Modifying the steric and electronic character within Re(I)-phenanthroline complexes for electrocatalytic CO2 reduction

Article abstract of DOI:10.1016/j.ica.2018.04.008

We have synthesized and characterized a series of fac-[Re(R₂phen)(CO)₃Cl] complexes (R₂phen = 2,9-disubstituted-1,10-phenanthroline) that function as electrocatalysts for the reduction of CO₂ to CO. The 2,9-disubstituted phenanthroline ligands contain proton (phen), methyl (2,9-Me₂phen), trimethylphenyl (Mes₂phen), or trimethoxyphenyl ((2,4,6-tmp)₂phen and (3,4,5-tmp)₂phen) groups as steric and electronic modifiers to provide insight into factors impacting catalytic activity. Cyclic voltammograms (CVs) recorded in CO₂-saturated CH₃CN or DMF solutions reveal that following two-electron reduction and chloride dissociation to form the active [Re(R₂phen)(CO)₃]^? intermediate, current enhancement indicative of CO₂ reduction to CO was observed and confirmed by controlled potential electrolysis (CPE). Using current enhancement values (i_cat/i_p, where i_cat and i_p are the current response under CO₂ and N₂, respectively) to estimate catalytic activity, it was observed that catalysts with more cathodic Re^I/0 potentials displayed greater activity, in accord with an electronic effect driving catalysis. Interestingly, the trimethoxyphenyl-substituted complexes Re((2,4,6-tmp)₂phen) and Re((3,4,5-tmp)₂phen) showed different degrees of activity in DMF (i_cat/i_p = 14.8 and 6.0, respectively) and CH₃CN (i_cat/i_p = 8.5 and 7.7, respectively). These results suggest that while electronics dominate the observed catalytic activity within this fac-[Re(R₂phen)(CO)₃Cl] architecture, factors such as sterics and solvent play an important role as well.

Full text of DOI:10.1016/j.ica.2018.04.008

Accepted Manuscript
Research paper
Modifying the steric and electronic character within Re(I)-phenanthroline com-
plexes for electrocatalytic CO2 reduction
Bertrand J. Neyhouse, Travis A. White
PII:
S0020-1693(18)30066-5
https://doi.org/10.1016/j.ica.2018.04.008
ICA 18199
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
15 January 2018
5 April 2018
6 April 2018
Please cite this article as: B.J. Neyhouse, T.A. White, Modifying the steric and electronic character within Re(I)-
phenanthroline complexes for electrocatalytic CO2 reduction, Inorganica Chimica Acta (2018), doi: https://doi.org/
10.1016/j.ica.2018.04.008
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Modifying the steric and electronic character within
Re(I)-phenanthroline complexes for electrocatalytic CO2
reduction
Bertrand J. Neyhouse and Travis A. White*
Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio
University, Athens, Ohio 45701, United States
Abstract: We have synthesized and characterized a series of fac-[Re(R2phen)(CO)3Cl]
complexes (R2phen = 2,9-disubstituted-1,10-phenanthroline) that function as electrocatalysts for
the reduction of CO2 to CO. The 2,9-disubstituted phenanthroline ligands contain proton (phen),
methyl (2,9-Me2phen), trimethylphenyl (Mes2phen), or trimethoxyphenyl ((2,4,6-tmp)2phen
and (3,4,5-tmp)2phen) groups as steric and electronic modifiers to provide insight into factors
impacting catalytic activity. Cyclic voltammograms (CVs) recorded in CO2-saturated CH3CN or
DMF solutions reveal that following two-electron reduction and chloride dissociation to form the
active [Re(R2phen)(CO)3]− intermediate, current enhancement indicative of CO2 reduction to CO
was observed and confirmed by controlled potential electrolysis (CPE). Using current
enhancement values (icat/ip, where icat and ip are the current response under CO2 and N2,
respectively) to estimate catalytic activity, it was observed that catalysts with more cathodic ReI/0
potentials displayed greater activity, in accord with an electronic effect driving catalysis.
1

Interestingly, the trimethoxyphenyl-substituted complexes Re((2,4,6-tmp)2phen) and Re((3,4,5-
tmp)2phen) showed different degrees of activity in DMF (icat/ip = 14.8 and 6.0, respectively) and
CH3CN (icat/ip = 8.5 and 7.7, respectively). These results suggest that while electronics dominate
the observed catalytic activity within this fac-[Re(R2phen)(CO)3Cl] architecture, factors such as
sterics and solvent play an important role as well.
Keywords: electrocatalytic CO2 reduction, rhenium(I) catalyst, phenanthroline, sterics,
electronics
1. Introduction
Carbon dioxide is a thermodynamically stable, kinetically inert product of carbon-based fuel
combustion that can serve as a raw material to produce high-value commodities [1, 2]. CO2
reduction products such as carbon monoxide (CO) and formic acid (HCOOH) are valuable
compounds; CO is a starting material in the Fischer-Tropsch process to produce synthetic fuels
and HCOOH has been proposed as a method for storing hydrogen (H2) fuel in the liquid phase.
Given the energetically-demanding nature for the 1e− reduction of CO2 to form the CO2−• radical
anion (E0′ = −1.90 V vs. NHE, pH = 7, 25 °C), accessing the 2e−/2H+ reduction pathway provides
a more thermodynamically accessible route to produce CO (E0′ = −0.53 V vs. NHE) [2].
Molecular electrocatalysts are attractive options given the ability to fine-tune the steric and
electronic environment surrounding the catalytically-active center with the aim of producing
rapid and selective catalysts [2-7]. Furthermore, catalysts possessing multiple redox-active
components can be designed to deliver multiple electrons and protons to CO2, avoiding high-
energy intermediates that may hinder catalysis.
2

