Enhanced Electrochemical CO2 Reduction by a Series of Molecular Rhenium Catalysts Decorated with Second-Sphere Hydrogen-Bond Donors

Article abstract of DOI:10.1021/acs.inorgchem.0c00154

A series of rhenium(I) fac-tricarbonyl complexes containing pendent arylamine functionality in the second coordination sphere have been developed and studied as electrocatalysts for carbon dioxide (CO₂) reduction. Aniline moieties were appended at the 6 position of a 2,2′-bipyridine (bpy) donor in which the primary amine was positioned at the ortho- (1-Re), meta- (2-Re), and para- (3-Re) sites of the aniline substituent to generate a family of isomers. The relationship between the catalyst structure and activity was explored across the series, and the catalytic performance was compared to that of the benchmark catalyst Re(bpy)(CO)₃Cl (ReBpy). Catalysts 1-Re, 2-Re, and 3-Re outperform the benchmark catalyst both in anhydrous acetonitrile and with added trifluoroethanol (TFE) as an external proton source. In the presence of TFE, the aniline-substituted catalysts convert CO₂ to carbon monoxide (CO) with high Faradaic efficiencies (≥89%) and have superior turnover frequencies (TOFs) relative to ReBpy (72.9 s^-1), with 2-Re having the highest TOF of the series at 239 s^-1, a value that is twice that of the next most active catalyst. TOFs of 123 and 109 s^-1 were observed for the ortho- and para-substituted aniline complexes (1-Re and 3-Re), respectively. Indeed, catalytic activities vary widely across the series, showing a high sensitivity to the position of the amine functionality relative to the rhenium active site. IR and UV-vis spectroelectrochemical experiments were conducted on the aniline-substituted systems, revealing important differences between the catalysts and mechanistic insight.

Full text of DOI:10.1021/acs.inorgchem.0c00154

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Article
Enhanced Electrochemical CO2 Reduction by a Series of Molecular
Rhenium Catalysts Decorated with Second-Sphere Hydrogen-Bond
Donors
Kallol Talukdar,† Sayontani Sinha Roy,† Eva Amatya, Elizabeth A. Sleeper, Pierre Le Magueres,
and Jonah W. Jurss*
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ABSTRACT: A series of rhenium(I) fac-tricarbonyl complexes containing
pendent arylamine functionality in the second coordination sphere have
been developed and studied as electrocatalysts for carbon dioxide (CO2)
reduction. Aniline moieties were appended at the 6 position of a 2,2′-
bipyridine (bpy) donor in which the primary amine was positioned at the
ortho- (1-Re), meta- (2-Re), and para- (3-Re) sites of the aniline
substituent to generate a family of isomers. The relationship between the
catalyst structure and activity was explored across the series, and the
catalytic performance was compared to that of the benchmark catalyst
Re(bpy)(CO)3Cl (ReBpy). Catalysts 1-Re, 2-Re, and 3-Re outperform
the benchmark catalyst both in anhydrous acetonitrile and with added
trifluoroethanol (TFE) as an external proton source. In the presence of
TFE, the aniline-substituted catalysts convert CO2 to carbon monoxide
(CO) with high Faradaic efficiencies (≥89%) and have superior turnover frequencies (TOFs) relative to ReBpy (72.9 s−1), with 2-
Re having the highest TOF of the series at 239 s−1, a value that is twice that of the next most active catalyst. TOFs of 123 and 109
s−1 were observed for the ortho- and para-substituted aniline complexes (1-Re and 3-Re), respectively. Indeed, catalytic activities
vary widely across the series, showing a high sensitivity to the position of the amine functionality relative to the rhenium active site.
IR and UV−vis spectroelectrochemical experiments were conducted on the aniline-substituted systems, revealing important
differences between the catalysts and mechanistic insight.
multiproton processes. However, product selectivity is also
required in the presence of a proton source where hydrogen
evolution is often a competing side reaction that reduces
efficiency.7 The development of well-defined homogeneous
electrocatalysts for reducing CO2 at a low overpotential and
with high product selectivity is still a significant scientific
challenge.5,8,9
Nature achieves catalytic reactions with high efficiency and
selectivity by employing tightly regulated environments within
metalloenzymes. Metalloenzymes such as Ni,Fe-carbon mon-
oxide dehydrogenase10 and formic acid dehydrogenase11 are
known to efficiently catalyze CO2 reduction to carbon
monoxide (CO) or formate through cooperative bimetallic
CO2 activation and/or by stabilization of a metal carboxylate
INTRODUCTION
■
Global energy consumption continues to increase with ∼80%
of this energy sourced from nonrenewable fossil fuels. The
combustion of fossil fuels releases airborne pollutants and
copious amounts of carbon dioxide (CO2) into the
atmosphere, which has contributed to climate change and
other environmental problems.1−3 Indeed, CO2 is a green-
house gas whose atmospheric concentration has risen to levels
not seen in at least 800,000 years.4 Renewable energy sources
offer a clean alternative to fossil fuels, but they are intermittent
and geographically diffuse. Thus, energy storage is required to
power society night and day and to meet the energy demands
of dense population centers. To address this challenge,
renewable electricity can be converted into chemical energy
by reducing CO2 into energy-rich fuels (or fuel precursors) as
part of a closed carbon cycle.5,6 However, large overpotentials
are often required to overcome activation barriers for CO2
conversion, and more efficient catalysts are needed to
effectively reduce CO2 into value-added products.
Received: January 16, 2020
The direct one-electron reduction of CO2 to the highly
•−
energetic CO2 radical anion can be circumvented with
catalysts capable of mediating lower-energy multielectron/
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intermediate with optimally positioned amino acid residues in
the second coordination sphere of the active site. Noncovalent
second-coordination-sphere interactions are recognized for
their importance in directing multielectron/multiproton
catalysis.6,12 Synthetically, noncovalent interactions are difficult
to harness in a reliable manner, often causing unwanted
structures and perturbation of function.13−15 Significant effort
continues to be exerted by the scientific community to append
second-sphere functionality onto the ligand framework of
catalysts to provide specific and reproducible intramolecular
interactions to improve catalyst performance. Inspired by
nature, a variety of synthetic systems have been developed with
different functional groups in the secondary coordination
sphere of the catalytic active site.12 Iron porphyrin complexes
decorated with phenolic16 or quaternary ammonium17 groups
are prototypical examples that have dramatically improved the
performance metrics for electrocatalytic CO2 reduction relative
to the parent catalyst, both thermodynamically and kinetically.
