Homogeneous Electrochemical Reduction of CO2 to CO by a Cobalt Pyridine Thiolate Complex

Article abstract of DOI:10.1021/acs.inorgchem.9b03056

The chemical and electrochemical reduction of CO₂ to value added chemicals entails the development of efficient and selective catalysts. Synthesis, characterization and electrochemical CO₂ reduction activity of a air-stable cobalt(III) diphenylphosphenethano-bis(2-pyridinethiolate)chloride [{Co(dppe)(2-PyS)₂}Cl, 1-Cl] complex is divulged. The complex reduces CO₂ under homogeneous electrocatalytic conditions to produce CO with high Faradaic efficiency (FE > 92%) and selectivity in the presence of water. Through detailed electrochemical investigations, product analysis, and mechanistic investigations supported by theoretical calculations, it is established that complex 1-Cl reduces CO₂ in its Co(I) state. A reductive cleavage leads to a dangling protonated pyridine arm which enables facile CO₂ binding through a H-bond donation and facilitates the C-O bond cleavage via a directed protonation. A systematic benchmarking of this catalyst indicates that it has a modest overpotential (～180 mV) and a TOF of ～20 s^-1 for selective reduction of CO₂ to CO with H₂O as a proton source.

Full text of DOI:10.1021/acs.inorgchem.9b03056

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Article
Homogeneous Electrochemical Reduction of CO2 to CO by a Cobalt
Pyridine Thiolate Complex
Md Estak Ahmed, Atanu Rana, Rajat Saha, Subal Dey,* and Abhishek Dey*
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ABSTRACT: The chemical and electrochemical reduction of CO2
to value added chemicals entails the development of efficient and
selective catalysts. Synthesis, characterization and electrochemical
CO2 reduction activity of a air-stable cobalt(III) diphenylphos-
phenethano-bis(2-pyridinethiolate)chloride [{Co(dppe)(2-PyS)2}-
Cl, 1-Cl] complex is divulged. The complex reduces CO2 under
homogeneous electrocatalytic conditions to produce CO with high
Faradaic efficiency (FE > 92%) and selectivity in the presence of
water. Through detailed electrochemical investigations, product
analysis, and mechanistic investigations supported by theoretical
calculations, it is established that complex 1-Cl reduces CO2 in its
Co(I) state. A reductive cleavage leads to a dangling protonated
pyridine arm which enables facile CO2 binding through a H-bond
donation and facilitates the C−O bond cleavage via a directed protonation. A systematic benchmarking of this catalyst indicates that
it has a modest overpotential (∼180 mV) and a TOF of ∼20 s−1 for selective reduction of CO2 to CO with H2O as a proton source.
the polypyridyl-based catalysts, fac-M(CO)3 bpy (where M =
Re and Mn) are selective toward CO generation at moderate
overpotentials. Structural improvisations in the bpy ligand of
these catalysts, keeping the basic core of these catalysts intact,
led to activation of low-energy pathways substantially lowering
the overpotential (additional energy required to the
thermodynamic threshold) without compromising high
rates.24,25,39−41
INTRODUCTION
■
An urgent search for alternative clean renewable energy
sources has encouraged the scientific community to develop
new technologies for mitigating CO2 to value-added products
through intermittent energy sources, e.g., solar, wind, and so
on.1,2 CO2 can be reduced to produce various products which
are not only important for storing carbon but also provides a
way to valorise CO2 present in the atmosphere upon capture,
e.g., CO, HCOOH, CH4, CH2CH2, C2H6, and so on.3−5 In
biological methanogenesis, CO2 is converted to CH4. Similarly,
CO2 is converted to CO industrially and hydrogenated to
produce liquid fuel via Fischer−Tropsch synthesis.6 From the
early 1980s, several complexes of first or second row transition
metal complexes have been investigated for electrochemical
CO2 reduction.4,7 (Figure 1). Several metalo-phthalocya-
nines8,9 and metalo-porphyrins10,11 as well as di- to poly-
pyridyl complexes of rhenium,12 ruthenium, rhodium, and
iridium were developed.13−15 Apart from these, macrocyclic
complexes of cobalt and nickel evolved as electrocatalysts for
CO2 reduction almost at the same time.16,17 Among the recent
developments, Ni-cyclam,18,19 Fe-porphyrin,20−22 fac-Mn-
(CO)3(R-bpy)Br,23−25 and Co/Fe/Ni-polypyridine com-
plexes5,26−33 have shown promising activity for electrochemical
CO2 reduction. Some catalysts are reported to produce
Most of the molecular catalysts reported to show 2e−
reduction of CO2 with moderate to high overpotentials.5,28,33
Tuning the selectivity of CO2 reduction between CO and
HCOOH is a major challenge in these catalysts.42,43 Since CO2
reduction requires a source of protons (CO2 + 2e− + 2H+ →
CO + H2O), competitive H2 evolution adds to the complexity
of the problem and effectively bifurcates away the implemented
electrons from the carbon-based products. Although the
selectivity for HCOOH production against H2 generation is
explained based on the intrinsic hydricity of a metal hydride
intermediate formed during catalytic turnover in combination
of the choice of proton sources. Till date, the literature is
lacking any rationalization to the selectivity over CO
Received: October 18, 2019
HCOOH selectively.30−32,34,35 Recently, Saveant and Robert
et al. modified the Fe-porphyrin to include second sphere
́
functionality, e.g., polymethyoxy, polyhydroxy, and trimethy-
+
lammonium (−NMe3 ), and those are efficient and durable
molecular catalysts for CO2 reduction to CO.33,36−38 Among
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Figure 1. Metal thiolate complexes known via electrochemical CO2 reduction. (top) Enzymatic active site of Mo−Cu CODH (left) and Ni−Fe
CODH (right). (A−C) Some representative synthetic complexes investigated for CO2 reduction. (A) Various kinds of first row transition metal
based macrocyclic complexes; (B) different bipyridine based complexes. (C) First row transition metal based thiolate complexes.
