysis (Figure 6.11). Analysis of the computed molecular
227

orbitals of [FeL
1
]
0
reveal significant overlap of the Fe d z
2
with that of ligand-based orbitals,
delocalizing charge and leading to decreased Lewis basicity of the generate [FeL
1
]
0
and likely
limiting reactivity with H
+
and CO2.

Figure 6.11 High-resolution X-ray photoelectron spectroscopy spectra of Ni 2p and of a washed
post-electrolysis glassy carbon electrode and bare electrode. The post-electrolysis glassy carbon
electrode investigated here was generated upon performing an electrolysis with a glassy carbon
electrode for 2 hours at -2.90 V under CO2 and in a DMF solution with 1.3 M TFE and 0.1 M
[nBu4N][PF6], and subsequently washed with clean DMF.
Moreover, analysis of the computationally optimized structure of [NiL
1
]
0
suggest
significant distortion within the macrocyclic ring, signifying sufficient instability of the
electrolytically generated [NiL
1
]
0
, which is exhibited in the decomposition of [NiL
1
]
0
observed in
electrolysis experiments. Lastly, comparing the relative orbital energies of the HOMOs of [FeL
1
]
0
,
[CoL
1
]
0
, and [NiL
1
]
0
, the SOMOs of [CoL
1
]
0
lie higher in energy compared to those of the metal
analogues (Figure 6.12). These higher energy HOMOs/SOMOs may be sufficiently reactive to
reduce substrates such as CO2, whereas the HOMOs of [FeL
1
]
0
and [NiL
1
]
0
would be relatively
too low in energy, and either produce an inert species (as is the case of [FeL
1
]
0
) or result in
deleterious side reactions (as is the case of [NiL
1
]
0
).
228

Figure 6.12 Molecular orbital scheme of complexes [FeL
1
]
0
, [CoL
1
]
0
, and [NiL
1
]
0
at the 6-
31+G*/B3LYP level of theory.

6.3 Conclusion
This report focuses on the investigation of the effects of ligand functionalization and metal
identity on the activity of a set of CO2 reduction catalysts using DFT computational methods. A
series of DFT calculations were performed on 4,4'-NH
2
-Re, 5,5'-NH
2
-Re, 6,6'-NH
2
-Re to probe
the electronic implication of amine position on the bipyridine ligand. These calculations indicate
that the 6,6'-Re derivative displays greater electron density on the Re center compared to the other
functional analogues, which results in greater activity towards the reduction of CO 2. Additionally,
a series of DFT calculations were performed of ML
1
complexes with iron or nickel metal centers
to study the influence of transition metal identity on the reactivity of these macrocyclic complexes
towards the CO2RR. The HOMOs of [FeL
1
]
0
and [NiL
1
]
0
were found to be lower in energy than
229

those calculated for [CoL
1
]
0
suggesting the origin of inactivity of these complexes towards CO2
reduction. Moreover, the orbital character of [FeL
1
]
0
was found to be ill-suited towards small
molecule reduction, and computational results of [NiL
1
]
0
suggest significant instability of the
complex, corroborated by experimental observations.
For future work, additional study of [FeL
1
]
0
and [NiL
1
]
0
is warranted to investigate the
origin of inactivity towards the CO2RR. Calculating the CO2-adduct structures can be potentially
helpful in understanding the thermodynamic barriers involved in the CO 2 reduction chemistry.
Moreover, the presence of an axial base in the form of pyridine has been observed to alter the
reactivity of these complexes towards small molecule reduction. Being this the case, further
computational studies into the effects of axial bases on reactivity will provide important
information towards further optimizing these complexes towards the CO 2RR.
6.4 Experimental Methods
6.4.1 Density Functional Theory (DFT)
All calculations were run using the Q-CHEM program package.
11
Level of theory for section 6.2.1
was determined based on previous a computational report on the 5,5'-NH
2
Re system.
10
Geometry
optimizations were run with restricted DFT calculations at the M06 level of theory with a
composite basis set. The Pople 6-31G* basis set was used for H,C,N, and O atoms and the Hay–
Wadt VDZ (n + 1) effective core potentials and basis sets (LANL2DZ) were used for Cl and Re
atoms as they were similarly employed in Re-bpy based systems reported in the literature.
10,12–19