Within the scope of molecular electrocatalysts for CO2 reduction, several architectures have
proven to be active, selective, and capable of synthetic modifications, with a few examples
including tetraazamacrocycles [8-13], phosphines [14-16], and diimines [4, 17]. Early reports
illustrating the selective reduction of CO2 to CO by fac-[Re(bpy)(CO)3Cl] (bpy = 2,2′-
bipyridine) established the foundation for this Re(I)-NN architecture (NN = bidentate diimine
ligand) to be further studied [18, 19]. Cathodic cyclic voltammograms typically result in multi-
electron reduction and monodentate ligand loss (e.g. halide, CH3CN, triflate) to afford a
coordinately-unsaturated, five-coordinate [Re(NN)(CO)3]− intermediate readily available to
transfer electrons to the CO2 substrate. Scheme 1 highlights a proposed catalytic cycle for CO2
reduction, which proceeds via two-electron reduction to activate the catalyst. Multiple research
groups have established the non-innocent nature of the bipyridine ligand by incorporating a
range of substituents to emphasize the effect of steric and/or electronic contributions within the
primary and secondary coordination spheres towards catalysis [20-28].
Scheme 1. One proposed catalytic cycle for CO2 reduction using fac-[Re(diimine)(CO)3Cl]
complexes.
3

As shown in Scheme 1, the proposed catalytic cycle involves coordination of CO2 to the
[Re(NN)(CO)3]−
five-coordinate
intermediate
to
form
a
metallocarboxylate
[Re(NN)(CO2)(CO)3]− intermediate. The metallocarboxylate (Re-CO2) undergoes rapid
protonation to form a metallocarboxylic acid structure, [Re(NN)(CO2H)(CO)3] (Re-CO2H), that
can be further protonated to evolve water and a tetracarbonyl rhenium species, [Re(NN)(CO)4]+
(Re-CO), prior to CO evolution [29-34]. Recent efforts to enhance activity of Re(I)-bipyridine
and Mn(I)-bipyridine congeners have focused on modifying the secondary coordination sphere
with Brønsted acidic substituents (e.g. hydroxyphenyl, imidazolium) [35-39]. This approach
stabilizes the metallocarboxylate intermediate via intramolecular hydrogen bonding, resulting in
a weakened C–O bond that rapidly dissociates water following protonation. A recent publication
further extended the secondary coordination sphere concept in a Mn(I)-bipyridine to include
methoxyphenyl Lewis bases which were capable of stabilizing the proposed metallocarboxylic
acid intermediate that forms in the presence of weak Brønsted acids (e.g. 2,2,2-trifluoroethanol,
phenol) [40]. While multiple ligand architectures deviating from substituted Re(I)-bipyridines
have been recently reported to function as CO2 reduction electrocatalysts [35, 41-46],
surprisingly few reports exist describing Re(I)-phenanthroline analogues as catalysts [47-49].
Herein we report the synthesis, characterization, and electrocatalytic CO2 reduction activity
for a series of Re(I) catalysts of the design fac-[Re(R2phen)(CO)3Cl], where R2phen is a 2,9-
disubstituted-1,10-phenanthroline ligand (Figure 1). The substituents varied from possessing no
steric bulk and minimal electronic contribution towards the Re(I) catalytic site (Re(phen)) to
possessing steric hindrance and electron-donating substituents with possible weak hydrogen-
bonding interactions (Re((2,4,6-tmp)2phen)). In place of the more commonly used 2,2′-
bipyridine architecture, substituted phenanthrolines were chosen given that the ligand’s
4

backbone can be easily modified [50] to incorporate surface-anchoring substituents for
heterogeneous catalysis [51-53]. Therefore, we sought to investigate the catalytic activity of the
1,10-phenanthroline architecture using various substituents at the 2 and 9 positions with the aim
of future ligand expansion.
Figure 1. Structures of ligands and the corresponding synthesis of Re(I) electrocatalysts.
2. Experimental Details
2.1. Materials
All
materials
were
used
as
received
unless
otherwise
indicated.
Pentacarbonylchlororhenium(I) (98%) and 2,4,6-trimethylphenylboronic acid (95%) were
purchased from Acros Organics. 1,10-Phenanthroline (97%), 2,9-dichloro-1,10-phenanthroline
(95+%), 2,4,6-trimethoxyphenylboronic acid (98%), and 3,4,5-trimethoxyphenylboronic acid
(98%) were purchased from Ark Pharm. 2,9-Dimethyl-1,10-phenanthroline (98%) was purchased
from Alfa Aesar. Tetrakis(triphenylphosphine)palladium(0) (98%) was purchased from
Oakwood Chemical. Toluene, hexanes, CH2Cl2, CH3CN, and DMF solvents were purchased
5

from Fisher Scientific. It should be noted that the amount of H2O reported to be present in
CH3CN and DMF solvents (0.003% and 0.02%, respectively) is a lower limit as the solvents
were not extensively dried prior to analysis and atmospheric moisture may increase H2O
concentrations. Tetrabutylammonium hexafluorophosphate (98%), purchased from Alfa Aesar,
was recrystallized using ethanol prior to electrochemical analyses and dried in a 110 °C oven.
2,9-Dimesityl-1,10-phenanthroline (Mes2phen) [54], fac-[Re(phen)(CO)3Cl] (Re(phen)), and
fac-[Re(2,9-Me2phen)(CO)3Cl] (Re(2,9-Me2phen)) [20, 52] were synthesized following
previously established procedures.
2.2. Instrumentation
13
1H and C NMR spectra were collected at 298 K using a Bruker Ascend spectrometer (500
MHz for 1H and 125 MHz for 13C). Chemical shifts (ppm) were referenced to the residual solvent
peak; CDCl3 (1H NMR) δ = 7.26 ppm, DMSO-d6 (13C NMR) δ= 39.42 ppm [55]. High-resolution
electrospray ionization mass spectrometry (HR-ESI-MS) was performed using a Thermo Fisher
Scientific Q Exactive Plus hybrid quadrapole – Orbitrap mass spectrometer in positive mode.
Samples were dissolved in HPLC-grade CH3CN and a few drops of aqueous formic acid (0.1%)
were added prior to injection. Elemental analysis (C, H, N combustion) was performed by
Atlantic Microlabs, Inc. (Norcross, GA). Absorption spectra were recorded using an Agilent
Cary 8454 diode array UV-visible spectrophotometer (1 nm resolution, 0.5 s integration time) in
a 1 × 1 cm quartz cuvette (Starna Cells, Atascadero, CA) at room temperature. Molar
absorptivity values were measured from gravimetrically prepared samples. IR spectra were
collected in transmission mode using a Shimadzu IRAffinity-1S FT-IR spectrophotometer
(average of 10 scans, 1 cm−1 resolution), whereby samples were dissolved in CH2Cl2 solvent and
added to a liquid IR cell equipped with CaF2 windows.
6