generation vs HCOOH or H2 production, more accurately M−
CO2n− formation versus M−H formation.44−46 To envisage the
general protocol for designing product selective catalyst,
further study of new complexes is highly desirable, although
some of the previous reported complexes have proved to
catalyze efficiently either of these processes. However, most of
these complexes can only function at sufficiently high
overpotentials as well as slower rates. Hence, the search for
new cheap catalysts that can catalyze the processes at lower
overpotentials with higher selectivity and rates is still ongoing.
The metalloenzymes like carbon monoxide dehydrogenases
(CODHs), nitrogenases (N2-ases), and formate dehydro-
genases which efficiently catalyze CO2 reduction are comprised
of thiolate and sulfide ligands around the metals.47−54
Unfortunately, metal thiolate complexes have not been
investigated for CO2 reduction except for some synthetic
Fe−S clusters which proved to be inefficient.55,56 Very
recently, some Ni and Mo complex with synthetically
challenging bioinspired dithiolate ligands were reported to
catalyze CO2 reduction and found inefficient in terms of
overpotentials as well as product selectivity.57−59 Our group
reported a CODH inspired Co pyridine dithiolate complex
that reduces CO2 to CO selectively at very low overpotentials
in the presence of water as the proton source.60 The electron-
rich thiolate ligand in this complex not only contributes to
activation of Co(I) state for CO2 activation (there by lowers
the overpotential) but also transfers the proton to a rate-
determining Co(III)−COOH intermediate, selectively to a
hydroxyl group, resulting in fast and selective CO generation.
In this manuscript, CO2 reduction by a new cobalt(III)-
bis(2-thiopyridinato)-diphenylphosphenoethane chloride com-
plex (1-Cl) is described. This complex can be synthesized from
readily available chemicals. A detailed electrochemical
investigation establishes that this complex, once reduced, is
capable of catalyzing CO2 reduction to CO in the presence of
water as a proton source. Detailed electrochemical analysis and
B
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a
Scheme 1. Synthesis and Molecular Structure of 1-Cl
a
(left) Synthetic procedure of complex 1-Cl. (right) Molecular structure of the cationic part of 1-Cl determined by single-crystal X-ray diffraction.
The H atoms and Cl− counterion are omitted for clarity.
theoretical calculations help us to gain insight into the high
selectivity for CO production over HCOOH and H2.
performed in CH3CN in the presence of 0.1 M n-
tetrabutylammonium perchlorate (TBAP) as supporting
electrolyte, and the data showed a reversible 1e− redox process
at −1.18 V. A second irreversible 1e− reduction process
appeared at −1.95 V vs Fc+/0 (black line in Figure 2, left). The
peak to peak separation of 73 mV of first reduction process
compared to 67 mV of ferrocene indicate that the process at
−1.18 V is a 1e− redox process. We attribute that this redox
process as the CoI I I / I I redox process as tris-
pyridiniumthiolatozinc(II) complex does not show any redox
event in this potential range. The reduction potential value
matches well with the previously reported bipyridine
analogues.66 The second irreversible process at −1.95 V was
assigned as a 1e− reduction process of Co(II) → Co(I). The
irreversibility of the process is attributed as the dissociation of
one pyridine moiety from the metal center as the process
becomes quasi-reversible at high scan rates (>300 mVs−1)
(Figure S5a). Similar dangling of a pyridine arm upon
reduction was previously reported for nickel-pyridinethiolate
complexes.61,67 Variation of the scan rates shows that the peak
currents (Ip) associated with both of the redox events vary
linearly with the square root of the scan rates (Figure S5b) and
follow Randles−Sevcik diffusion equation, Ip = 0.4463(F/
RT)1/2np3/2FAD1/2[C0]ν1/2, where Ip is the peak current, F is
Faraday’s constant (F = 96 485 C mol−1), R is the universal gas
constant (R = 8.314 J K−1 mol−1), T is temperature (T = 300
K), np is the number of electrons transferred, A is the active
surface area of the electrode, D is the diffusion coefficient of
the complex, [C0] is the concentration of the catalyst, and ν is
the scan rate (V/s). Using the equation, the electrochemical
diffusion coefficients of 1-Cl and 1 are determined to be 4.5 ×
10−6−4.2 × 10−6 cm2 s−1 for first and second reduction
processes (see the Supporting Information). An almost similar
slope in the Ip versus ν1/2 plot for both of these processes
indicates homogeneous behavior of the complex in electrolytic
solution. The homogeneous electrocatalysis has been inves-
tigated in CH3CN in the presence of H2O. The result shows
almost no change in both redox events (blue line in Figure 2,
left) except for a 30 mV positive shift in 1/1− redox process,