All optimized geometries were verified as stable minima with frequency calculations at the same
level of theory. The M06 functional was used for section 6.2.1 as it provides reduced Hartree–
Fock exchange contributions and includes empirical fitting for accuracy in organometallic systems
230

in Re-bpy based systems as observed in the literature.
10,20
Single point energy calculations were
run with a larger 6-311G** basis for H, C, N, and O atoms and solvation was treated with COSMO
(dielectric constant of 37.5 for acetonitrile).
21
Kohn–Sham orbital images are presented with
isovalues of 0.05 for clarity.
Level of theory for section 6.2.2 was determined based on previous a computational report on the
CoL
1
system.
8
Geometry optimizations were run with unrestricted DFT calculations at the B3LYP
level of theory using a 6-31+G* basis set.
12,13,22–25
The B3LYP functional and 6-31+G* basis set
was used throughout this study based on the prior successful application of this functional to
mechanistic modeling of the cobalt complex.
8
Solvation effects were considered using the SMD
implicit solvent model, with dimethylformamide as the modeled solvent.
26
All optimized
geometries were verified as stable minima with frequency calculations at the same level of theory.
Single point energy calculations were run with a larger 6-311++G** basis. Kohn–Sham orbital
images are presented with isovalues of 0.05 for clarity. TD-DFT calculations were performed using
B3LYP level of theory using a 6-31+G* basis set, using the SMD implicit solvent model, with
dimethylformamide as the modeled solvent. Ten excited states were considered and calculated for
both singlet and triplet transitions.
22

6.4.2 Coordinates of DFT-computed structures (Charge and spin specified by first two digits
respectively for each structure. Atomic coordinates listed in an order of the X, Y, then Z
coordinate values, respectively.)
4,4'-NH
2
Re
0 1

C 2.3184391 -0.1419925 0.4492281
C 1.0880455 -2.5010929 -0.2023490
C 1.5704955 -0.6573855 -2.1352302
C 0.2183121 2.3047418 -1.4843546
H 0.9518813 1.9752385 -2.2157084
C -0.3612425 3.5499920 -1.5898250
231

C -1.3129495 3.9384780 -0.6363496
N -1.8941848 5.1716123 -0.6670349
C -1.6167018 3.0223599 0.3792902
H -2.3473461 3.2977462 1.1354411
C -0.9929620 1.7861124 0.4125024
C -1.2662297 0.7813914 1.4607719
C -2.1870566 0.9873218 2.4750689
H -2.7648501 1.9073392 2.5251987
C -2.3958609 0.0009687 3.4479262
N -3.2768363 0.1951570 4.4727949
C -1.6341534 -1.1718756 3.3495712
C -0.7405068 -1.3014201 2.3091405
H -0.1439564 -2.2048584 2.2030691
Cl -1.5737833 -1.1393677 -1.4609998
N -0.0714615 1.4311196 -0.5101563
N -0.5454072 -0.3586348 1.3768036
O 3.3013467 0.1669918 0.9928320
O 1.2859779 -3.6292868 -0.0161784
O 2.0766631 -0.6065828 -3.1784060
Re 0.6950941 -0.6356829 -0.4276193
H -3.5644504 -0.6034363 5.0192942
H -3.9712871 0.9217321 4.3819198
H -1.8180072 5.7322816 -1.5028149
H -2.7110460 5.3417380 -0.0994570
H -0.0803638 4.2151675 -2.4027044
H -1.7472939 -1.9740899 4.0749973
5,5'-NH
2
Re
0 1

C 2.2953087 -0.1158783 0.5082069
C 1.0958605 -2.4797086 -0.1893620
C 1.6171210 -0.6084590 -2.1021687
C 0.2027772 2.2921865 -1.4940691
H 0.9232209 1.9507558 -2.2356055
C -0.3874434 3.5549377 -1.6129916
C -1.3091300 3.9192268 -0.6246086
H -1.7980902 4.8922953 -0.6648035
C -1.5992050 3.0354596 0.3953822
H -2.3242285 3.3176889 1.1533768
C -0.9803187 1.7857754 0.4438180
C -1.2564907 0.7801112 1.4700100
C -2.1608129 0.9659286 2.5161205
H -2.7042537 1.9021008 2.6132758
232