2.3. Electrochemical measurements
Electrochemical measurements were performed using a BASi Epsilon EClipseTM
electrochemical analyzer (Bioanalytical Systems Inc.; West Lafayette, Indiana, USA). Cyclic
voltammograms (CV) were obtained using a standard three-electrode configuration with a glassy
carbon (3 mm diameter) working electrode, a Pt wire auxiliary electrode, and a Ag/AgCl (3 M
NaCl(aq)) reference electrode encapsulated in a glass tube with a Vycor® tip. Ferrocene (Fc) was
added to the cell following each set of CV experiments, and potentials were referenced against
the ferrocenium/ferrocene (Fc+/Fc) couple (CH3CN solvent: E1/2(Fc+/Fc) = +0.45 V vs. Ag/AgCl,
ΔEp = 90 mV; DMF solvent: E1/2(Fc+/Fc) = +0.56 V vs. Ag/AgCl, ΔEp = 120 mV). Typical CVs
were recorded with the following conditions: 0.5 mM complex in CH3CN or DMF solvent, 0.1
M n-Bu4NPF6 supporting electrolyte, 200 mV/s scan rate, N2 or CO2 atmosphere, and 298 K
temperature.
Controlled potential electrolysis (CPE) experiments were performed using a two-
compartment H-cell separated by a fine porosity glass frit (Pine Research Instrumentation;
Durham, North Carolina, USA). The auxiliary compartment consisted of a large surface area
platinum disk auxiliary electrode that was submerged in a 25 mL, N2- or CO2-saturated
electrolyte solution (0.1 M n-Bu4NPF6). The working compartment contained a 45 ppi (pore per
linear inch) vitreous reticulated carbon working electrode and a Ag/AgCl (3 M NaCl(aq))
reference electrode encapsulated in a glass tube with a Vycor® tip, both of which were
submerged in a 25 mL, N2- or CO2-saturated electrolyte solution (0.1 M n-Bu4NPF6) containing
0.5 mM Re(I)-diimine catalyst. The working compartment was rapidly stirred to ensure efficient
mass transport throughout the solution. After 1 h electrolysis at −2.75 V vs. Fc+/Fc, a 100 μL
aliquot of the working compartment’s headspace was sampled using a Hamilton sample lock
7

GASTIGHT® syringe and injected into a Shimadzu GC-2014 gas chromatograph (ultra high
purity He carrier gas) with a packed ShinCarbon ST SilicoSmooth stainless steel column (2 m
long × 1/8 in. o.d. × 2.0 mm i.d.; 80/100 mesh) and a Shimadzu TCD-2014 thermal conductivity
detector. The GC conditions were as follows: injector temp, 200 °C; column temperature, 64 °C;
detector temperature, 200 °C; detector current, 120 mA; and gas flow, 20 mL/min. The evolution
of CO from the CPE experiments was confirmed by comparing the retention times with a CO gas
standard.
2.4. Synthesis of 2,9-Bis-(2,4,6-trimethoxyphenyl)-1,10-phenanthroline ((2,4,6-tmp)2phen)
Ligand (2,4,6-tmp)2phen was prepared following synthetic procedures for the dimesityl-
substituted analogue (Mes2phen) [54]. In a round bottom flask, 2,9-dichloro-1,10-phenanthroline
(0.225 g, 0.903 mmol) and 2,4,6-trimethoxyphenylboronic acid (0.440 g, 2.08 mmol) were
suspended in 18.4 mL of toluene and 7.9 mL of 2 M Na2CO3(aq), then purged with argon for 15
min. A catalytic amount of Pd(PPh3)4 (0.1 eq, 0.104 g, 0.090 mmol) was added and the reaction
mixture was heated at reflux for 48 hours. The biphasic reaction mixture was cooled to room
temperature, and the organic layer was separated from the aqueous layer and set aside. The
aqueous layer was washed with dichloromethane and the organic fractions were combined, dried
using anhydrous MgSO4, and filtered. The solvent was removed via rotary evaporation and the
remaining solid was suspended in cold diethyl ether and filtered. Yield = 0.336 g (0.656 mmol,
1
72.6%). H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.20 (d, 2H, J = 7.9 Hz; 4, 7), 7.78 (s,
2H; 5, 6), 7.61 (d, 2H, J = 8.2 Hz; 3, 8), 6.23 (s, 4H; meta), 3.85 (s, 6H; para OCH3), 3.73 (s,
13
12H; ortho OCH3). C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 161.09, 158.40, 154.78,
145.42, 135.63, 127.03, 126.13, 126.10, 112.43, 91.07, 55.65, 55.42. HR-ESI(+)-MS: [M + H]+
8

