Activation of Co(I) State in a Cobalt-Dithiolato Catalyst for Selective and Efficient CO2 Reduction to CO

Article abstract of DOI:10.1021/acs.inorgchem.8b00450

Reduction of CO₂ holds the key to solving two major challenges taunting the society - clean energy and clean environment. There is an urgent need for the development of efficient non-noble metal-based catalysts that can reduce CO₂ selectively and efficiently. Unfortunately, activation and reduction of CO₂ can only be achieved by highly reduced metal centers jeopardizing the energy efficiency of the process. A carbon monoxide dehydrogenase inspired Co complex bearing a dithiolato ligand can reduce CO₂, in wet acetonitrile, to CO with ～95% selectivity over a wide potential range and 1559 s^-1 rate with a remarkably low overpotential of 70 mV. Unlike most of the transition-metal-based systems that require reduction of the metal to its formal zerovalent state for CO₂ reduction, this catalyst can reduce CO₂ in its formal +1 state making it substantially more energy efficient than any system known to show similar reactivity. While covalent donation from one thiolate increases electron density at the Co(I) center enabling it to activate CO₂, protonation of the bound thiolate, in the presence of H₂O as a proton source, plays a crucial role in lowering overpotential (thermodynamics) and ensuring facile proton transfer to the bound CO₂ ensuring facile (kinetics) reactivity. A very covalent Co(III)-C bond in a Co(III)-COOH intermediate is at the heart of selective protonation of the oxygen atoms to result in CO as the exclusive product of the reduction.

Full text of DOI:10.1021/acs.inorgchem.8b00450

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Activation of Co(I) State in a Cobalt-Dithiolato Catalyst for Selective
and Efficient CO Reduction to CO
2
†
†
Subal Dey, Md Estak Ahmed, and Abhishek Dey*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, India
*
S Supporting Information
ABSTRACT: Reduction of CO holds the key to solving two major challenges taunting the societyclean energy and clean
2
environment. There is an urgent need for the development of efficient non-noble metal-based catalysts that can reduce CO
2
selectively and efficiently. Unfortunately, activation and reduction of CO can only be achieved by highly reduced metal centers
2
jeopardizing the energy efficiency of the process. A carbon monoxide dehydrogenase inspired Co complex bearing a dithiolato
−
1
ligand can reduce CO , in wet acetonitrile, to CO with ∼95% selectivity over a wide potential range and 1559 s rate with a
2
remarkably low overpotential of 70 mV. Unlike most of the transition-metal-based systems that require reduction of the metal to
its formal zerovalent state for CO reduction, this catalyst can reduce CO in its formal +1 state making it substantially more
2
2
energy efficient than any system known to show similar reactivity. While covalent donation from one thiolate increases electron
density at the Co(I) center enabling it to activate CO , protonation of the bound thiolate, in the presence of H O as a proton
2
2
source, plays a crucial role in lowering overpotential (thermodynamics) and ensuring facile proton transfer to the bound CO2
ensuring facile (kinetics) reactivity. A very covalent Co(III)−C bond in a Co(III)-COOH intermediate is at the heart of selective
protonation of the oxygen atoms to result in CO as the exclusive product of the reduction.
1
0
11
INTRODUCTION
phthalocyanines or porphyrins were studied for the same
purpose with limited chemical analysis of the reduction
product. Fischer and Eisenberg used Co and Ni complexes of
various tetra-azamacrocycle as an electrocatalyst and got CO as
■
An ongoing, exhaustive, and urgent search for clean and
sustainable energy sources has progressively shifted the focus of
the scientific community toward the development of efficient
12
the major product along with H2 as a minor product.
Molecular systems utilizing rare-earth metals for CO reduction
were utilized as well.
ways for the mitigation of CO , the “greenhouse gas”, to useful
2
1
−3
2
chemicals.