C -2.3835164 -0.0396398 3.4348953
H -3.0950196 0.1046515 4.2464139
C -1.6964666 -1.2534978 3.3110330
C -0.7973808 -1.3654342 2.2435198
H -0.2358190 -2.2867166 2.0926755
Cl -1.5249919 -1.1291314 -1.5276811
H 0.4167018 3.9822195 -3.4375734
H -0.6904559 5.1290344 -2.8766639
H -1.2313054 -3.0532312 4.1821443
H -2.3784149 -2.1268284 5.0376895
N -0.0806695 1.4466379 -0.5051062
N -0.5873954 -0.3886006 1.3634089
N -0.0299813 4.4041228 -2.6345316
N -1.9115097 -2.3074649 4.1607929
O 3.2646117 0.1922045 1.0748118
O 1.2891217 -3.6071852 0.0050907
O 2.1425439 -0.5412716 -3.1345456
Re 0.6979872 -0.6151954 -0.4160279
6,6'-NH
2
-Re

0 1

C -1.9776843 -0.3424515 3.1380150
C -3.0080422 0.0730631 0.7514934
C -1.2161923 -2.0153513 1.0638805
C -0.5727462 0.4441651 -1.7104135
C 0.0535356 1.1887952 -2.7292452
H -0.3888699 1.2044295 -3.7235533
C 1.2116593 1.8682430 -2.4404028
H 1.7070880 2.4568291 -3.2083651
C 1.7671585 1.7757942 -1.1642159
H 2.7000533 2.2800418 -0.9385828
C 1.1015533 1.0408677 -0.1998932
C 1.6679060 0.8079689 1.1416959
C 2.9023737 1.3042541 1.5226393
H 3.4664935 1.9630010 0.8734564
C 3.4161221 0.9401877 2.7667695
H 4.3751640 1.3336522 3.0980281
C 2.7197096 0.0633334 3.5634408
H 3.1072494 -0.2677896 4.5244675
C 1.4712026 -0.4127916 3.1206986
N -0.0801804 0.4195422 -0.4579610
N 0.9396912 -0.0015568 1.9551076
N -1.6659903 -0.3067018 -2.0016662
H -2.2097970 -0.0719744 -2.8184155
233

H -2.1684832 -0.7380397 -1.2381981
N 0.7659856 -1.2885735 3.8822245
H 0.0148760 -1.8059113 3.4455394
H 1.2353053 -1.7364860 4.6542443
Re -1.2065796 -0.1338168 1.3849357
O -2.4342855 -0.4525415 4.1959593
O -4.0869636 0.2148297 0.3487693
O -1.2263475 -3.1674950 0.8805973
Cl -1.0984095 2.3782319 1.7626348
[FeL
1
]
+2

2 1

Fe 0.0816835 0.0253259 -0.0257678
H 0.6625872 4.0336520 -0.8930686
C 1.6299433 2.2999862 -1.1553103
N 1.6380048 0.9650881 -0.9287392
O 0.2076503 1.1658250 1.6208879
C 1.1105969 -1.9899218 1.9010636
C -2.3827368 -2.1020041 2.7919445
H -2.1907408 -2.5948441 3.7397134
C -2.7262485 -0.8644285 0.3771322
C -3.3669493 1.9885552 -1.6980651
H -4.3777907 1.6673753 -1.9290813
C -1.6522105 3.6555485 -1.5686734
H -1.2751854 4.6584188 -1.7413247
C -0.8040017 2.6773486 -1.0336721
C -0.4411231 -2.1924053 -1.9152634
C -3.6543307 -2.0624529 2.2358230
H -4.4970040 -2.5072469 2.7574493
C -3.8350855 -1.4462803 1.0033200
H -4.8140596 -1.3715642 0.5408845
C -2.9485892 3.2948225 -1.9146527
H -3.6253786 4.0250258 -2.3491002
C 1.2336530 1.4708525 2.2725058
N -1.4729710 -0.9092542 0.8866248
C 3.7636095 2.2478470 -2.2495065
H 4.5860229 2.7432912 -2.7576954
C -1.3181464 -1.5373583 2.0756820
C 2.6803719 2.9819027 -1.7828215
H 2.6122638 4.0547237 -1.9324600
N -1.1941029 1.4051987 -0.7935352
N 0.5156049 3.0381921 -0.7565542
H -3.8782664 -0.3342710 -1.1724020
234