m/z = 513.2022 (calc. m/z = 513.2020). Elemental analysis calculated for C30H28N2O6: 70.29%
C, 5.52% H, 5.47% N. Found: 69.53% C, 5.45% H, 5.54% N.
2.5. Synthesis of 2,9-Bis-(3,4,5-trimethoxyphenyl)-1,10-phenanthroline ((3,4,5-tmp)2phen)
Ligand (3,4,5-tmp)2phen was prepared using the same procedure as described for (2,4,6-
tmp)2phen, except 2,9-dichloro-1,10-phenanthroline (0.350 g, 1.40 mmol) and 3,4,5-
trimethoxyphenylboronic acid (0.750 g, 3.50 mmol) were used. Yield = 0.467 g (0.911 mmol,
65%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.31 (d, 2H, J = 8.2 Hz; 4, 7), 8.09 (d, 2H,
J = 8.2 Hz; 3, 8), 7.80 (s, 2H; 5, 6), 7.70 (s, 4H; ortho), 4.05 (s, 12H; meta OCH3), 3.96 (s, 6H;
para OCH3). 13C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 154.98, 153.39, 145.14,
139.77, 137.25, 134.51, 127.77, 126.09, 120.12, 105.77, 60.27, 56.52. HR-ESI(+)-MS: [M + H]+
m/z = 513.2021 (calc. m/z = 513.2020). Elemental analysis calculated for C30H28N2O6·H2O:
67.98% C, 5.66% H, 5.28% N. Found: 67.85% C, 5.75% H, 5.23% N.
2.6. Synthesis of fac-[Re(Mes2phen)(CO)3Cl] (Re(Mes2phen))
An aliquot of Re(CO)5Cl (0.0493 g, 0.138 mmol) and Mes2phen ligand (0.0579 g, 0.139
mmol) were added to a 25 mL round bottom Schlenk flask equipped with a magnetic stir bar.
Toluene (5 mL) was added and the mixture was deoxygenated for 10 min using Ar while at room
temperature. The mixture was then heated at reflux for 6 h in the dark while under a blanket of
Ar. After 15 min, the white mixture changed to yellow and eventually yellow solids began to
precipitate from the reaction. After 6 h, the resulting yellow mixture was cooled to room
temperature and the yellow solids were collected by vacuum filtration, followed by rinsing with
hexanes. Yield = 0.0595 g (0.0824 mmol, 60%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) =
8.52 (d, 2H, J = 8.2 Hz; 4, 7), 8.05 (s, 2H; 5, 6), 7.71 (d, 2H, J = 8.2 Hz; 3, 8), 7.03 (s, 4H; meta),
2.38 (s, 6H; CH3), 2.20 (s, 6H; CH3), 2.00 (s, 6H; CH3). 13C NMR (125 MHz, 298 K, DMSO-d6):
9

δ (ppm) = 192.15, 163.86, 147.33, 140.00, 138.96, 135.47, 134.86, 129.79, 128.49, 128.08,
127.64, 127.25, 20.83, 20.75, 19.79. FTIR (CH2Cl2) ν(CO): 2024 cm−1, 1925 cm−1, 1889 cm−1.
HR-ESI(+)-MS: [M + Na]+ m/z = 745.1243 (calc. m/z = 745.1244), [M] m/z = 722.1348 (calc.
m/z = 722.1341), [M – Cl]+ m/z = 687.1665 (calc. m/z = 687.1658). Elemental analysis
calculated for C33H28N2O3ClRe·H2O: 53.56% C, 4.05% H, 3.78% N. Found: 53.49% C, 3.97%
H, 3.86% N.
2.7. Synthesis of fac-[Re((2,4,6-tmp)2phen)(CO)3Cl] (Re((2,4,6-tmp)2phen))
The target complex was prepared using the same procedure as described above, except
Re(CO)5Cl (0.0141 g, 0.0340 mmol) and (2,4,6-tmp)2phen ligand (0.0202 g, 0.0394 mmol) were
1
used. Yield = 0.0227 g (0.0277 mmol, 81%). H NMR (500 MHz, 298 K, CDCl3): δ (ppm) =
8.41 (d, 2H, J = 8.2 Hz; 4, 7), 7.94 (s, 2H; 5, 6), 7.73 (d, 2H, J = 8.2 Hz; 3, 8), 6.29 (d, 2H, J =
2.0 Hz; meta), 6.24 (d, 2H, J = 2.0 Hz; meta), 3.89 (s, 6H; OCH3), 3.79 (s, 6H; OCH3), 3.66 (s,
6H; OCH3). 13C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 194.31, 162.94, 159.08, 158.58,
158.15, 147.41, 138.36, 129.16, 128.48, 126.94, 112.75, 91.61, 90.63, 56.19, 55.53, 55.26. FTIR
(CH2Cl2) ν(CO): 2018 cm−1, 1913 cm−1, 1890 cm−1. HR-ESI(+)-MS: [M – Cl]+ m/z = 783.1340
(calc. m/z = 783.1347). Elemental analysis calculated for C33H28N2O9ClRe: 48.43% C, 3.46% H,
3.42% N. Found: 48.46% C, 3.74% H, 3.30% N.
2.8. Synthesis of fac-[Re((3,4,5-tmp)2phen)(CO)3Cl] (Re((3,4,5-tmp)2phen))
The target complex was prepared using the same procedure as described above, except
Re(CO)5Cl (0.0495 g, 0.138 mmol) and (3,4,5-tmp)2phen ligand (0.0706 g, 0.138 mmol) were
used. Yield = 0.0986 g (0.120 mmol, 87%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.55
(d, 2H, J = 8.2 Hz; 4, 7), 8.06 (s, 2H; 5, 6), 7.95 (d, 2H, J = 8.2 Hz; 3, 8), 6.99 (s, 2H; ortho),
13
6.87 (s, 2H; ortho), 3.95 (s, 6H; OCH3), 3.94 (s, 6H; OCH3), 3.90 (s, 6H; OCH3). C NMR (125
10

