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Design, synthesis, and study of polypyridine based molecular and heterogenized molecular electrocatalysts for CO₂ reduction

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Design, synthesis, and study of polypyridine based molecular and heterogenized molecular electrocatalysts for CO₂ reduction

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Content DESIGN, SYNTHESIS, AND STUDY
OF POLYPYRIDINE BASED MOLECULAR AND
HETEROGENIZED MOLECULAR ELECTROCATALYSTS
FOR CO
2
REDUCTION

by

Damir Popov

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

December 2019

2
ACKNOWLEDGEMENTS

I would like to thank my advisor, Professor Smaranda C. Marinescu, for her continued
support and availability to assist me with challenging research problems over the last six
years. She has always offered me an opportunity to explore my first attempts at
independent research that were better aligned with research interests of my own. I would
also like to thank Professor Ralf Haiges for his kindness and willingness to contribute his
time to teach me the theory and techniques of X-ray crystallography, which is one of the
most powerful tools for structure determination. I would also like to express my gratitude
to Professors Mark E. Thompson, Travis J. Williams, and Malancha Gupta for their
willingness to serve in my dissertation committee and providing me with directions
throughout my time at USC. And finally, I would like to thank the current and former
Marinescu group members, and the active members of SACNAS@USC for providing me
with their time, emotional support, and advice during my doctoral studies.

3
TABLE OF CONTENTS
Acknowledgements .................................................................................................................................................... 2
Table of Contents ........................................................................................................................................................ 3
List of Figures ............................................................................................................................................................... 4
List of Schemes ............................................................................................................................................................. 7
List of Tables ................................................................................................................................................................. 7
Chapter 1. Immobilization of Molecular Catalysts for Electrocatalytic CO2 Reduction .... 8

1.1. Electrocatalytic CO
2
Reduction: General Introduction ..................................................................... 9
1.2. Immobilization of Molecular Catalysts for Electrocatalytic CO
2
Reduction .......................... 10
1.3. 2D and 3D Porous Architectures for Electrochemical CO
2
Reduction .................................... 11
1.4. Surface Modification Using Non-Tetraporphyrin Active Sites ................................................... 14
1.5. Conclusions ................................................................................................................................................... 17
1.6. References .......................................................................................................................................................... 17
Chapter 2. A 2,2'-Bipyridine-Containing Covalent Organic Framework Bearing
Rhenium (I) Tricarbonyl Moieties for CO2 Reduction ..................................................................... 22

2.1. Introduction ................................................................................................................................................... 23
2.2. Results and Discussion ............................................................................................................................. 24
2.3. Conclusions ................................................................................................................................................... 61
2.4. Experimental Section ................................................................................................................................. 62
2.4.1 General considerations ...................................................................................................................... 62
2.4.3. Physical methods ............................................................................................................................... 64
2.4.4. Computational Methods .................................................................................................................. 65
2.4.5. Synthesis of Re(2,2'-bpy-5,5'-diamine)(CO)
3
Cl (1) .............................................................. 65
2.4.6. Synthesis of Re(2,2'-bpy-5,5'-diamine)(CO)
3
(MeCN)(OTf) (1
OTf
) .................................. 66
2.4.7. Synthesis of Zn(2,2 -bpy-5,5 -diamine)(CH
3
COO)
2
·H
2
O .......................................... 67
2.4.8. Typical synthesis of COF-2,2 -bpy-Re ................................................................................... 67
2.4.9. Electrode fabrication (carbon ink method) ................................................................................ 68
2.5. Electrochemical methods ......................................................................................................................... 68
2.6. References ..................................................................................................................................................... 70
Chapter 3. Synthesis and Electrochemical CO
2
Reduction Activity of a Ruthenium (II)
Terpyridine Complex Featuring Ancillary Hydroxyl Groups ........................................................... 73

3.1. Introduction ................................................................................................................................................... 74
3.2. Results and Discussion ............................................................................................................................. 75
3.3. Conclusions ................................................................................................................................................... 95
3.4. Experimental Section ................................................................................................................................. 96
3.4.1. General Considerations .................................................................................................................... 96
3.4.3. Synthesis of [Ru(dhtp)(CO)
2
(Cl)](Cl) (3) .................................................................................. 97
3.4.4. Physical Methods ............................................................................................................................... 98
3.4.5. Experimental Methods ..................................................................................................................... 98
3.5. References ..................................................................................................................................................... 99
Bibliography ............................................................................................................................................................. 101

4
LIST OF FIGURES
Figure 2.1. X-ray crystal structure of 1•DMF ................................................................................................ 25
Figure 2.2. CO stretching region of the ATR-FTIR spectrum of 1 .............................................. 28
Figure 2.3. CO stretching region of the FTIR of a pressed pellet of 1 with KBr ............................... 29
Figure 2.4. Visualization of the three characteristic carbonyl stretching modes in Re-2,2'-
bipyridine(CO)
3
Cl. The character of the three carbonyl modes in 1 are identical to these .............. 30
Figure 2.5. XPS analysis of 1 .............................................................................................................................. 32
Figure 2.6. CVs of 1 (0.5 mM) in 0.1 M [nBu
4
N][PF
6
] in MeCN under Ar (black) and CO
2
(red)
at a scan rate of 100 mV/s. .................................................................................................................................... 33
Figure 2.7. Cyclic voltammograms data (1
st
, 2
nd
, and 3
rd
scans) of 1 (0.5 mM) in MeCN solution
containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere of N
2
...................................................................... 34
Figure 2.8. Cyclic voltammograms of 0.5 mM of 1 in an MeCN solution containing 0.1 M
[nBu
4
N][PF
6
] under an atmosphere of N
2
at scan rates ranging from 100 to 1000 mV/s ................ 35
Figure 2.9. Cyclic voltammogram data of 1 (0.5 mM) in MeCN solution containing 0.1 M
[nBu
4
N][PF
6
] under an atmosphere of N
2
. ....................................................................................................... 36
Figure 2.10. Plot showing the peak cathodic current density at –2.11 V (left) and –2.47 V (right)
vs. Fc
+/0
as a function of the square root of the scan rate ............................................................................ 36
Figure 2.11. Controlled potential electrolysis of 1 under N
2
(black) and under CO
2
(red).
Conditions: 1 (0.5 mM) in 0.1 M solution of [nBu
4
N][PF
6
] in MeCN, measured at a potential of –
2.57 V vs. Fc
+/0
.......................................................................................................................................................... 38
Figure 2.12. FTIR of a solution of 1 before (blue) and after (light-brown) CPE (conditions: 1 (0.5
mM) in MeCN solution containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere of CO
2
at –2.57 V
versus Fc
+/0
) ................................................................................................................................................................ 39
Figure 2.13. Molecular orbital diagrams and frontier orbital images for Re-2,2'-
bipyridine(CO)
3
Cl (HOMO, LUMO; blue) and 1 (HOMO', LUMO'; red) ........................................... 41
Figure 2.14. Molecular orbital diagrams and frontier orbital images for Re-2,2'-
bipyridine(CO)
3
Cl (HOMO-2, HOMO-3; blue) and 1 (HOMO-2', HOMO-3'; red) ......................... 42
Figure 2.15. Cyclic voltammogram scan rate dependence of 0.5 mM Zn(5,5'-diamino-2,2'-bpy)
complex in an MeCN solution containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere of N
2
.......... 43
Figure 2.16. Cyclic voltammograms of 1 (0.5 mM) in an MeCN solution containing 0.1 M
[nBu
4
N][PF
6
] under an atmosphere of N
2
at varying concentrations of nBu
4
NCl (TBACl). .......... 44
Figure 2.17. Overlay of cyclic voltammetry data of 1 and of its chloride abstracted analog (0.5
mM concentration each) in MeCN solution containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere
of N
2
. ............................................................................................................................................................................. 45
Figure 2.19. Overlay of the FTIR spectra of COF-2,2'-bipyridine-Re with various amounts of
incorporated rhenium (I) tricarbonyl moieties ................................................................................................ 49
Figure 2.20. Overlay of the FTIR spectra of COF-2,2'-bipyridine-Re that displays 29.38 wt % Re
incorporation (purple) and of COF-2,2'- bipyridine (red). .......................................................................... 50
Figure 2.21. FTIR of Re(CO)
5
Cl in the CO stretching region. ................................................................ 50
Figure 2.22. HR XPS analysis of COF-2,2'- bipyridine -Re that displays 15.39 wt % Re.. ........... 52

5
Figure 2.23. X-ray photoelectron spectroscopy (XPS) analysis of COF-2,2'- bipyridine -Re that
displays 29.38 wt % Re.. ........................................................................................................................................ 53
Figure 2.25. Overlay of the experimental PXRD patterns of COF-2,2'-bipyridine-Re with 0.00
(red), 0.21 (orange), 0.65 (yellow), 1.86 (green) and 15.39 (blue) wt % Re incorporation. ........... 54
Figure 2.26. Polarization curves of 2 in a 0.1 M [nBu
4
N][PF
6
] acetonitrile solution under N
2

(black) and CO
2
(red) at a scan rate of 10 mV/s. ........................................................................................... 56
Figure 2.27. Controlled potential electrolysis of 2 in an atmosphere of CO
2
(red) and N
2
(black);
conditions: 0.1 M [nBu
4
N][PF
6
] in MeCN, measured at a potential of –2.8 V vs. Fc
+/0
. ................. 57
Figure 2.28. (a) Polarization curves of the composites based on COF-2,2'-bipyridine, rhenium
precursor, Re(CO)
5
Cl, carbon black and polyvinylidene fluoride by themselves, and 2 (red) under
CO
2
; conditions: 0.1 M [nBu
4
N][PF
6
] in MeCN, scan rate = 10 mV/s. (b) Controlled potential
electrolysis of the composites based on COF-2,2'-bipyridine, rhenium precursor, carbon black and
PVDF by themselves, and 2 under CO
2
; conditions: 0.1 M [nBu
4
N][PF
6
] in MeCN, measured at a
potential of –2.8 V vs. Fc
+/0
................................................................................................................................... 58
Figure 2.29. (a) Polarization curves of the postcatalysis solution (solution after CPE) measured
with a clean glassy carbon electrode and 2 under of CO
2
; conditions: 0.1 M [nBu
4
N][PF
6
] in
MeCN, scan rate = 10 mV/s. (b) Controlled potential electrolysis of the postcatalysis solution
(solution after CPE) measured with a clean glassy carbon electrode and 2 under CO
2
; conditions:
0.1 M [nBu
4
N][PF
6
] in MeCN, measured at a potential of –2.8 V vs. Fc
+/0
......................................... 60
Figure 2.30. HR XPS analysis of 2 after a 1 h electrolysis experiment. ............................................... 61
Figure 3.1. Solid-state structure and atomic numbering scheme for 3·DMF. ........................... 76
Figure 3.2. FTIR of 3 in the form of a pellet with KBr .............................................................................. 79
Figure 3.3. Cyclic voltammogram of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere at a scan rate of 100 mV/s ............................................................. 80
Figure 3.4. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere. Solid curve: scan reversed after the second reduction,
dashed curve: scan reversed after the first reduction .................................................................................... 81
Figure 3.5. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere with and without added chloride source nBu
4
NCl ................ 82
Figure 3.6. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere at scan rates ranging from 50 to 1000 mV/s ........................... 82
Figure 3.7. Plot showing the peak cathodic current density at –2.13 V (left) and –2.44 V (right)
vs. Fc
+/0
as a function of the square root of the scan rate ............................................................................ 83
Figure 3.8. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere with and without added base nBu
4
NOH .................................. 84
Figure 3.9. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere and a carbon dioxide atmosphere at a scan rate of 100 mV/s
......................................................................................................................................................................................... 85
Figure 3.10. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) under a nitrogen atmosphere and with 5% of added water by volume under a carbon
dioxide atmosphere at a scan rate of 100 mV/s .............................................................................................. 85

6
Figure 3.11. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
]
(0.1 M) and 2 equivalents of added base nBu
4
NOH under a nitrogen atmosphere and a carbon
dioxide atmosphere at a scan rate of 100 mV/s .............................................................................................. 86
Figure. 3.12. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
at a potential of –2.37 V vs. Fc
+/0
. After the CPE, the
working electrode was rinsed (3×10 mL DMF) and CPE curve (a dashed line) was measured with
it in a fresh DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
at a potential of –2.37 V
vs. Fc
+/0
. Top right: black curve represents the CPE of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under N
2
at a potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of 3 (0.5 mM) in
a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the CPE ................. 87
Figure. 3.13. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added 2 equivalents of base nBu
4
NOH at a
potential of –2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10 mL DMF)
and CPE curve (a dashed line) was measured with it in a fresh DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) with added 2 equivalents of base nBu
4
NOH under CO
2
at a potential of –
2.37 V vs. Fc
+/0
. Top right: black curve represents the CPE of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under N
2
with added 2 equivalents of base nBu
4
NOH at a
potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the CPE ....................................................................... 88
Figure. 3.14. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added 62 equivalents of H
2
O at a potential of –
2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10 mL DMF) and CPE
curve (a dashed line) was measured with it in a fresh DMF solution containing [nBu
4
N][PF
6
] (0.1
M) with added 62 equivalents of H
2
O under CO
2
at a potential of –2.37 V vs. Fc
+/0
. Top right:
black curve represents the CPE of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1
M) under N
2
with added 62 equivalents of H
2
O at a potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of
3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the
CPE ................................................................................................................................................................................ 90
Figure. 3.15. Left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added 62 equivalents of PhOH at a potential of
–2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10 mL DMF) and CPE
curve (a dashed line) was measured with it in a fresh DMF solution containing [nBu
4
N][PF
6
] (0.1
M) with 62 equivalents of PhOH under CO
2
at a potential of –2.37 V vs. Fc
+/0
. Top right: black
curve represents the CPE of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1 M)
under N
2
with added 62 equivalents of PhOH at a potential of –2.37 V vs. Fc
+/0
............................... 93
Figure. 3.16. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added PhOH (5 % by mass) at a potential of –
2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10 mL DMF) and CPE
curve (a dashed line) was measured with it in a fresh DMF solution containing [nBu
4
N][PF
6
] (0.1
M) with added PhOH (5 % by mass) under CO
2
at a potential of –2.37 V vs. Fc
+/0
. Top right:
black curve represents the CPE of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1
M) under N
2
with added PhOH (5 % by mass) at a potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of
3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the
CPE ................................................................................................................................................................................ 94