CO can be reduced electrochemically through
2
13−15
Recent years have seen a rapid
two-, four-, six-, and eight-electron reduction pathways in
gaseous and solution phases to useful gaseous as well as liquid
fuels like carbon monoxide (CO), formic acid or formate (in
basic solution), oxalic acid or oxalate (in basic solution), ethane,
growth in reports of CO reduction by transition-metal
2
4
,16−19
complexes.
e.g., Mn, Fe, Co, Ni) have been in the focus recently. Most of
the first-row transition-metal catalysts reported in the literature
Among these the first-row transition metals
(
4
−6
methane, ethanol, and methanol.
In nature CO is reduced
2
−
+
can activate CO only in their formal “0” or “(−) ve” oxidation
by 8e and 8H to methane (CH ), through a process called
2
4
states (M0 ). Initial reports demonstrated that Ni(cyclam)
/n−
methanogenesis, which can, in principle, be used as a fuel to
7
can reduce CO to CO in aqueous solution on Hg cathode as a
working electrode with high selectivity.
reduced Ni(cyclam) is the active species for the catalysis, with 0
M ) formal oxidation state of the nickel center. It was
derive energy from. CO can also be reduced to CO, and
2
2
20,21
The two-electron
further reduction, by hydrogenation, can produce liquid fuels
by Fischer−Tropsch synthesis.
Homogeneous transition-metal-based catalysts have been
8
0
(
9
−11
utilized since 1970s for CO reduction.
Cobalt or nickel
2
phthalocyanines were shown to have electrocatalytic CO2
reduction properties. Later on sulfonated cobalt or nickel
Received: February 20, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00450
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Inorganic Chemistry
Article
Figure 1. Active site structure of CODH (left). Schematic presentation of the 2,6-dithiolatomethylpyridine-dppe cobalt(II) complex (1) (middle)
and its X-ray crystallographically determined structure (right).
a
Scheme 1
a
(
top) Synthetic procedure for 2,6-dithiolatomethylpyridinium hydrochloride (L1). (below) Synthetic procedure for 2,6-dithiolatomethylpyridine-
dppe cobalt(II) complex (1).
determined that in the case of Ni(cyclam), CO release from
[
(generally 0 or lower), which requires substantially more
21
Ni(cyclam)(CO)]+ is the rate-determining step. Later,
energy to reduce CO than what is thermodynamically
2
3
0
+
−
another Ni(macrocycle) was used during the electrocatalysis
required. Thus, the CO + 2H + 2e = CO + H O process
2
2
2
1
with an E0
as a CO scavenger to increase the catalytic current.
= −0.52 V versus normal hydrogen electrode
CO2/CO
Manganese bipyridyl tricarbonyl complexes could also reduce
(NHE) can only be achieved at potentials lower than −1.0 to
−1.5 V. This additional energy in an electrocatalytic system is
referred to as the overpotential. Large overpotentials are
detrimental to the efficiency of the catalytic process and have
deterred development of good CO2 reduction catalysts.
Alternatively, the large electron density on the reduced metal
CO to CO in the presence of Lewis acid in organic medium,
2
and these complexes are generally active in their formal −1
−
I
22,23
(
M ) oxidation state in monomeric form.
Metallo-
porphyrins (Fe, Co) can reduce CO in organic and aqueous
2
medium to useful chemicals in the presence of phenol as a
2
4−27
proton source.
Recently an iron porphyrin complex with
center, necessary for CO binding and activation, also makes it
2
+
four quaternary alkyl ammonium moiety have been reported to
susceptible to protonation by H , which is also needed for the
6
−1
31,32
achieve 1 × 10 s turnover frequency (TOF) at 220 mV
overpotential when a mildly acidic phenol is used as proton
donors, and this complex likely represents the most active
reduction of CO , resulting in H formation.
Protonation of
2
2
a reduced metal center produces a metal-hydride species that
−
+
tends to reduce CO to HCOOH, another 2e /2H reduction
product of CO2.
2
28
33−36
homogeneous CO reduction electrocatalyst. However, all of
Thus, control over the competitive metal
2
these porphyrinato complexes reduced CO2 at formal 0
hydride formation pathway, which will lead to H or HCOOH,
2
oxidation state.