MHz, 298 K, DMSO-d6): δ (ppm) = 193.60, 163.38, 153.03, 152.69, 147.03, 139.35, 139.29, 137.20,
129.87, 127.36, 127.08, 107.75, 60.15, 56.32, 55.90. FTIR (CH2Cl2) ν(CO): 2022 cm−1, 1924 cm−1, 1884
cm−1. HR-ESI(+)-MS: [M – Cl]+ m/z = 783.1344 (calc. m/z = 783.1347). Elemental analysis
calculated for C33H28N2O9ClRe: 48.43% C, 3.46% H, 3.42% N. Found: 48.15% C, 3.53% H,
3.47% N.
3. Results and Discussion
The Re(I) phenanthroline electrocatalysts were originally designed to provide varied steric
bulk, electronic contribution, and potential weak hydrogen bonding sites towards the Re metal
center with the aim of evaluating their respective effects on electrocatalytic CO2 reduction.
Compared to the unsubstituted 1,10-phenanthroline (phen), inclusion of methyl (2,9-Me2phen)
and trimethylphenyl (Mes2phen) substituents at the 2 and 9 positions introduces steric bulk near
the metal center. These added substituents also enhance the electron density on the
phenanthroline ligand, inducing a small cathodic shift in the reduction potentials of both the
R2phen0/− and ReI/0 redox couples (vide infra). Trimethoxyphenyl ((2,4,6-tmp)2phen and (3,4,5-
tmp)2phen) substituents were selected to not only provide steric bulk and an electronic
contribution, but also to introduce Lewis basic sites capable of weak hydrogen bonding to
stabilize the proposed metallocarboxylic acid intermediate (Re-CO2H). However, this work
showed that the electronic effects dominate the observed catalytic activity.
3.1. Synthesis and characterization
The new trimethoxyphenyl-substituted phenanthroline ligands were synthesized in good
yield using well-established Suzuki-Miyaura cross-coupling reaction conditions [54, 56]. 1H and
13C NMR spectra of (2,4,6-tmp)2phen and (3,4,5-tmp)2phen (Supporting Information, Figures
S1 – S4) agree well with the expected splitting patterns and chemical shifts for the symmetric
11

(C2v), trimethoxyphenyl-substituted architecture. High-resolution ESI-MS (Figures S5 and S6)
and elemental analysis further corroborate the proposed molecular weight and C, H, N elemental
composition, respectively.
The fac-[Re(R2phen)(CO)3Cl] complexes were prepared by combining the substituted-
phenanthroline ligands with Re(CO)5Cl (1:1 ratio) in toluene and heating at reflux (Ar atm, dark,
1
6 h) to afford bright yellow solids. H NMR spectra for the new Re(Mes2phen), Re((2,4,6-
tmp)2phen), and Re((3,4,5-tmp)2phen) complexes (see Supporting Information) show three
proton resonance signals ranging from δ = 7.6 – 8.6 ppm (two doublets and one singlet in each
spectrum), consistent with a symmetrically-substituted phenanthroline backbone coordinated to
the ReI(CO)3Cl core. Furthermore, the Re((2,4,6-tmp)2phen) and Re((3,4,5-tmp)2phen)
trimethoxyphenyl-substituted structures display the expected downfield OCH3 singlets (δ = 3.65
– 3.95 ppm) compared to the trimethylphenyl-substituted Re(Mes2phen) CH3 singlets located
upfield (δ = 2.00 – 2.40 ppm). Three OCH3 and CH3 signals are observed for the
trimethoxyphenyl and trimethylphenyl complexes, respectively, which further attest to their
symmetric architecture. More specifically, the three carbonyl substituents are in a facial
arrangement around the Re metal center with Cs symmetry (one mirror plane bisecting the
phenanthroline ligand).
FTIR spectra for all Re(I) complexes (Figure S16) were obtained in CH2Cl2 solvent and
agree well with a fac-[Re(R2phen)(CO)3Cl] geometry of Cs symmetry whereby three carbonyl
stretching vibrations (order of energy: A′(1) > A′(2) > A″) are observed in the range of ν(CO) =
2024 – 1884 cm−1 [57]. As shown in Table 1, the new Re(Mes2phen), Re((2,4,6-tmp)2phen),
and Re((3,4,5-tmp)2phen) complexes possess features in agreement with the structurally-similar
fac-[Re(dap)(CO)3Cl] (dap = 2,9-di-anisyl-1,10-phenanthroline), which displays three CO
12

stretching vibrations at ν(CO) = 2022, 1924, and 1884 cm−1 when measured in CH2Cl2 solvent
[58]. Of note is the shift to lower energy of the A′(1) symmetric vibration in Re((2,4,6-
tmp)2phen) (ν(CO) = 2018 cm−1). This shift is indicative of increased electron density at the
Re(I) center, resulting from enhanced π-backbonding between the Re(dπ) and CO(π*) molecular
orbitals, thus weakening the C–O vibration. As described below, methoxy substituents at the
ortho and para positions of the phenyl ring contribute electron density into the phenanthroline
ligand, and ultimately to the Re metal center.
Table 1. Carbonyl vibrational frequencies ν(CO) for fac-[Re(R2phen)(CO)3Cl] complexes a
Complex
A′(1)
2024
2024
2024
2018
2022
2022
A′(2)
1921
1916
1925
1913
1924
1924
A″
Re(phen)
1898
1896
1889
1890
1884
1884
Re(2,9-Me2phen)
Re(Mes2phen)
Re((2,4,6-tmp)2phen)
Re((3,4,5-tmp)2phen)
fac-[Re(dap)(CO)3Cl] b
a Measured in CH2Cl2 solvent; values reported as cm−1. b Reference [58].
The new trimethylphenyl and trimethoxyphenyl complexes possess UV and visible light
absorbing properties similar to previously reported Re(I)-diimine carbonyl complexes [59-61].
As illustrated in Figure 2, the UV region is dominated by intense R2phen-based intraligand (1IL)
π→π* transitions (λabs ~ 264 nm), whereby the inclusion of substituted aryl groups at the 2 and 9
positions on 1,10-phenanthroline extend the UV light absorbing character past 350 nm. The
broad, featureless transition extending into the visible region is assigned to the
Re(dπ)→R2phen(π*) metal-to-ligand charge transfer (1MLCT) transition for each complex. The
13