C 2.7125080 0.2746614 -1.3774901
N 1.3483039 -1.3677251 0.7203049
C 3.7806644 0.8722399 -2.0605335
H 4.6149863 0.2624649 -2.3925056
C -2.4690321 1.0853954 -1.1132691
N -0.0396914 -1.6580666 2.6140111
H 3.5964105 -1.5105335 -1.5867446
C 3.3413115 -2.7356088 0.5021664
H 4.2202306 -2.9848732 -0.0840964
N 2.7833904 -1.0955516 -1.1422698
C 3.0833709 -3.3437324 1.7228732
H 3.7517115 -4.1083068 2.1085065
N -2.9306475 -0.2006397 -0.8331433
H -0.0598309 -2.1077138 3.5240056
C 2.4666262 -1.7385215 0.0494649
O -0.1463828 -0.9867342 -1.7449663
C 1.9603871 -2.9612884 2.4459429
H 1.7095401 -3.4259276 3.3941375
N 1.2249803 2.2298758 3.3673942
H 2.2248639 1.1198413 1.9690206
C 2.4603975 2.5296209 4.0862956
H 2.6169838 3.6135123 4.1229425
H 2.3971568 2.1469853 5.1112788
H 3.3082389 2.0616268 3.5807997
C -0.0067545 2.7876635 3.9180304
H -0.1635520 2.4058969 4.9334333
H 0.0716351 3.8798842 3.9603505
H -0.8503270 2.5060872 3.2883645
H -0.5685010 -2.8727356 -1.0674068
N -0.6249240 -2.7532422 -3.1099578
C -0.9422706 -4.1731636 -3.2362893
H -1.0006927 -4.6318210 -2.2465182
H -1.9043071 -4.2968882 -3.7462153
H -0.1650298 -4.6782323 -3.8209120
C -0.5006582 -1.9959720 -4.3521458
H -1.4174437 -2.1102099 -4.9410621
H -0.3410394 -0.9420421 -4.1257019
H 0.3439643 -2.3775494 -4.9377608
[FeL
1
]
+2

2 3

Fe 0.0868374 0.0557814 -0.0369668
H 0.6278197 4.0748691 -0.8758867
235

C 1.6249803 2.3544655 -1.1102895
N 1.6432197 1.0129730 -0.9126937
O 0.2254776 1.3260390 1.8423771
C 1.1185156 -2.0094090 1.8572941
C -2.3824556 -2.1219948 2.7275884
H -2.1921447 -2.6280497 3.6685786
C -2.7209067 -0.8572567 0.3252552
C -3.3737506 1.9959417 -1.7534787
H -4.3781837 1.6682355 -2.0017946
C -1.6746573 3.6808500 -1.5907338
H -1.3061221 4.6884471 -1.7530026
C -0.8228301 2.7073034 -1.0529742
C -0.4936171 -2.3513794 -2.1172694
C -3.6516201 -2.0748082 2.1701812
H -4.4956696 -2.5254801 2.6842545
C -3.8285061 -1.4468084 0.9423754
H -4.8055579 -1.3712550 0.4763572
C -2.9611942 3.3069960 -1.9572035
H -3.6377687 4.0326843 -2.3994049
C 1.2621639 1.6147384 2.4743877
N -1.4673029 -0.9024138 0.8403594
C 3.8067067 2.3582760 -2.1070126
H 4.6431919 2.8770474 -2.5663565
C -1.3142073 -1.5521543 2.0199124
C 2.6948487 3.0648362 -1.6705739
H 2.6173492 4.1399577 -1.7968853
N -1.2100073 1.4314151 -0.8243463
N 0.4869525 3.0761036 -0.7586630
H -3.8672049 -0.3382087 -1.2334604
C 2.7480176 0.3484264 -1.3330447
N 1.3742160 -1.3613864 0.6951636
C 3.8339875 0.9780271 -1.9530382
H 4.6896046 0.3876930 -2.2646660
C -2.4791347 1.0993676 -1.1570605
N -0.0405813 -1.6974638 2.5594297
H 3.6530179 -1.4210403 -1.5815274
C 3.3724318 -2.7125027 0.4681554
H 4.2592587 -2.9470824 -0.1120179
N 2.8299475 -1.0269901 -1.1363197
C 3.0987852 -3.3484704 1.6720509
H 3.7640422 -4.1207990 2.0474042
N -2.9259672 -0.1896429 -0.8826105
H -0.0687148 -2.1571819 3.4642099
C 2.5034900 -1.7056232 0.0297262
O -0.1465982 -1.1604283 -2.0055961
C 1.9673503 -2.9872993 2.3922998
236