From their inception, fac-Re(bpy)(CO)3Cl (ReBpy) cata-
lysts have gained considerable attention due to their excep-
tional selectivity toward CO2 reduction over proton reduc-
tion.18−20 Efforts have been made to enhance the activity of
this class of catalysts by incorporating thiourea,21 imidazo-
lium,22 hydroxyl,23 and amine24 functionalities as well as
variable peptide linkages25 into the second coordination
sphere. We note that bimetallic complexes with well-defined
distances between Re(bpy) units have also been synthesized to
access cooperative modes of CO2 activation and/or to
accumulate multiple reducing equivalents to enhance the
multielectron reduction of CO2.26−28
RESULTS AND DISCUSSION
■
Synthesis and Structure of the Complexes. Ligands
L1−L3 were prepared via palladium-catalyzed Suzuki cross-
coupling of 6-bromo-2,2′-bipyridine and the appropriate
boronic acid or boronate ester with good-to-excellent yields
(Figure 2).26,29 In all cases, pure products were obtained by
silica gel chromatography. Metalation was achieved by
refluxing the ligands with 1 equiv of the metal precursor
Re(CO)5Cl in anhydrous toluene for 12 h under a N2
atmosphere.26,30 The complexes were purified by washing
them with low-polarity organic solvents. Detailed synthetic
procedures are described in the Supporting Information (SI).
All three complexes are air- and moisture-stable in the solid
state. The ligands and complexes were characterized by
elemental analysis, 1H NMR, and high-resolution mass
spectrometry.
Crystals of 1-Re and 2-Re in the form of small yellow
needles were grown from slow ether diffusion into a
concentrated solution of each complex in acetonitrile
(CH3CN) or N,N-dimethylformamide (DMF) at low temper-
ature. Several attempts to grow X-ray-quality crystals of 3-Re
were unsuccessful. In theory, a pair of conformers could form
during the synthesis of both 1-Re and 2-Re. However, only
one isomer (amine syn to the chloro ligand) was observed in
1
each case by room temperature H NMR and from the solid-
state crystal structures. This may be credited to electrostatic
interactions, the steric demand of the aniline moieties, and
hindered rotation around the C−C bond of the pendant
aminophenyl group and central pyridine ring. Interestingly,
similar rhenium bipyridyl complexes with o-methoxyphenyl or
o-benzoic acid rings at the 6 position of the bipyridine ligand
produced the anti isomer, where the functional group is
oriented away from the chloride donor.31,32 Thermal ellipsoid
plots of 1-Re and 2-Re are shown in Figure 3, and selected
bond lengths and angles are reported in Table 1.
In this work, we sought to investigate the effect of pendant
aniline groups on catalytic CO2 reduction in a series of
substituted Re(bpy)(CO)3Cl compounds (Figure 1). By
Both structures depict slightly distorted octahedral geo-
metries around the rhenium(I) center with three carbonyls in a
facial arrangement, a κ2 bipyridyl ligand, and an anionic chloro
donor. In 1-Re, the dihedral angle between the aminophenyl
ring and adjacent pyridyl ring is 74.25°. The dihedral angle
(64.58°) of 2-Re is smaller, as expected given the reduced
steric demand of this isomer, and similar to the dihedral angle
(68.53°) of a related complex containing an unsubstituted
phenyl group at the 6 position of 2,2′-bipyridine.33
Figure 1. Structures of the catalysts studied in this work.
The Re−N2 bond distances in both 1-Re and 2-Re are
0.055 Å longer than that of the adjacent, unsubstituted
rhenium−pyridine bond distance (Re−N1) due to the steric
bulk imposed by the aniline substituents. Unlike ReBpy, the
bipyridine units in 1-Re and 2-Re are not coplanar with the
Re(CO)2 plane formed by the trans-carbonyl donors.34
systematically altering the position of the primary amine on
the aniline moiety at the 6 position of each 2,2′-bipyridine
donor, we have developed a series of positional isomers to
correlate the reactivity of each catalyst with the distance
between the pendant amine and rhenium active site.
Figure 2. Synthesis of isomeric aniline-substituted bipyridine ligands and their corresponding rhenium tricarbonyl compounds.
B
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Figure 3. Crystal structures of 1-Re (left) and 2-Re (right) with thermal ellipsoids rendered at the 50 and 70% probability levels, respectively.
Hydrogen atoms and outer-sphere solvent molecules have been omitted for clarity.
observed. The Re−Cl bond length is shorter in 2-Re than in
1-Re, but both have slightly longer Re−Cl bonds relative to the
unsubstituted ReBpy complex. The distances between the
pendant amine nitrogen atom and the chloro ligand are
3.989(4) and 6.412(2) Å for 1-Re and 2-Re, respectively.
Electrochemical Studies under N2. To assess the redox
behavior of the complexes under noncatalytic conditions, cyclic
voltammetry was performed in N2-saturated anhydrous
CH3CN solutions containing a 0.5 mM complex and 0.1 M
Bu4NPF6 as the supporting electrolyte (Figure 4). All reported
potentials are referenced versus the ferrocenium/ferrocene
(Fc+/0) redox couple.
As observed with ReBpy, the three aniline-substituted
isomers show a quasi-reversible one-electron-reduction event,
followed by a more negative irreversible one-electron reduction
in the potential window studied. The reduction potentials and
diffusion coefficients of each complex are summarized in Table
2. On the basis of previous reports on ReBpy-type
Table 1. Selected Bond Distances and Angles of 1-Re, 2-Re,
and ReBpy
a
selected bond distances and angles
1-Re
2-Re
ReBpy34
Re1−N1
Re1−N2
2.162(3)
2.217(4)
1.932(4)
1.907(6)
1.900(5)
2.490(1)
74.6(1)
2.166(2)
2.221(2)
1.940(3)
1.920(2)
1.899(3)
2.4733(6)
75.09(7)
64.58
2.173(6)
2.176(6)
1.932(7)
1.938(9)
1.919(7)
2.460(2)
74.9(2)
Re1−C17
Re1−C18
Re1−C19
Re1−Cl1
N1−Re1−N2
b
dihedral angle
torsion angle
74.25
−10.6(6)
c
−9.6(3)
0.1(8)
a
b
All bond distances are reported in angstroms (Å). Dihedral angle
between the aminophenyl ring and adjacent pyridyl ring. Torsion
c
angle between the pyridine rings.
Torsion angles of −10.6(6)° and −9.6(3)° between the
pyridine rings of 1-Re and 2-Re, respectively, are also
Figure 4. CVs of 1-Re, 2-Re, 3-Re, and ReBpy at 0.5 mM concentrations in CH3CN/0.1 M Bu4NPF6 under N2 (black), CO2 (red), N2 with 4%
TFE (green), and CO2 with 4% TFE (blue). Scan rate = 100 mV s−1; glassy carbon working electrode.