The key challenges involved in CO reduction are (a) the
becomes a necessary, albeit daunting, task and has presented
itself as a formidable challenge in the area. Spectroscopic
investigations established the presence of at least two
intermediates in the reaction of iron(0) porphyrins with
2
binding of CO to a metal center via the C atom instead of the
2
more Lewis acidic O atom and (b) weakening (activating) the
37
II
2−
strong C−O bonds by backbonding into the C−O π*
CO2, namely, an Fe −CO species, stabilized by hydrogen
2
2
9
II
orbitals. Reduced transition-metal centers are known to
bonding, followed by an Fe −COOH species. While the
II
2−
bind CO through the carbon atom, where the high-energy
proton transfer to the Fe −CO species can be achieved by
2
2
II
filled d-orbitals of the metal activate CO by back bonding as
well. However, the high electron density needed to do so
requires the metal to be reduced to low formal oxidation states
methanol, heterolytic C−O bond cleavage of the Fe −COOH
species can only be achieved by a relatively stronger proton
donor like PhOH or organic acids, and weak proton source
2
B
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Figure 2. (a) Overlay of the CV at different conditions (0.5 mM of 1 in CH CN, scan rate: 0.2 V/s, GC working electrode, 0.1 M nBu NClO as a
3
4
4
supporting electrolyte). The black one is in dry acetonitrile; purple one is in the presence of 0.11 M H O in acetonitrile; green one is in the presence
2
of CO in dry acetonitrile; and the blue one is in the presence of both 0.11 M H O and CO . (b) CVs of Co(II/I) redox process of complex 1 at
2
2
2
different scan rate (2 mM in dry CH CN). (inset) i versus square root of scan rate plot. (c) Linear sweep voltammetry of 0.5 mM solution of 1 in
3
p
CO -saturated CH CN with increasing concentration of H O (scan rate: 0.2 V/s, GC working electrode). (d) Overlay of the CVs at different
2
3
2
concentration of CO in wet acetonitrile (scan rate: 0.2 V/s, GC working electrode. (inset) Catalytic peak current vs square root of % of CO plot.
2
2
3
7
leads to HCOOH. Unfortunately, use of very strong acid
protonates the Fe(0) center and catalyzes competitive hydro-
reduction of CO to CO under mildly acidic conditions. In the
2
absence of H O the complex does not show any catalytic
2
3
8
gen evolution. A few factors have thus been deemed to be
activity in CO -saturated acetonitrile solution. But when H O is
2
2
important in determining the selectivity of CO reduction, for
added, a large catalytic current is observed, which suggests the
2
example, back-donation capacity and hydricity of the metal
catalytic CO reduction in the presence of a Lewis acid. A
2
centers, pK of the acid source, and the site of protonation of a
substantial amount of potential advantages is observed due to
activation of Co(I) state, which could be quite impressive to
further catalyst design.
a
33,38,39
M-COOH intermediate.
Choice of metal to tune the
selectivity is as essential as ligand framework.
In nature, CO to CO reduction is catalyzed by the thiolate-
2
ligated Ni center in the active site (Figure 1, left) of carbon
RESULTS AND ANALYSIS
■
7
,40
monoxide dehydrogenase (CODH). CODH works near the
Synthesis and Characterization. The synthetic scheme of
the ligand and the complex is given in Scheme 1. The ligand
2,6-dithiolmethylpyridinium hydrochloride (L1) is synthesized
separately from 2,6-dihydroxymethylpyridine by previously
reported procedures (Scheme 1, top). L1 is deprotonated
with sodium methoxide in tetrahydrofuran (THF) and then is
thermodynamic potential (−0.52 V vs standard hydrogen
40−42
electrode (SHE)) of CO reduction.
The nickel center of
2
the C-cluster binds CO at Ni(0) state, and the adjacent iron
2
40,43
48
weakens the C−O bond to facilitate C−O bond cleavage.
Thus far, no first-row transition-metal thiolate catalyst has been
shown to activate CO in its formal +1 oxidation state and/or
added to a suspension of Co(dppe)Cl in THF to immediately
2
2
utilize a bound thiolate ligand acting as a proton source in CO2
result in a reddish-brown homogeneous solution. After 4 h of
stirring, filtration and removal of the solvent yields the complex
1. The purity of the complex is checked by elemental analysis,
reduction. A nickel complex containing a NiS motif, mimicking
4
the molybdopterin, has been recently reported, which can
reduce CO2 to formate with minor amounts of carbon
monoxide and hydrogen with an overpotential of 340 mV in
1
and the H NMR shows that it is a paramagnetic species as may
be expected for a Co(II) compound in solution (Figure S1).