7
LIST OF SCHEMES
Scheme 2.1. Synthesis of 1. .................................................................................................................................. 24
Scheme 2.2. Synthesis of COF-2,2'-bipyridine-Re (2) through post-metallation. ............................ 48
Scheme 3.1. Synthesis of [Ru(dhtp)(CO)
2
(C)](Cl) (3) ................................................................................ 75

LIST OF TABLES
Table 2.1. Crystal data and structure refinement for 1. ............................................................................... 26
Table 2.2. Selected bond lengths (Å) for complex 1. .................................................................................. 27
Table 2.3. Calculated carbonyl stretching mode frequencies from DFT calculations for Re-2,2'-
bipyridine(CO)
3
Cl and 1 (bpy=2,2'-bipyridine) ............................................................................................. 30
Table 2.4. A comparison between select experimental (averaged) and calculated bond lengths for
complex 1. For Re-2,2'-bipyridine(CO)
3
Cl (bpy=2,2'-bipyridine) the values are taken from
previous reports ......................................................................................................................................................... 31
Table 2.5. CPE results for Complex 1. ............................................................................................................. 38
Table 2.6. Reduction potentials for a selection of Re(CO)
3
Cl complexes bearing various 2,2'-
bipyridine ligands ..................................................................................................................................................... 40
Table 2.7. Electrochemical data for 2. .............................................................................................................. 57
Table 3.1. Selected Interatomic Distances (Å) for 3·DMF ........................................................................ 77
Table 3.2. Crystal data and structure refinement for 3 ................................................................................ 78
Table 3.3. Controlled potential electrolysis data for 3 (0.5 mM) in 0.1 M TBAPF
6
solution in
DMF, CO
2
, –2.37 V vs. Fc
+/0

with various Brønsted acids and bases added externally. .................. 95

8

CHAPTER 1

Immobilization of Molecular Catalysts for Electrocatalytic CO
2
Reduction

A portion of this chapter has appeared in print:

Popov, D. A.; Luna, J. M.; Orchanian, N. M.; Haiges, R.; Downes, C. A.; Marinescu, S. C. “A 2,2'-Bipyridine-Containing
Covalent Organic Framework Bearing Rhenium (I) Tricarbonyl Moieties for CO
2
Reduction”, Dalton Trans. 2018, 47,
17450-17460. DOI: 10.1039/C8DT00125A

9
1.1. Electrocatalytic CO
2
Reduction: General Introduction
Carbon dioxide (CO
2
), a greenhouse gas that is released by both natural and
artificial processes, has received attention as an abundant, economical, and
renewable C
1
feedstock that can be converted to higher-energy products.
1-5

Production of renewable chemical fuels from CO
2
reduction can provide a way of
storing the electricity generated from solar and wind power in the form of chemical
bonds, which can counteract the intrinsic intermittency of renewable energy
sources.
6
In addition, the conversion of CO
2
to chemical fuels would positively
impact the global CO
2
balance.
2
One promising route to the production of renewable chemical fuels is the
electrocatalytic reduction of CO
2
. A variety of molecular catalysts have been
studied for this process.
2-5, 7-9
Molecular catalysts are attractive because control
over the first, second, and outer coordination spheres of the ligand environment
allows for tuning of their chemical properties, such as reduction potentials,
catalytic activity, selectivity.
Of particular promise are rhenium 2,2'-bpy complexes. 2,2'-bpy ligands are
common ligands in coordination chemistry and form stable well-defined
complexes.
10, 11
Such systems can undergo multiple reduction events with redox
equivalents stored on both the metal centre and the ligand due to redox non-
innocence of 2,2'-bpy.
12, 13
Due to their high selectivity for CO
2
reduction over
proton reduction, rhenium 2,2'-bpy catalysts have been extensively studied for the
electrochemical reduction of CO
2
to CO.
14-16

10
1.2. Immobilization of Molecular Catalysts for Electrocatalytic CO
2
Reduction
Deployment of large-scale electrocatalytic devices requires the development of
synthetic strategies for the immobilization of molecular catalysts to electrode
surfaces. Achieving practically relevant performance is an end goal for the
development of catalysts for electrocatalytic CO
2
reduction. It was suggested that
current densities exceeding 100 mA/cm
2
are necessary for CO production.
17
CO is
the most common product in electrocatalytic CO
2
reduction with catalysts based on
transition metals, and can be converted to liquid fuels using the Fischer-Tropsch
industrial process. Heterogenization of molecular catalysts has emerged as a
promising strategy that combines the favourable properties of molecular
homogeneous systems, such as high selectivity and tunability, with the stability
and robustness associated with heterogeneous catalysts. Demonstrated methods for
the immobilization of molecular CO
2
reduction electrocatalysts onto carbon-based
electrodes include casting methods
15, 18-21
, the use of pyrene groups
22-24
, covalent
attachment
25-27
, electropolymerization
15, 28-32
, and incorporation of catalytic units
into extended frameworks
33-36
. Chemically linking molecular catalyst active sites
into 2D and 3D architectures, covalent organic frameworks (COFs) and metal
organic frameworks (MOFs), respectively, is another promising option for several
reasons. Immobilization of molecular electrocatalysts via MOFs and COFs is an
attractive strategy because the permanent porosity of these materials allows for
rapid substrate diffusion and access to an unprecedented number of active sites
leading to enhanced catalytic activity in comparison with solution-based molecular
electrocatalysts.
34-47
Additionally, site-isolation of the molecular catalysts

11
incorporated in MOFs and COFs results in enhanced stability of these systems and
allows for extended catalytic performance. Linking molecular complexes into rigid
structures such as COFs and MOFs reduces catalyst aggregation, a common
deactivation pathway for molecular catalysts. Lastly, in theory, incorporating
molecular catalysts into COFs and MOFs allows one to engineer the 3D
environment surrounding the active site in a fashion similar to an active site in
enzymes, thus mimicking nature.

1.3. 2D and 3D Porous Architectures for Electrochemical CO
2
Reduction
Presented are relevant examples of MOFs and COFs investigated for
electrocatalytic CO
2
reduction. MOFs are crystalline structures containing building
blocks connected to metal ions via organic linkers. Known homogeneous
molecular catalysts, iron tetrakis(4-carboxyphenyl)porphyrins (FeTCO
2
PP), were
incorporated into MOF-525 through hexa zirconium (IV) nodes and studied for
electrocatalytic CO
2
reduction.
34
The observed current density in acetonitrile was
2.3 mA/cm
2
, which is lower than that of FeTCO
2
PP molecular catalyst showing 12
mA/cm
2
current densities. This lower activity for the heterogeneous material was
attributed to limited charge transport within the MOF. In order to improve this
material, one needs to improve the MOF’s conductivity and stability. A MOF
based on cobalt tetrakis(4-carboxyphenyl)porphyrin (CoTCO
2
PP) molecules
connected via aluminum ion linkages was investigated as a catalyst for
electrocatalytic CO
2
reduction.
36
Current densities of around 1 mA/cm
2
were
achieved at the optimum MOF thickness of 30-70 nm. The authors reported that

12
the electrocatalytic CO
2
reduction occurred under conditions of mass transport
limitations. A copper tetrakis(4-carboxyphenyl)porphyrin (CuTCO
2
PP) based
MOF was also investigated for electrocatalytic CO
2
reduction.
48
This material has a
short lifetime, as after 15 min of electrolysis, the morphology of the material
changes from crystalline nanosheets to partially amorphous structures. In addition,
according to X-ray diffraction, peaks corresponding to various Cu oxides and
hydroxides were also observed. Despite these changes in the structure under
operating conditions, the catalyst showed faradaic efficiencies for formate of up to
68.4 %. It was hypothesized that under operating conditions, the catalyst existed in
the form of CuTCO
2
PP units covalently bound to copper oxide clusters.
Considering previous studies on copper porphyrins that have shown their
demetallation under electrocatalytic CO
2
reduction conditions,
49
it is also possible
that CuTCO
2
PP units become demetallated during the reaction.
Electrical conductivity within MOFs has been consistently reported as a limiting
factor a good electrocatalytic CO
2
reduction performance.
50, 51
This causes the
apparent rates at the metal centers to be at least an order of magnitude lower than
those of their molecular homogeneous analogues.
52, 53

COFs are crystalline materials, which contain building blocks and structural
linkages connected via covalent bonds. This is in contrast to MOFs, in which
organic/organometallic building blocks are bound to metal ions via coordination
bonds.
54, 55
Unlike MOFs, COFs possess high charge carrier mobility due to π-
conjugation within COF sheets and π-π stacking between the sheets.
35

13
COFs synthesized from cobalt tetraaminophenyl porphyrin (CoTAPP) subunits
connected via imine linkages, COF-366-Co and COF-367-Co, were investigated
for electrochemical CO
2
reduction.
35
The authors achieved a CO
2
to CO reduction
current of up to 3.2 mA/cm
2
at –0.67 V vs. RHE. Only 4-8% of the deposited Co
sites were electrochemically accessible. It was also shown that, upon changing
from a shorter linker molecule (COF-366-Co) to a longer linker molecule (COF-
367-Co), rates were higher when calculated with respect to the deposited Co sites,
but lower with respect to electroactive Co sites. The authors characterized charge
transport through the COF thin films using direct current conductivity
measurements, which yielded a value of 1×10
–6
S/cm, which is in the conductivity
range of a semiconductor. In addition, COF-366-Co was modified with various
functional groups, such as –OMe and –F.
56
The COF containing one fluorine group
resulted in the COF with the most electron-deficient cobalt atom. This also led to
an increased activity for electrocatalytic CO
2
reduction, which is inconsistent with
other studies.
57
Another claim made by the authors is that electron-withdrawing
groups improved activity because removing electron density from the cobalt
centers in COF-366 facilitates the initial reduction of Co(II) to Co(I), even though
it was shown that this transition is not rate-limiting.
35
A COF based on CoTAPP connected via rhenium bipyridine containing linkages
was also explored for electrochemical CO
2
reduction.
58
While the separate subunits
of this COF material, the rhenium bipyridine complex and CoTPP, were both
possessing a high selectivity toward CO with faradaic efficiencies exceeding 80%,
the COF material itself, COF-Re_Co, showed a poor faradaic efficiency of 18.2%.

14
A similar material reported in this study, COF-Re_Fe, displayed an even smaller
faradaic efficiency of less than 2%. The authors suggested that the diffusion of the
reactants, i.e. mass transport, was limited. In addition, the incorporation of both
rhenium bipyridine and CoTPP could lead to the competition between these active
sites for electrons. Lastly, the authors achieved a partial metallation of the
porphyrin units. The presence of the unmetallated porphyrin units (TPP), known to
be selective for the hydrogen evolution reaction,
59
could contribute to the low
faradaic efficiency for CO observed.

1.4. Surface Modification Using Non-Tetraporphyrin Active Sites
Non-tetraphenyl porphyrin active sites are of interest as well. One of the well-
known molecular catalysts for both electrocatalytic and unsensitized photocatalytic
conditions is rhenium (I) bipyridine tricarbonyl complex.
60, 61
Strategies have been
developed for the incorporation of Re-2,2'-bpy catalytic units into heterogeneous
structures for the immobilization on carbon-based supports and the selected and
relevant examples are presented here. The non-covalent attachment of the Re-2,2'-
bpy complexes containing pyrene groups onto highly oriented pyrolytic graphite
(HOPG) electrodes was presented by Gray et al.
22
The resulting device was shown
to be active towards the electrochemical reduction of CO
2
to CO, however, long-
term performance was limited due to leaching of the rhenium active sites into the
solution. Similarly, an analogous Mn-2,2'-bpy complex was anchored via a pyrene
unit to a carbon nanotube electrode yielding an assembly that displays high activity
for electrocatalytic CO
2
reduction under fully aqueous conditions with TONs of up

15
to 1790 ± 290 for CO and up to 3920 ± 230 for formate.
24
. Covalent attachment is
exemplified by graphite-conjugated Re-2,2'-bpy (GCC-Re) catalysts, where the
authors used the o-quinone moieties commonly found on the edge planes of
graphite to condense site-selectively under mild conditions with the o-
phenyldiamino units of a modified Re-2,2'-bpy complex.
27
The resulting GCC-Re
surfaces exhibit high turnover numbers (> 12,000) and turnover frequencies
exceeding the activity of the soluble molecular analogue. Controlled current
electrolysis of GCC-Re revealed sustained catalytic activity at 1 mA/cm
2
for 1.4(3)
h, followed by rapid deactivation. To our knowledge, the only example of Re-2,2'-
bpy incorporation into a MOF for ERC was developed by Sun and coworkers. The
monolithic Re-based MOF thin film was deposited onto conductive fluorine doped
tin oxide (FTO) by liquid-phase epitaxy.
62
The device demonstrated high current
densities that exceeded 2 mA/cm
2
,
34, 36
however the current densities in a CO
2
-
saturated solution gradually dropped off, which was attributed to degradation of
the MOF. Additionally, metal 2,2'-bpy catalysts have been incorporated into MOFs
for photocatalytic H
2
evolution, photocatalytic carbon dioxide reduction, and
electrocatalytic water oxidation.
47, 63-68
However, electrocatalytic CO
2
reduction
was not reported in these studies.
Heterogenization of rhenium bipyridine molecular active sites has been
successfully applied to a wide range of surfaces through attachments involving
both covalent and noncovalent interactions.
22, 69
The functional groups chosen to
perform this attachment limited the scope of available substrates.
Electropolymerization, on the other hand, allows for generation of films on a