C
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a
prominent oxidation wave discussed above upon cycling
through both reductions, where the redox couple becomes
less reversible. The smaller oxidation feature paired with the
prominent peak only appears, on this time scale, when cycling
through the second reduction. This observation is in
agreement with earlier electrochemical experiments on
ReBpy-type complexes, and thus the less prominent wave of
each complex is tentatively assigned to the oxidation of
[Re0(bpy•−)(CO)3]−.39,40
At more positive potentials (−0.3 to −0.5 V), a small
characteristic oxidation feature is observed, which is assigned
to the oxidation of neutral Re0−Re0 dimeric species based on
earlier reports involving [Re(bpy)(CO)3]2 complexes.44,45
This peak is larger at fast scan rates and appears upon cycling
through the second reduction. In comparison to the electro-
chemical behavior of ReBpy, we conclude that only a small
fraction of [Re0(bpy)(CO)3]• undergoes dimerization in the
aniline-substituted complexes because of the added steric bulk
of the ligand structure.
Table 2. Reductive Peak Potentials and Diffusion
Coefficients (D) of 1-Re, 2-Re, 3-Re, and ReBpy from Cyclic
Voltammetry of a 0.5 mM Complex in Anhydrous CH3CN/
0.1 M Bu4NPF6 Solutions under a N2 Atmosphere
b
catalyst
Ep1,c (V)
Ep2,c (V)
D (cm2 s−1
)
1-Re
2-Re
3-Re
ReBpy
−1.81
−1.73
−1.84
−1.79
−2.22
−2.14
−2.24
−2.15
1.31 × 10−5
1.18 × 10−5
1.36 × 10−5
1.72 × 10−5
a
Potentials are versus Fc+/0, rounded to the nearest 0.01 V, and
b
measured at v = 100 mV s−1. Glassy carbon disk working, platinum
wire counter, and silver wire quasi-reference electrodes.
complexes,18,35 the first reduction is bipyridine-based and
forms an anionic ligand-centered radical species, [ReI(bpy•−)-
(CO)3Cl]− (where bpy is used generically to encompass the
substituted bipyridyl ligands). This species can undergo slow
chloride loss through a ligand-to-metal charge transfer, in
which the six-coordinate bipyridine π-radical species is favored
over the neutral five-coordinate rhenium-centered radical
species,36 or by rapid dissociation of Cl− at the second
reduction. The second irreversible process is assigned to a
mostly metal-based reduction (ReI to Re0), resulting in a five-
coordinate anionic species that is best described as
[Re0(bpy•−)(CO)3]−, which can activate CO2.37,38 These
assignments are supported by infrared spectroelectrochemical
(IR-SEC) experiments (vide infra).
Cyclic voltammograms (CVs) under a wide range of scan
rates show that the reductive peak currents (ip,c) vary linearly
versus the square root of the scan rate for all four complexes
(Figures S16−S19), consistent with diffusion-controlled
homogeneous systems. The diffusion coefficients (D) were
determined by the Randles−Sevcik equation (eq 1) using the
slopes of linear fits associated with the first reduction.46
1/2
nFvD
RT
i
y
j
j
j
z
z
z
ip = 0.4463nFAC
(1)
k
{
The first reduction (Ep1,c) of the complexes varies by 110
mV. Similar reduction potentials for 1-Re and ReBpy are
In this equation, ip is the current response, F is Faraday’s
constant, A is the surface area of the electrode (0.07 cm2 for
the electrode used in this study), n is the number of electrons
associated with the redox event, C is the concentration of the
catalyst in solution, v is the scan rate, R is the ideal gas
constant, and T is the temperature in Kelvin.
observed for this process at −1.81 and −1.79 V versus Fc+/0
,
respectively, while 3-Re has the most cathodic initial reduction
at −1.84 V. Complex 2-Re has the most positive initial
reduction of the series (80, 110, and 60 mV more positive than
1-Re, 3-Re, and ReBpy, respectively) at −1.73 V. The metal-
centered reductions of 2-Re and ReBpy occur at very similar
potentials, whereas Ep2,c of 1-Re and 3-Re are 70 and 90 mV
more negative than the parent ReBpy complex, respectively.
The meta-substituted aminophenyl group clearly has the
smallest electronic effect on the complex, resulting in minor
changes in the reduction potentials relative to the parent
complex. In contrast and consistent with resonance arguments,
the ortho- and para-substituted aminophenyl groups provide
additional electron density to the ligand framework through a
resonance-donating effect and are responsible for the cathodi-
cally shifted reduction potentials.
Spectroelectrochemistry under N2. IR-SEC has been a
powerful tool for analyzing electrochemical reactions in real
time and providing insight into the underlying mechanisms of
rhenium and manganese tricarbonyl catalysts.9 An optically
transparent thin-layer electrochemical (OTTLE) cell47 was
used to monitor changes in the carbonyl stretching region of
IR spectra of catalyst solutions under an applied potential. The
experimental details are provided in the SI.
In their initial state, 1-Re, 2-Re, and 3-Re show three
carbonyl stretches (νCO) that are characteristic of fac-Re(CO)3
complexes (Table 3). The C−O stretching frequencies of the
The return scan of each complex is relatively complicated
due to possible dimer formation/cleavage, chloride dissocia-
tion, and solvent coordination.39−41 For 1-Re, only one
oxidation peak at −1.74 V corresponding to the first reduction
is observed in the return scan at 0.1 V s−1. At the same scan
rate, 2-Re shows a prominent oxidation peak at −1.69 V and a
smaller feature at −1.56 V; 3-Re exhibits similar behavior with
a prominent peak at −1.76 V and a smaller feature at −1.65 V.
The resolution between these two peaks becomes more
conspicuous at faster scan rates (Figures S13−S15). Indeed, 1-
Re begins to show an analogous oxidation at −1.6 V, appearing
as a shoulder on the prominent peak, at a scan rate of 1 V s−1.
In contrast, upon isolation of the first reduction event, only
one reversible oxidation peak is observed at around −1.7 V for
all three complexes, which is assigned to the oxidation of one-
electron-reduced species [ReI(bpy•−)(CO)3Cl]−.35,42,43 The
potential of this oxidation is analogous to that of the
Table 3. Carbonyl Stretches of the Complexes under Study
catalyst
νCO (cm−1
)
solvent
1-Re
2-Re
3-Re
ReBpy48
2023, 1919, 1895
2022, 1919, 1892
2022, 1919, 1891
2021, 1914, 1897
CH3CN
CH3CN
CH3CN
CH3CN
aniline-substituted complexes are very similar to those of
ReBpy.48 Our IR-SEC experiments were limited to 1-Re and
2-Re due to the poor solubility of 3-Re in CH3CN. CVs of 1-
Re and 2-Re in the OTTLE cell show two reduction waves
similar to the CVs performed with a glassy carbon disk
electrode. The C−O stretching frequencies observed for these
compounds under reducing conditions are summarized in
Table 4.
D
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consistent with previous IR-SEC studies.23,48 This species is
a result of rapid ligand exchange of the radical anion in a
coordinating solvent (CH3CN). Notably, this intermediate was
not observed with 1-Re, where labilization of the chloride
ligand results in a five-coordinate neutral species. The steric
bulk of the ortho-amine functionality presumably favors
formation of the five-coordinate species over the six-coordinate
solvent-bound complex. Likewise, increased electron density
from the conjugated o-NH2 group to the bipyridine donor
could limit the formation of such an intermediate.