which might originate from the greater solvation of anionic
species due to an increase in the solvent polarity due to the
addition of water.68,69
RESULTS AND ANALYSIS
■
Synthesis and Characterization. Complex 1-Cl is
synthesized in two steps. Addition of a acetonitrile solution
containing 2 equiv of pyridine-2-thiol to a suspension of
presynthesized Co(dppe)Cl2 in acetonitrile, anaerobically,
results in a brown solution. Then, 2 equiv of triethyl amine
was added to deprotonate the thiols. Finally, aerobic oxidation
of the yellow brown solution and subsequent purification gives
yellowish green crude complex 1-Cl in good yield (∼76%)
(Scheme 1, left). The synthesis can be achieved in
tetrahydrofuran (THF) as well (yield ∼ 67%). A similar
protocol was previously used for synthesizing tris-pyridine-
thiolatonickel complex by Eisenberg et al.61 Complex 1-Cl is
characterized by UV−vis, 1H NMR, ESI-MS, and single-crystal
X-ray diffraction. The UV−vis spectra in CHCl3 show
1
absorption bands at 284, 318, 365 nm (Figure S2). The H
NMR data in CDCl3 ensures that the complex is fully
diamagnetic in solution (Figure S3). X-ray quality yellowish
green needlelike single crystals are grown by slow diffusion of
diethyl ether in a concentrated solution of 1-Cl in dichloro-
methane. The X-ray crystal structure is shown in Scheme 1
(right). In all the characterizations, it is confirmed that
complex 1-Cl exists in monomeric octahedral Co(III) low spin
form. As all of the ligands in 1-Cl are bidentate, they bind the
cobalt in an η2-fashion. The pyridines bind cis to each other,
whereas the thiolate ligations are trans to each other. The dppe
unit completes the coordination to yield the monomeric
octahedral complex. The bond distances and bond angles are
given in the Table 1. The Co−S bond distances (2.24−2.28 Å)
Table 1. Geometrical Parameters for Complex 1-Cl
Determined by X-ray Crystallography
bonds
bond distances (Å)
bond angles
bond angle values (deg)
Co1−S1
Co1−S2
Co1−P1
Co1−P2
Co1−N1
Co1−N2
2.2794(19)
2.258(2)
2.222(2)
2.211(2)
1.948(5)
1.952(5)
S1−Co1−N1
S2−Co1−N2
P1−Co1−P2
72.61(14)
72.71(15)
86.81(8)
Electrochemical CO2 Reduction. The catalytic activity of
complex 1-Cl is investigated in the presence of CO2 and H2O.
The CV of complex 1-Cl almost remains unchanged in
presence of CO2 except a small increase in the current
associated with CoII/I process (red line in Figure 2a). We
assume that the interaction of CO2 with the Co(I) center of 1−
is similar to the previously reported Co(dppe)PyS2 complex.
As H2O is added to the electrolyte solution, a faradaic process
arises and saturates at around −1.5 V (green line in Figure 2a).
are in good agreement with other reported low spin Co(III)−
thiolato complexes,62 and the Co−N bond distances (1.95−
1.96 Å) and N−Co−S bond angles (72.3−72.6 deg) are also in
good agreement with other low-spin Fe and Ni pyridine
thiolate complexes.63−65
Electrochemical Investigation. Under Argon. The
homogeneous cyclic voltammetry (CV) of complex 1-Cl is
C
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Figure 2. Electrochemical data for 1-Cl. (left) Overlay of the CVs of complex 1-Cl in CH3CN under Ar (black line), CO2 (red line), H2O (blue
line), and both together (green line). (right) Linear sweep voltammetry of 1-Cl with increasing concentration of water under CO2 atmosphere.
(Conditions: 0.5 mM of 1-Cl; 0.1 M TBAP as supporting electrolyte, glassy carbon as working electrode, Pt as counter electrode, scan rate 100
mVs−1).
This catalytic process increases with addition of water (Figure
2b, right) and is not observed in the absence of CO2. The large
positive shift of the catalytic process in the presence of CO2
and H2O together can be due to protonation of the pyridine
which dissociates upon reduction facilitating the further
reduction of 1−. Such ligand protonation assisted positive
potential shift in a reduction process has already been observed
for several pyridine thiolate complexes of Fe(II), Co(II), and
Ni(II).60,61,70−72 Specially trispyridinethiolatoNickel(II) com-
plex shows catalytic turnover for H+ reduction via pyridine
protonation in presence of acetic acid (pKa = 23.5 in
CH3CN).67 Notably, previous reports by Morris et al. showed
that depending on the solvent used the acidity of the
pyridinium proton can be tuned.73 Addition of H2O in CO2-
saturated acetonitrile generates H2CO3 through the equili-
brium reaction CO2 + H2O ↔ H2CO3, that can assist the
protonation of one of the pyridine ligands dissociated from the
metal center providing the open site to bind the substrate, i.e.,
either CO2 or H+. The product analysis corresponding to the
catalytic process is discussed below. The process at −1.5 V vs
Fc+/0 is second-order with respect to H2O, as the catalytic
current (Icat) varies linearly with concentration of H2O until
the concentration of H2O reaches a limiting value of 2.4 M.