16
broader substrate scope,
70-73
ranging from platinum disk electrodes
31
to a variety of
semiconductor materials.
74
These modified electrodes were shown to exhibit high
rates, stabilities, and faradaic efficiencies of 90% with respect to CO. Despite their
promising properties, the vinyl group utilized for polymerization introduced
undesirable side reactions, such as the formation of Re-Re and Re-C bonds via
radical-radical coupling imparted by the formation of vinyl radicals and the
flexibility of the methylene group.
Polymers with conjugated backbones present an alternative to methylene spaced
polymers generated by vinyl polymerization. Polymers with conjugated backbones
display structural rigidity
75
and several studies on rigid metallopolymers report that
this class of materials displays properties promising for photocatalytic
applications.
76
In particular, studies on the conjugated poly([2,2'-bipyrdine]-5,5'-
diyl) and related metallopolymers demonstrate that they feature unique
phophysical properties, including intraligand π- π* transition and metal-to-ligand
charge transfer (MLCT) bands, which facilitate photocatalytic H
2
-evolving
activity.
77, 78
Examples include rigid rhenium bipyridine metallopolymers with
aryleneethynylene architectures were reported to exhibit π- π* transitions as well
as dπ(Re)- π
polymer
charge transfer bands.
79
An analogous rigid polymeric material
generated from an alkyne-substituted rhenium bipyridine complex was
investigated for its electrocatalytic CO
2
reduction activity. These structures
performed with a faradaic efficiency of 33% for CO, which is low relative to their
vinyl-polymerized analogues.
80
Surface immobilized polymers with a
poly(Re(CO)
3
Cl[2,2'-bipyridine]-5,5'-diyl) structure grown on graphite rods show

17
electrocatalytic reduction of CO
2
to CO demonstrating high stability and faradaic
efficiencies.
81
In the same study these polymers were utilized in the modification
of TiO
2
electrodes to produce a catalyst with a promising photocatalytic
performance.

1.5. Conclusions
In summary, 2D COFs and 3D MOFs containing atomically precise active sites
which resemble their molecular analogs is a promising strategy to increase the
number of active sites while maintaining their tenability. In general, these
pioneering studies show immobilization of molecular complexes at high loadings
while maintaining catalysis characteristics similar to the molecular analogs.
Studies continue in this area to investigate complex phenomena occurring within
these frameworks, which include the effect of morphology of the framework on the
exposure of active sites, reactant availability, and electronic conductivity, as well
as the effect of the incorporation into an extended network on the electronic
structure of the active site.

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22

CHAPTER 2

A 2,2'-Bipyridine-Containing Covalent Organic Framework Bearing Rhenium (I)
Tricarbonyl Moieties for CO
2
Reduction

A portion of this chapter has appeared in print:

Popov, D. A.; Luna, J. M.; Orchanian, N. M.; Haiges, R.; Downes, C. A.; Marinescu, S. C. “A 2,2'-Bipyridine-Containing
Covalent Organic Framework Bearing Rhenium (I) Tricarbonyl Moieties for CO
2
Reduction”, Dalton Trans. 2018, 47,
17450-17460. DOI: 10.1039/C8DT00125A

23
2.1. Introduction
We sought to investigate crystalline porous frameworks, in particular covalent
organic frameworks (COFs), as tunable materials for electrocatalysis.
1
These
materials are advantageous because they can be easily constructed from molecular
building blocks, thus enabling the control of the arrangement of catalytic active
sites within the COF structure. Moreover, the pores surrounding the active sites
would provide access for the substrate, in this case CO
2
, to the catalytic site. Here,
we show the incorporation of catalytic rhenium 2,2'-bpy active sites into a COF. In
this study, we use a previously reported COF (COF-2,2'-bpy) derived via the
modified Schiff base reaction between 1,3,5-triformylphloroglucinol and 5,5'-
diamino-2,2'-bpy, which combines reversible and irreversible Schiff base
reactions.
1, 2
COF-2,2'-bpy is the product of proton tautomerism, which yields a
framework that displays remarkable stability in water as well as strong acids and
bases. Therefore, the resulting framework can serve as a platform for catalysis
under harsh conditions, which was demonstrated in the study of COF-2,2'-bpy-
supported Co catalyst for the electrocatalytic oxygen evolution reaction (OER).
3

This catalyst demonstrated high stability (24 h) and efficiency (95%).
With this in mind, a Re-2,2'-bpy complex is proposed, where the 2,2'-bpy fragment
is modified with amino groups in the 5,5' positions as reactive groups for COF
synthesis. These amino groups are installed with the aim of using them in the
integration of 1 into a 2D covalent organic framework via a solvothermal Schiff
base condensation reaction. The synthesis, characterization and electrocatalytic
properties of the resulting complex, Re(5,5'-diamino-2,2'-bpy)(CO)
3
Cl, 1, towards

24
CO
2
reduction is presented. The effects of the amino groups on the reduction
behavior of the complex are probed using DFT. Two different synthetic pathways,
a direct Schiff base condensation reaction starting from 1 and post-metallation of
COF-2,2'-bpy, are explored for the integration of 1 into a COF structure. The
resulting materials are studied as electrocatalysts for CO
2
reduction.
2.2. Results and Discussion
Treatment of 2,2'-bipyridine-5,5'-diamine with Re(CO)5Cl in a mixture of toluene
and methanol under reflux leads to the formation of a yellow powder (Scheme 2.
1). A
1
H NMR spectrum of the yellow powder displays a set of signals that is
shifted downfield with the respect to the
1
H NMR signals of 2,2'-bipyridine-5,5'-
diamine. The observation of a deshielding effect of the Re(CO)
3
Cl moiety upon
metallation points to the formation of 1.

Scheme 2.1. Synthesis of 1.

Crystals of single X-ray quality have been obtained by vapor diffusion of diethyl
ether to a saturated solution of 1 in DMF. A solid state structure of 1·DMF, where
N
N
NH
2
NH
2
+
Re(CO)
5
Cl
N
N
NH
2
NH
2
Re
CO
CO
Cl
CO
1
toluene/methanol
1 h
- 2 CO
reflux

25
1 contains one molecule of DMF per molecule, reveals equatorial coordination of
the 2,2'-bipyridine-5,5'-diamine ligand and the facial arrangement of the three
carbonyl ligands, which is typical Re(CO)
3
Cl complexes bearing ligands of the
2,2'-bipyridine class (Figure 2.1, Tables 2.1 and 2.3).

Figure 2.1. X-ray crystal structure of 1•DMF (DMF molecule is not shown for clarity)
with ellipsoids set at 50% probability level. Selected hydrogen atoms bonded directly to
the 5,5'-diamino-2,2'-bpy backbone have been omitted for clarity.

26
Table 2.1. Crystal data and structure refinement for 1.

x C
16
H
17
ClN
5
O
4
Re
Formula weight 564.99
Crystal system monoclinic
Space group C
2/c

a (Å) 26.2689(18)
b (Å) 8.5420(6)
c (Å) 17.5006(12)
α (°) 90
β (°) 98.2270(10)
γ (°) 90
V (Å
3
) 3886.5(5)
Z 8
D
calc
(g/cm
3
) 1.931
µ (Mo Kα) (mm
–1
) 6.422
F (000) 2176
Reflections collected 46175
Independent reflections 5911 [R(int) = 0.0292]
R
1
(I > 2σ(I)) 0.0163
wR
2
(all data) 0.0317
Goodness-of-fit (GOF) on F
2
1.029

27
Table 2.2. Selected bond lengths (Å) for complex 1.
Bond Bond length (Å)
Re(1)-C(1) 1.899(18)
Re(1)-C(2) 1.9252(16)
Re(1)-C(3) 1.9249(18)
Re(1)-Cl(1) 2.4848(4)
Re(1)-N(1) 2.1755(13)
Re(1)-N(2) 2.1707(13)
C(1)-O(1) 1.144(2)
C(2)-O(2) 1.150(2)
C(3)-O(3) 1.151(2)

A FTIR spectrum of 1 was collected using an ATR attachment (Figure 2.2). This
spectrum shows 2 major peaks at 2011 and 1891 cm
–1
and two minor peaks at
1919 and 1836 cm
–1
. The major peaks are typical of Re(CO)
3
Cl complexes bearing
ligands of the 2,2'-bipyridine class, and the peak at 2011 cm
–1
is assigned to the
stretching mode of the axial CO ligand. The broad frequency at 1891 cm
–1
is a
fundamental band that corresponds to the in-phase and out-of-phase stretching
modes of the equatorial CO ligands, and the minor peaks at 1919 and 1836 cm
–1

are due to coupling caused by Fermi resonance.

28

Figure 2.2. CO stretching region of the ATR-FTIR spectrum of 1.

An FTIR of a sample that is prepared as a pellet 1 of KBr shows displays two
major frequencies at 2015 and 1896 cm
–1
that are characteristic of CO stretching
frequencies (Figure 2.3). Besides these features, there are additional shoulders at
2022, 1929, and 1845 cm
–1
. Comparison with Figure 2.2 suggests that 1 undergoes
halogen exchange with KBr during the pellet preparation. Therefore, use of KBr in
the pellet preparation with 1 should be avoided.

29

Figure 2.3. CO stretching region of the FTIR of a pressed pellet of 1 with KBr.

In order to gain computational insight for the assignment of the carbonyl stretching
modes we performed frequency calculations at the M06 level of theory with a
hybrid basis set (6-31G* for C, H, N, O and LANL2DZ for Cl, Re). The calculated
carbonyl stretching frequencies for Re(2,2'-bipyridine)(CO)
3
Cl and 1 are shown in
Table 2.3, and the stretching vectors for these modes are depicted in Figure 2.4.
The relative energy ordering and the character of these three modes are unchanged
in 1 which provides theoretical support for the assignment of the experimental
FTIR values.

30
Table 2.3. Calculated carbonyl stretching mode frequencies from DFT calculations for
Re-2,2'-bipyridine(CO)
3
Cl and 1 (bpy=2,2'-bipyridine). Calculations were performed
using the M06 functional with the 6-31G* basis set for H, C, N, and O atoms and the
LANL2DZ effective core potential and basis set for Cl and Re atoms.
Experimental Frequencies (cm
-1
) Calculated Frequencies (cm
-1
)
Carbonyl
Stretching
Mode
[1]

Re-2,2'-bpy(CO)3Cl
16
1 Re-2,2'-bpy(CO)3Cl 1
a'1 2025 2015 2129 2125
a'' 1918
1896
[2]

2050 2042
a'2 1902 2030 2027
[1] See visualization of carbonyl stretching modes in Figure 2.4. [2] Stretching
modes a” and a'
2
appear coalesced in Figures 2.2 and 2.3.

Figure 2.4. Visualization of the three characteristic carbonyl stretching modes in Re-2,2'-
bipyridine(CO)
3
Cl with pink vectors indicating nuclear motion. The character of the three
carbonyl modes in 1 are identical to these.

A donor strength of 2,2'-bipyridine-5,5'-diamine relative to 2,2'-bipyridine is
expected to decrease the Re-N bond lengths due to the stronger nitrogen donors,
while the carbonyl C-O bond length should increase due to stronger π-backbonding

31
interactions with Re metal center. According to Table 2.4 it can be inferred that the
electron density at Re is largely unchanged by the inclusion of electron donating
amino groups in the meta-positions as neither the Re-N nor C-O bond lengths are
perturbed. However, there is a consistent decrease in the experimental and
calculated carbonyl stretching frequencies upon inclusion of the amino groups in
the meta-positions, suggesting that there is a subtle increase in π -backbonding
from rhenium to the carbonyl ligands, which was less evident from the structural
parameters.
Table 2.4. A comparison between select experimental (averaged) and calculated bond
lengths for complex 1. For Re-2,2'-bipyridine(CO)
3
Cl (bpy=2,2'-bipyridine) the values
are taken from previous reports.
4
Calculations were performed using the M06 functional
with the 6-31G* basis set for H, C, N, and O atoms and the LANL2DZ effective core
potential and basis set for Cl and Re atoms.

Experimental Bond Length (Å) Calculated Bond Length (Å)
Bond Re-2,2'-bpy(CO)3Cl 1 Re-2,2'-bpy(CO)3Cl 1
Re-N 2.175(7) 2.173(2) 2.203 2.204
Re-C(eq) 1.925(10) 1.925(3) 1.922 1.920
C-O (eq) 1.150(13) 1.151(3) 1.159 1.160

To gain more structural insight for 1 we performed high-resolution XPS studies on
1 dropcast on carbon tape used as a support (Figure 2.5). In the Re 4f region, one
set of peaks of binding energies of 43.4 and 40.9 eV corresponding to Re 4f
5/2
and
Re 4f
7/2
, respectively. The other two regions represent the Cl 2p and the N 1s
regions. The Cl 2p region reveals two peaks of binding energies of 198.9 and 197.1

32
eV which are assigned to Cl 2p
1/2
and Cl 2p
3/2
levels, respectively. The N 1s region
shows one peak at 398.8 eV which was assigned to N 1s level.