Table 4. Experimental νCO Frequencies of Different
Intermediates Detected by IR-SEC Experiments under N2
a
complex
1-Re (cm−1
)
2-Re (cm−1
)
[Re(L)(CO)3Cl]
2023, 1919, 1895
2006, 1895, 1879
2022, 1919, 1892
2000, 1889, 1863
2007, 1892, 1871
1986, 1868, 1848
1946, 1848(br)
[Re(L)(CO)3Cl]•−
[Re(L)(CO)3(CH3CN)]•
[Re(L)(CO)3]•
1984, 1865, 1853
1947, 1847(br)
[Re(L)(CO)3]−
[Re(L)(CO)3(CH3CN)]−
1992 (overlapped)
a
The IR spectra are shown in Figure 5. Experimental νCO frequencies
Interestingly, holding the potential just after the first
reduction wave of 2-Re concurrently forms the five-coordinate
radical [Re(L2)(CO)3]• species and the two-electron-reduced
[Re(L2)(CO)3]− intermediate as a minor species. This
behavior is rationalized by the expected close reduction
potentials of pristine 2-Re and the neutral species [Re(L2)-
(CO)3(MeCN)]• in light of similar observations of related
rhenium complexes.23,48 The peaks corresponding to the two-
electron-reduced species grow in intensity when the potential
is held for a longer time. A shoulder is also observed at 1992
cm−1 and tentatively assigned to the two-electron-reduced six-
coordinate [Re(L2)(CO)3(CH3CN)]− species.23,48,50 The
lower-energy peak of this solvento species is likely obscured
by the broad peak of [Re(L2)(CO)3]−.
The νCO frequencies of the proposed intermediates en route
to generating the catalytically active species [Re(L)(CO)3]−
for both 1-Re and 2-Re and some related complexes are
presented in Table S2. In 1-Re, formation of the active species
[Re(L1)(CO)3]− was only observed at the second reduction
potential. In contrast, the active species of 2-Re was generated
at the first reduction wave. CVs performed under catalytic
conditions at a slower scan rate also support this observation
(vide infra).
of related complexes are summarized in Table S2.
When the applied potential is held at the first reduction
peak, the initial carbonyl signals start to diminish and a new set
of lower-energy peaks begin to emerge. For 1-Re, the νCO
modes of the new species are red-shifted by ∼20 cm−1 relative
to the initial complex (Figure 5). This change in energy is
consistent with a one-electron ligand-based reduction, and we
assign this species to [Re(L1)(CO)3Cl]•−, in accordance with
previous studies.18,48,49 Holding the potential at the tail of the
first reduction peak forms the chloride-dissociated neutral
species [Re(L1)(CO)3]•, which results in another ∼20−30
cm−1 red shift of the C−O vibrational modes. Scanning toward
more negative potentials (onset of the second reduction wave)
quickly gives rise to peaks corresponding to the five-coordinate
[Re(L1)(CO)3]− species at the expense of [Re(L1)(CO)3]•.
The two-electron-reduced intermediate [Re(L1)(CO)3]− is
widely proposed to be the catalytically active species that is
responsible for CO2 activation.18,38,48 Although the C−O
absorption bands of this species are overlapped by [Re(L1)-
(CO)3]• in the IR spectrum, the high-energy band of
[Re(L1)(CO)3]− at 1947 cm−1 (with a characteristic ∼40
cm−1 shift relative to [Re(L1)(CO)3]•) and a broad lower-
energy band at 1847 cm−1 are clearly visible in the
spectrum.44,45
Notably, no dimeric intermediate was detected for any of
our complexes during IR-SEC studies, presumably because of
the steric demand of the aminophenyl substituents. However,
in oxidative scans of CVs at fast scan rates, evidence of a
dimeric species in small quantities was observed. This
discrepancy prompted us to perform UV−vis spectroelec-
trochemistry (UV−vis SEC) to determine whether Re−Re
bond formation was occurring under reducing conditions as
the dimer has a characteristic absorption band centered around
IR-SEC of 2-Re shows slightly different behavior relative to
1-Re. At the first reduction potential, the C−O stretching
frequencies shift to lower energies corresponding to [Re(L2)-
(CO)3Cl]•−. However, this radical anion rapidly converts into
a new species denoted by a shift in the carbonyl vibrational
modes to higher energy, which is ascribed to the neutral
CH3CN-bound intermediate [Re(bpy)(CO)3(CH3CN)]•,
Figure 5. IR-SEC reduction of 1-Re (left) and 2-Re (right) under N2. The assigned species are color-coded as follows: Re(L)(CO)3Cl (black),
[Re(L)(CO)3Cl]•− (red), [Re(L)(CO)3(CH3CN)]• (pink), [Re(L)(CO)3]• (green), [Re(L)(CO)3]−, and [Re(L)(CO)3(CH3CN)]− (blue),
where L = bipyridyl ligand.
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800 nm.36,44 Experimental details of the UV−vis SEC studies
are given in the SI.
At an applied potential corresponding to the first reduction,
a new absorption band at 521 nm appears in the UV−vis
spectra of both 1-Re and 2-Re (Figure 6). This absorption is
work (Figure 4). Catalysts 1-Re, 2-Re, and ReBpy show
crossing of the forward and return sweeps under CO2
atmosphere, indicating that adsorption to the electrode surface
or depletion of the substrate in the diffusion layer of the
electrode may be taking place.53,54 In general, the amino-
phenyl-substituted catalysts exhibit higher catalytic currents
relative to ReBpy. At a scan rate of 100 mV s−1, 1-Re, 2-Re, 3-
Re, and ReBpy show 8.8-, 11.0-, 6.6-, and 4.4-fold current
enhancements as measured by the ratio of the catalytic current
(icat) relative to the reductive peak current observed in the
absence of substrate (ip). Complex 2-Re shows the largest icat/
ip value from the series with a catalytic half-wave potential
(Ecat/2) that is 50 mV more positive than that of ReBpy and
100 mV more positive than those of 1-Re and 3-Re.
Importantly, this indicates that the catalytic activity in this
series depends on factors other than mere catalytic over-
potentials, since 3-Re would be expected to be the most active
catalyst of the series on the basis of thermodynamic driving
force for CO2 reduction.
Figure 6. UV−vis SEC reduction of 1-Re (left) and 2-Re (right)
under N2. The assigned species are color-coded as follows: pristine
complexes (black); one-electron-reduced species (red); two-electron-
reduced species (blue).
The ratio of the catalytic current (icat) and corresponding
reductive peak current in the absence of a substrate (ip) serves
as a useful gauge to compare the activity of different catalysts.