Thus, we choose the H2O concentration 2.4 M for determining
the catalytic parameters and for mechanistic investigation.
Different mixture of CO2 and Ar gas is used and the data
showed that the Icat at −1.5 V vs Fc+/0 varies linearly with
square root of CO2 concentration, suggestive of first-order
kinetics (Figure S6). Thus, the catalytic process at −1.5 V vs
Fc+/0 is second- and first-order with respect to H2O and CO2,
respectively. These results are crucial for mechanistic analysis
of the reaction and will be discussed in a later section of the
article.
each applied potential using a Hg-pool electrode with 2.7 cm2
electrode surface area, and the head space gas is analyzed by
gas chromatography (GC) using a thermal conductivity
detector (TCD) to identify the product (Figure S7). The
background catalysis is monitored under the same conditions
without adding the complex in the electrolyte solution prior to
performing the electrolysis. The results show that without the
complex Hg-pool electrode itself produces very little CO or H2
at these potentials even after 20 min electrolysis. The amount
of charge consumed is substantially higher compared to the
background when complex 1-Cl is added in the electrolyte
solution. Electrolysis at −1.1 or −1.2 V does not yield CO (in
GC) more than the background after 20 min electrolysis.
Similarly, it does not show any peak corresponding to
1
HCOOH in the H NMR (or in ion chromatography (IC)),
so the process at −1.1 V is not catalytic. The peak intensities of
CO and H2 in GC increase when a potential of −1.3 V was
applied. Even within 5 min of electrolysis at −1.3 V, substantial
production of CO is observed. These data suggest that the
second catalytic process is responsible for CO2 reduction. As
the applied potential is further lowered, the amount of charge
consumed over bulk electrolysis (BE) experiments at same
duration increases almost linearly. The results showed that the
amount of H2 release also increased at more cathodic potential.
The ratio of CO/H2 changes from 17.3:1 to 6.5:1 on changing
the potential from −1.3 to −1.6 V (Figure S8). The faradic
yield (FY) for the electrocatalytic process is determined by the
displacement of water in an inverted buret set up from
electrolysis at −1.5 V over 2 h of electrolysis and is 91 6%
(Table S2). The electrolyzed solution is analyzed by 1H NMR,
and no chemical shift value corresponding to HCOOH is
observed. These results clearly suggest that complex 1-Cl is a
very efficient catalyst for CO2 reduction to CO in the presence
of H2O as proton source. The electrolysis is performed with
glassy carbon plate electrode as well at −1.5 V for 2 h, a total
FY of 92 4% is obtained (Figure S10). To determine the
product selectively, the head space is analyzed at the end of
electrolysis and it shows ∼12:1 ratio of CO/H2 at the end of 2
Controlled Potential Electrolysis (CPE) and Product
Analysis. As discussed earlier, there are two cathodic processes
in the presence of both CO2 and H2O. A series of electrolysis
experiments are performed at different potentials between
−1.1 to −1.6 V vs Fc+/0 having an interval of 0.1 V between
D
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Figure 3. Catalytic Tafel plot using TOFmax value obtained from scan rate dependent on CVs under optimized catalytic conditions. The catalytic
parameters are compared with other reported CO producing catalysts and other thiolate catalysts. Other catalysts or composites are represented
with respective colors. Proton source used during the catalysis are mentioned in the figure with the same color code. TFE = trifluoroethanol.
h of electrolysis, so complex 1-Cl catalyzes CO2 reduction to
CO with high selectivity irrespective of the electrode used. The
high selectivity for CO production is very similar to the
previously reported cobalt-pyridinethiolate catalyst60 and very
different from other reported metal thiolate catalyst, i.e.,
[Ni(qpdt)2]1− catalyst, which is more selective for CO2
reduction to HCOOH.58 A rinse test experiment was
performed on the glassy carbon electrode after 1 h of
electrolysis in a CO2-saturated wet acetonitrile solution
containing 2.4 M H2O without the catalyst showed no
catalytic wave which indicates that the catalytic activity is
originating from soluble complex 1-Cl (Figure S11).
Determination of Catalytic Efficiency. The efficiency of
electrocatalysts depends on their overpotential, inherent turn
over frequencies (TOFs, cycles completed in a second), turn
over numbers (TONs, maximum number of cycles possible
from 1 mol of catalyst during controlled potential electrolysis),
and rate of the catalysis, i.e., maximum TOF (TOFmax). Icat/Ip
is a direct estimate of the TOF although more detailed analysis
is required for a more accurate determination of TOF.36 The
TOFmax can be determined from catalytic response obtained at
various scan rates. If the catalytic currents do not vary at
different scan rates, which is the case here (Figure S12), then
than our previously reported cobalt-pyridinedithiolate catalyst,
but it functions almost at same potentials as the previous
one.60 Moreover the TOF from CPE experiment is determined
to be 17.6 s−1 (see the Supporting Information) which is in
reasonable agreement with the TOFmax determined from CV.