H 1.7081087 -3.4744680 3.3266411
N 1.3031892 2.3745217 3.5738755
H 2.2449666 1.2444664 2.1537891
C 2.5619832 2.6468599 4.2604516
H 2.7448831 3.7269886 4.2942597
H 2.5199232 2.2646875 5.2870392
H 3.3858843 2.1611873 3.7320179
C 0.0983368 2.9583457 4.1537366
H -0.0418605 2.5844645 5.1748301
H 0.1962752 4.0495134 4.1894749
H -0.7653008 2.6890437 3.5455549
H -0.6213294 -2.9923393 -1.2341394
N -0.7413158 -2.9772183 -3.2757279
C -1.1128628 -4.3876786 -3.3099651
H -1.1623322 -4.7839325 -2.2927141
H -2.0921895 -4.5085872 -3.7876392
H -0.3708741 -4.9592562 -3.8799262
C -0.6193815 -2.2961917 -4.5595249
H -1.5580474 -2.3890154 -5.1178320
H -0.3982165 -1.2420780 -4.3909237
H 0.1862999 -2.7490347 -5.1500897
[NiL
1
]
+2
2 3

Ni 0.0917961 -0.0923900 -0.3662822
H 0.7330798 4.0305938 -0.9348120
C 1.6941678 2.3070042 -1.2723528
N 1.7075017 0.9653781 -1.1094246
C 1.2152270 -1.9062000 1.7741039
C -2.2704909 -1.9619168 2.7784278
H -2.0466138 -2.4305299 3.7311424
C -2.7176446 -0.7733865 0.3504221
C -3.3729194 2.0964300 -1.7004334
H -4.4027270 1.8127047 -1.8926659
C -1.6005251 3.7163249 -1.6104624
H -1.2085537 4.7141396 -1.7774688
C -0.7594401 2.7085167 -1.1249195
C -0.5235167 -2.3577461 -2.1250582
C -3.5659530 -1.9085664 2.2803640
H -4.3894876 -2.3223265 2.8552131
C -3.8032465 -1.3145135 1.0453176
H -4.8050661 -1.2245026 0.6381105
C -2.9175711 3.3913511 -1.9162718
237

H -3.5877667 4.1474035 -2.3150974
N -1.4422541 -0.8498479 0.7992172
C 3.8571818 2.2933428 -2.3063502
H 4.6904295 2.8098834 -2.7741644
C -1.2306162 -1.4406826 1.9967520
C 2.7605682 3.0113845 -1.8442913
H 2.6989517 4.0890866 -1.9555713
N -1.1847670 1.4456456 -0.9027659
N 0.5736594 3.0302676 -0.8589512
H -3.9192501 -0.2343458 -1.1627682
C 2.8011857 0.2888832 -1.5299329
N 1.4383772 -1.3559599 0.5589888
C 3.8845309 0.9113268 -2.1599312
H 4.7369382 0.3206145 -2.4793566
C -2.4812586 1.1611309 -1.1629987
N 0.0698454 -1.5480749 2.4843484
H 3.6735520 -1.5038497 -1.7571211
C 3.4667938 -2.6637963 0.3980688
H 4.3556092 -2.9154919 -0.1714112
N 2.8619895 -1.0881617 -1.3090225
C 3.2180137 -3.2190607 1.6473213
H 3.9045013 -3.9485371 2.0674374
N -2.9574236 -0.1152673 -0.8584159
H 0.0832360 -1.9240926 3.4276830
C 2.5690543 -1.7114911 -0.0964021
O -0.2125266 -1.1386024 -2.0648051
C 2.0919939 -2.8270526 2.3614778
H 1.8634958 -3.2460744 3.3360507
H -0.6018980 -2.9661259 -1.2182346
N -0.7776956 -3.0026815 -3.2560165
C -1.1194151 -4.4241064 -3.2525500
H -1.1258646 -4.8019766 -2.2279878
H -2.1105251 -4.5671973 -3.6965296
H -0.3827374 -4.9830805 -3.8395405
C -0.7132218 -2.3429959 -4.5582135
H -1.6691374 -2.4684766 -5.0780302
H -0.5076846 -1.2819047 -4.4212028
H 0.0806695 -2.7974474 -5.1614609
[FeL
1
]
0
0 1