According to the following electroanalytical equations (eqs 2
and 3), the catalytic rate constant (kcat) and turnover frequency
(TOF) are proportional to (icat/ip)2 under steady-state
conditions (where np is 1, the number of electrons transferred
in the noncatalytic reduction, and ncat is 2 for CO2 reduction to
CO, the number of electrons transferred in the catalytic
reaction).54
assigned to the one-electron-reduced species, consistent with
previous reports on related compounds.36,42,51 Holding the
potential at the second reduction gives rise to a new absorption
feature with a λmax value of 560 nm for 1-Re and 566 nm for 2-
Re, which are comparable to previously reported two-electron-
reduced rhenium bipyridyl complexes.26,36,42 Notably, the
characteristic absorption band for the Re−Re dimer is absent
in the spectra of both complexes.
In previous studies, the Re0−Re0 dimer was also not
observed during IR-SEC experiments with ReBpy in
CH3CN.48 It was proposed that [Re(bpy)(CO)3]• reacts too
quickly with the coordinating solvent for dimerization to
occur.39,48 It has also been shown spectroscopically that
substitution with bulky tert-butyl groups at the 4 and 4′
positons of ReBpy slows down or prevents dimerization.18,44
We reason that substitution at the 6 position of bipyridine will
have a similar effect. Finally, the SEC and cyclic voltammetry
experiments operate on different time scales.23,48 This may be
a contributing factor behind the absence of dimer in the SEC
studies.
Electrochemical Studies under CO2. Following the
studies under an inert atmosphere, cyclic voltammetry was
performed in CO2-saturated anhydrous CH3CN solutions to
assess the catalytic activity of 1-Re, 2-Re, and 3-Re for CO2
reduction. The first redox event for all three complexes remains
largely unchanged in the presence of substrate. Scanning
toward more negative potentials gives rise to catalytic waves
near the second reduction process (Figure 4). This behavior is
comparable to that of ReBpy. From the IR-SEC results, 2-Re is
expected to form a small amount of reactive intermediate for
CO2 binding and reduction at the first reduction potential,
which would allow catalysis to occur at a lower overpotential.
Although this was not observed in CVs conducted at higher
scan rates, the catalytic current does initiate at the first
reduction of 2-Re at slow scan rates (Figure S20). This
behavior is not as prominent with the other complexes studied
here, particularly 3-Re and ReBpy. We note that similar results
were obtained with ruthenium polypyridyl complexes by ligand
modification, which altered the catalytic cycle and enabled
catalysis at a lower overpotential.52
kobs = kcat[CO2]
(2)
2
2
3
i
y
z
z
z
z
z
z
Fvnp i
y
z
z
z
z
z
ji
0.4463
j
cat
j
j
j
j
j
j
TOF = kobs
=
j
j
j
RT
k
n
ip
cat
{
(3)
k
{
Cyclic voltammetry as a function of the scan rate was
performed in N2- and CO2-saturated solutions (Figure S21).
From these data, icat/ip values were obtained at each scan rate
and TOFs were calculated using eq 3, which are plotted versus
the scan rate in Figure S22. Although “S-shaped” catalytic
waves were not observed with these systems, scan-rate-
independent TOF values were obtained and are reported to
allow a comparison to literature values. The estimated TOFs
determined are 31.7, 98.8, 30.1, and 14.6 s−1 for 1-Re, 2-Re, 3-
Re, and ReBpy, respectively.
Deviations from ideal “S-shaped” catalytic waves in the CVs
of these catalysts prompted us to apply foot-of-the-wave
analysis (FOWA; eq 4). From this electroanalytical expression,
kcat can be determined and used to find the maximum TOF
(TOFmax) under saturation conditions using eq 5.55 In this
method, the onset of the catalytic wave is analyzed in lieu of
the peak catalytic current to avoid nonideal behavior, such as
substrate depletion and catalyst deactivation, in calculating the
rate of catalysis. The catalytic rate constant (kcat) was
determined from the linear portions of the slopes of plots of
i/ip versus 1/[1 + exp{(F/RT)(E − Ecat)}], where Ecat is the
reduction potential of the catalyst associated with catalysis.
The plots are shown in Figure S23, and the maximum TOFs
obtained by FOWA are 45.5, 127, 58.8, and 21.2 s−1 for 1-Re,
2-Re, 3-Re, and ReBpy, respectively. Table 5 contains Ecat/2
,
icat/ip, and TOF values determined using both methods for all
four catalysts. Noticeably, the TOF values determined by the
two methods are comparable and display the same trend. In
The overpotentials and catalytic current increases are
considerably different for all four catalysts investigated in this
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catalysis for each complex, as is evident by increases in the icat/
ip values and the calculated TOFs in the presence of 4% TFE,
which are summarized in Table 6. The same trend in catalytic
Table 5. Summary of the Results of Electrocatalysis in CO2-
Saturated Anhydrous CH3CN
a
b
c
catalyst
Ecat/2 (V)
icat/ip
TOF (s−1
)
TOF (s−1
)
1-Re
2-Re
3-Re
ReBpy
−2.17
−2.07
−2.17
−2.12
8.8
11.0
6.6
31.7
98.8
30.1
14.6
45.5
127
58.8
Table 6. Summary of the Results of Electrocatalysis in CO2-
Saturated CH3CN in the Presence of 4% TFE as the Proton
Source
a
4.4
21.2
b
c
catalyst
Ecat/2 (V)
icat/ip
TOF (s−1
)
TOF (s−1
)
a
Potentials are referenced versus Fc+/0 couple. The catalytic half-wave
1-Re
2-Re
3-Re
ReBpy
−2.05
−1.98
−2.04
−1.98
14.2
25.0
14.3
13.4
123
239
109
72.9
47.5
138
69.7
potential (Ecat/2), catalytic current (icat), and corresponding peak
current in the absence of a substrate (ip) were measured by cyclic
voltammetry at v = 100 mV s−1 in anhydrous CH3CN/0.1 M
Bu4NPF6. The reported TOF values have a standard deviation of
<5%. TOF calculated using eq 3. TOFmax obtained from FOWA
(eqs 4 and 5), where Ecat/2 was used for Ecat in eq 4.
47.5
b
c
a
Potentials are referenced versus Fc+/0 couple. The catalytic half-wave
potentials (Ecat/2), catalytic currents (icat), and corresponding peak
currents in the absence of a substrate (ip) were measured by cyclic
voltammetry at v = 100 mV s−1 in CH3CN/0.1 M Bu4NPF6 solutions
containing 4% TFE. Reported TOF values have a standard deviation
of <5%. TOF calculated using eq 3. TOFmax obtained from FOWA
(eqs 4 and 5), where Ecat/2 was used for Ecat in eq 4.
general, the aminophenyl-substituted catalysts show faster rates
than that of ReBpy, while 2-Re is the most active system
overall with a TOF that is 2.8, 2.2, and 6 times higher than the
TOFs for 1-Re, 3-Re, and ReBpy, respectively.
b
c
RT
Fvnp
2.24
3 ncatkcat[CO2]
activity is observed under protic conditions with 2-Re, again,
showing the best performance with a TOF of 239 s−1. Catalysts
1-Re and 3-Re also exhibit better performances than the
benchmark ReBpy system with TOFs of 123 and 109 s−1,
respectively.