The X-ray quality single crystals were grown from slow
diffusion of THF and hexane. The single-crystal X-ray
diffraction (XRD) data show that the complex 1 crystallized
in Co(II) form in a slightly distorted square pyramidal
geometry with an open coordination site. The Co−S
(2.229°) and Co−N (1.967°) bond distances are in good
4
4
acetonitrile. The paramount importance of thiolate proto-
nation in reactivity and redox potential is well-established in
45−47
several natural enzymes like hydrogenases and ferredoxins.
In this present report a bioinspired 2,6-dithiomethylpyridino
cobalt(dppe) (dppe = bis(diphenylphosphinoethane)) complex
1) was synthesized, and this complex is capable of efficient
(
C
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Figure 3. (left) FOW plot of complex 1 to determine the second-order rate at the kinetic region. (right) Catalytic Tafel plot using the TOFmax value
obtained from scan-rate-dependent CVs under optimized catalytic conditions. The red stars designate the TOF determined from BE.
agreement with other high-spin cobalt(II) thiolate complexes
known in the literature (Table S1).
a sealed electrochemical cell containing CO -saturated 0.5 mM
solution of 1 on Hg-pool electrode with surface area of 27 cm .
2
49
2
Electrochemical Investigations and Product Analysis.
The cyclic voltammogram (CV) of 1 using glassy carbon (GC)
electrode shows two quasi-reversible redox processes at −1.65
The chromatograms obtained from GC after electrolyzing the
solution at −1.6 V (Figure S5) and −2.2 V (Figure S6) versus
+/0
Fc suggest that the complex 1 reduces CO to CO at both
2
+/0
V (ΔE = 240 mV) and −2.12 V (ΔE = 270 mV) versus Fc
,
potentials. Trace amounts of H (3−5%) were also observed in
p
p
2
which corresponds to Co(II/I) and Co(I/0) redox processes in
acetonitrile (0.1 M nBu NClO supporting electrolyte; Figure
both cases. To further characterize the electrocatalytic process,
bulk electrolysis is performed over a range of potential from
4
4
+/0
2a, black). The CV corresponding to Co(II/I) process was
−1.2 to −1.6 V versus Fc for the same duration of time
recorded at higher concentration (2 mM) of 1 to obtain data at
different scan rates (Figure 2b), and the current versus square
root of scan rate shows a linear relationship (Figure 2b, inset)
as expected, and the diffusion coefficient is determined to be
(Figure S7, left), and the data suggest that the complex 1
reduces CO to CO (Figure S7, right) with a faradaic yield of
2
95 ± 2% (Table S2) at all potentials, and the data indicate that
CO to CO reduction rate (volume of CO produced) increases
2
1
.2 × 10− cm s using Randles−Sevcik eq (Figure S2).
7
2
−1
on lowering the applied potential (Figure S7, right). No H was
2
Addition of CO2 and H O (0.11 M) separately in the
detected at lower overpotentials, and only at higher over-
2
electrolytic solution shows a very little change on Co(I/0)
process but no effect on the Co(II/I) process (Figure 2a,
purple & green). However, when the CV of 1 is recorded in a
potential could minor amounts of H be detected (Figure S7,
2
right). Similarly, no HCOOH was detected in Fourier
transform infrared (FTIR) measurements performed with the
electrolytic solution after bulk electrolysis experiments (Figure
CO -saturated acetonitrile solution in the presence of 0.11 M
2
+/0
H O, the redox event at −1.65 V shifts to −1.27 V versus Fc
Figure 2b blue). The 380 mV positive shift in the first cathodic
S8). Thus, the results confirm that the CO is reduced to CO
2
2
(
predominantly by complex 1 in both Co(I) and Co(0) states.