Figure 2.5. XPS analysis of 1. (a) N 1s core level XPS spectrum; (b) Cl 2p core level
XPS, and (c) Re 4f core level

Cyclic voltamograms (CVs) of 1 (0.5 mM) were recorded in 0.1 M solutions of
[nBu
4
N][PF
6
] in acetonitrile (MeCN) using glassy carbon electrodes under a
nitrogen atmosphere (Figure 2.6). Multiple CV scans of 1 display similar current
density values suggesting that the surface deposition is not taking place during a
CV scan (Figure 2.7). Two irreversible reduction features are observed at –2.11 V
and –2.47 V vs. Fc
+/0
. These reductions stay irreversible either upon increasing the
scan rate (Figure 2.8), or reversing the potential after the first reduction (Figure
2.9). The cathodic peak current densities at –2.11 V and –2.47 V vs. Fc
+/0
are

33
directly proportional to the square root of the scan rate (Figure 2.10), as expected
for a freely diffusing species in solution obeying the Randles-Sevcik equation.

Figure 2.6. CVs of 1 (0.5 mM) in 0.1 M [nBu
4
N][PF
6
] in MeCN under Ar (black) and
CO
2
(red) at a scan rate of 100 mV/s.

34

Figure 2.7. Cyclic voltammograms data (1
st
, 2
nd
, and 3
rd
scans) of 1 (0.5 mM) in MeCN
solution containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere of N
2
.

35

Figure 2.8. Cyclic voltammograms of 0.5 mM of 1 in an MeCN solution containing 0.1
M [nBu
4
N][PF
6
] under an atmosphere of N
2
at scan rates ranging from 100 to 1000 mV/s.

36

Figure 2.9. Cyclic voltammogram data of 1 (0.5 mM) in MeCN solution containing 0.1
M [nBu
4
N][PF
6
] under an atmosphere of N
2
. Black curve: scan reversed after second
reduction, red curve: scan reversed after first reduction.

Figure 2.10. Plot showing the peak cathodic current density at –2.11 V (left) and –2.47 V
(right) vs. Fc
+/0
as a function of the square root of the scan rate. The cathodic peak current
density was obtained from the cyclic voltammetry data of 1 (0.5 mM) in MeCN solution
containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere of N
2
. The cathodic peak current
density increases linearly with the square root of the scan rate indicative of a freely-
diffusing species obeying the Randles-Sevcik equation.

37
CVs of 1 under a CO
2
atmosphere exhibit a ~4 fold increase in current density near
the second reduction feature, suggesting that a catalytic reaction is taking place
(Figure 2.6). Addition of Brønsted acids such as water, methanol, trifluoroethanol,
and phenol did not improve the catalytic performance. Controlled potential
electrolysis (CPE) of 1 (0.5 mM) was performed at –2.57 V vs. Fc
+/0
in 0.1 M
solutions of [nBu
4
N][PF
6
] in MeCN under a CO
2
atmosphere (Figure 2.11).
Charge in the amount of 9.2 coulombs was consumed after 1h, and analysis of the
gas mixture confirmed production of CO with a faradaic efficiency of 99% and a
TON of 100,800 (Table 2.5) as described in Experimental Section. A negligible
amount of CO was observed under a nitrogen atmosphere, suggesting that the CO
2

is the source of the CO and not the carbonyl groups of 1. Negligible amounts of
carbonate were observed by FTIR suggesting that the overall reaction is a 2-
electron, 2-proton reduction of CO
2
to CO.
5
At the negative reduction potentials
needed for catalysis, it has been reported that MeCN and [nBu
4
N][PF
6
] can serve
as O-acceptors: a proton can be extracted from MeCN
6
or from [nBu
4
N][PF
6
]
through Hoffman degradation.
7
The FTIR spectra of the electrolysis solutions
before and after the CPE studies display identical peaks (Figure 2.12), suggesting
that 1 is stable under the electrocatalytic conditions.
8-10

38

Figure 2.11. Controlled potential electrolysis of 1 under N
2
(black) and under CO
2
(red).
Conditions: 1 (0.5 mM) in 0.1 M solution of [nBu
4
N][PF
6
] in MeCN, measured at a
potential of –2.57 V vs. Fc
+/0
.

Table 2.5. CPE results for Complex 1. Overall TON is calculated as mol
CO
/mol
catalyst
.
TOF
CPE
(s
-1
) is calculated as described previously. TON
CPE
is calculated by multiplying
TOF
CPE
(s
-1
) with the time for CPE studies (3600 s) (see “TOF
CPE
calculations from
controlled potential electrolysis” section below for details).

Time,
min
Charge
(C)
Faradaic Efficiency,
%
Overall
TON
TOF
CPE
TON
CPE
60 9.200 99 2 28 100,800

39

Figure 2.12. FTIR of a solution of 1 before (blue) and after (light-brown) CPE
(conditions: 1 (0.5 mM) in MeCN solution containing 0.1 M [nBu
4
N][PF
6
] under an
atmosphere of CO
2
at –2.57 V versus Fc
+/0
).

The reduction events of 1 occur at potentials more negative than those of any
previously reported 2,2'-bipyridine molecular catalysts bearing rhenium tricarbonyl
moieties (Table 2.6).
11, 12
In order to understand this difference, we performed DFT
calculations to explore the electronics of 1. Relative energies for the Kohn-Sham
molecular orbitals of Re-2,2'-bipyridine(CO)
3
Cl and 1, along with the illustrations
of the respective HOMOs and LUMOs, are presented in Figure 2.13. According to
this diagram, the overall HOMO and LUMO character for 1 is not drastically
perturbed compared to that of Re-2,2'-bipyridine(CO)
3
Cl. The LUMO retains its
2,2'-bipyridine π* character, and the HOMO exhibits the Re-Cl π* character.
Although the character of these orbitals is not perturbed, the LUMO is evidently
destabilized by the increased antibonding interactions in the π-framework of the
2,2'-bipyridine-5,5'-diamine ligand. As the additional electrons would populate this
destabilized, higher-lying LUMO, this destabilization is the likely cause for the

40
increased reduction potentials of 1 relative to Re-2,2'-bipyridine(CO)
3
Cl. The
relative energy of the corresponding 2,2'-bipyridine-5,5'-diamine π-bonding orbital
was also shown to lie higher in energy than that of Re-2,2'-bipyridine(CO)
3
Cl
(Figure 2.14), confirming that the highest occupied orbital based on 2,2'-
bipyridine-5,5'-diamine is similarly destabilized upon introduction of the amino
groups.
Table 2.6. Reduction potentials for a selection of Re(CO)
3
Cl complexes bearing various
2,2'-bipyridine ligands.
11
The reduction potentials for complex 1 are from this study and
the rest are from reference 11. The literature values were reported relative to .the
saturated calomel electrode (SCE). For the purposes of comparison, these values were
converted to the Fe
3+/2+
couple of Fc by adding a value of –0.400 V to the potential
reported and rounded to the second digit past the point.

Complex 1
st
reduction potential
(V vs. Fc
+/0
)
2
nd
reduction potential
(V vs Fc
+/0
)
Re-2,2'-bpy(4,4'-COOH)(CO)
3
Cl -1.34 -2.13
Re-2,2'-bpy(CO)
3
Cl -1.74 -2.13
Re-2,2'-bpy(4,4'-Me)(CO)
3
Cl -1.83 -2.17
Re-2,2'-bpy(4,4'-tBu)(CO)
3
Cl -1.85 -2.23
Re-2,2'-bpy(4,4'-OMe)(CO)
3
Cl -1.89 -2.26
1 -2.11 -2.47

41

Figure 2.13. Molecular orbital diagrams and frontier orbital images for Re-2,2'-
bipyridine(CO)
3
Cl (HOMO, LUMO; blue) and 1 (HOMO', LUMO'; red). 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.

42

Figure 2.14. Molecular orbital diagrams and frontier orbital images for Re-2,2'-
bipyridine(CO)
3
Cl (HOMO-2, HOMO-3; blue) and 1 (HOMO-2', HOMO-3'; red).
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.

These predictions are consistent with experimental evidence provided that the
observed reduction events are based on 2,2'-bipyridine. In order to demonstrate
this, we synthesized Zn(2,2'-bipyridine-5,5'-diamine)(CH
3
COO)
2
·H
2
O by treating
zinc acetate with 2,2'-bipyridine-5,5'-diamine in a mixture of water and methanol
at room temperature. The white solid generated as a result is a Zn analog of 1. A
1
H NMR spectrum of this solid displays a peak at 1.79 ppm, corresponding to the
methyl protons in the acetate group (6H), and peaks corresponding to the 2,2'-

43
bipyridine-5,5'-diamine ligand environment. The electrochemical properties of the
zinc complex were explored (Figure 2.15). Two reduction events are observed at –
2.31 V and –2.71 V vs. Fc
+/0
, suggesting that the reduction feature exhibited by 1
(Figures 2.6-2.9) are ligand-based reductions. This is common for related 2,2'-
bipyridine complexes bearing Re(CO)
3
Cl fragments.

Figure 2.15. Cyclic voltammogram scan rate dependence of 0.5 mM Zn(5,5'-diamino-
2,2'-bpy) complex in an MeCN solution containing 0.1 M [nBu
4
N][PF
6
] under an
atmosphere of N
2
. Scan rates vary from 100 to 1000 mV/s.

According to published mechanistic studies of CO
2
reduction with Re-2,2'-
bipyridine(CO)
3
Cl, it is common for this and related 2,2'-bipyridine based
Re(CO)
3
Cl complexes to dissociate Cl
–
following the electron transfer step.
5, 13-21

Upon the electron transfer, the species, [Re-2,2'-bipyridine(CO)
3
Cl]
–
, exists in
equilibrium with the neutral complex, [Re-2,2'-bipyridine(CO)
3
], and the
dissociated Cl
–
. As the first reduction even in 1 appears to be irreversible at various
scan rates (Figure 2.8), chloride dissociation is an irreversible process on the time
scale of the CV experiment. This indicates complete chloride dissociation in 1,and

44
it is attributed to the more cathodic potential of the first reduction caused by the
destabilization of the LUMO (vide supra).
To probe this observed irreversibility changes in the CV upon addition of nBu
4
NCl
were monitored. The addition of this external chloride source is expected to shift
the equilibrium away from the dissociation of chloride, causing an increase in the
reversibility of the first reduction feature. This, however, is not observed even in
the presence of a large excess of chloride (100 mM nBu
4
NCl), supporting that the
observed chloride dissociation is indeed an irreversible process (Figure 2.16).

Figure 2.16. Cyclic voltammograms of 1 (0.5 mM) in an MeCN solution containing 0.1
M [nBu
4
N][PF
6
] under an atmosphere of N
2
at varying concentrations of nBu
4
NCl
(TBACl).

We have also made a chloride-abstracted analog of 1, Re-2,2'-bipyridine-5,5'-
diamine(CO)
3
(MeCN)(CF
3
SO
3
–
) (1
OTf
), from 1 and silver triflate. A
1
H NMR of
this compound shows a broad peak at 2.09 ppm (3H) indicative of the coordinated
acetonitrile ligand along with the peaks associated with the 2,2'-bipyridine-5,5'-

45
diamine ligand environment. The reduction events observed in 1
OTf
are more
positive that the ones in 1, by approximately 200 mV and 190 mV for the first and
the second reductions, respectively (Figure 2.17), consistent with other previously
reported Re(CO)
3
(CF
3
SO
3
–
) 2,2'-bipyridine based complexes.
12, 22
The first
reduction feature in 1
OTf
does not exhibit an increase in reversibility upon
increasing scan rate (Figure 2.18), suggesting that the acetonitrile ligand is
irreversibly lost following the first reduction on the CV timescale. Therefore, both
the experiments utilizing nBu
4
NCl and the chloride-abstracted version of 1,
support that the first reduction and the subsequent ligand loss (Cl
–
and MeCN) is
an irreversible process.

Figure 2.17. Overlay of cyclic voltammetry data of 1 and of its chloride abstracted
analog (0.5 mM concentration each) in MeCN solution containing 0.1 M [nBu
4
N][PF
6
]
under an atmosphere of N
2
.

46

Figure 2.18. Cyclic voltammogram scan rate dependence of the Cl-abstracted
analog 1 (0.5 mM) in an MeCN solution containing 0.1 M [nBu4N][PF6] under an
atmosphere of N2. Scan rates vary from 100 to 1000 mV/s.

Following the successful electrocatalytic reduction of CO
2
to CO using 1, we
explored attempts to integrate 1 into a COF by a modified Schiff base
condensation reaction.
2, 3, 23
In this reaction the initial imine formation is followed
by an irreversible keto-enol tautomerism, leading to a two-dimensional COF
featuring excellent stability in water, strong acid, and bases. Inspired by this, we
reasoned that 1 could induce COF formation similarly to the structurally related
2,2'-bipyridine-5,5'-diamine, which was successfully used as a building block in
COF formation to form COF-2,2'-bipyridine.
3, 23
Unfortunately, attempts to
directly utilize 1 in COF synthesis were unsuccessful and lead to unidentifiable
mixtures.

47
Since COF-2,2'-bipyridine contains 2,2'-bipyridine fragments, we proposed that
post-metallation
24-26
of this framework using Re(CO)
5
Cl could result in the
incorporation of Re(CO)
3
Cl fragments, effectively heterogenizing a molecular
catalyst of the Re-2,2'-bipyridine(CO)
3
Cl type. We prepared COF-2,2'-bipyridine,
which is an orange crystalline material featuring an intense peak at 2θ =3.6°in the
powder X-ray diffraction (PXRD) pattern, corresponding to the [100] reflection.
3,
23
Treatment of COF-2,2'-bipyridine with Re(CO)
5
Cl in toluene under reflux for 3
days results in formation of a dark-red powder, COF-2,2'-bipyridine-Re (Scheme
2.2). We performed solvent exchange on this compound to remove the excess
starting materials. In addition, by varying the starting Re(CO)
5
Cl/COF-2,2'-
bipyridine ratio we can modify the amount of the rhenium catalyst incorporated
into the framework. Using ratios of 0.01, 0.05, 0.15, 1.0, and 1.5 results in the
formation of COF-2,2'-bipyridine-Re samples with rhenium weight percentages of
0.21, 0.65, 1.86, 15.39, and 29.38, respectively, as determined by inductively
coupled plasma optical emission spectroscopy (ICP-OES) studies of the digested
dark-red powders. To probe whether molecular, well-defined, Re centers exist
within COF-2,2'-bipyridine-Re, FTIR and XPS studies were performed.