The peak catalytic current (icat) for an electrocatalytic
process that involves heterogeneous electron transfer can be
expressed by the following electroanalytical equation:64
i
ip
=
Ä
Å
Å
Å
É
Ñ
F
RT
Ñ
Ñ
1 + exp (E − E )
Å
Ñ
cat
(4)
(5)
Å
Ç
Ñ
Ö
TOFmax = kcat[CO2]
Effect of Brønsted Acids in CO2 Reduction. Next, the
catalytic activity of these systems was assessed in the presence
of added external Brønsted acids in order to facilitate the
proton-coupled reduction of CO2. A wide range of proton
sources have been used in the literature that have produced
lower overpotentials (η) while enhancing the catalytic activity
of various systems in organic solvents. However, it has also
been observed, in the presence of proton sources, that the
selectivity is compromised for some catalysts that favor H2
evolution over CO2 reduction due, in part, to the comparable
standard potentials of these reactions.7,56−58 We examined four
weak Brønsted acids of different pKa values in CH3CN: water
(H2O; pKa = 38−41),59 methanol (pKa = not reported),
trifluoroethanol (TFE; pKa = calcd 35.8),60 and phenol (pKa =
27.2).61 In the presence of these acids, changes to the current
response or peak shapes in the CVs of each catalyst under N2-
saturated conditions are minor, suggesting that the catalysts
will have insignificant proton reduction activity in the potential
window studied. Conversely, significant increases in the
current are observed with the rhenium catalysts when CO2 is
introduced (Figure S24). The catalytic current begins at the
onset of the first reduction in the presence of a proton source,
which is markedly different from the anhydrous conditions
(Figure 4).
Of the proton sources examined, TFE gives the highest icat/ip
value for the aniline-substituted catalysts and was chosen for
the following experiments. A linear increase in the catalytic
current as a function of the TFE concentration is observed
initially with 1-Re and 2-Re, before reaching a maximum icat/ip
value at 4% TFE and subsequently dropping to lower values
(Figure S30). This behavior is consistent with previous reports
involving rhenium- and manganese-based electrocata-
lysts.22,62,63
The TOFs of each catalyst in CH3CN/4% TFE solutions
were estimated via cyclic voltammetry by applying the two
electroanalytical methods described earlier (Figures S25−S27).
The addition of an external proton source improved the rate of
icat = ncatFA[cat](Dkcat[S]y)1/2
(6)
where [S] is the substrate concentration and [cat] is the
catalyst concentration. This equation was used to investigate
the reaction order with respect to the catalyst and substrate
concentrations and to gain insight into the catalytic
mechanism. Cyclic voltammetry was performed to record the
current response (icat) under variable concentrations of CO2,
TFE, and catalyst by systematically changing the concentration
of one species at a time. First, the catalyst concentration was
varied from 0 to 0.5 mM in CO2-saturated CH3CN. A linear
relationship is observed in plots of the peak catalytic current
(icat) from each CV versus the concentration of catalyst,
revealing a first-order dependence on the catalyst (Figure S28).
The maximum catalyst concentration was restricted to 0.5 mM
to avoid suspensions of the catalyst at higher concentrations in
CH3CN. Next, CVs were obtained where the CO2
concentration was varied in CH3CN solutions containing 0.5
mM catalyst. A first-order dependence on [CO2] was
determined (where y = 1) as the catalytic current increases
linearly as a function of [CO2]1 (Figure S29). The
concentration of TFE was also varied from 0 to 4% (v/v) in
CO2-saturated CH3CN with 0.5 mM catalyst and showed a
linear increase in the plots of icat versus [TFE], indicating a
second-order dependence on the acid ( [TFE]2 ) (Figure
S30). The rate law can be summarized as follows: rate =
k[cat][CO2][TFE]2, which indicates that the second proto-
nation event, resulting in C−O bond cleavage and the
elimination of H2O, is the rate-limiting step in this 2H+/2e−
reduction of CO2 to CO.26,57
Controlled Potential Electrolysis (CPE). Bulk electrolysis
experiments were performed on the complexes using an
airtight electrochemical cell with applied potentials corre-
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sponding to their peak catalytic currents in order to quantify
products and to assess catalyst stability. The experiments were
conducted with and without an external proton source to
determine the efficiency, stability, and selectivity under both
conditions. The experimental details are provided in the SI.
The CPE results are summarized in Table 7 and shown
graphically in Figure 7. Representative charge versus time plots
are given in Figures S31 and S32.
often observed, where carbonate is formed as a coproduct of
CO2 reduction (2CO2 + 2e− → CO + CO32−). Here, 1 equiv
of CO2 is reduced to CO and the other CO2 molecule acts as
an “oxide acceptor” to generate CO32−. Barium triflate was
added to the filtered solutions following electrolyses, which
caused significant precipitation of BaCO3 for all three aniline-
substituted catalysts, confirming the reductive disproportiona-
tion pathway under anhydrous conditions.65−67 Moreover,
analysis of the electrolyzed solution by FT-IR spectroscopy
showed the emergence of strong bands at 1647 and 1683 cm−1,
a
Table 7. Summary of 1 h CPEs under Different Conditions
−
2−
which can be assigned to HCO3 /CO3 species (Figure
b
H+ source
Eappl (V)
charge (C)
FECO (%)
S33).68,69
catalyst
1-Re
4% TFE
none
4% TFE
none
4% TFE
none
4% TFE
none
−2.10
−2.20
−2.05
−2.15
−2.15
−2.20
−2.12
−2.15
5.04 0.10
1.04 0.03
7.66 0.23
1.68 0.07
5.54 0.08
1.40 0.04
3.65 0.31
1.17 0.05
89
38
93
83
98
52
92
62
3
2
2
2
2
4
3
2
A proton source was added to solutions to facilitate the
proton-coupled reduction of CO2, and large catalytic current
enhancements were observed in the presence of TFE along
with higher FEs for CO2-to-CO conversion. Under these
conditions, carbonate was not observed as a coproduct,
consistent with the following reaction: CO2 + 2H+ + 2e− →
CO + H2O. The FEs for the four catalysts using TFE as a
proton source are exceptional and range from 89 to 98% for
CO production.
Rinse tests were performed after each CPE experiment to
determine if any heterogeneous material had deposited onto
the working electrode.70 The post-CPE working electrodes did
not show any sign of deposition, which suggests that the
catalysts are homogeneous and do not adsorb to the electrode
during bulk electrolysis experiments.