Since the electrocatalysis of the Co(I) state has much lower
overpotential, the rest of the investigation is focused on that.
The shift of the Co(II/I) potential at low H CO concentration
process in the presence of CO and H O is likely due to
2
2
protonation of the Co(I) bound electron-rich thiolate ligand
due to the formation of H CO in CO -saturated wet
2
3
2
2
3
50
acetonitrile (Figure 2a, blue). Similar positive shift in
Co(II/I) potential due to thiolate protonation has been
reported for cobalt-dithiolene complexes by Eisenberg et al.
and reduction of CO at higher concentration is attributed to
2
thiolate-assisted protonation first CO reduction mechanism as
2
reported for polymethoxy-bipyridyl manganese tricarbonyl
5
1−53
31,32,54
and later supported by theoretical calculations.
Upon
complex by Carter et al.
Control potential electrolysis
addition of more water into the CO -saturated electrolytic
of a solution of complex 1, CO , and H O in CH CN using
2
2 2 3
solution, a steady increase in the electrocatalytic current (I ) is
both Hg-pool (Figure S9) and GC electrode (Figure S10)
indicates that the complex is stable through the duration of the
electrolysis experiment. Slight decay of current is observed in
the case of Hg-pool electrode, which suggests decay of the
catalyst likely due to the higher affinity of the thiol groups
present in the ligand for Hg. Rinse test experiment performed
cat
observed with an onset of −1.17 V (where I /I > 1), which
cat
p
1
/2
saturates at −1.46 V (half peak potential, E
= −1.29 V)
cat
2
with current density ∼1.1 mA/cm (Figure 2c). This current is
attributed to the electrocatalytic CO reduction by the Co(I)
2
state of the complex. The catalytic current shows linear
correlation with the concentration of water (Figure 2c) as well
after 30 min of electrolysis in a CO -saturated wet acetonitrile
2
as with the concentration of CO (Figure 2d, inset). The same
current is observed when bicarbonate and equivalent amount of
solution without the catalyst showed no catalytic wave
indicating that the catalyst does not get absorbed on the GC
electrode (Figure S11).
2
trifluoroacetic acid is used as the source of CO (Figure S3).
2
These results suggest that complex 1 activates CO in Co(I)
Kinetic and Thermodynamic Parameters. The catalytic
2
state and reduces it. The process at −1.46 V saturates at 2.1 M
parameters for CO reduction for the first cathodic process is
2
H O concentration, and further H O addition only enhances a
determined at −1.46 V, where the faradic current has an S
2
2
catalytic current at −2.2 V representing electrochemical CO2
shape. The pseudo-first-order rate of the catalysis (TOF ) is
max
reduction by complex 1 in its Co(0) state, which reaches up to
determined by the equation I /I = 4.484 × (RT/
cat
p
2
1/2
1/2 −1/2 33
a current density of greater than 10 mA/cm at 3.6 M H O
F) (TOFmax
)
ν
.
The TOFmax for CO reduction to
2
2
(
Figure S4). Thus, the complex 1 can reduce CO in both its
CO is determined from the scan rate dependence and found to
be 1559 ± 8 s− (Figure S12). The second-order rate of the
same process is also determined by foot-of-the-wave (FOW)
2
1
Co(I) and Co(0) states.
Headspace gas analysis was performed by gas chromatog-
5
−1 −1
raphy (GC) fitted with thermal conductivity detector (TCD) in
analysis (Figure 3, left) to be 2.53 × 10 M
s
(Figure
D
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Figure 4. (left) Linear sweep voltammetry of 0.5 mM solution of 1 in CO -saturated CH CN with increasing concentration of D O. (right)
2
3
2
Concentration of Bronsted acid vs catalytic current plot for the first catalytic process.
1
9,27
S13).
The rate of this complex, which utilizes water as a
estimated from the relative slopes to be 5.41 ± 0.4 (Figure
source of proton, thus compares quite favorably to those
reported for the best iron porphyrin complex using water as a
source of proton. The stability of the complex is checked
before and the after electrolysis by UV−vis spectroscopy
Figure S14). On a glassy carbon working electrode no decay is
S16). Thus, the rds of CO reduction catalyzed by 1 involves
2
protonation.