48

Scheme 2.2. Synthesis of COF-2,2'-bipyridine-Re (2) through post-metallation.

The FTIR spectra of all the COF-2,2'-bipyridine-Re samples confirmed the
presence of CO stretches at 2023 and 1907 cm
–1
(Figure 2.19). The former is
assigned to the stretching mode of the axial CO ligand, and the latter is assigned to
the in-phase and out-of-phase stretching modes of the equatorial CO ligands. Due
to the similar energies of these stretching modes, the latter peak is unresolved. This
data agrees with the FTIR of 1, suggesting that Re(CO)
3
Cl sites retain their
molecular nature within the heterogeneous framework. CO stretches were not
observed in the FTIR spectrum of COF-2,2'-bipyridine (Figure 2.20). In addition,
the CO stretches in the FTIR spectrum of Re(CO)
5
Cl are distinct from those of
COF-2,2'-bipyridine-Re, indicating that Re(CO)
3
Cl moieties coordinated to 2,2'-
bipyridine are the source of the CO stretches observed in Figure 2.19.

49

Figure 2.19. Overlay of the FTIR spectra of COF-2,2'-bipyridine-Re with various
amounts of incorporated rhenium (I) tricarbonyl moieties, such as: 0.21 (orange), 0.65
(yellow), 1.86 (green) and 15.39 (blue), and 29.38 (purple) wt % Re.

50

Figure 2.20. Overlay of the FTIR spectra of COF-2,2'-bipyridine-Re that displays 29.38
wt % Re incorporation (purple) and of COF-2,2'- bipyridine (red).

Figure 2.21. FTIR of Re(CO)
5
Cl in the CO stretching region.

HR XPS studies of COF-2,2'-bipyridine-Re with rhenium weight percentages (wt
% Re) of 15.39 and 29.38 confirm the presence of Re, Cl, and N (Figure 2.22 and
2.23). A set of peaks with binding energies of 44.3 and 41.9 eV was observed in
the Re 4f region. These peaks correspond to Re 4f
5/2
and Re 4f
7/2
levels,

51
respectively. These are also similar to the set of peaks observed for 1 in the Re 4f
region, and to those of other reported heterogenized Re(CO)
3
Cl moieties.
27, 28
This
suggests that this moiety is present within the heterogeneous COF structure. The
presence of Cl and N is confirmed by studying the Cl 2p and N 1s regions. The Cl
2p region shows two peaks with binding energies of 198.9 and 198.2 eV,
corresponding to Cl 2p
1/2
and Cl 2p
3/2
levels, respectively. The N 1s region shows
one peak corresponding to the only level in this region. HR XPS of Re(CO)
5
Cl
(Figure 2.24) demonstrates distinct electronic environments for both the Re 4f and
Cl 2p regions from that of COF-2,2'-bipyridine-Re. Therefore, we conclude that
Re-2,2'-bipyridine fragments are incorporated into the framework, based on the
combination of the FTIR and HR XPS data for COF-2,2'-bipyridine-Re.

52

Figure 2.22. HR XPS analysis of COF-2,2'- bipyridine -Re that displays 15.39 wt % Re:
(a) N 1s core level XPS spectrum; (b) Cl 2p core level XPS spectrum; (c) Re 4f core level
XPS spectrum.

53

Figure 2.23. X-ray photoelectron spectroscopy (XPS) analysis of COF-2,2'- bipyridine -
Re that displays 29.38 wt % Re. (a) N 1s core level XPS spectrum; (b) Cl 2p core level
XPS spectrum; (c) Re 4f core level XPS spectrum.

Figure 2.24. HR XPS analysis of the rhenium precursor, Re(CO)
5
Cl: (a) Cl 2p
core level XPS spectrum; (b) Re 4f core level XPS spectrum.

To probe whether the structural integrity of COF-2,2'-bipyridine was maintained
upon post-metallation, we collected PXRD patterns for all the samples of COF-

54
2,2'-bipyridine-Re and COF-2,2'- bipyridine. Except for the 29.39 wt % Re sample,
all the COF-2,2'-bipyridine-Re samples retained the crystalline structure of the
original COF-2,2'-bipyridine as evidenced by retention of the peak at 2θ = 3.6°.
The loss of the peak in the 29.39 wt % Re sample is attributed to the lack of long-
range order, including exfoliation and pore misalignment, due to a high occupancy
of the 2,2'- bipyridine sites by Re(CO)
3
Cl fragments.

Figure 2.25. Overlay of the experimental PXRD patterns of COF-2,2'-bipyridine-Re with
0.00 (red), 0.21 (orange), 0.65 (yellow), 1.86 (green) and 15.39 (blue) wt % Re
incorporation.

The permanent porosity of COFs facilitating substrate diffusion is an attractive
property of these materials. In order to investigate the surface areas of the
frameworks developed in this study, Brunauer-Emmett-Teller (BET)
measurements were performed. The surface areas of COF-2,2'-bipyridine-Re
samples containing 15.39 and 29.38 wt % Re were found to be 192.7 and 150.0
m
2
/g, respectively. These values are smaller than the surface area found for COF-

55
2,2'-bipyridine supported Co material (450 m
2
/g).
3
The small pore size of the
resulting COF-2,2'-bipyridine-Re could lead to poor diffusion of CO
2
throughout
the material, thus inhibiting electrocatalysis.
Having observed the successful incorporation of Re(CO)
3
Cl fragments into COF-
2,2'-bipyridine, we explored the efficiency of the resulting materials as catalysts
for electrocatalytic CO
2
reduction. Considering that COF materials are generally
insulating,
29-31
and that an analogous Co catalyst hosted by COF-2,2'-bipyridine
was integrated with a highly conductive carbon-based material,
3
we generated
COF-2,2'-bipyridine-Re composites with carbon black, a conductive additive, and
polyvinylidene fluoride (PVDF), a polymeric binder. CVs collected using a glassy
carbon electrode modified with these composites in a 0.1 M [nBu
4
N][PF
6
] solution
in MeCN under a N
2
atmosphere do not display any redox features. A similar
behavior for the surface-bound Re-2,2'-bipyridine species was reported by
Surendranath et al.
28
Minimal current increases were observed by CV under a CO
2
atmosphere for the
composites based on COF-2,2'-bipyridine-Re samples containing 0.21, 0.65, 1.86,
and 15.39 wt % Re, which possessed the crystallinity of the parent COF-2,2'-
bipyridine. In addition, CO was not detected by GC following electrolysis with
these composites. However, the CV under a CO
2
atmosphere for the composites
based on COF-2,2'-bipyridine-Re samples containing 29.38 wt % Re (2) displays
currents reaching 13.2 mA at a potential of –2.8 V vs. Fc
+/0
(Figure 2.26). In
comparison, 2 displays a current of 4.8 mA at the same potential under a N
2

56
atmosphere. This current enhancement for 2 observed under a CO
2
atmosphere
suggests that a catalytic reaction is taking place.

Figure 2.26. Polarization curves of 2 in a 0.1 M [nBu
4
N][PF
6
] acetonitrile solution under
N
2
(black) and CO
2
(red) at a scan rate of 10 mV/s.

CPE of 2 was performed in a 0.1 M [nBu
4
N][PF
6
] solution in MeCN under a CO
2

atmosphere at –2.8 V vs. Fc
+/0
for 1 h (Figure 2.27). During this time, aliquots of
the gaseous headspace were taken at the 30 min and 60 min intervals. These
aliquots were analyzed by GC, which confirmed the formation of CO, with an 81%
faradaic efficiency at 30 min and 57% at 60 min (Table 2.6). The amounts of CO
detected at 30 and 60 min were the same, suggesting low stability of the catalyst
under prolonged electrolysis (Table 2.7). The CPE profile exhibits a large drop-off
in current similar to that of the surface-bound Re-2,2'-bipyridine species was
reported by Surendranath et al.
28
CPE experiments with composites based on COF-
2,2'-bipyridine, Re(CO)
5
Cl, and carbon black/PVDF under a CO
2
atmosphere led
to negligible amounts of CO detected, suggesting that none of these is a catalyst

57
for electrocatalytic CO
2
reduction to CO (Figure 2.28). Together, these data
indicate that 2 is responsible for electrocatalytic CO
2
reduction observed.

Figure 2.27. Controlled potential electrolysis of 2 in an atmosphere of CO
2
(red) and N
2

(black); conditions: 0.1 M [nBu
4
N][PF
6
] in MeCN, measured at a potential of –2.8 V vs.
Fc
+/0
.

Table 2.7. Electrochemical data for 2.

Time, min Volume
(CO), mL
Charge passed,
coulombs
Faradaic Efficiency,
%
Total TON
30 0.376 3.746 81 51
60 0.377 5.267 57 51

58

Figure 2.28. (a) Polarization curves of the composites based on COF-2,2'-bipyridine
(purple), rhenium precursor, Re(CO)
5
Cl (blue), carbon black and polyvinylidene fluoride
(PVDF) by themselves (black), and 2 (red) under CO
2
; conditions: 0.1 M [nBu
4
N][PF
6
]
in MeCN, scan rate = 10 mV/s. (b) Controlled potential electrolysis of the composites
based on COF-2,2'-bipyridine (purple), rhenium precursor (blue), carbon black and
PVDF by themselves (black), and 2 (red) under CO
2
; conditions: 0.1 M [nBu
4
N][PF
6
] in
MeCN, measured at a potential of –2.8 V vs. Fc
+/0
.

The instability of 2 was confirmed by comparing the CVs before and after CPE
(Figure 2.28). This limited stability is common for other heterogenized Re-2,2'-
bipyridine molecular catalysts.
27, 28, 29
There are several possible deactivation
pathways in 2. One of them is leaching of Re into solution during the CPE studies.
This deactivation pathway was ruled out, because ICP-OES analysis of the
solution generated after the CPE experiments detected negligible amounts of
rhenium in solution. Electrolyzing this post-catalysis solution using a clean glassy
carbon electrode under CO
2
atmosphere showed background currents (Figure
2.29). These currents are typical of those of the bare glassy carbon electrode in
fresh electrolyte solution and did not lead to the formation of CO. Moreover, we
collected the HR XPS spectra of 2 following electrolysis experiments, which
demonstrated the persistence of the signals in the Re 4f region at the same binding

59
energies as those observed for 2 prior catalysis (Figure 2.30). These data combined
indicate that the Re active sites are retained after catalysis and that leaching of Re
into solution is not responsible for the loss of catalytic activity. Another possible
degradation pathway could be attributed to the degradation of the framework or a
composite as a whole, limiting its long-term catalytic activity.
In addition, weak non-covalent interactions between 2 and the electrode surface
limit the ability of 2 to achieve high electrocatalytic activity. Moreover, PVDF, a
polymeric binder used to make 2, swells in MeCN, disrupting the electrical
integration between the catalyst, carbon black, and the electrode surface. This
results in a surface assembly with poor conductivity and charge transfer. Low
porosity as indicated by low surface areas of the resulting materials prevents
efficient mass transport of the substrate, CO
2
, to the rhenium active sites, further
inhibiting catalysis. Overall, poor mass and charge transport within our composites
are suspected to be the main factors limiting the electrocatalytic activity and
leading to the poor performance of the COF-2,2'-bipyridine composites for
electrocatalytic CO
2
reduction.

60

Figure 2.29. (a) Polarization curves of the postcatalysis solution (solution after CPE)
measured with a clean glassy carbon electrode (black) and 2 (red) under of CO
2
;
conditions: 0.1 M [nBu
4
N][PF
6
] in MeCN, scan rate = 10 mV/s. (b) Controlled potential
electrolysis of the postcatalysis solution (solution after CPE) measured with a clean
glassy carbon electrode (black) and 2 (red) under CO
2
; conditions: 0.1 M [nBu
4
N][PF
6
] in
MeCN, measured at a potential of –2.8 V vs. Fc
+/0
.

61

Figure 2.30. HR XPS analysis of 2 after a 1 h electrolysis experiment in 0.1 M
[nBu
4
N][PF
6
] acetonitrile solution at -2.8 V vs Fc
0/+
: (a) N 1s core level XPS spectrum;
(b) Cl 2p core level XPS spectrum; (c) Re 4f core level XPS spectrum.

2.3. Conclusions
In summary, we report here the synthesis of Re(2,2′-bpy-5,5′- diamine)(CO)
3
Cl
(1), and its investigation towards the electrocatalytic CO
2

reduction. Complex 1
reduces CO
2

to CO at −2.57 V with a faradaic efficiency of 99% during 1 hour of
electrolysis. DFT studies were employed to investigate the electronic structure of 1
and predicted that the LUMO of 1 is destabilized relative to that of Re(2,2′-
bpy)(CO)
3
Cl. This destabilization perturbs the electrochemical behavior of 1 as
the complex displays more negative and irreversible reduction features in

62
comparison to Re(2,2′-bpy)(CO)
3
Cl. This, in turn, leads to irreversible chloride
dissociation as the population of the destabilized higher-lying LUMO necessitates
a more negative reduction potential.
Efforts to heterogenize 1 by incorporation into the COF-2,2′-bpy framework
containing unmetallated 2,2′-bpy moieties led to the isolation of COF-2,2′-bpy-Re.
A series of FTIR and XPS experiments showed the presence of well-defined
rhenium sites in a single homogeneous environment. The ratio of Re incorporation
is varied between 0.21 and 29.38 wt% Re. When COF-2,2′-bpy-Re is utilized as a
composite with carbon black and PVDF, only the sample with the highest rhenium
concentration, 29.38 wt% Re, was shown to be active for ERC to CO at −2.8 V.
Following a 30 min CPE, CO was produced with a faradaic efficiency of 81%. The
electrocatalytic activity of the composite was diminished after 30 min of
electrolysis, and attributed to inhibited mass transport and substrate diffusion.