The determination of the overpotential is complicated for CO2
reduction in the presence of H2O. Recently, Mayer and Appel
et al. derived an easy way to determine ECO /COstandard from the
2
thermodynamic cycle of the reductive process which is given
74
standard
0
by ECO /CO
= ECO /CO − 0.059 × pKa vs Fc+/0
.
2
2
Assuming that the source of the proton under the electro-
chemical condition is in situ generated H2CO3 (pKa = 17.03),
as described by Artero, Fontecave, and co-workers,75 the
standard reduction potential for CO2 to CO production is
determined to be ECO /COstandard(H2CO3, CH3CN) = −1.12 V
2
vs Fc+/0. Using these values, the lowest overpotential for
complex 1-Cl is determined to be 180 mV as CO is detected
even at −1.3 V. This value is almost at the same range as some
of the best-known molecular catalysts for electrochemical
reduction of CO2 to CO.60 The cationic −NMe3 substituted
+
iron porphyrin catalyst shows 100−280 mV overpotential to
reach optimal activity.38 The quaterpyridine cobalt or iron
complex works at 200−400 mV overpotentials.76 The
bioinspired thiolato complex [Ni(qpdt)2]1− catalyst needs
500 mV overpotential to catalyze CO2 reduction to produce
HCOOH as a major product along with CO and H2 as minor
the TOFmax can be determined from the equation Icat/Ip=
25,30
4.484(RT/F)1/2(TOFmax
)
1/2ν−1/2
.
Accordingly, the
TOFmax for the second catalytic process, i.e., CO2 reduction
is determined to be 27 3 s−1, which is substantially lower
E
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Figure 4. Kinetic dependence of complex 1-Cl. (left) Overlay of the linear sweep scan at increasing concentration of H2O and D2O in CO2
saturated CH3CN with 0.5 mM of 1-Cl. The x-axis is offset by 1.2 V for D2O for clarity. (right) Icat/Ip vs concentration of Bronsted acid plot (here
H2O or D2O).
Scheme 2. DFT Computed Potential Energy Surface for the Reduction of CO2 to CO by the Co(I) State of Complex 1-Cl
products.58 To directly compare the catalytic activity of 1-Cl
with other metal thiolate complexes, we determine the catalytic
Tafel Plot (Figure 3). The plot shows that 1-Cl compares well
with the best known first row transition metal catalysts in
terms of overpotential. To determine the stability of the
catalyst, we determined the limiting TON from the CPE at
Mechanistic Consideration. From Electrochemistry. The
rate of CO2 reduction increases with increasing concentrations
of the catalyst (Figure S15) and it shows a linear dependence.
Icat is related to its catalytic components by the equation Icat =
ncatFA[cat](DKsub[sub])1/2 ([sub] denotes the substrate
concentration). Hence, the catalytic activity is first-order with
catalyst concentration. As mentioned earlier, the catalytic
current varies linearly with the square-root of CO2 partial
pressure, suggesting a first order kinetic dependence of CO2.
Also, the current varies linearly with the concentration of H2O
as is expected for a second order rate dependence.30 Taken
together, the rate of CO2 reduction can be expressed as rate =
k × [catalyst] × [CO2] × [H2O]2, where k is the rate constant.
To get further insight into the rate-determining step (rds,
which is the slowest step during catalytic turnover), the
catalytic activity is determined in D2O instead of H2O. The
−1.5 V to be 75
4 over 2 h of electrolysis with Hg-pool
electrodes (2.7 cm2) and almost 22% of the complex is
degraded (Figure S13) (likely due to high affinity of Hg for
thiols; see the Supporting Information). However, with a glassy
carbon plate electrode, only 4% degradation of 1-Cl was
observed (Figure S14) and hence TON is determined to be
much higher (>1 × 105). These data support that complex 1-
Cl is a sustainable catalyst for CO2 reduction to CO at very
low overpotential at fast rates.
F
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data show that the catalysis proceeds at slower rates and shows
a second-order dependence on D2O (Figure 4). Primary
the metal by stabilizing the electron-rich oxygen atoms of CO2.