38
The CV and UV−vis absorption data of 1 in the presence of
CO or H O do not show any change implying that CO and
2
2
2
(
H O by themselves do not interact with either Co(II) or Co(I)
2
observed after 1 h of bulk electrolysis (BE), and a turnover
number (TON) ≫ 1 × 10 is estimated. After 2 h of
electrolysis with Hg-pool electrode ∼27% of the complex
decays (likely due to high affinity of Hg for thiols) yielding a
TON of only 300 (detailed calculation in Supporting
Information).
state of 1 (Figure 1a, purple & green lines, and Figure S17).
6
Yet, when both CO and H O are present in solution, the
2
2
Co(II/I) CV shifts by more than 300 mV. This implies that 1
gets protonated in the presence of H O and CO in its Co(I)
2
2
state. Since H O and CO forms H CO in CH CN, and the
2
2
2
3
3
lone pairs on the bound thiolates are available in 1, it is likely
The determination of the overpotential for the electro-
that the H CO formed protonates the thiolates. Similar shift in
2
3
catalytic CO reduction in the presence of water in organic
the Co(II/I) process is observed in the presence of trifluoro-
acetic acid (TFA) in case of electrocatalytic proton reduction
(Figure S18). Of course, in the presence of TFA, there is no
CO reduction, and H evolution is observed instead. The
2
55
solvents (e.g., acetonitrile) is nontrivial. Addition of H O in
2
CO -saturated acetonitrile generates H CO (pK = 17.03 in
2
2
3
a
2
7,33
CH CN), which plays the role of proton donor.
Hence,
3
2
2
0
0
E
0
can be determined by the equation E = −0.12 −
results suggest that the CO does not bind Co(I) on its own; it
CO2/CO
2
.059 × pK as developed by Appel & Mayer, to be ca. −1.12 V
versus Fc
binds only when the thiolate(s) is(are) protonated.
Theoretical Structure−Function Correlation to Reac-
tivity. Geometrically optimized density functional theory
a
+
/0 56
.
Thus, according to the Appel and Mayer
estimates, complex 1 is able to generate CO at very high rates
with as low as 340 mV overpotential (peak maximum at −1.46
(DFT) calculations are used to model the CO binding to
2
+
/0
vs Fc ). Note that the thermodynamic potential of CO to
Co(I) state of 1. The optimized geometry of 1 agrees quite well
2
CO conversion can also been determined by Savea
considering the thermodynamic cycle for the conversion of
́
nt et al. by
with the crystal structure (Table S1). The calculations indicate
that CO binding to a Co(I) state leads to dissociation of the
2
CO into CO in a solvent S and in the presence of an acid HA.
phosphine arm (Figure 5a, Co···P = 3.3°) trans to CO . On the
2
2
With this approach, the standard thermodynamic potential
determined in CH CN solvent in the presence of H O is
contrary when one of the thiolates is protonated a stable
minimum is observed, where the CO is bound to the Co(I)
3
2
2
0
27
E
= −0.650 V versus NHE or −1.41 V versus
Fc (EAg/AgCl = 0.210 V vs NHE and E
vs Ag/AgCl). According to this complex 1 can reduce CO to
center (Figure 5b). The proton is localized on one of the
oxygen atoms of the bound CO2 in this structure. The
optimized Co−C and C−O bond lengths are 1.98° and 2.30°,
and the O−C−O angle of 118° shows substantial bending from
CO2/CO,CH3CN
/0
+
+/0
(Fc ) = 0.550 V
CH3CN
5
7
2
CO at only 50 mV overpotential. In any case, we can conclude
that complex 1 can produce CO from CO with very low
180° in CO and is close to that of free formate suggesting
2
2
overpotential.