Fe 0.0680899 0.1245215 0.0997632
H 0.6350635 4.0398817 -0.8290232
238

C 1.5884780 2.2985955 -1.1463037
N 1.5361505 0.9508176 -0.9261152
O 0.2225604 1.3311109 1.8189483
C 1.0646787 -1.9584875 1.9044507
C -2.3939904 -2.2002258 2.6916880
H -2.2118739 -2.7269438 3.6243906
C -2.6876818 -0.9050970 0.3045092
C -3.3517474 1.9136425 -1.7377776
H -4.3517942 1.5607520 -1.9746186
C -1.6531224 3.5992223 -1.6667252
H -1.2792106 4.5994167 -1.8674875
C -0.8149104 2.6673296 -1.0549411
C -3.6618178 -2.1738035 2.1023755
H -4.5092248 -2.6588532 2.5781824
C -3.8054420 -1.5090465 0.8843975
H -4.7633951 -1.4553837 0.3748527
C -2.9574611 3.2244728 -2.0081412
H -3.6416994 3.9325007 -2.4664182
C 1.2676899 1.5994420 2.4350180
N -1.4501697 -0.8865675 0.8713808
C 3.7332681 2.1971080 -2.2432634
H 4.5742052 2.6736566 -2.7384249
C -1.3364765 -1.5443772 2.0591645
C 2.6354400 2.9455336 -1.8016656
H 2.5799016 4.0188479 -1.9622533
N -1.1668168 1.3743892 -0.7987818
N 0.4889886 3.0461695 -0.6775558
H -3.7155902 -0.4987007 -1.3681846
C 2.6075606 0.2435731 -1.3938467
N 1.2513586 -1.3287030 0.7056896
C 3.7113071 0.8174534 -2.0240373
H 4.5295849 0.1790935 -2.3459506
C -2.4375441 1.0329497 -1.1563164
N -0.0609026 -1.5565947 2.6523169
H 3.2855986 -1.6070776 -1.7654132
C 3.2706819 -2.6784768 0.4611000
H 4.1353282 -2.9171425 -0.1523986
N 2.5191162 -1.1571485 -1.2709610
C 3.0433345 -3.3189373 1.6810661
H 3.7277838 -4.0746356 2.0550134
N -2.7995573 -0.3142690 -0.9678508
H -0.0846750 -2.0238243 3.5536285
C 2.3590376 -1.7253392 0.0087716
C 1.9047095 -2.9509318 2.4082010
H 1.6604988 -3.4246466 3.3551676
N 1.3297803 2.3541876 3.5459875
239

H 2.2426199 1.2233133 2.0967052
C 2.5986803 2.6118565 4.2143448
H 2.7932995 3.6904402 4.2561090
H 2.5748512 2.2210552 5.2391146
H 3.4111114 2.1256692 3.6682959
C 0.1379929 2.9406923 4.1448661
H 0.0044662 2.5618232 5.1657767
H 0.2387759 4.0320689 4.1876517
H -0.7348322 2.6802472 3.5453993

[NiL
1
]
0
0 1

Ni 0.1936679 0.1499224 0.0385013
H 0.2900590 4.1275085 -0.9086323
C 1.5601995 2.5825290 -0.9679591
N 1.6774139 1.2407232 -0.8297735
C 1.2320828 -1.9039411 1.7010993
C -1.8941648 -1.6307710 3.3423506
H -1.5819092 -1.9688751 4.3257605
C -2.5477939 -0.7510805 0.8202951
C -2.9254419 1.1098681 -2.3754033
H -3.8015870 0.5721347 -2.7293617
C -1.3452351 2.9167236 -2.4922460
H -0.9322791 3.8055645 -2.9624636
C -0.8008573 2.4586047 -1.3016328
C -3.2267428 -1.6868267 2.9296176
H -3.9944012 -2.0601540 3.6024557
C -3.5600968 -1.2798212 1.6476524
H -4.5834900 -1.3108017 1.2862304
C -2.4337580 2.2307649 -3.0563834
H -2.8944056 2.5695676 -3.9793536
N -1.2377089 -0.7150245 1.1908235
C 3.7777028 2.8178376 -1.8471183
H 4.5865792 3.4278787 -2.2408342
C -0.9356895 -1.2063997 2.4157760
C 2.5963408 3.4160162 -1.3994243
H 2.4436686 4.4899451 -1.4433226
N -1.2211912 1.3347832 -0.6339912
N 0.2975206 3.1339874 -0.6951113
H -3.8608953 -0.5241443 -0.7005101
C 2.8511827 0.6821843 -1.2392212
N 1.6082097 -1.0251550 0.7119128
240