methyl-substituted Re(2,9-Me2phen) (not shown to minimize overcrowding) absorbs at λabs
=
370 nm, in agreement with a previously reported value [60]. Re(Mes2phen) (λabs = 375 nm; ε =
3,140 M−1cm−1) displays similar absorption characteristics as Re(phen) (λabs = 370 nm; ε = 3,900
M−1cm−1), indicating that trimethylphenyl substituents do not strongly modify the electronic
structure within this architecture. Similarly, the 1MLCT for the trimethoxyphenyl analogues tails
into the visible region and is somewhat overshadowed by the encroaching R2phen π→π*
transitions, making it difficult to assign a λabsmax value. However, based on electrochemical data
(vide infra) and analogous complexes, the lowest energy absorption for Re((2,4,6-tmp)2phen)
and Re((3,4,5-tmp)2phen) is also 1MLCT in nature.
Figure 2. Electronic absorption spectra of complexes Re(phen) (─), Re(Mes2phen) (─),
Re((2,4,6-tmp)2phen) (─), and Re((3,4,5-tmp)2phen) (─) in CH3CN. Inset displays the
normalized absorption profile for the corresponding free ligands (phen (─ ─), Mes2phen (─ ─),
(2,4,6-tmp)2phen (─ ─), and (3,4,5-tmp)2phen (─ ─)) over the same range of wavelengths.
14

3.2. Electrochemical properties under N2 atmosphere
Electrochemical properties of the new trimethylphenyl- and trimethoxyphenyl-substituted
complexes were first analyzed under an inert atmosphere (N2) to better understand how sterics
and electronics affect the redox characteristics. CVs of all Re(I) complexes were recorded in N2-
saturated CH3CN and DMF solvents (Figures 3 and 4, respectively), and a summary of the
pertinent redox potentials is listed in Table 2. As expected, the complexes display a Re-based
HOMO and diimine-based LUMO, with similar redox characteristics to those for structurally-
related fac-[Re(NN)(CO)3Cl] complexes (NN = substituted 2,2′-bipyridine or 1,10-
phenanthroline ligand) [62, 63].
Figure 3. Cyclic voltammograms of Re(phen) (purple lines), Re(2,9-Me2phen) (green lines),
Re(Mes2phen) (black lines), Re((2,4,6-tmp)2phen) (red lines), and Re((3,4,5-tmp)2phen) (blue
lines) measured in CH3CN solvent under a N2 atmosphere. Conditions: 0.5 mM Re(I) complex,
0.1 M n-Bu4NPF6 electrolyte, 200 mV/s scan rate, RT. The arrow indicates the scan direction.
15

Figure 4. Cathodic cyclic voltammograms of Re(phen) (purple lines), Re(2,9-Me2phen) (green
lines), Re(Mes2phen) (black lines), Re((2,4,6-tmp)2phen) (red lines), and Re((3,4,5-
tmp)2phen) (blue lines) measured in DMF solvent under a N2 atmosphere. Conditions: 0.5 mM
Re(I) complex, 0.1 M n-Bu4NPF6 electrolyte, 200 mV/s scan rate, RT. The arrow indicates the
scan direction.
Anodic CVs collected in CH3CN (Figure 3) illustrate how ligand substitution modifies the
ReI/II chemical reversibility and oxidation potential. At ν = 200 mV/s, the unsubstituted
a
Re(phen) displays a chemically irreversible ReI/II oxidation at Ep = +0.99 V vs. Fc+/Fc. This is
attributed to the rapid dimerization of two [ReII(phen)(CO)3Cl]+ species via a chloride bridge,
which undergoes disproportionation and attack by
a
solvent molecule forming
[ReI(phen)(CO)3(CH3CN)]+ and [ReIII(phen)(CO)3Cl2]+ [63]. Methyl substituents in Re(2,9-
Me2phen) introduce enough steric bulk to increase the chemical reversibility of the ReI/II couple
by inhibiting the rapid coordination of a solvent molecule. Further increasing the steric bulk with
16

Re(Mes2phen) and Re((2,4,6-tmp)2phen) produces a chemically reversible ReI/II couple at E =
½
+0.93 and +0.79 V, respectively, in line with the steric contribution inhibiting solvent
coordination [63]. Furthermore, the presence of methyl and methoxy substituents at the ortho
and para positions of the phenyl ring adds electron density into the phenanthroline ligand,
destabilizing the Re(dπ)-based HOMO as evident by the less positive E (ReI/II). The 3,4,5-
½
trimethoxyphenyl-substituted complex, Re((3,4,5-tmp)2phen), displays a quasi-reversible couple
a
at E = +0.89 V, followed by an irreversible wave at Ep = +1.07 V, the latter of which may
½
involve irreversible oxidation of the meta-positioned methoxy groups on the phenyl ring [64].
The change in reversibility and the shift to a more positive potential compared to the 2,4,6-
substituted analogue emphasizes the profound steric and electronic impact of ortho vs. meta
positions on the phenyl ring. The meta-positioned methoxy groups withdraw electron density
from the ring system, while also providing smaller steric bulk than the ortho methyl and methoxy
substituents in Re(Mes2phen) and Re((2,4,6-tmp)2phen), respectively.
In both CH3CN and DMF solvents, cathodic scans reveal the rich and complex
electrochemistry also observed for previously reported Re(I) complexes [4, 20, 62]. This
discussion will refer primarily to the properties observed in CH3CN, but similar trends are
observed in DMF solvent. The first redox couple is assigned to the quasi-reversible R2phen0/−
reduction. Subsequent processes involve competing mechanisms that undergo Cl− dissociation
and ReI/0 reduction (ECE mechanism; E = electron transfer, C = chemical step) or an irreversible
ReI/0 reduction and rapid Cl− dissociation (EEC mechanism). Electron donor effects from the
methyl (Re(2,9-Me2phen)) and trimethylphenyl (Re(Mes2phen)) substituents cathodically shift
the R2phen0/− reduction (E (R2phen0/−) = −1.83 and −1.82 V, respectively) compared to the
½
unsubstituted analogue Re(phen) (E (R2phen0/−) = −1.74 V). Interestingly, the methyl and
½
17