electron transfer to the bound CO from the Co(I) center
2
Mechanistic Analysis. The linear correlation of the
resulting in a Co(III)-COOH electronic structure. Note that
there is a strong H-bonding between the bound −COOH and
the thiolate sulfur (S···O = 2.96°). There are two bonding
electrocatalytic CO reduction current with H O concentration
2
2
(
Figure 4, right) implies that H O (or H CO ) is involved in
2 2 3
the transition state of the rate-determining step (rds). The
interactions between the Co and the bound CO ; a σ-bonding
2
2
2
catalyst concentration (Figure S15) and CO concentration-
between an empty Co d and a filled C sp orbital (Figure 5c)
2
z
dependent catalysis (Figure 2d) suggest that the catalysis is
first-order with respect to both. To obtain further insight, CO2
and a π-backbonding interaction between the occupied Co t2
orbital and an unoccupied CO π* (Figure 5d).
reduction is performed under varying concentration of D O. As
On the basis of the experimental evidence a plausible
mechanism for CO reduction can be forwarded, where CO
2
2
may be expected, the CO reduction current increases linearly
2
2
with increasing D O concentration (Figure 4, left). The slope
binds the protonated Co(I) complex, produced upon reduction
at the electrode, and gets reduced to CO after a subsequent
protonation from H CO . The CO then dissociates and results
2
of the line is proportional to the rate constant (kinetic isotope
2
effect (KIE) = kCO2, H/kCO2, D = (slopeH2O/slopeD2O) ) and is
2
3
much lower in D O than in H O (Figure 4, right) implying that
in the Co(III) complex, which is then reduced again at the
2
2
there is a H/D isotope effect on the rate. The k /k ratio is
electrode to resume the catalytic cycle (Figure 6, left). The rate-
H
D
E
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Article
covalent bonding between Co(III) and carbon center of the
Co(III)-COOH intermediate imparts selectivity to CO2
reduction resulting in almost exclusive formation of CO. The
proposed mechanism for CO reduction by complex 1 is, in
2
essence, similar to that of cyanocobalamin, where the Co(I)
+
state binds the incoming CH3 group, reduces it by two
−
electrons to CH , and then transfers it to an incoming acylium
3
electrophile.
CONCLUSION
In conclusion, a Co complex bearing thiolate ligands
predominantly reduces CO to CO in its Co(I) state resulting
■
2
in less than 100 mV overpotential. The two electron-rich
thiolates play dual role in the process. One of the thiolates
increases electron density at the Co center allowing Co(I) state
to bind CO by backbonding. The other is protonated under
2
natural experimental conditions (H O + CO ), raises the
2
2
formal potential of the Co(II/I) process lowering the
overpotential of CO reduction, and helps the reduction of
2
CO by acting as a local proton source. The large covalent
2
charge transfer from the −COOH to the Co(III) in a proposed
Co(III)-COOH intermediate species prohibits protonation of
the carbon center precluding the formation of HCOOH.
Rather the protonation of the OH center of this intermediate
leading to CO release is likely to be the rate-determining step
with a k /k value of ∼5.4. Using a dithiolate ligand framework
Figure 5. DFT-optimized structure of the (a) Co(I)-CO complex and
2
2
(
b) thiol-protonated Co(I)-CO2 complex, (c) dz orbital of the
Co(III)-COOH complex, and (d) occupied Co t orbital backbonding
2
to the unoccupied CO π*.
H
D
to both increase electron density on the metal and act as a
proton source may be a general strategy to catalyze low-
potential redox reactions that require multiple protons and
multiple electrons.
determining step is likely to be the protonation of the Co(III)-
COOH, which has a H/D isotope effect of ∼5.41. The site of
protonation of the Co(III)-COOH is crucial in determining the
selectivity between HCOOH and CO as products. In the
absence of a preorganized second sphere network ascertaining
protonation of either the C or the O, the greater than 93%
Faradaic yield for CO is apparently surprising. However,
normal population analysis of the ground-state wave function of
the DFT-optimized Co(III)-COOH species reveals that the
carbon center bears a charge of 0.38, whereas the oxygen
METHODS
■
General Procedures. Syntheses and electrochemical investigations
were performed under inert atmosphere or Ar glove box. All the
solvents, cobalt chloride hexahydrate (CoCl ·6H O), and HBr (48% in
2
2
H O) were purchased from local vendors and used after purification
2
whenever it was required. Bis(diphenylphosphinoethane) (dppe) and
potassium thioacetate were bought from Sigma-Aldrich. 2,6-
Pyridinedimethanol was purchased from Across Chemicals. These
chemicals were used as obtained. The electrodes used for
homogeneous electrochemistry were purchased from Pine Instruments
and used after polish for every use. Further experimental details and
characterization details are given in the Supporting Information.