C 3.9010448 1.4398819 -1.8019392
H 4.8008334 0.9344064 -2.1398772
C -2.3272348 0.7258245 -1.1745993
N 0.4272026 -1.3252639 2.7262773
H 3.8328100 -1.0403737 -1.6360770
C 2.7982976 -2.9124293 -0.3005662
H 3.4690876 -3.2494289 -1.0874596
N 3.0392603 -0.6743147 -1.1218314
C 2.3474606 -3.7880249 0.6960893
H 2.6264087 -4.8371838 0.6875119
N -2.9202168 -0.2733077 -0.4164015
H 0.5514718 -1.7973671 3.6175388
C 2.4471293 -1.5643693 -0.2382016
C 1.5451742 -3.2538391 1.7208678
H 1.1803207 -3.8736692 2.5361648
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Abstract (if available)

Abstract With increases global population projected, an ever-increasing energy supply will be needed to satiate coupled energy demand. To combat this issue, generating energy from renewable carbon-neutral (RCN) sources has been proposed to compensate for an every-increasing demand for energy while avoiding long term ecological issues associated with anthropogenic combustion of fossil fuels. Implementation of renewable energy sources is limited, due to their intermittent nature, in addition to major energy-consuming sectors such as transportation heavy reliance on non-renewable liquid-based fuels. Synthetic fuels are novel energy storage platform where electricity derived from renewable energy sources is used to electrochemically convert abundant small molecules to value-added products, in which energy is stored in the form of chemical bonds. These products can then be employed as a means of energy storage during peak renewable energy supply and employed towards the liquid-dependent transportation sector, while also providing a framework for large scale energy commodization and provide a renewable source for chemical feedstocks.

To effectively implement this strategy at the required scales necessary for sustainable applications, a key research goal is to develop Earth-abundant electrocatalysts that drive these conversion efficiently at high rates under mild conditions. In biological settings, evolution has provided highly efficient catalytic systems that can efficiently produce the targeted products under mild conditions while employing earth-abundant components. As a result, research into the incorporation of common structural motifs found in enzymatic active sites into synthetic catalytic systems demands attention as means to produce similarly active Earth-abundant catalytic systems.

In this dissertation, several electrocatalytic systems towards electrochemical H2 production and CO2 reduction were developed and/or further studied. Chapter 1 presents a general overview of global energy demand and supply, energy storage through synthetically derived fuels, fundamentals of both hydrogen evolution and CO2 reduction, and catalysts design principles incorporating design elements from enzymatically-derived systems. In Chapter 2, building blocks of a series of cobalt-dithiolene derived metal organic frameworks active towards electrocatalytic hydrogen evolution are isolated as homogenous multimetallic complexes using a phosphine capping unit. These complexes are further studied using electrochemical means to further investigate the electronic coupling between redox active sites present in the analogous materials. In Chapter 3, a cobalt phosphino-thiolate complex is investigated as a selective electrocatalyst towards CO2 reduction to formate. Employing thiolate-based moieties similarly observed in enzymatic acitve sites, this complex exhibits robust stability, with formate selectivities as high as 91%. Chapter 4 explores functionalization of the core cobalt phosphino-thiolate complex through modification of the ligand scaffold and variation of the metal center identity as a means of tuning electrocatalytic activity and selectivity. Chapter 5 investigates utilizing a highly-conjugated multimetallic cobalt phosphino-thiolate complex towards CO2RR. Lastly, Chapter 6 employs computational techniques to further study the effects of ligand functionalization and metal identity on established CO2 reduction electrocatalysts.

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Asset Metadata

Creator Intrator, Jeremy A. (author)

Core Title Cobalt phosphino-thiolate complexes: applications towards electrocatalytic small molecule conversion to synthetic fuels

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2022-12

Publication Date 10/03/2023

Defense Date 08/15/2022

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemical fuel,CO₂ reduction,homogeneous electrocatalysis,MOFs,multimetallic.,OAI-PMH Harvest,renewable energy,solar energy conversion

Format theses (aat)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Marinescu, Smaranda (committee chair), Malmstadt, Noah (committee member), Williams, Travis (committee member)

Creator Email intrator@usc.edu,jeremy.intrator@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC112114340

Unique identifier UC112114340

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Legacy Identifier etd-IntratorJe-11261

Document Type Dissertation

Format theses (aat)

Rights Intrator, Jeremy A.

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Source 20221017-usctheses-batch-986 (), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

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