trimethylphenyl substituents influence the electronics within the phenanthroline π-system to a
similar degree as evidenced by the nearly identical R2phen0/− potential; however, the
trimethylphenyl exhibits a more negative shift in the ReI/0 reduction, suggesting the latter
contributes more strongly to the σ-donating component (i.e. inductive effects).
Introduction of methoxy Lewis basic groups in the ortho and para positions in Re((2,4,6-
c
tmp)2phen) display more negative reduction potentials (E (R2phen0/−) = −1.94 V; Ep (ReI/0) =
½
−2.49 V) than Re(Mes2phen), indicating the increased electron donating character as a result of
the methoxy substituents. In contrast, the presence of methoxy groups at the meta position in
Re((3,4,5-tmp)2phen) lead to less negative reduction potentials (E (R2phen0/−) = −1.76 V;
½
c
Ep (ReI/0) = −2.17 V) that are comparable to Re(phen), which further confirms the electron
withdrawing character of meta-substituted methoxy substituents. The steric crowding and
electronic push-pull effect are expected to contribute to the observed electrocatalytic CO2
reduction within this substituted-phenanthroline architecture.
Table 2. Redox properties of Re(I) electrocatalysts in CH3CN and DMF solvents a
c
E (ReI/II) / V
E (R2phen0/−) / V
Ep (ReI/0) / V b
½
½
Complex
CH3CN
+0.99 c
+0.99
CH3CN
DMF
−1.75
−1.83
−1.83
−1.99
−1.77
CH3CN
DMF
−2.23
−2.28
−2.43
−2.58
−2.48
Re(phen)
−1.74
−1.83
−1.82
−1.94
−1.76
−2.11
−2.16
−2.30
−2.49
−2.17
Re(2,9-Me2phen)
Re(Mes2phen)
+0.93
Re((2,4,6-tmp)2phen)
Re((3,4,5-tmp)2phen)
+0.79
+0.89 d
a Reported E values were measured using 0.5 mM Re(I) catalyst, 0.1 M n-Bu4NPF6 electrolyte,
½
RT, N2 atm, and 200 mV/s scan rate. Potentials referenced against the Fc+/Fc reversible redox
b
c
c
a
couple. Reported Ep for an irreversible reductive process. Reported Ep for an irreversible
oxidative process. d Reported E for a quasi-reversible oxidative process.
½
18

3.3. Electrocatalytic activity under CO2 atmosphere
The electrocatalytic activity for all five fac-[Re(R2phen)(CO)3Cl] complexes was determined
by CV analysis in both CO2-saturated CH3CN and DMF solvents. The results are shown in
Figure 5 for complexes Re(Mes2phen), Re((3,4,5-tmp)2phen), and Re((2,4,6-tmp)2phen), and
all additional CVs are provided in the Supporting Information (Figures S17 – S27). For all
complexes, an increase in cathodic current is observed at the ReI/0 redox wave and is
characteristic of catalytic CO2 reduction, indicating that the two-electron reduced Re-
phenanthroline species, [Re0(R2phen−•)(CO)3]−, is the dominant mechanism to enable rapid
catalytic turnover. To confirm the formation of CO gas, controlled potential electrolysis (CPE)
was performed by applying −2.75 V vs Fc+/Fc under a CO2-saturated atmosphere and sampling
the headspace above the electrolysis solution.
19

Figure 5. Cyclic voltammograms measured under CO2 atm in CH3CN (a) and DMF (b). CVs of
Re(Mes2phen) (black), Re((2,4,6-tmp)2phen) (red), and Re((3,4,5-tmp)2phen) (blue) using 0.5
mM Re(I) catalyst, 0.1 M n-Bu4NPF6 electrolyte, 200 mV/s scan rate, RT.
In order to compare the electrocatalytic CO2 reduction activity between each complex, the
current response ratio (icat/ip) was determined from the current for a non-catalytic one-electron
reduction under N2 (ip) and the catalytic current under CO2 (icat) [65]. The calculated icat/ip values
are tabulated in Table 3 for each complex in CH3CN and DMF. The methyl-substituted Re(2,9-
Me2phen) exhibits a small increase of catalytic activity compared to Re(phen), indicating the
methyl substituent’s minimal electronic and steric effects. The sterically bulky, electron donating
trimethylphenyl groups in Re(Mes2phen) further enhance CO2 reduction activity, likely the
result of the more negative ReI/0 reduction displayed in the redox characteristics under N2. In
both CH3CN and DMF solvents, a small increase in activity occurs when methoxy groups are
introduced at the meta and para positions in Re((3,4,5-tmp)2phen), accompanied by a small
shift to less negative potential for the catalytic wave. By changing the position of the methoxy
20

groups from meta to ortho, Re((2,4,6-tmp)2phen) displays an additional increase in the icat/ip
ratio compared to Re((3,4,5-tmp)2phen) when analyzed in either DMF or CH3CN. This
indicates that the strong electron donating effects of ortho/para vs. meta/para-substituted
methoxy groups, evidenced by the redox properties under N2, play a significant role in the
catalytic enhancement.
Table 3. Electrocatalytic activity of Re(I) catalysts a
DMF
icat/ip
4.1
CH3CN
icat/ip
4.2
Complex
Re(phen)
Re(2,9-Me2phen)
Re(Mes2phen)
4.9
4.6
5.4
6.8
Re((2,4,6-tmp)2phen)
Re((3,4,5-tmp)2phen)
14.8
6.0
8.5
7.7
a Values were determined from CVs collected using 0.5 mM Re(I) catalyst, 0.1 M n-Bu4NPF6 in
DMF or CH3CN electrolyte system, room temperature, CO2 atm, and 200 mV/s scan rate. icat/ip
values were determined from the ratio of the peak catalytic current response under CO2
atmosphere (icat) and the current response for a non-catalytic one-electron reduction under N2
atmosphere (ip).
3.4. Solvent contribution to catalytic activity
During the electrocatalytic analyses under CO2 atmosphere, we observed varying degrees of
catalytic enhancement in CH3CN and DMF solvents depending on which Re catalyst was used,
and therefore must comment on this phenomenon. A comparison of the catalytic activity in DMF
and CH3CN for each Re catalyst is provided in Table 3. Note that the absolute value of the
currents and icat/ip ratios between solvents should be compared with caution due to differences in
21