Electrochemical Measurements. Cyclic Voltammetry. All
electrochemical experiments were performed using a CH Instruments
−
centers bear −0.34 and −0.29 e charges, respectively. The
strong covalent bonding between the Co(III) and the C
depletes the latter of electron density, which is localized on the
more electronegative oxygen centers of the bound anionic
−
COOH ligand (Figure 6, right). Logically the next protonation
happens on the oxygen resulting in CO and not on the C,
thereby precluding the formation of HCOOH. Thus, the strong
Figure 6. (left) Proposed mechanistic cycle of CO reduction by complex 1. (right) Charge distribution contour of S-protonated COOH bound
2
Co(III) form.
F
DOI: 10.1021/acs.inorgchem.8b00450
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
model CHI710D Bipotentiostat Electrochemical Analyzer). Refer-
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00450.
■
ence electrodes and Teflon plate material evaluating cell (ALS Japan)
were purchased from CH Instruments. A Pt wire electrode was used as
a counter electrode. The measurements were made against a leakproof
Ag/AgCl aqueous reference electrode (saturated KCl). Anaerobic
experiment was performed in glove box or within a custom-made four-
neck electrochemical cell by thoroughly degassing the whole setup
with Ar gas. The glassy carbon electrode was used as working
electrode, which was freshly polished to get rid of all the
contaminations before each single use.
*
S
Additional cyclic voltammetry data, control experiments,
NMR data, and optimized coordinates (PDF)
Accession Codes
Homogeneous Electrochemistry. Anaerobic solution (0.5 mM)
of complex 1 (in ACN) with 0.1 M tetrabutylammonium perchlorate
CCDC 1582073 contains 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
(
TBAP) was taken in a four-neck two-compartment electrochemical
cell that was degassed by Ar gas prior to electrochemical experiment.
Then CO gas was bubbled into the solution for 1−1.5 h, and then the
2
electrochemical data were collected. To determine the diffusion
constant of complex 1, a 2 mM solution was used. The experiments
were done in different solvents like dimethylformamide (DMF),
dichloromethane (DCM), THF, and different working electrodes
1
EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: icad@iacs.res.in.
(
GCE and Pt), and very similar data were obtained. Rinse test was
performed after bulk electrolysis experiments and did not show any
adsorbed material on the electrode. The catalyst was fully soluble in 3
M H O in CH CN up to concentrations less than or equal to 3 mM.
ORCID
2
3
Abhishek Dey: 0000-0002-9166-3349
Author Contributions
Bulk Electrolysis and Gas Collection. The bulk electrolysis (BE)
experiment for gas analysis was done in a custom-made two-
compartment water jacket three-electrode electrochemical cell brought
from PINE with 27 cm2 Hg-pool working electrode, which was
connected to an inverted buret for potential dependent product
analysis. The gas evolved during BE was collected into the buret by
vertical displacement of water. The amount of gas evolved was
measured from the volume of water displaced during the experiments.
†
S.D. and M.E.A. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
2
To check the stability of the catalyst BE was performed on a 2.8 cm
This research is funded by DST-SERB Grant No. EMR/2016/
008063. E.A. acknowledges CSIR-SRF, and S.D. acknowledges
IACS Institute Fellow Scheme. Mr. N. Pal and Dr. R. Saha are
greatly acknowledged for their assistance in structure
determination.
2
Hg-pool electrode or a 0.1 cm GCE in the custom-made four-neck
two-compartment electrochemical cell with CO -saturated 10 mL of
2
0
.5 mM CH CN solution of the complex 1 containing 2.1 M H O.
3
2
Gas Detection by Gas Chromatography. The gas evolved
during BE was detected by using GC instrument of model no. 7890B
G3440B), fitted with thermal conductivity detector (TCD). Gas (400
(
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