A similar bisdithiolato complex has been recently shown to be
very efficient and selective for CO2 reduction where
protonation of the thiolate arm was proposed.60 A similar
protonation of the thiolates here leads to a CO2 binding energy
of 26.8 kcal/mol which is much higher than the CO2 binding
energy of the pyridyl protonated complex and is unlikely to be
catalytically active. The CO2 binding step is followed by proton
transfer from the pyridyl nitrogen to the oxygen atom of the
bound CO2 in the [1-CO2-H] species which is computed to be
uphill by 4.5 kcal. The transition state (TS1, Scheme 2) for the
process is very low energy and involves one imaginary mode at
571 cm−1 which involves transfer of the proton from the
pyridyl nitrogen to the CO2 oxygen with simultaneous
shortening of the Co−C bond. The Co−C bond shortens
from 1.98 Å in [1-CO2-H] to 1.95 Å in [1-COOH]. The
Mulliken charge on the C atom of the [1-COOH] moiety is
0.38, and on O atoms, the charges are −0.34 and −0.57,
respectively. This ascertains the second protonation on the O
atom of the elongated C−O bond facilitating the release of a
H2O molecule and precludes any possibility of isomerization to
produce HCOOH. The mode of binding and the ensuing
protonation step has been proposed to be key to the observed
product selectivity, i.e., only CO and no HCOOH.60
kinetic isotope effect (kH/kD) of 6.28
0.5 is determined
(Figure S16) indicating that a proton is directly involved in the
rds.30,60 In analogy to the CodppePyS2 complex, it could be
proposed that either the formation of Co−COOH inter-
mediate or the second protonation of the Co−COOH species
could be the rds.60 As the first faradaic process around −1.1 to
−1.2 V has already proved to be noncatalytic, this reduction
process is likely associated with pyridine protonation to
generate a Co(II)−PyH+ (which we denote as 1-H) species.
Such ligand protonation associated noncatalytic increment of
current was also previously observed by nickel-pyridine thiolate
complex or nickel-amido complex catalyzed electroreduction of
proton to H2.70 We conclude that first reduction to Co(II)
species which follows the pyridine protonation by the in situ
generated H2CO3 facilitates the further reduction to Co(I) to
more positive potential. Then, the CO2 interacts with the
Co(I) center and gets reduced to CO. Hence, this catalytic
process can be represented as an ECEC mechanism. For
detailed insights into the mechanistic steps involved density
functional theory (DFT) calculations are performed and are
presented in the next section.
DFT Calculations and Structure−Function Correlation.
Geometry-optimized DFT calculations are performed to
elucidate the energy and barriers in the different steps involved
in the catalytic cycles and presented in Scheme 2. Furthermore,
structure−function correlations are developed to explain the
selectivity. To test the reliability of the method, an
optimization of the cationic part for 1-Cl is performed, and
the optimized structural parameters show good agreement with
the crystal structure (Table S3). From the experimental data
and previous literature reports, it is evident that one of the
pyridine ligands opens up on the reduction of 1 to 1− state,
and the electrochemical data also suggest that all the
components are important for the activity. From the reversible
nature of Co(III/II) redox couple even after scanning beyond
the irreversible second reduction process, it can be assumed
that the resting structure of 1+ is regenerated upon completion
of the redox cycles. This is also confirmed from the unchanged
UV−vis data after CPE indicating that 1+ remains unaltered
even after catalytic turnovers. The redox-triggered dangling
pyridine moiety provides the binding site for exogeneous CO2
,and after completion of catalytic turnover, it comes back to the
original oxidized form. Note that for further discussions, it is
assumed that all the Co(I) and Co(II) intermediates involved
during the catalytic turnovers are high-spin and Co(III)
intermediates are low-spin.
Further protonation of the pyridyl arm of [1-COOH] from
bulk proton generates [1-COOH-H]+ intermediate that is
exothermic by −16.4 kcal/mol relative to the [1-CO2-H]
intermediate. This [1-COOH-H]+ intermediate releases H2O
to give [1-CO]+. The H2O elimination step passes through a
transition state (TS2) which is 12.9 kcal/mol uphill. The TS2
involves a single imaginary frequency at 342 cm−1 which
involves a proton transfer from the pyridyl arm to the C−OH
oxygen with subsequent cleavage of the C−OH bond. The
computed TS2 barrier of 12.9 kcal/mol compares reasonably
well with the 13.8 kcal/mol barrier estimated from the
experimental rate of 122 s−1 using Eyring equation. We invoke
that this step is the rate-determining step because it fits all the
experimental data, i.e., rate linear with CO2 and H2O and a kH/
kD of 6.28 0.5. Finally, CO release and regeneration of the
oxidized catalyst is required for catalytic turn overs. Our
calculation suggests that CO release from [1-CO]+ species is
spontaneous and exothermic by −10.2 kcal/mol to regenerate
1+ which can re-enter the catalytic cycle. The possibility that
the pyridyl arm may also translocate proton from bulk solvent
cannot be eliminated.
CONCLUSION
■
In conclusion, complex 1-Cl is an active electrocatalyst for
CO2 reduction to CO with high selectivity that operates at low
overpotentials. The redox triggered hemilability of the pyridine
ligand provides a local proton source and dictates the CO2
binding to the reduced metal center through carbon center to
produce CO. The anionic thiolate ligands increase the electron
density on the Co(I) and effectively catalyzes the CO2
reduction. Benchmarking of this catalyst proved that it
functions at low overpotential with high rates. The mechanistic
investigations reveal the following: (a) Protonation of the
pyridine moiety shifts the CoII/I potential more positive
without compromising the electron density on the metal-
dithiolate unit required to activate CO2 which effectively
lowers the overpotential. (b) The Co−COOH bond covalency
plays a role in determining the selectivity for CO2 reduction.