2-Re
3-Re
ReBpy
a
CPEs were performed with 0.5 mM catalyst concentration in CO2-
saturated CH3CN/0.1 Bu4NPF6 solutions using a glassy carbon rod
working electrode. Accumulated charges and FEs are reported as the
b
average of three runs. Applied potentials (vs Fc+/0) correspond to the
peak catalytic current observed for each system.
Structure and Activity. External Brønsted acids can have
dramatic effects on the catalytic mechanism, especially when
second-coordination-sphere functionality is present in the
ligand framework. It was recently demonstrated in a
manganese tricarbonyl catalyst with pendant ether groups
and in macrocyclic cobalt catalysts with second-sphere amine
groups that these groups can hydrogen-bond to the metal−
carboxylic acid intermediate and/or orient a hydrogen-bonding
network with Brønsted acid molecules to help stabilize the
intermediate and accelerate C−O bond breaking and the
elimination of H2O.63,71 The aniline-substituted complexes
reported here are also expected to organize hydrogen-bonding
interactions around the active site. Indeed, the crystal structure
of 2-Re shows the presence of a DMF solvent molecule that is
hydrogen-bonded to the meta-amine group (Figure S34),
which suggests that 2-Re can orient Brønsted acid molecules
toward the metal-bound substrate during catalysis, effectively
increasing the local concentration of protons to enhance the
activity.
In this work, the distance between the pendent amine and
rhenium active site of each catalyst is well-defined and was
systematically varied by preparing the ortho-, meta-, and para-
substituted isomers to probe the structure−activity relation-
ship. We anticipated that 1-Re would have the highest activity
within the series given the proximity of its amine to the active
site, enabling direct and/or short-range hydrogen-bonding
interactions during catalysis. Conversely, 3-Re was expected to
have modest improvements relative to ReBpy because its
amine is oriented away from the metal center and limited to
mediating long-range hydrogen-bonding networks or partic-
ipating in intermolecular interactions. The expectation for 3-
Re was borne out by the data. Surprisingly, however, the
activities of 1-Re and 3-Re are very similar and significantly
lower than that of 2-Re, which shows the fastest catalysis by a
factor of 2 and at the lowest overpotential under both
anhydrous and protic conditions.
Figure 7. FEs from CPEs with 0.5 mM catalyst in CO2-saturated
CH3CN/0.1 M Bu4NPF6 solutions under anhydrous conditions
(gray) or with 4% TFE (red).
In these experiments, CO was found to be the sole carbon-
containing product, and no H2 was detected. These results
demonstrate that all four catalysts strongly favor CO2
reduction over H+ reduction. In anhydrous solutions, ReBpy
has a Faradaic efficiency (FE) of 62
2%. This value is in
agreement with previous reports.56 Likewise, in the absence of
TFE, 1-Re, 2-Re, and 3-Re also show selective CO production
with FEs of 38 2%, 83 2%, and 52 4%, respectively.
Interestingly, 2-Re operates with a significantly higher FE
compared to the other complexes in this study. The total
amount of charge passed after 1 h for each system follows the
same trend, with 2-Re (1.68 0.07 C) accumulating the most
charge and 1-Re passing the least amount of charge (1.04
0.03 C). Notably, catalysis with 1-Re slows down after ∼2 h,
whereas 2-Re and 3-Re exhibit sustained catalysis for longer
periods of time. This observation is consistent with the
deactivation of 1-Re during catalysis. In the absence of a
proton source, the reductive disproportionation of CO2 is
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The shortest distance between the amine functionality and
metal center is found in 1-Re. Under anhydrous conditions, 1-
Re performs poorly during CPE experiments relative to its
isomeric counterparts, and catalysis slows down after ∼4
turnovers. This behavior indicates probable deactivation of 1-
Re during catalysis in the absence of a proton source. In fact, κ3
chelation of ligand L1 with first- and second-row transition
metals is known in the literature, where L1 acts as an N,N′,N″-
tridentate ligand and coordinates in a distorted meridional
fashion.72 During the catalytic cycle (following chloride
dissociation from the precatalyst), 1-Re likely forms a
deactivated species in which the −NH2 nitrogen atom binds
to the open coordination site needed for substrate activation
and catalysis, which may explain the lower than expected
activity of this system. A related compound [Re(bpy)-
(CO)3(NH2Ph)]+ bearing an aniline ligand has also been
reported in the literature.73 In addition, Mn(κ2-N,N′-
terpyridine)(CO)3Br was recently investigated as an electro-
catalyst for CO2 reduction, where the bidentate terpyridine
shifts to a κ3-N,N′,N″ coordination mode during catalysis,
ultimately leading to catalyst deactivation.74 Several attempts
to synthesize the proposed deactivated species of 1-Re were
unsuccessful in our laboratory. We note that, in the presence of
TFE, catalysis with 1-Re is sustained for a longer period of
time and was comparable to that the other catalysts within the
series.
On the basis of the results of IR-SEC experiments, we
hypothesize that 2-Re forms the catalytically active species at a
lower potential than its aniline-substituted counterparts, giving
2-Re a unique advantage during catalysis. This electronic
effect, along with the position of its hydrogen-bond-donating
amine group in the second coordination sphere, which is close
enough to enhance catalysis while avoiding deactivation, makes
2-Re the best catalyst of the series and a significantly more
active catalyst relative to ReBpy.
Comparison to Related Catalysts. Despite being known
in the literature for more than 3 decades, installing second-
coordination-sphere functionalities in fac-Re(bpy)(CO)3-type
catalysts is a relatively new avenue of research. Previously,
researchers appended phenolic groups, secondary and tertiary
amines, thiourea moieties, and positively charged imidazolium
groups to ReBpy-based catalysts, which aimed to stabilize the
metal carboxylate and/or metal−carboxylic acid intermediates
or to promote proton-transfer events that lead to C−O bond
cleavage (Figure 8 and Table 8).8,9,75
Inspired by the high activity of ReBpy in room temperature
ionic liquid,76 Nippe and co-workers have incorporated
imidazolium functionalities into the bipyridine framework to
assess the intramolecular effect of these cationic substituents on
catalysis (1).22 Similar icat/ip values relative to ReBpy were
observed but at a ∼170 mV more positive potential. The
decrease in overpotential was attributed to an intramolecular
hydrogen-bonding interaction between the imidazolium C2−
H hydrogen atom and Cl− ligand, which was proposed to
accelerate Cl− dissociation following the first reduction. A
higher icat/ip value is observed in the presence of an added
Brønsted acid.22
Marinescu and co-workers recently reported a series of
rhenium complexes featuring pendant secondary and tertiary
amines at the 6 and 6′ positions of 2,2′-bipyridine.24 In
contrast to our system, the reported complexes have the
amines directly attached to the bipyridine ligand. Overall, the
monosubstituted complexes (2a and 2b) outperform their
Figure 8. Related rhenium catalysts for CO2 reduction with pendant
functionality in the second coordination sphere.