2.4. Experimental Section
2.4.1 General considerations
All manipulations of air and moisture sensitive materials were conducted under a
nitrogen atmosphere on a dual manifold Schlenk line. The glassware was oven-
dried prior to use. Acetonitrile and toluene were degassed with nitrogen and passed
through activated alumina columns and stored over 4 Å Linde-type molecular
sieves. Deuterated solvents were dried over 4 Å Linde-type molecular sieves prior
to use.
1
H NMR spectra were acquired at room temperature using Varian
spectrometers and referenced to the residual
1
H resonances of the deuterated

63
solvent (
1
H: CDCl
3
, d 7.26 ppm; DMSO-d
6
, δ 2.50 ppm). Elemental analyses
were performed by Robertson Microlit Laboratories, Ledgewood, New Jersey. All
the chemical reagents purchased from commercial vendors and were used without
further purification. Commercially available tetrabutylammonium
hexafluorophosphate ([nBu
4
N][PF
6
]) was recrystallized from hot methanol prior to
use. Compounds 2,2'-bpy-5,5'-diamine
31, 33
and 2,4,5-trihydrobenzene-1,3,5-
tricarbaldehyde,
34
and the porous framework COF-2,2'-bpy
5
were prepared
according to the reported literature procedures. The isolation of 2,2'-bpy-5,5'-
diamine from the corresponding dihydrochloride was performed with slight
modifications from the reported literature procedures.
33
The isolated
dihydrochloride was dissolved in the minimal amount of water and the solution
was treated with concentrated ammonia. This treatment allowed for the formation
of needle-like crystals of the diamine (70 %).
2.4.2. TOF
CPE
calculations from controlled potential electrolysis
35
Equation (1) was used to determine TOF from CPE data, as previously reported.
9, 10
This
equation assumes that electron transfer to the catalyst is fast, obeying the Nernst law. In
eq 6, i is the stable current transferred during CPE (i =charge*FE/time, C/s), F is
Faraday’s constant (F=96 485 C/mol), A is the surface area of the working electrode (A =
3 cm
2
for CPE), k
cat
is the overall rate constant of the catalytic reaction, D is the diffusion
coefficient (~1 × 10
–5
cm
2
/s

), [cat] is the concentration of the catalyst without substrate
([cat] = 0.5 mM = 5 × 10
–7
mol/cm
3
). The value of D = ~ 1 × 10
–5
cm
2
/s was chosen based
on the previous assessment of D for Re(2,2'-bpy(4,4'-tBu)(CO)
3
Cl and Mn(2,2'-bpy(4,4'-
tBu)(CO)
3
Br which was found to be 1.1 × 10
–5
cm
2
/s.
11, 36

64
!"#= !
!"#
=
!
!
!
!
!
!
!!"#
!

2.4.3. Physical methods
Powder X-ray diffraction (PXRD) was performed on a Rigaku Ultima IV X-ray
diffractometer in reflectance parallel beam/parallel slit alignment geometry. The
measurement employed Cu Ka line focused radiation at 1760 W (40 kV, 44 mA)
power and a Ge crystal detector fitted with a 0.6 mm radiation entrance slit.
Samples were mounted on zero-background sample holders. Samples were
observed using a 1° 2q step scan from 2.0° to 60.0° with an exposure time of 11.4 s
per step. No peaks could be resolved from the baseline for 2q > 35°.
Inductively coupled plasma optical emission spectroscopy (ICP-OES)
measurements were performed using a Thermo Scientific iCAP 7000 ICP-OES. A
1000 ppm rhenium standard in nitric acid (Sigma Aldrich) was used to construct a
calibration plot.
XPS data were collected using a Kratos AXIS Ultra instrument. The
monochromatic X-ray source was the Al Ka line at 1486.6 eV, directed at 35° to
the sample surface (55° off normal). Emitted photoelectrons were collected at an
angle of 35° with respect to the sample surface (55° off normal) by a hemispherical
analyzer. The angle between the electron collecting lens and the X-ray source is
71°. Low-resolution survey spectra were acquired between binding energies of 1-
1100 eV. Higher-resolution detailed scans, with a resolution of ~ 0.8 eV, were
collected on individual XPS lines of interest. The sample chamber was maintained
at < 2 ´ 10
–9
Torr. The XPS data were analyzed using the CasaXPS software.

65
Brunauer−Emmett−Teller (BET) was collected using a Nova 2200e surface area
and pore size analyzer (Quantachrome Instruments, Inc.). Materials were degassed
for 2 h at 150 °C in vacuum before measurement.

2.4.4. Computational Methods
All calculations were run using the Q-CHEM program package.
37
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.
38-45
All optimized geometries were
verified as stable minima with frequency calculations at the same level of theory.
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.
46
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).
47
Kohn-Sham orbital images are
presented with isovalues of 0.05 for clarity.

2.4.5. Synthesis of Re(2,2'-bpy-5,5'-diamine)(CO)
3
Cl (1)
A 100 mL oven dried 3-neck flask fitted with a reflux condenser, adaptors, and a
stir bar was allowed to cool down under vacuum. The flask was then refilled with
nitrogen and charged with Re(CO)
5
Cl (109 mg, 0.30 mmol) and toluene (45 mL).
A warm (50 °C) solution of 5,5'-diamino-2,2'-bpy (55.9 mg, 0.30 mmol) in

66
methanol (5 mL) was cannula transferred to the 3-neck flask. The reaction mixture
was refluxed for 1 h. The color of the reaction mixture changed from light to bright
yellow characteristic of the formation of the Re(CO)
3
moiety. The reaction mixture
was allowed to cool down to room temperature and then placed in the freezer (–
29°C) overnight. The precipitate formed and was collected on a glass frit, washed
with 1 mL of cold methanol and dried under vacuum to afford a bright yellow solid
(141 mg, yield = 95%).
1
H NMR (400 MHz, DMSO-d
6
), δ 8.22 (d, 2H,
4
J = 2.4
Hz), 7.96 (d, 2H,
3
J = 8.8 Hz), 7.21 (dd, 2H,
3
J = 8.8 Hz,
4
J = 2.4 Hz), 6.20 (s, 4H).
Anal. Calcd for C
13
H
10
ClN
4
O
3
Re: C, 31.74; H, 2.05, N, 11.39. Found: C, 31.94; H,
1.97; N, 11.35. X-ray quality crystals were grown by vapor diffusion of a diethyl
ether/dimethylformamide mixture at room temperature over the course of 1 day.

2.4.6. Synthesis of Re(2,2'-bpy-5,5'-diamine)(CO)
3
(MeCN)(OTf) (1
OTf
)
A reported procedure was followed.
22
A 3-neck flask fitted with a reflux
condenser, adaptors, and a stir bar was charged with Re(2,2′-bpy-5,5′-
diamine)(CO)
3
Cl (1) (24.9 mg, 0.051 mmol), AgOTf (13.0 mg, 0.051 mmol), and
acetonitrile (5 mL). The

flask was covered with aluminium foil and the reaction
mixture was allowed to stir under reflux for 24 h, during which a white precipitate
formed. The supernatant was filtered, and diffusion with diethyl ether resulted in
the formation of amber

crystals, that were washed and dried (24.0 mg, yield =
79%).
1
H NMR (400 MHz, MeCN-d
3
) δ 8.33 (d, 2H), 7.85 (d, 2H), 7.34 (dd, 2H),
5.05 (s, 4H), and 2.09 (s, 3H, CH
3
CN). Anal. Calcd for C
16
H
13
F
3
N
5
O
6
ReS: C,
29.72; H, 2.03; N, 10.83. Found: C, 30.12; H, 2.04; N, 10.86.

67

2.4.7. Synthesis of Zn(2,2′-bpy-5,5′-diamine)(CH
3
COO)
2
·H
2
O
A solution of 2,2′-bpy-5,5′-diamine (9.3 mg, 0.050 mmol) in 1 mL of MeOH was
added to a vial with a solution of Zn (CH
3
COO)
2
·2H
2
O (21.9 mg, 0.100 mmol) in
1 mL of water at room temperature. The reaction mixture was stirred for 1 h.
Methanol was allowed to evaporate over the period of 2 days, leading to the
appearance of colourless crystals, that were washed with water and dried under
vacuum at 100 °C

(16.3 mg, 84%).
1
H NMR (400 MHz, DMSO-d
6
) δ 8.01 (d,
2H), 7.97 (d, 2H), 7.23 (dd, 2H), 6.09 (s, 4H), and 1.79 (s, 6H, CH
3
COO). Anal.
Calcd for C
14
H
18
N
4
O
5
Zn: C, 43.37; H, 4.68; N, 14.45. Found: C, 43.30; H, 4.36;
N, 14.32.
2.4.8. Typical synthesis of COF-2,2′-bpy-Re
Solid COF-2,2′-bpy (87.1 mg) was transferred to a 100 mL round bottom flask
equipped with a magnetic stir bar and a reflux condenser. A solution of Re(CO)
5
Cl
(108.6 mg, 1.5 equivalents) in toluene (10 mL) was added to the reaction

mixture
and heated at reflux temperature for 3 days. A color change was observed from
red-orange to dark-red. The resulting powder was collected on a medium porosity
frit and washed with acetone (3 × 10 mL), dichloromethane (3 × 10 mL),
tetrahydrofurane (3 × 10 mL), and acetone (3 × 10 mL). Additionally, the powder
was washed with copious amounts of acetone to remove unreacted starting
materials. The resulting powder was dried at 150 °C under high vacuum for 4 h.
According to ICP-OES the sample contains 29.38 mass% of Re.

For other wt%

68
Re, the synthesis was performed analogously to the original synthesis of COF-2,2′-
bpy-Re (see above), with the exception that the following equivalents of the
rhenium precur-sor were used: 0.01, 0.05, 0.15, 1.0 equivalents, or 0.7, 3.6,
10.9, and 72.4 mg, respectively. According to ICP-OES the samples contain 0.21,
0.65, 1.86, and 15.39 wt% Re, respectively.

2.4.9. Electrode fabrication (carbon ink method)
Glassy carbon plate electrodes (6 cm × 1 cm × 0.3 cm, Tokai Carbon Co., Ltd.)
were polished with MicroPolish Powder 0.05 micron (CH Instruments, Inc.),
rinsed with Millipore water and acetone, and dried. A carbon black/PVDF paste
was prepared by combining 70 mg of the commercial carbon black material
(Vulcan XC-72R; Fuel Cell Store), 10 mg poly(vinylidene fluoride) (Sigma-
Aldrich), 20 mg COF-2,2'-bpy-Re and 4 mL of freshly distilled N-methyl-2-
pyrrolidone (NMP). The resulting mixture was sonicated for 3 hours. Acetonitrile
(10 µL) was added to the 6 cm × 1 cm electrode surface area, followed by the
addition of 40 µL of the carbon black/PVDF paste per 2.5 cm
2
. The electrode was
allowed to dry under vacuum for 2 hours.

2.5. Electrochemical methods
Electrochemistry experiments were carried out using a Pine WaveDriver 20
Bipotentiostat. The electrochemical experiments were performed in a three-
electrode configuration electrochemical cell under N
2
or CO
2
saturated
atmospheres in 0.1 M tetrabutylammonium hexafluorophosphate acetonitrile

69
solution using glassy carbon electrodes as the working electrode. Cyclic
voltammetry experiments of 1 (0.5 mM) were carried out in a single compartment
cell using a 3 mm glassy carbon electrode as the working electrode, platinum wire
purchased from Alfa Aesar as the auxiliary electrode, and silver wire as the
reference electrode. Controlled potential electrolysis (CPE) experiments of 1 and
all electrochemical measurements for COF-2,2'-bpy and COF-2,2'-bpy-Re were
conducted in a two-chambered H cell. The first chamber held the working and
reference electrodes in 50 mL of 0.1 M tetrabutylammonium hexafluorophosphate
in acetonitrile. The second chamber held the auxiliary electrode in 20 mL of 0.1 M
tetrabutylammonium hexafluorophosphate in MeCN. The two chambers were
separated by a fine porosity glass frit. The reference electrode (silver wire) was
placed in a separate compartment charged with 0.1 M tetrabutylammonium
hexafluorophosphate in acetonitrile, and connected by a Vycor tip. Glassy carbon
plate electrodes (6 cm × 1 cm × 0.3 cm; Tokai Carbon USA) were used as the
working and auxiliary electrodes. All electrochemical experiments presented here
were referenced relative to ferrocene (Fc) with the Fe
3+/2+
couple at 0.0 V.
Decamethylferrocene (Fc*) with the Fe
3+/2+
couple at –0.48 V was used as an
internal standard.
Gas analysis for controlled potential electrolysis experiments were performed
using 2 mL sample taken from the headspace of the electrochemical cell and
injected into a gas chromatography instrument (Shimadzu BID-2010 plus series
gas chromatograph) equipped with a BID detector and a 2 m × 1 mm Restek
ShinCarbon ST Micropacked column. Faradaic efficiencies were determined by

70
dividing the measured CO produced by the amount of CO expected based on the
charge passed during the bulk electrolysis experiment. For each species the
controlled-potential electrolysis measurements were performed at least twice,
leading to similar behavior. The reported Faradaic efficiencies and mmol and mL
of CO produced are average values.