(c) Low barrier (computed) proton transfer from the pyridine
The catalysis begins with the interaction of CO2 with [1open
-
NH] species (Scheme 2) that is generated by 1e− reduction of
the protonated Co(II) species in the presence of in situ
generated H2CO3 as discussed earlier. A similar protonation
feature was observed before for a Ni-imidothiol complexes in
the presence of proton source.
To initiate the CO2 reduction, CO2 is allowed to bind the
open site in [1open-NH] which leads to the formation of [1-
COO-NH] species, and this step is endergonic by 11.6 kcal/
mol. Substantial bending of O−C−O bond angle (131.5°) in
this metal-bound CO2 bound entity is suggestive of a strong
charge transfer from reduced Co(I) center to one of the π*
orbital of CO2 molecule. Additionally, the H-bonding donated
by protonated pyridine to one of the oxygen atoms of the CO2
moiety should help the CO2 binding through the C center to
G
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Inorg. Chem. XXXX, XXX, XXX−XXX

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pubs.acs.org/IC
Article
min), the sample was collected directly from head space. Throughout
the electrolysis period, the electrochemical solution was stirred.
Gas Detection by Gas Chromatography. The gas evolved
during BE was detected by using a GC instrument (model no. 7890B
(G3440B), serial no. CN14333203) fitted with a TCD; 400 μL of gas
was syringed out by a gas-tight syringe and was injected into the inlet
of the GC.
CO2 Percentage Dependence Using Mass Flow Meter. The
CO2 partial pressure dependence experiments were done by using
CVG Technocrafts India mass flow meter purchased from Chemix.
Mass flow rate of the gases were controlled manually during the
experiments using a gas regulator. In this experiment we used two
different gas flow meters with full setup (one for CO2 and another for
Ar). The outlet from these two flow meters were passed through
degassed bulk CH3CN in a closed vessel tightened with a rubber
septum. The outlet coming from this closed vessel was connected to
the electrochemical cell. During the CO2 concentration dependence
experiment, different CO2/Ar gas mixtures were used. During the
experiments, the defined gas mixture is purged through the
electrochemical solution for 20 mints before recording data. Once
the data are collected, the solution is purged again with N2 for 30 min
to remove the dissolved CO2 and then another portion of the gas
mixture is purged.
DFT Calculations. The geometry of all compounds is optimized
in gradient-corrected BP86 Functional in unrestricted formalism using
Gaussian 03 version C03. All the atoms are optimized using 6-31G(d)
basis set. An energy minimum is confirmed by performing frequency
calculation on the fully optimized structure using the same basis set
used for optimization to ensure no imaginary mode is present for all
these compounds. The transition states show only one imaginary
frequency along the reaction coordinates. The final energy
calculations were performed using 6- 311+G(d) basis set on all
atoms in PCM model using acetonitrile as a solvent and convergence
criterion of 10−10 Hartree. The energies reported are Gibbs free
energies which were calculated for all the hypothetical models from
the final optimized geometries and frequencies and are corrected for
zero-point energies.
Preparation of Bis(diphenylphosphino)ethane Dichloro
Cobalt(II). To a solution of anhydrous cobalt(II) dichloride (1.29
g, 10 mmol) in THF (40 mL) was added a solution of
bis(diphenylphosphino)ethane (3.99 g, 10 mmol) in THF (40 mL).
The color of the reaction mixture changed rapidly from blue to dark
green, and the resulting reaction mixture was stirred overnight. The
dark green suspension was filtrated and washed with diethyl ether.
After a long time under vacuum, a green powder was obtained (4.94 g,
9.3 mmol). Yield 93%.
moiety selectively to the oxygen results in facile (experimental)
C−OH bond cleavage to form CO. These features impart
cobalt 1-Cl complex with the ability to reduce CO2 to CO
selectively, at very low overpotential with high rates.
EXPERIMENTAL DETAILS
■
General Procedure. Electrochemical investigations were per-
formed under inert atmosphere in a N2 Glove Box from MBRAUN.
All the solvents used were purchased and used after distillation over
drying agents. Hexahydrated cobalt chloride (CoCl2.6H2O) was
purchased from Spectrochem Pvt. Ltd. (India). Bis-
(diphenylphosphinoethane) (dppe) and 2-pyridinethiol were bought
from Sigma-Aldrich. Glassy carbon, Pt, and Ag/AgCl (saturated KCl)
electrodes were purchased from Pine Instruments. The supporting
electrolyte n-tetrabutylamonium perchlorate salt was bought from
Sigma-Aldrich and used without purification. Caution: As perchlorate
salts are explosive, they should be handled with care. UV−vis absorption
data were recorded in an Agilent technologies spectrophotometer
model 8453 fitted with a diode-array detector. All the NMR spectra
were recorded on the Bruker DPX-400 or DPX-500 spectrometer at
room temperature. The mass spectra are recorded by QTOF Micro
YA263 instrument. X-ray single-crystal data were collected at 100 K
on a Bruker D8VENTURE Microfocus diffractometer equipped with
PHOTON II Detector, with Mo Kα radiation (λ = 0.710 73 Å),
controlled by the APEX3 (v2017.3−0) software package. Raw data
were integrated and corrected for Lorentz and polarization effects
using the Bruker APEX II95/APEX III program suite. Absorption
corrections were performed using SADABS. All the structures were
solved by direct methods and were refined against all data in the
reported 2θ ranges by full-matrix least-squares on F2 with the
SHELXL program suite98 using the OLEX 299 interface. Hydrogen
atoms at idealized positions were included during the final
refinements of each structure. The OLEX 2 interface was used for
structure visualization, analysis of bond distances and angles, and
drawing ORTEP100,101 plots. The CO and H2 gases evolved during
controlled potential electrolysis were detected on a GC instrument
(model no. 7890B (G3440B), serial no. CN14333203) fitted with a
TCD.