Table 8. Catalytic Performance of Previously Reported
Rhenium Complexes Containing Second-Coordination-
a
Sphere Functionality
Ecat/2
FECO
(%)
TOF
catalyst
conditions
dry CH3CN
(V)
i
cat/ip
(s−1
NR
)
ref
1
−1.87
NR
NR
4.7
12
22
22
b
2−3 M H2O/
70
∼270
CH3CN
c
c
2a
2b
3
4a
4b
5a
5b
5c
2 M TFE/CH3CN −2.06
2 M TFE/CH3CN −2.32
73
58
89
88
70
84
17
40
18.3
29.5
NR
NR
NR
10.3
4.7
NR
NR
24
24
21
23
23
78
78
78
d
dry CH3CN
5% H2O/CH3CN
5% H2O/CH3CN
dry DMF
dry DMF
dry DMF
NR
NR
NR
−2.59
−2.32
−2.37
3040
NR
NR
NR
NR
NR
5
a
Catalyst structures are presented in Figure 8. NR = not reported.
CPE was done in 9.4 M H2O/CH3CN. Ecat. TOFmax value was
b
c
d
reported.21
disubstituted counterparts in terms of stability, TON, and FE.
No TOF values were reported in the study, but the icat/ip
values in anhydrous CH3CN and in the presence of TFE
clearly indicate that catalyst 2b mediates the fastest catalysis
within the series. However, the Ecat values are more negative
than that of ReBpy due to the added electron density from the
amine groups. The addition of a Brønsted acid improves the
catalytic current response and allows moderate FEs ranging
from 51 to 73% for the reported complexes.24
Neumann and co-workers have reported a ReBpy derivative
that has a thiourea moiety tethered in the second coordination
sphere (3).21 The thiourea tether features an electron-
withdrawing −PhCF3 group, which increases the acidity of
the hydrogen atoms and enhances the probable interaction
with CO2 molecules. Interestingly, thiourea in this catalyst acts
as a local proton source during the catalytic cycle, and the
addition of an external proton source inhibits catalysis.21 This
is in contrast to our system and other reported catalysts
utilizing amine or amide functional groups in the second
coordination sphere because these groups tend to form
hydrogen bonds with the catalytic intermediates or neighbor-
ing Brønsted acid molecules instead of donating protons.71,77
Nervi and co-workers reported two rhenium tricarbonyl
catalysts containing 1,3-dihydroxyphenyl (pdbpy) and 3,4,5-
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trihydroxyphenyl (ptbpy) groups attached to the 6 position of
a 4-phenyl-2,2′-bipyridine framework.23 Although TOF or kcat
values were not reported, apparent icat/ip values of ∼2.2 and
∼1.5 were obtained for 4a and 4b, respectively, in anhydrous
CH3CN. IR-SEC experiments under argon with 4a reveal the
reductive deprotonation of one of the hydroxyl substituents
after the first reduction and subsequent formation of a Re−O
bond. Upon reduction of this intermediate, a doubly
deprotonated anionic intermediate is formed. IR-SEC under
CO2-saturated conditions shows that the deprotonated
intermediate with a Re−O bond is the active catalytic species.
The catalysts also have significantly different FEs in the
absence or presence of external Brønsted acids. In anhydrous
solutions, 4a has a FE for CO production of 49%, whereas the
FE for 4b is 70%.23 This is similar to our findings, where meta-
substituted 2-Re performed better than ortho-substituted 1-Re
in anhydrous solutions.
White and co-workers recently reported a series of rhenium
complexes with methoxy-substituted 2,9-diphenyl-1,10-phe-
nanthroline ligands where methoxy groups are systematically
positioned around the phenyl groups in the ortho, meta, and
para positions (5a, 5b, and 5c).78 In contrast to our results, an
o-methoxy-substituted rhenium complex showed the best
catalytic performance, which was attributed to the formation
of a more nucleophilic rhenium center that can activate CO2
more effectively relative to the other isomers but at the expense
of a higher overpotential.78
Experimental details, synthetic procedures, character-
ization of the rhenium compounds, crystal data and
refinement details, and electrochemical and SEC results
(PDF)
Accession Codes
CCDC 1976889 and 1976890 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Jonah W. Jurss − Department of Chemistry and Biochemistry,
University of Mississippi, University, Mississippi 38677, United
States; orcid.org/0000-0002-2780-3415; Email: jwjurss@
olemiss.edu
Authors
Kallol Talukdar − Department of Chemistry and Biochemistry,
University of Mississippi, University, Mississippi 38677, United
States; orcid.org/0000-0001-5480-9276
Sayontani Sinha Roy − Department of Chemistry and
Biochemistry, University of Mississippi, University, Mississippi
38677, United States; orcid.org/0000-0002-2663-2810
Eva Amatya − Department of Chemistry and Biochemistry,
University of Mississippi, University, Mississippi 38677, United
States
Elizabeth A. Sleeper − Department of Chemistry and
Biochemistry, University of Mississippi, University, Mississippi
38677, United States
Pierre Le Magueres − Rigaku Americas Corporation, The
Woodlands, Texas 77381, United States
CONCLUSION
■
The synthesis, characterization, and electrocatalytic CO2
reduction activity of a novel series of fac-Re(CO)3 complexes
have been reported here and compared to the unsubstituted
benchmark ReBpy catalyst. The bipyridyl ligand was decorated
with aniline-based hydrogen-bond donors, and the distance
between the −NH2 group and the catalytic center of each
complex was varied systematically across the series. Catalysts
1-Re, 2-Re, and 3-Re are highly selective for CO2 reduction
with FEs of ≥89% for CO evolution in the presence of a
proton source. Notably, the TOF of 2-Re is significantly better,
no less than twice as high, compared to the rest of the catalysts
investigated here in both anhydrous and protic conditions. IR-
SEC and cyclic voltammetry experiments suggest that 2-Re can
access the catalytically active two-electron-reduced species at
lower overpotentials. Moreover, 1-Re suffers from deactivation,
as indicated by its low stability during bulk electrolysis
experiments. We reason that the relatively short distance
between the −NH2 group of 1-Re and the rhenium active site
results in coordination of the amine to the reduced metal
center during catalysis and subsequent deactivation of the
catalyst. The position of the second-coordination-sphere
functionality relative to the active site must be carefully
considered in order to enhance the catalytic activity while
avoiding catalyst deactivation and/or unintended electronic
effects. Ongoing efforts in our laboratory include the
methylation of these aniline groups to access quaternary
ammonium groups to investigate the influence of through-
space charge interactions on catalysis.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00154
Author Contributions
†K.T. and S.S.R. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation for generous
support through seed grant funding OIA-1539035 and a
CAREER Award CHE-1848478.
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