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Hehre, H. Schaefer, J. Kong, A. Krylov, P. Gill and M. Head-Gordon, Phys.
Chem. Chem. Phys., 2006, 8, 3172.
(38) R. Ditchfield, W. Hehre and J. Pople, J. Chem. Phys., 1971, 54, 724.
(39) P. Harihara and J. Pople, Theor. Chim. Acta, 1973, 28, 213.
(40) W. Hehre, R. Ditchfield and J. Pople, J. Chem. Phys., 1972, 56,
2257.
(41) D. Feller, J. Comput. Chem., 1996, 17, 1571.
(42) K. Schuchardt, B. Didier, T. Elsethagen, L. Sun, V. Gurumoorthi, J.
Chase, J. Li and T. Windus, J. Chem. Inf. Model., 2007, 47, 1045.
(43) J. Towns, T. Cockerill, M. Dahan, I. Foster, K. Gaither, A.
Grimshaw, V. Hazlewood, S. Lathrop, D. Lifka, G. Peterson, R. Roskies, J.
Scott and N. Wilkins-Diehr, Comput. Sci. Eng., 2014, 16, 62.
(44) P. Hay and W. Wadt, J. Chem. Phys., 1985, 82, 270.
(45) W. Wadt and P. Hay, J. Chem. Phys., 1985, 82, 284.
(46) Y. Zhao and D. Truhlar, Theor. Chem. Acc., 2008, 120, 215.
(47) A. Klamt and G. Schuurmann, J. Chem. Soc., Perkin Trans. 1, 1993,
799.

73

CHAPTER 3

Synthesis and Electrochemical CO
2
Reduction Activity of a Ruthenium (II) Terpyridine
Complex Featuring Ancillary Hydroxyl Groups

74
3.1. Introduction
Improvements for transition metal catalysts can be realized by the addition of non-
innocent functional groups in the second coordination sphere of the ligand
environment.
1, 2
Generally, these functional groups include pendant amines,
3, 4

ancillary phenolic groups,
5-7
and positively charged quaternary amines.
8
Examples
of the “state-of-the-art” catalysts for CO production reported so far are Re and
Mn(2,2'-bpy)(CO)3X (2,2'-bpy = 2,2'-bipyridine, X = halide, or a labile ligand, or
a pseudohalide) hydroxyphenyl substituted Fe porphyrins, and Ni(cyclam)
2+

(cyclam = 1,4,8,11-tetraazocyclotetradecane).
5, 9-13

In general, molecular catalysts based on ruthenium have shown lower activities
than these “state-of-the-art” catalysts.
14, 15
Under reducing conditions, complexes
of the type Ru(2,2'-bpy)(CO)2Cl2 are known to form dimers or to polymerize on
the cathode via the formation of Ru-Ru bonds. Bulky substituents on 2,2'-bpy such
as mesityl- were used to prevent the dimerization.
16

Utilizing 2,2’:6’,2’’-terpyridine (tp) as a ligand platform as an alternative to 2,2'-
bpy ligands with bulky substituents can provide a way of supressing dimerization
in Ru complexes. This ligand was shown to stabilize the Mn metal center in its
complex with a Mn(I) tricabonyl moiety.
17
This enabled an alternate pathway at
lower reduction potentials, which is reminiscent of Mn(I) tricabonyl complexes
bearing 2,2'-bpy with bulky substituents such as mesityl-.
18

We were interested a tp-type ligand system featuring two hydroxyl functional
groups in the 6,6’ positions of tp prepared previously.
19
Use of pendant proton
relays in the second coordination sphere is instrumental in improving ECR

75
catalysts.
1-4, 20-23
Being a proton responsive ligand, the dihydroxyl terpyridine
(dhtp) is attractive in this context. It can provide proton donors and acceptors due
to the tautomerism between its hydroxyl- and pyridone tautomers. This ligand
features 2-hydroxypyridine fragments, which are important in modulating efficient
CO
2
hydrogenation/dehydrogenation pathways,
24, 25
and other CO
2

transformations.
26
This moiety is occasionally found in transition metal complexes
investigated for electrocatatalytic CO
2
reduction.
27, 28
We report here the
preparation, characterization, and electrocatalytic CO
2
reduction reactivity of the
Ru complex based on dhtp.

3.2. Results and Discussion
Reaction of [Ru(CO)
2
Cl
2
]
n
with 1 equiv of 6,6''-dihydroxy 2,2':6'-2''-terpyridine
(dhtp) in methanol under reflux (Scheme 3.1) followed by solvent evaporation and
recrystallization from a mixture of methanol and water (3:1 ratio) led to the
formation of yellow bright crystals of 3.

Scheme 3.1. Synthesis of [Ru(dhtp)(CO)
2
(Cl)](Cl) (3)

N
N
N
OH OH
+
[Ru(CO)
2
Cl
2
]
n
N
N
N
OH OH
Ru
CO
Cl
OC
MeOH
reflux
[Ru(dhtp)(CO)
2
Cl]
+
Cl
–
(3)
+
Cl
–

76
The solid state structure of 3·DMF together with its atomic numbering scheme is
depicted in Figure 3.1, which reveals an octahedral coordination geometry around
the ruthenium (II) metal center with the terpyridine nitrogen atoms occupying 3
equatorial positions, and the chloride ligand occupying one axial position. The
mutually cis carbonyl ligands occupy the remaining axial and equatorial positions.
Selected interatomic distances and refinement parameters for 3 are listed in Tables
3.1 and 3.2, respectively. FTIR of 3 in the form of a pellet with KBr pellet shows
two major frequencies in the CO region at 2069 and 2004 cm
–1
, and two minor
frequencies at 2038 and 1952 cm
–1
most likely occurring due to the Fermi
resonance (Figure 3.2). The presence of two major frequencies in the CO region
confirms the integrity of the dicarbonyl coordination environment of the ruthenium
center after ligand binding.

Figure 3.1. Solid-state structure and atomic numbering scheme for 3·DMF, DMF
omitted for clarity.

77
Table 3.1. Selected Interatomic Distances (Å) for 3·DMF

Bond Bond length (Å)
Ru(1)-C(1) 1.9167(19)
Ru(1)-C(2) 1.9307(17)
Ru(1)-Cl(1) 2.4269(6)
Ru(1)-N(1) 2.1026(14)
Ru(1)-N(2)
Ru(1)-N(3)
2.0268(14)
2.1089(14)
C(1)-O(1) 1.094(2)
C(2)-O(2) 1.134(2)

78
Table 3.2. Crystal data and structure refinement for 3

Chemical formula C
20
H
18
Cl
2
N
4
O
5
Ru
Formula weight 566.35
Crystal system triclinic
Space group P –1
a (Å) 6.8860(17)
b (Å) 12.358(3)
c (Å) 14.030(3)
α (°) 68.210(3)
β (°) 76.506(4)
γ (°) 78.934(4)
V (Å
3
) 1070.6(5)
Z 2
D
calc
(g/cm
3
) 1.757
µ (Mo Kα) (mm
–1
) 1.023
F (000) 568
Reflections collected 18225
Independent reflections 6387 [R(int) = 0.0297]
R
1
(I > 2σ(I)) 0.0250
wR
2
(all data) 0.0581
Goodness-of-fit (GOF) on F
2
1.048

79

Figure 3.2. FTIR of 3 in the form of a pellet with KBr

CVs of 3 in a 0.1 M solution of [nBu
4
N][PF
6
] in dimethylformamide (DMF) under
a N
2
atmosphere displayed 3 irreversible reduction events at –1.77 V, –2.13, and –
2.44 V vs. Fc
+/0
, termed 1
st
, 2
nd
, and 3
rd
reductions, respectively, and 1 oxidation
event at –1.79 V vs. Fc
+/0
(Figure 3.3). The oxidation event at –1.79 V vs. Fc
+/0
is
not observed (Figure 3.4) when scan is reversed after the 2
nd
reduction, suggesting
that this oxidation event corresponds to the oxidation of species formed at the 3
rd

reduction
.
The CV of 3 is typical of an EEC mechanism where chloride
dissociation is the chemical step due to the agreement with data published for
Re(CO)
3
Cl complexes of various 2,2'-bipyridine derivatives in DMF.
27, 29
The
formation of these species depends on the concentration of chloride anion as the
addition of a large excess of a chloride source (nBu
4
NCl) leads to the
disappearance of the oxidation event at –1.79 V vs. Fc
+/0
in the CV (Figure 3.5).
This suggests that the reduced species formed at the 3
rd
reduction exists in
equilibrium between Cl
–
and other species, which are oxidized at –1.79 V vs. Fc
+/0
.

80
Therefore, this equilibrium is likely a dissociation reaction. Moreover, none of the
redox events exhibited increased reversibility upon increasing the scan rate (Figure
3.6), and the cathodic peak current densities at the 2
nd
and 3
rd
reductions are
directly proportional to the square root of the scan rate (Figure 3.7), as expected
for a freely diffusing species in solution obeying the Randles-Sevcik equation.

Figure 3.3. Cyclic voltammogram of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere at a scan rate of 100 mV/s.

81

Figure 3.4. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere. Solid curve: scan reversed after the
second reduction, dashed curve: scan reversed after the first reduction.

82

Figure 3.5. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere with (blue curve) and without (black
curve) added chloride source nBu
4
NCl.

Figure 3.6. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere at scan rates ranging from 50 to 1000
mV/s.

83

Figure 3.7. Plot showing the peak cathodic current density at –2.13 V (left) and –2.44 V
(right) vs. Fc
+/0
as a function of the square root of the scan rate. The cathodic peak current
density was obtained from the cyclic voltammetry data of 3 (0.5 mM) in DMF solution
containing 0.1 M [nBu
4
N][PF
6
] under an atmosphere of N
2
. The cathodic peak current
density increases linearly with the square root of the scan rate indicative of a freely
diffusing species obeying the Randles-Sevcik equation.

Considering 3 contains two hydroxyl groups, the addition of two equivalents of a
strong organic base is expected to chemically deprotonate 3 in-situ. As a result of
the addition of nBu
4
NOH, serving as an organic base, a bright yellow solution of 3
in DMF containing 0.1 M of [nBu
4
N][PF
6
] turned dark-green under a N
2

atmosphere. In addition, the CV of the green solution does not exhibit the broad 1
st

reduction typical of the yellow solution, suggesting this broad reduction is due to
reductive deprotonation of 3 (Figure 3.8). Similar observations were reported by
Fujita et al. for Re and Ru complexes containing 2,2'-bipyridine decorated with
hydroxyl groups. This led to the assignment of the 1
st
reduction as a reductive
deprotonation of 3. Moreover, the CV of chemically deprotonated 3 exhibits a
reduction event at –2.13 V vs. Fc
+/0
, identical to the 2
nd
reduction in CVs of 3, and
an oxidation event at –1.75 V vs. Fc
+/0
.

84

Figure 3.8. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere with (green curve) and without (black
curve) added base nBu
4
NOH.

The CV of 3 under a CO
2
atmosphere (Figure 3.9) displays a slight current
increase at the 2
nd
reduction followed by a further current increase peaking at –
2.35 V vs. Fc
+/0
. This suggests that a catalytic reaction is taking place. While the
magnitude of the current exhibited by 3 under a CO
2
atmosphere is not affected by
the addition of H
2
O (5% by volume), the peak potential shifts more positively and
observed at –2.30 V vs. Fc
+/0
(Figure 3.10). Addition of PhOH affects neither the
current nor the peak potential exhibited by 3 under a CO
2
atmosphere.
Additionally, the chemically deprotonated 3 exhibits a similar behavior with the
current under a CO
2
atmosphere peaking at –2.37 V vs. Fc
+/0
(Figure 3.11).

85

Figure 3.9. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere (black curve) and a carbon dioxide
atmosphere (red curve) at a scan rate of 100 mV/s.

Figure 3.10. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under a nitrogen atmosphere (black curve) and with 5% of added
water by volume under a carbon dioxide atmosphere (blue curve) at a scan rate of 100
mV/s.

86

Figure 3.11. Cyclic voltammograms of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) and 2 equivalents of added base nBu
4
NOH under a nitrogen
atmosphere (green curve) and a carbon dioxide atmosphere (maroon curve) at a scan rate
of 100 mV/s.

Controlled potential electrolysis of 3 was performed in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under CO
2
atmosphere (Figure 3.12). 1.853 coulombs of
charge was consumed after 1 hour of electrolysis. Analysis of the gas mixture
confirmed the production of CO with a faradaic efficiency (FE) of 91 % and a
TON of 15000 (Table 3.3), as described in the Experimental Section. We expected
the addition of a strong organic base such as nBu
4
NOH to decrease the activity of
3 toward CO
2
by removing protons from the reaction system. Contrary to this, the
presence of 2 equiv of nBu
4
NOH during the CPE led to a larger amount of charge
(4.021 C) and a higher TON with respect to CO production (48000), albeit a
smaller FE (74 %) (Figure 3.13, Table 3.3). The introduction of 2 equiv of the
base, whose formula nBu
4
NOH·30H
2
O introduces 60 equiv of H
2
O into the
reaction system along with 2 equiv of H
2
O forming during the reaction between

87
the base and the catalyst. The water generation can contribute to the increase in the
stability and decrease in efficiency.

Figure. 3.12. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF
solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
at a potential of –2.37 V vs. Fc
+/0
.
After the CPE, the working electrode was rinsed (3×10 mL DMF) and CPE curve (a
dashed line) was measured with it in a fresh DMF solution containing [nBu
4
N][PF
6
] (0.1
M) under CO
2
at a potential of –2.37 V vs. Fc
+/0
. Top right: black curve represents the
CPE of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under N
2
at a
potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of 3 (0.5 mM) in a DMF solution containing
[nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the CPE.