ELECTROCHEMICAL MEASUREMENTS
■
Cyclic Voltammetry. All electrochemical experiments were
performed using a CH Instruments (model CHI710D Biopotentiostat
Electrochemical Analyzer). Reference electrodes waswere purchased
from CH Instruments. A Pt wire was used as a counter electrode and
was purchased from CH Instruments. The measurements were made
against a leak-proof Ag/AgCl aqueous reference electrode (saturated
KCl). Anaerobic experiments were performed either in glove box or
within a 4-necked custom-made electrochemical cell by thoroughly
degassing the whole set up with Ar/N2 gas depending on availability.
The glassy carbon electrode was used as working electrode and was
freshly polished to get rid of all the contaminations out before each
single use.
Preparation of Cobalt(III)-bis(2-thiopyridinato)-diphenyl-
phosphenoethane Chloride Complex. To a suspension of
Co(dppe)Cl2(530 mg, 1 mmol) in acetonitrile was added a solution
of pyridine-2-thiol (222.32 mg, 2 mmol) in acetonitrile in N2
atmosphere, and the resulting mixture was stirred for few minutes.
Then, triethylamine (280 μL, 2 mmol) was added to the brown
mixture. The yellowish brown color solution was stirred in aerobic
condition. After 4 h, the solvent was evaporated by rotary evaporator
to reduce the volume to 10 mL. Diethyl ether was added to the
mixture to crash out the compound, and the mixture was filtered and
then dried to obtain a solid precipitate. The desired yellowish green
solid compound (575 mg, 0.766 mmol) was obtained in good yield.
1H NMR (CDCl3): δ 8.19 (2H, d), 7.52 (10H, s), 7.30 (4H, t), 7.12
Homogeneous Electrochemistry. A 0.5 mM anaerobic solution
of complex 1-Cl (in freshly distilled acetonitrile) with 0.1 M TBAP
were taken in a 4-necked two-compartment electrochemical cell
which was degassed by N2 gas prior to electrochemical experiment.
Then, CO2 gas was bubbled into the solution for 0.5−1 h, and the
electrochemical data were collected. The H2O dependence was done
by incremental addition of degassed deionized H2O in the
electrochemical solution by an air-tight syringe.
Controlled Potential Electrolysis and Gas Collection. The
CPE experiment was done in a custom-made two-compartment,
three-electrode electrochemical cell with a 2.7 cm2 Hg-pool working
electrode which was connected to an inverted buret for gas collection.
The gas evolved during BE was collected into the buret by vertical
displacement of water during long-term electrolysis, ca. 1−2 h. The
amount of gas evolved was measured from the volume of water
displaced during the experiments. For short-term electrolysis (20
(8H, m), 6.85 (2H, t), 6.33 (2H, d), 2.19 (4H, s). ESI-MS data (M)+
= 722.108 (m/z). Elemental Analysis: Found: C, 61.92; H, 5.51; N,
3.64. Calcd: C, 61.78; H, 5.45; N, 3.69.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03056.
H
https://dx.doi.org/10.1021/acs.inorgchem.9b03056
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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CCDC 1960041 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
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data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
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AUTHOR INFORMATION
Corresponding Authors
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■
̈
Subal Dey − School of Chemical Sciences, Indian Association for
the Cultivation of Science, Kolkata 700032, India;
orcid.org/0000-0002-5626-4879; Email: skdey85@
gmail.com
́
Electrochemical Reduction of Carbon Dioxide by Iron(0) Porphyrins.
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Abhishek Dey − School of Chemical Sciences, Indian Association
for the Cultivation of Science, Kolkata 700032, India;
orcid.org/0000-0002-9166-3349; Email: icad@iacs.res.in
Authors
Md Estak Ahmed − School of Chemical Sciences, Indian
Association for the Cultivation of Science, Kolkata 700032,
India; orcid.org/0000-0002-0817-2099
Atanu Rana − School of Chemical Sciences, Indian Association
for the Cultivation of Science, Kolkata 700032, India;
orcid.org/0000-0002-2397-5869
Rajat Saha − Department of Chemistry, Kazi Nazrul University,
Asansol, Paschim Bardhaman 713340, India
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research is funded by SERB grant EMR/008063 and
DST/TMD/HFC/2K18/90 from Department of Science and
Technology, Government of India. R.S. acknowledges the
TARE scheme from the Department of Science and
Technology, Government of India. M.E.A. and A.R. acknowl-
edge IACS Institute Fellow Scheme.
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