solvent properties such as CO2 solubility and diffusion rates. However, the trend in current
enhancement ratios (icat/ip) within a solvent series provides useful insight into the solvent’s role
in electrocatalysis. A 2.5x increase in the icat/ip ratio is observed for Re((2,4,6-tmp)2phen)
compared to Re((3,4,5-tmp)2phen) in DMF (icat/ip = 14.8 vs. 6.0, respectively), but only a small
increase is observed in CH3CN (icat/ip = 8.5 vs. 7.7, respectively). This significant difference in
activity based on the solvent used and the proximity of the methoxy substituents to the Re active
site highlights how the electrocatalysts’ design and solvent system should be carefully
considered.
A recent publication reporting fac-[Re(pyridine-oxazoline)(CO)3Cl] complexes as CO2
reduction electrocatalysts showed how the Lewis basicity of the solvent (i.e. the donor number,
DN) greatly affects the rate of catalysis, whereby the calculated rate constant for CO2 reduction
decreased as the solvent’s DN increased [42]. It was suggested that stronger Lewis basic solvents
such as DMF and DMSO (DN = 26.6 and 29.8, respectively) more effectively hydrogen bond
with the metallocarboxylic acid intermediate than CH3CN (DN = 14.1) and/or compete more
strongly for available protons, which serve to protonate the metallocarboxylate intermediate.
Both processes are expected to suppress rapid CO2 reduction activity and the study emphasizes
how the Lewis basicity of a solvent can dominate the observed reactivity. Within our study, only
Re((2,4,6-tmp)2phen) displayed a significant current enhancement using the stronger Lewis
basic solvent DMF compared to the weaker Lewis basic solvent CH3CN. We originally
considered weak intramolecular hydrogen bonding between the proximal methoxy substituents
and a metallocarboxylic acid intermediate (Re-CO2H) being capable of suppressing the strong
Lewis basicity of DMF as a solvent for electrocatalysis. Hydrogen-bonding intermediates
typically lower the energetic barrier for CO2 reduction and result in an anodic shift of the
22

catalytic wave; however, our CVs in DMF still display relatively cathodic potentials for
catalysis. The significant activity increase in DMF for Re((2,4,6-tmp)2phen) compared to
Re(Mes2phen) and Re((3,4,5-tmp)2phen) does suggest a solvent-mediated pathway which
couples the electronic and Lewis basic character of the proximal methoxy substituents with the
solvent. However, the exact mechanism by which the solvent influences catalysis requires further
investigation and these results warrant such analysis.
4. Conclusions
We have synthesized and characterized a new series of fac-[Re(R2phen)(CO)3Cl] complexes
containing varying degrees of steric bulk and electronic contribution. The complexes were
characterized by multiple physical methods (1H NMR, 13C NMR, FTIR, and UV-Vis
spectroscopies, ESI-MS, and elemental analysis) to establish their structural properties.
Moreover, the Re(I)-diimine complexes were analyzed using cyclic voltammetry in the absence
and presence of CO2 substrate to determine the impact of sterics and electronics on the
electrochemical properties and electrocatalytic activity. In general, increasing the electronic
contribution into the phenanthroline ligand from the 2 and 9 positions cathodically shifts the
R2phen- and Re-based redox couples, while increasing the steric bulk near the Re metal center
increases the reversibility of the ReI/II oxidation. In both CH3CN and DMF, the electronics
appear to dominate the observed electrocatalytic CO2 reduction activity with the electron-
donating aryl-substituted complexes, Re(Mes2phen), Re(2,4,6-tmp)2phen), and Re(3,4,5-
tmp)2phen, displaying increased current enhancement at more negative potentials with respect to
the unsubstituted Re(phen) analogue. Interestingly, Re((2,4,6-tmp)2phen) displayed a much
higher current enhancement compared to the bulky Re(Mes2phen) and Re(3,4,5-tmp)2phen
catalysts in DMF solvent, but not in CH3CN, indicating that solvent plays an important role in
23

catalysis. Ongoing studies in our lab include synthesizing and comparing the analogous
hydroxyphenyl substituents to investigate the effect of intramolecular hydrogen bonding with
weak Brønsted acids and a metallocarboxylate intermediate, as well as pursing solvent effects on
Re(I)-diimine electrocatalysts for CO2 reduction.
Appendix A. Supplementary Information.
Supplementary data including HR-ESI(+)-MS data, 1H NMR spectra, 13C NMR spectra, FTIR
spectra, and cyclic voltammograms associated with this article can be found, in the online
version, at
*Corresponding Author
Mailing Address: Department of Chemistry & Biochemistry, 100 University Terrace, 136
Clippinger Laboratories, Athens, Ohio 45701, USA
E-mail address: whitet2@ohio.edu (T. A. White)
ORCID
Travis A. White: 0000-0002-6200-4416
Bertrand J. Neyhouse: 0000-0001-6747-8197
Author Contributions
The manuscript was written through contributions of all authors and all authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
Declarations of interest: none
Acknowledgements
24

T.A.W. thanks Ohio University for start-up funds supporting this research. Acknowledgements
are given to Mr. Brian Hivick (Prof. Hao Chen’s research group, Ohio University) for his
assistance with obtaining HR-ESI(+)-MS data, Prof. Howard Dewald (Ohio University) for
assistance with bulk electrolysis electrodes, and Prof. Kyle Grice (DePaul University) for fruitful
discussions.
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Highlights:
 New Re(I)-phenanthroline electrocatalysts for CO2 reduction were synthesized
 Substituent modifications indicate electronics dominate observed catalytic activity
 CH3CN and DMF solvents display varying degrees of catalytic enhancement
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