88

Figure. 3.13. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF
solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added 2 equivalents of base
nBu
4
NOH at a potential of –2.37 V vs. Fc
+/0
. After the CPE, the working electrode was
rinsed (3×10 mL DMF) and CPE curve (a dashed line) was measured with it in a fresh
DMF solution containing [nBu
4
N][PF
6
] (0.1 M) with added 2 equivalents of base
nBu
4
NOH under CO
2
at a potential of –2.37 V vs. Fc
+/0
. Top right: black curve represents
the CPE of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under N
2

with added 2 equivalents of base nBu
4
NOH at a potential of –2.37 V vs. Fc
+/0
. Bottom:
CVs of 3 (0.5 mM) in a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the CPE.

To probe the influence of water, we added 62 equiv of water to 3 (0.5 mM) in a
solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
. After the CPE, an even
larger amount of charge (7.493 C) was consumed compared to the two previous
experiments. The current was stable throughout 1 hour of the experiment with a
TON of 130,000 producing 0.61 mL of CO (Figure 3.14 and Table 3.3). This
larger activity compared to the acid-free experiment is surprising, considering the

89
addition of PhOH did not lead to increased current densities by CV. However,
similar trends were reported by Fujita et al.,
27
where, despite the attenuation of the
CV current, the addition of H
2
O led to the production of larger amounts of CO.
Comparing this experiment with the experiment in Figure 3.13 suggests that the
presence of organic base decreases the activity and stability of 3 under
electrochemical CO
2
reduction conditions.

90

Figure. 3.14. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF
solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added 62 equivalents of H
2
O
at a potential of –2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10
mL DMF) and CPE curve (a dashed line) was measured with it in a fresh DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) with added 62 equivalents of H
2
O under CO
2
at a
potential of –2.37 V vs. Fc
+/0
. Top right: black curve represents the CPE of 3 (0.5 mM) in
a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under N
2
with added 62 equivalents of
H
2
O at a potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the CPE.

Addition of a 62 equiv of PhOH to 3 (0.5 mM) in a solution containing
[nBu
4
N][PF
6
] (0.1 M) under CO
2
led to the production of CO with a faradaic
efficiency of 97 % (Figure 3.15 and Table 3.3). However, the amount of charge
(3.165 C) and the amount of CO (0.38 mL) were smaller than in the presence of
the same number of equivalents of H
2
O. It is also obvious from the CPE curve that

91
a significant drop-off in current down to the background current occurs within the
first 20 min of the CPE. We attributed this observation to the depletion of PhOH
during electrolysis. An excess of PhOH (5 % by mass) was added to 3 (0.5 mM) in
a solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
and the CPE was measured
(Figure 3.16). The observed current is very stable as compared to the experiment
with 62 equiv of added PhOH, and lead to the production of 0.78 mL of CO with a
faradaic efficiency of 94 % and a TON of 220000 (Table 3.3.). In the presence of
excess PhOH, 3 features a higher stability and larger amount of CO than under any
other conditions tested in this work. Wash tests under each conditions tested were
performed and did not indicate the deposition of any catalytically active species for
electrochemical CO
2
reduction. Where necessary, control experiments were
performed under a nitrogen atmosphere demonstrating H
2
evolving activity with
faradaic efficiencies not exceeding 36 %. Carbon containing products were not
detected during these experiments, suggesting that the CO produced is from CO
2

and not from the carbonyl groups present in 3. Additionally, the currents under N
2

were somewhat unstable with the exception of the experiment in the presence of
excess PhOH. This suggests that these conditions promote the stabilization of 3
under reducing conditions, which could be due to more matching pKa values of the
complex and added acid. We were not able to find pKa data for H
2
O and PhOH
DMF, but so we used data reported in DMSO (Bordwell pKa data). According to
these data, pKa(H
2
O) and pKa(PhOH) in DMSO are 31.2 and 18.0, respectively.
This trend likely holds true in DMF as well considering that both DMF and DMSO
are both polar aprotic solvents. Phenol is a stronger acid and its pKa is closer to the

92
range of 4.4 to 7.3 pKa units typical of the complexes containing 2-
hydroxypyridine fragment,
27
thus is a more potent proton source capable of
stabilizing 3.

93

Figure. 3.15. Left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF
solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added 62 equivalents of PhOH
at a potential of –2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10
mL DMF) and CPE curve (a dashed line) was measured with it in a fresh DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) with 62 equivalents of PhOH under CO
2
at a potential
of –2.37 V vs. Fc
+/0
. Top right: black curve represents the CPE of 3 (0.5 mM) in a DMF
solution containing [nBu
4
N][PF
6
] (0.1 M) under N
2
with added 62 equivalents of PhOH
at a potential of –2.37 V vs. Fc
+/0
.

94

Figure. 3.16. Top left: controlled potential electrolysis (CPE) of 3 (0.5 mM) in a DMF
solution containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
with added PhOH (5 % by mass) at
a potential of –2.37 V vs. Fc
+/0
. After the CPE, the working electrode was rinsed (3×10
mL DMF) and CPE curve (a dashed line) was measured with it in a fresh DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) with added PhOH (5 % by mass) under CO
2
at a
potential of –2.37 V vs. Fc
+/0
. Top right: black curve represents the CPE of 3 (0.5 mM) in
a DMF solution containing [nBu
4
N][PF
6
] (0.1 M) under N
2
with added PhOH (5 % by
mass) at a potential of –2.37 V vs. Fc
+/0
. Bottom: CVs of 3 (0.5 mM) in a DMF solution
containing [nBu
4
N][PF
6
] (0.1 M) under CO
2
before and after the CPE.

95
Table 3.3 Controlled potential electrolysis data for 3 (0.5 mM) in 0.1 M TBAPF
6

solution in DMF, CO
2
, –2.37 V vs. Fc
+/0

with various Brønsted acids and bases added
externally.

External Acid/Base Charge, C V (CO), mL V (H
2
), mL
no acid/base 1.853 0.21 0
2 equiv NBu
4
OH·30H
2
O 4.021 0.37 0
62 equiv H
2
O 7.493 0.61 0.11
62 equiv PhOH 3.165 0.38 0.038
5 mass% PhOH 6.735 0.78 0.087

External Acid/Base FE (CO), % FE (H
2
), % TON
CPE
*
no acid/base 91 0 15000
2 equiv NBu
4
OH·30H
2
O 74 0 48000
62 equiv H
2
O 65 6 130000
62 equiv PhOH 97 5 51000
5 mass% PhOH 94 5 220000

* See 3.4.3 for details.

3.3. Conclusions
A molecular Ru carbonyl complex based on 6,6''-dihydroxy 2,2':6'-2''-terpyridine
was synthesized and characterized using NMR, FTIR, and single crystal X-ray
crystallography. The complex is active toward electrochemical CO
2
reduction to
CO. While, the presence of organic base decreases the activity and stability of 3
under electrochemical CO
2
reduction conditions, the presence of excess PhOH
increases both the metrics, suggesting that PhOH promotes CO
2
reduction activity,
which is agreement with the general effect of Brønsted acids on this reaction.

96
3.4. Experimental Section
3.4.1. General Considerations
All manipulations of air and moisture sensitive materials were conducted under a
nitrogen atmosphere in a Vacuum Atmospheres glovebox or on a dual manifold
Schlenk line. The glassware was oven-dried prior to use. Water was deionized with
the Millipore Synergy system (18.2 MW·cm resistivity) and placed under vacuum
and refilled with nitrogen (10 ×). Excluding water, all other solvents used were
degassed with nitrogen and passed through activated alumina columns and stored
over 4Å Linde-type molecular sieves. Oligomeric metal precursor [Ru(CO)
2
Cl
2
]
n

and the ligand 6,6''-dihydroxy 2,2':6'-2''-terpyridine (dhtp)
19
were synthesized
according to literature procedure. Proton NMR spectra were acquired at room
temperature using a Varian 400-MR 2-Channel spectrometer and referenced to the
residual
1
H resonances of the deuterated solvent (
1
H: dmso, δ 2.50 ppm). All other
chemical reagents were purchased from commercial vendors and used without
further purification.

3.4.2. TOF
CPE
calculations from controlled potential electrolysis
31
Equation (1) was used to determine TOF from CPE data, as previously reported.
3, 4

This equation assumes that electron transfer to the catalyst is fast, obeying the
Nernst law. In eq 6, i is the stable current transferred during CPE (i
=charge*FE/time, C/s), F is Faraday’s constant (F=96 485 C/mol), A is the surface
area of the working electrode (A = 2.8 cm
2
for CPE), k
cat
is the overall rate
constant of the catalytic reaction, D is the diffusion coefficient (~2 × 10
–6
cm
2
/s

),

97
[cat] is the concentration of the catalyst without substrate ([cat] = 0.5 mM = 5 ×
10
–7
mol/cm
3
). The value of D = ~ 2.8 × 10
–6
cm
2
/s was assessed using the Randles-
Sevcik equation based on the data in Figure 3.6 and 3.7.
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3.4.3. Synthesis of [Ru(dhtp)(CO)
2
(Cl)](Cl) (3)
This synthesis was performed under an atmosphere of dry nitrogen gas using
standard Schlenk techniques. The reaction mixture containing 94.3 mg (0.356
mmol) of dhtp, 81.1 mg of 0.356 mmol) of [Ru(CO)
2
Cl
2
]
n
,
30
and 25 mL of dry
methanol was refluxed for 24 h in dark. The reaction mixture was then allowed to
cool down to room temperature and then filtered through Celite. Afterwards, the
solvent was removed from the filtrate by rotary evaporation yielding 122.4 mg
(yield = 70%) of light amber colored crude material. The pure material was
obtained from the crude material by recrystallization from a hot mixture of
methanol and water (3:1 ratio of methanol to water) and dried under high vacuum
yielding 44.9 mg (yield = 26%) of bright-yellow colored crystals.
1
H NMR (400
MHz, dmso-d
6
) δ (ppm) 11.95 (br s, 2H), 8.19-8.40 (m, 3H), 7.53 (app t, 2H), 7.44
(d, 2H), 6.50 (d, 2H). Anal. Calcd for C
17
H
11
Cl
2
N
3
O
4
Ru : C, 41.40; H, 2.25; N,
8.52. Found: C, 40.96; H, 2.33; N, 8.10.

98
3.4.4. Physical Methods
FT-IR spectra were acquired using a Bruker Vertex 80v spectrometer. Samples (2
mg) for analysis were mixed into a KBr (100 mg) matrix and pressed into pellets.

3.4.5. Experimental Methods
Electrochemistry experiments were carried out using a Pine potentiostat. Cyclic
voltammetry experiments were carried out in a single compartment cell under N
2

and CO
2
using a 3.0 mm diameter glassy carbon electrode as the working
electrode, platinum wire purchased from Alfa Aesar as the auxiliary electrode, and
silver wire as the reference electrode. Ferrocene was used as an internal standard in
all electrochemical experiments. Electrochemical experiments were carried out in
either 0.1 M TBAPF
6
DMF solutions.
Controlled potential electrolysis experiments were carried out in a two-chambered
H-cell. The first chamber held the working and reference electrodes in 50 mL of 3
(0.5 mM) in 0.1 M TBAPF
6
DMF solution. The second chamber contained the
counter electrode in 0.1 M TBAPF
6
DMF solution. The two chambers were
separated from each by a fine porosity frit. The reference electrode was placed in a
separate compartment and connected by a Vycor frit. Glassy carbon plate
electrodes (6 cm × 1 cm × 0.3 cm; Tokai Carbon USA) were used as the working
and auxiliary electrodes. Ferrocene was used as an internal standard for all
controlled potential electrolysis experiments. Using a gas-tight syringe, 2 mL of
gas were withdrawn from the headspace of the H cell and injected into a gas
chromatography instrument (Shimadzu GC-2010-Plus) equipped with a BID

99
detector and a Restek ShinCarbon ST Micropacked column. To determine the
Faradaic efficiency, the theoretical CO
2
amount based on total charge flowed is
compared with the GC-detected CO
2
produced from controlled-potential
electrolysis.

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Abstract (if available)

Abstract The conversion of CO₂ into higher energy products such as carbon-based fuels and feedstocks is an attractive strategy for mitigating the continuous rise in CO₂ emissions associated with the growing global energy demand. Molecular catalysts based on Re (I) and Ru (II) carbonyls supported on polypyridine ligands have been studied as catalysts for CO₂ capture and electrochemical conversion. Catalysts of this type lead to the production of CO, which is a precursor for synthetic carbon-based fuels. In this work Re-2,2'-bipyridine catalysts were heterogenized through integration into covalent organic frameworks and their electrochemical CO₂ reduction activity was studied. In addition, Ru(II) carbonyl fragments were supported on a terpyridine platform containing acidic hydroxyl groups in the proximal position to the metal center. Both the catalytic systems explored in this study were shown to perform CO₂ reduction to CO with moderate to high faradaic efficiencies.

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Asset Metadata

Creator Popov, Damir (author)

Core Title Design, synthesis, and study of polypyridine based molecular and heterogenized molecular electrocatalysts for CO₂ reduction

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 09/18/2019

Defense Date 05/29/2019

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

Tag carbon dioxide,carbon monoxide,catalysis,electrochemistry,fuels,OAI-PMH Harvest,synthesis

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Marinescu, Smaranda (committee chair), Gupta, Malancha (committee member), Haiges, Ralf (committee member)

Creator Email damir.a.popov@gmail.com,damirpop@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-220403

Unique identifier UC11673400

Identifier etd-PopovDamir-7817.pdf (filename),usctheses-c89-220403 (legacy record id)

Legacy Identifier etd-PopovDamir-7817.pdf

Dmrecord 220403

Document Type Dissertation

Rights Popov, Damir

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA