Electrocatalytic CO2 Reduction with a Half-Sandwich Cobalt Catalyst: Selectivity towards CO

Article abstract of DOI:10.1002/asia.201901805

We present herein a Cp*Co(III)-half-sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron-donating Cp* ligand (Cp=pentamethylcyclopentadienyl), the catalyst featured a proton-responsive pyridyl-benzimidazole-based N,N-bidentate ligand. Owing to the presence of a relatively electron-rich Co center, the reduced Co(I)-state was made prone to activate the electrophilic carbon center of CO₂. At the same time, the proton-responsive benzimidazole scaffold was susceptible to facilitate proton-transfer during the subsequent reduction of CO₂. The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half-sandwich CpCo(III)-based CO₂-reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70percent while the overpotential for CO production was found to be 0.78 V, as determined by controlled-potential electrolysis.

Full text of DOI:10.1002/asia.201901805

CHEMISTRY
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Accepted Article
Title: Electrocatalytic CO2 Reduction with Half-Sandwich Cobalt
Catalyst: Selectivity towards CO
Authors: Indresh Kumar Pandey, Abhishek Kumar, and Joyanta
Choudhury
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Chemistry - An Asian Journal
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Electrocatalytic CO Reduction with Half-Sandwich Cobalt
2
Catalyst: Selectivity towards CO
Indresh Kumar Pandey,[a]$ Abhishek Kumar,[a]$ and Joyanta Choudhury*[a]
+
Abstract: We present herein a Cp*Co(III)-half-sandwich catalyst
system for electrocatalytic CO reduction in aqueous acetonitrile
solution. In addition to an electron-donating Cp* ligand (Cp* =
pentamethylcyclopentadienyl), the catalyst featured proton-
(2e/2H ) reduction pathways as shown in Figure 1A. While
HCO
2
H is a well-known hydrogen-storage material,[3] CO is used
2
[
4]
in synthesizing fuel via the Fischer–Tropsch process. However,
under given reaction conditions, the competition depends
crucially on the combined effect of both the metal center and the
ligand backbone of the applied catalyst. A highly reduced
a
responsive pyridyl-benzimidazole-based N,N-bidentate ligand.
Owing to the presence of a relatively electron-rich Co center, the
reduced Co(I)-state was made prone to activate the electrophilic
electron-rich metal center may directly activate CO
2
forming ‘M-
’
adduct, break a C–O bond in the presence of H+ and
carbon center of CO
benzimidazole scaffold was susceptible to facilitate proton-transfer
during the subsequent reduction of CO . The above factors rendered
2
. At the same time, the proton-responsive
CO
2
liberate CO (equation i, Figure 1A). On the other hand, the metal
center, after its reduction, may form hydride by reacting with H .
+
2
the present catalyst active toward producing CO as the major
Depending on the hydricity of the generated metal hydride (M–
+
product over the other potential 2e/2H reduced product HCOOH, in
H) species (and also on the pK
a
of the proton donor), it may
+
contrast to the only known similar half-sandwich CpCo(III)-based
evolve H
Figure 1A) or may form HCO
formation of HCO
2
via further reaction with H (or H
2
O) (equation ii,
followed by
−
2
CO -reduction catalysts which produced HCOOH selectively. The
2
via reaction with CO
2
system exhibited Faradaic efficiency (FE) of ~70% while the
overpotential for CO production was found to be 0.78 V as
determined by controlled-potential electrolysis experiment.
2
H upon protonation (equation iii, Figure 1A). In
some cases, if the metal center is relatively electron-deficient,
’
the ‘M-CO
2
adduct may form ‘M-OCHO’ intermediate upon
protonation and further reduction, followed by formation of
HCO
2
H (equation iii, Figure 1A).[
5]
Introduction
One of the important contemporary challenges in the context of
energy and environment research is catalytic reduction of CO –
2
a renewable and economical C1 feedstock – to high-value
products and/or utilizable fuels.[1] The alarming issue of
escalating CO
to consumption of fossil fuels adds renewed relevance to the
above challenge. With regard to this, electrocatalytic CO
2
concentration (>400 ppm) in the atmosphere due
2
reduction strategy holds promise in view of its suitability toward
storing intermittent electrical energy and its modularity toward
converting CO
HCO H/HCO
−, C
2
to desirable products (such as CO,
2−
2
2
2
O
4
etc.) based on variables such as applied
of the proton donor
As a result, several transition metal complexes with a
wide variety of ligand scaffolds have been evolved as
homogeneous molecular electrocatalysts for CO
reduction.[1,2]
potential, electrocatalyst, electrolyte, pK
a
[1,2]
etc.
2
Significantly, the choice of relatively inexpensive and abundant
first-row transition metals (e.g., Mn, Fe, Co, Ni) in combination
with suitable ligands, is preferred over noble metal (e.g., Re, Ru,
Rh, Ir)-based counterparts, considering practicality and
sustainability.
2
An interesting point in the context of electroreduction of CO , is
the opportunity to the selective production of CO or
−
HCO
2
H/HCO
2
via the same net two-electron/two-proton
[
a]
$]
Dr. I. K. Pandey, A. Kumar, Dr. J. Choudhury
Organometallics & Smart Materials Laboratory
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhopal 462 066, India
Figure 1. [A] Reduction of CO
HCO
2
by 2e/2H+ pathways to produce CO and
-to-CO conversion via Co and CoI
0
2
H. [B] Known Co-catalysts for CO
2
E-mail: joyanta@iiserb.ac.in
These authors contributed equally.
I
states, and CO
present work.
2
2
-to-HCO H conversion via Co state. [C] Key features of the
[
Supporting information for this article is given via a link at the end of
the document.
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Focusing on Co as the potential metal candidate in this area,
primarily multidentate (commonly tetradentate, and rarely penta-
/
x
tridentate) N-donor ligand motifs, N (Figure 1B) were utilized
extensively to design a plethora of highly efficient homogeneous
molecular complexes for electrocatalytic CO
2
reduction in both
organic (with proton-donor additives) and aqueous media.[
1,2]
Notably, most of these Co-catalysts selectively produce CO via
activating CO in highly reduced state of the metal, often in
2
formal “Co(0)” oxidation state.[1,2] In contrast, recently, Dey
exploited an interesting proton-responsive SNS-donor based
2
,6-dithiomethylpyridino tridentate ligand (SNS) at Co center to
achieve a highly efficient CO-selective CO -electroreduction
catalysis at dramatically low overpotential, owing to the
activation and reduction of CO by formally “Co(I)” oxidation
state (Figure 1B).[6] On the other hand, Artero introduced a new
family of half-sandwich Co(III) complexes as molecular CO
electroreduction precatalysts which were found to produce
HCO H selectively at “Co(I)” state in dimethylformamide (DMF)-
2
2
2
-
2
water solvent mixture (Figure 1B).[7] This class of precatalysts
featured a Cp (Cp = cyclopentadienyl) ligand around Co,
additionally supported with a bisphosphine chelate (PP) having
pendant N–H groups. These aspects rendered the suitable
electronics and hydricity of the Co center, and easy availability
of the pendant protons, thus facilitating the route (iii) as shown in
Figure 2. Synthesis of complexes 1, 2 and 2-Me.
Figure 1A, toward selective formation of HCO
Artero’s HCO
H-selective half-sandwich Co(III)-complexes[7] and
our own interest in developing half-sandwich complex-based
CO
-reduction catalysts,[8] we asked whether the same “Co(I)”
2
H. Inspired by
2
2
state in this new class of half-sandwich Co(III) complexes, could
be exploited toward selective electrocatalytic production of CO.
To achieve the desired CO-selectivity, we relied upon a new
design comprising of Cp*Co(III) center surrounded with pyridyl-
benzimidazole-based N,N-bidentate ligand (Figure 1C). The Cp*
ligand (Cp* = pentamethylcyclopentadienyl) is relatively strong
electron-donating which would facilitate the reduced Co center
to bind the electrophilic carbon center of CO , while the
2
benzimidazole scaffold is proton-responsive which might
facilitate proton-transfer step(s).
Results and Discussion
The study was initiated with synthesizing the Cp*Co(III)-based
half-sandwich complex 1 as shown in Figure 2, from reaction of
the cobalt precursor, [Cp*Co(CO)I
pyridyl-benzimidazole ligand in CH Cl
iodide) metathesis with aqueous NH
Supporting Information for details). Complex
2
] and the corresponding
, followed by counteranion
PF in CH CN (see
was fully
2
2
(
4
6
3
1
characterized by 1H, C{ H} NMR spectroscopic, and high-
resolution electron-spray ionization mass spectrometric (HR-
ESIMS) techniques (for details, see Supporting Information).
Further, the molecular structure of 1 was determined by single-
crystal X-ray diffraction study. The crystal structure revealed a
13
1
monocationic Co(III) complex having one PF −
counteranion in
6
the molecule (Figure 3). The cobalt center consisted of the
expected bidentate N,N-chelating binding mode of the pyridyl-
benzimidazole ligand, with the pendant benzimidazole N–H
group intact. In fact, an intramolecular N−H···O hydrogen-
bonding interaction between this pendant benzimidazole N–H
Figure 3. (Top) X-ray structure of 1 (CCDC 1973864). Selected bond lengths
Å) and bond angles (°): N1‒Co1 = 1.973(9); N3‒Co1 = 1.992(8); I1–Co1 =
(
2.599(19); N1-Co1-N3= 81.8 (3). (Bottom) X-ray structure of
2 (CCDC
1973865). Selected bond lengths (Å) and bond angles (°): N2‒Co1 = 1.986(3);
group and the OH
crystallization step) at a distance of 1.850 Å was evident from
2
molecule (probably came from solvent during
N1‒Co1= 1.975(2); N4‒Co1 = 1.933(3); N2-Co1-N1= 81.33 (10). In structure 1
Cp*-H and in 2 Cp*-H as well as CH CN hydrogens are omitted for clarity.
3
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−
1
−1
the solid-state structure of the complex. The Cp* ligand was
range of scan rates, 25 mVs –500 mVs
under argon,
5
coordinated via η mode, resulting in a three-legged piano-stool
suggested a diffusion-controlled process.
geometry around Co, with the sixth coordination site occupied by
the iodo ligand. The Nbim-Co-N
8
1
p
bite angle in 1 was found to be
1.8(3)°, while the Co–Nbim, Co–N and Co–I distances were
.992(8) Å, 1.973(9) Å, and 2.599(19) Å respectively. At this
p
stage, anticipating a plausible interference from the iodide ligand
during future electrochemical and catalytic studies, due to its
reversible dissociation in polar solvent, we synthesized a new
complex 2, which is the iodo-free solvato version of complex 1
(
2
Figure 2). Thus, the cobalt precursor, [Cp*Co(CO)I ] was stirred
with 1.1 equiv. of the corresponding pyridyl-benzimidazole ligand
in the presence of 3 equiv. of the halide abstracting agent,
AgOTf in CH CN solution at ambient temperature for 24 h. The
3
solution eventually changed its color to red with concomitant
formation of a precipitate of AgI. Upon filtering out the precipitate
followed by evaporation of the resulting filtrate, a brick-red solid
was obtained which upon further purification by recrystallization
3
from a mixture of CH CN/diethylether furnished pure complex 2
as dicationic triflate salt. Notably, the corresponding N-Me
version of the complex 2 (2-Me) was also synthesized for
comparative control study (Figure 2). Full characterization of
both 2 and 2-Me was accomplished via similar NMR and HR-
ESIMS techniques (see Supporting Information). Furthermore,
Figure 4. [A] CVs of complexes 1, 2, and 2-Me at a scan rate of 50 mVs−1. [B]
Scan-rate (25 mVs−1 – 500 mVs ) dependent CVs of complex 2. [C] Variation
−1
III
II
II
I
of cathodic peak currents corresponding to the Co /Co and Co /Co redox
responses of complex vs square root of the scan rate. Experiment
2
the crystal structure of 2 disclosed a dicationic Co(III) complex
conditions: concentration of complexes
=
1
mM in CH
3
CN, supporting
n
−
electrolyte = 0.1 M solution of Bu
4
NPF
6
in CH
3
CN, working electrode= glassy
supported with two CF
3
SO
3
counteranions in the molecule
carbon (3 mm diameter), counter electrode= a Pt wire, reference electrode=
Ag/AgCl). Ferrocene (E1/2, Fc/Fc+ = 0.46 V vs. Ag/AgCl) was used as an
external calibration standard for the CV experiments.
(
Figure 3). The coordination geometry around the Co center was
similar to that found in complex 1, except the sixth coordination
site which contained now a CH CN ligand instead of the iodo
ligand. Additionally, in this case also, the intramolecular N−H···O
hydrogen-bonding interaction between the pendant
benzimidazole N–H group and the CF counteranion at a
SO −
distance of 1.945 Å was evident. Finally, the complex 2 featured
the Nbim-Co-N bite angle of 81.33(10)°, and the Co–Nbim, Co–N
and Co–NCH3CN distances of 1.975(2) Å, 1.986(3) Å, and
.933(3) Å respectively.
3
Complex 2 was next investigated toward the proposed activity
3
3
for electrocatalytic CO reduction. The CV trace of 2 measured
2
in CO -saturated dry CH CN solution under CO atmosphere
2
3
2
p
p
exhibited interesting property. The first reduction peak
essentially did not show any change but the second reduction
showed a positive (anodic) shift in comparison to the CV of 2 in
1
Next, electrochemical characterization of the complexes were
performed using cyclic voltammetric technique. The cyclic
voltammograms (CVs) of 1, 2 and 2-Me were recorded in an
Ar-saturated dry CH CN, followed by a small catalytic wave (red
3
trace vs black trace, Figure 5A). The shape of the peak at this
potential was associated with the loss of reversibility as well.
This observation near the Co /CoI reduction, suggested
II
argon-saturated CH
electrode and 0.1 M [ Bu
3
CN solution using a glassy carbon working
n
I
4
6
N][PF ] as supporting electrolyte under
interaction of the reduced Co catalyst with CO2 to form a
I
argon atmosphere (Figure 4A). All of the CVs featured two
reversible redox couples in the cathodic region. For the
complexes 1 and 2, the first peak appeared at E1/2 values of
plausible activated “Co -CO ” species followed by a weak
2
catalysis to convert CO .[ The catalysis was facilitated next by
10]
2
+
adding H O as proton (H ) source to the test solutions (Figure
2
−
0.95 V and −0.89 V vs. ferrocene/ferrocenium (Fc/Fc+),
respectively. The second redox peak for 1 appeared at −1.89 V
5B). Thus, the CV recorded in CO -saturated 5% H O/CH CN
2
2
3
(v/v) mixed solvent clearly showed a large enhancement of
catalytic current, achieving a current density of approx. 13.5
mA/cm2 (Figure 5B, blue trace). The large enhancement of
current density was corresponding to the catalytic reduction of
+
+
vs Fc/Fc while for 2 it was at −1.87 V vs Fc/Fc . Comparing the
electrochemical responses of similar ‘Cp*Co (LL)’-based
complexes (LL = bidentate ligands) reported in literature, the
observed two peaks in 1 and 2, were characterized for Co /Co
III
[
9]
III
II
CO to CO which was verified by bulk electrolysis and follow-up
2
II
I
and Co /Co redox couples. For the N-Me version 2-Me, the
product characterization by gas chromatography (GC) (see
corresponding two couples were quasi-reversible, and were
below, and Supporting Information). Significantly, when the CV
+
observed at −0.93 V and −1.98 V vs Fc/Fc respectively. Notably, was run in Ar-saturated 5% H O/CH CN (v/v) in the absence of
2
3
II
I
+
the proposed Co /Co potential was more negative in 2-Me than
CO , the corresponding H -reduction catalysis was observed but
2
that in 1 and 2, suggesting inefficient stabilization of highly
reduced Co state by the relatively more electron-rich N-Me
ligand. Finally, as per the Randles−Sevcik equation, the
at higher negative potential than the potential for catalytic CO2-
reduction (Figure 5C, red trace vs blue trace). Notably, the γ N–
H proton of the benzimidazole ligand in complex 2 seemed to
play an important role in the required proton transfer step for
CO2 reduction during catalysis in line with the previously
reported local-proton effect,[11] because the complex 2-Me did
I
relationship between the peak currents (i
p
) with the square root
1
/2
of the scan rate ( ) was evaluated for both the redox peaks
III
II
II
I
(
Co /Co and Co /Co ) in case of complex 2 (Figures 4B,C and
Supporting Information). The observed linear plot in the wide
not show any catalytic CO -reduction current under similar
2
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−
1
be 164 s . A controlled potential electrolysis (CPE) experiment
+
was performed at −2.19 V (vs Fc/Fc ) using a 1 mM solution of
catalyst 2 and 0.1 M solution of [ Bu
n
4
N][PF
6
] as supporting
2 3
electrolyte in CO -saturated CH CN solvent containing 5% (v/v)
H
2
O under CO
2
atmosphere (Figure 5G). GC analysis of the
produced gas mixture revealed the presence of CO and H
ratio of 83:17. Similar electrolysis at −2.39 V (vs Fc/Fc ) resulted
2
in a
+
in the CO:H
2
ratio of 79:21, while the catalysis at a lower
+
potential of −1.99 V (vs Fc/Fc ) led to the formation of traces of
H
2
only without any CO generation. For the electrolysis at −2.19
+
V (vs Fc/Fc ), only traces of formic acid were detected in the
liquid phase, when analysed by H NMR spectroscopy and ion-
1
exchange chromatography. A Faradaic efficiency (FE) of ~70%
was obtained in this experiment. The overpotential for the
present catalytic CPE results obtained by 2 was calculated to be
0
.78 V. A detailed study would be undertaken to optimize the
same through catalytic Tafel plots.
Conclusions
In summary, a new design of Cp*Co(III)-based half-sandwich
catalyst system was presented in this work for electrocatalytic
CO reduction in aqueous acetonitrile solution promoted by the
2
Co(I) state, driving the selectivity toward CO as the major
product. The Co(III) center in the catalyst was supported with an
electron-donating Cp* ligand and a proton-responsive pyridyl-
benzimidazole-based N,N-bidentate ligand. In effect, the
electron-rich reduced Co(I)-state expectedly preferred to activate
2
the electrophilic carbon center of CO , and convert it to CO. The
proton-responsive benzimidazole motif probably facilitated the
+
required proton-transfer during the 2e/2H reduction of CO
2
to
CO. Thus, for the present catalyst system, CO was the major
+
product over the other potential 2e/2H reduced product HCOOH,
in contrast to the only known similar half-sandwich CpCo(III)
catalysts which produced HCOOH selectively. The present
catalyst showed the Faradaic efficiency (FE) of ~70% while the
overpotential for CO production was found to be 0.78 V as
determined by controlled-potential electrolysis experiment.
Detailed investigation including mechanistic insight will focus on
optimizing the catalyst’s structure as well as the catalytic
conditions to improve the efficiency and selectivity further.
Figure 5. [A] CV of 1 mM solution of complex 2 in (black) Ar-saturated dry
CH
addition of H
reduction. [C] Comparative CVs of 2 in Ar- and CO
v/v) solvent system. [D] Comparative CVs of 2 (blue) and 2-Me (red) in CO
saturated CH CN containing 5% (v/v) under CO Experimental
mM in CH CN, supporting
CN, working electrode= glassy
3
CN under Ar, and (red) CO
2
-saturated CH
CN (v/v) for 2-catalyzed CO
-saturated 5% H O/CH CN
3 2
CN under CO . [B] Effect of
2
O (1%, 3% and 5%) to CH
3
2
2
2
3
(
2
-
3
H
2
O
2
.
conditions: concentration of complexes
=
1
3
n
electrolyte = 0.1 M solution of Bu
4
NPF
6
in CH
3
carbon (3 mm diameter), counter electrode= a Pt wire, reference electrode=
Ag/AgCl), scan rate = 50 mVs−1. Ferrocene (E1/2, Fc/Fc+ = 0.46 V vs Ag/AgCl)
was used as an external calibration standard for all the CV experiments. [E]
−
1
−1
CV plots for complex 2 (0.1 mM) at different scan rates (50 mVs –300 mVs ).
[
Experimental Section
F] Plot of normalized catalytic current (icat/i
p
) vs (scan rate)− for the catalytic
1/2
process by complex 2. Experimental conditions were same as mentioned
above. [G]. Plot of charge consumed vs time during the bulk electrolysis with
complex 2 (1 mM) at −2.19 V vs Fc/Fc+ using Hg pool electrode in CO
saturated CH CN solution containing 5% H O (v/v).
3 2
Procedure for synthesis of complexes 1, 2, and 2-Me
Synthesis of Complex 1:
2
Step-1: Synthesis of ligand precursor: The ligand precursor was
synthesized by a slightly modified method reported in literature.[
13]
A
conditions in CO
2
-saturated 5% H
2
O/CH
3
CN (Figure 5D). H+-
mixture
of
ortho-phenylenediamine
(2.3
mmol),
2-pyridine
transfer from the hydrogen-bonding assisted H
Co-bound CO might be a plausible scenario in line with Xiao’s
proposal with similar N–H-containing ligands around Ir
2
O molecules to
carboxaldehyde (2.3 mmol) and NH
4
Cl (0.70 mmol) were refluxed in 5
mL of ethanol for 5 h. After that the reaction mixture was cooled to room
temperature, followed by addition of cold water to it. Upon stirring the
mixture, a yellow precipitate appeared which was filtered off and washed
with water. Recrystallization of the crude product with a minimum amount
of ethanol containing 5 to 10 drops of water afforded the pure product as
light-yellow needle-like crystals (yield 80%). H NMR (500 MHz, CDCl
2
γ
center.[12] The maximum turnover frequency, TOFmax of this
catalytic system was roughly estimated from the linear
1
3
): δ
p
relationship between the normalized peak catalytic current (icat/i )
11.09 (s, 1H), 8.68 – 8.59 (m, 1H), 8.46 (d, J = 7.9 Hz, 1H), 7.87 (ddd, J
derived from scan-rate variable catalytic CV data and inverse
= 9.0, 6.7, 2.9 Hz, 1H), 7.45 (dd, J = 6.2, 2.7 Hz, 1H), 7.37 (ddd, J = 7.5,
4.8, 1.1 Hz, 1H), 7.32 – 7.27 (m, 1H) ppm. Step-2: Synthesis of Complex
1: The reaction mixture of ligand precursor (22.5 mg, 0.115 mmol) and
square root of scan rate (1/1/2) according to the formula
reported in literature (Figures 5E,F).[7] The TOFmax was found to
2
Cp*Co(CO)I (50.0 mg, 0.105 mmol) in dry dichloromethane was refluxed
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Chemistry - An Asian Journal
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FULL PAPER
under Ar atmosphere for 12 h. After 12 h, solvent was fully evaporated
and the crude solid obtained was dissolved in minimum amount of
few experiments including CPE, as well as helpful discussions.
We thank the reviewers for insightful comments.
acetonitrile followed by the addition of NH
Further 2 mL of H O was added to precipitate the desired complex. The
dark purple coloured precipitate obtained was washed with diethylether
for further purification (yield 51.3 mg, 74%). 1H NMR (400 MHz, CDCl
δ:9.23 (d, J = 5.50 Hz, 1H), 8.22 (d, J = 7.87 Hz, 1H), 7.84 (d, J = 7.64,
H), 7.80 (m, 1H), 7.73 (d, J = 7.91 Hz, 1H), 7.31 (t, J = 6.48, 1H), 7.20
4 6
PF (3 equiv. of solid crude).
2
2
Keywords: CO reduction • electrocatalysis • cobalt • half-
3
)
sandwich • CO.
1
13
1
(
dd, J = 9.04, 16.97 Hz, 2H), 1.53 (s, 15H). C{ H} NMR (101 MHz,
CDCl ): δ = 156.19, 149.17, 148.67, 141.17, 140.52, 136.34, 126.99,
26.64, 125.65, 124.86, 118.12, 94.78, 29.81, 11.46. HRMS (ESI,
[
1]
For selected recent reviews, see: a) C. Hepburn, E. Adlen, J.
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Ishitani, M. Robert, ACS Catal. 2017, 7, 70-88. c) K. E. Dalle, J.
Warnan, J. J. Leung, B. Reuillard, I. S. Karmel, E. Reisner, Chem. Rev.
3
1
+
24 3
positive ion): 516.0345 (calcd 516.0346 for [C22H N CoI] .
Synthesis of Complex 2:
The above-mentioned ligand precursor (22.5 mg, 0.115 mmol) as
prepared during the synthesis of complex 1, was dissolved in dry
acetonitrile in a Schlenk tube. Next, AgOTf (80.9 mg, 0.315 mmol) and
2019, 119, 2752-2875. d) R. Francke, B. Schille, M. Roemelt, Chem.
Rev. 2018, 118, 4631-4701. e) S. Fukuzumi, Y-M. Lee, H. S. Ahn, W.
Nam, Chem. Sci. 2018, 9, 6017-6034. f) J. M. Barlow, J. Y. Yang, ACS
Cent. Sci. 2019, 5, 580-588. g) N. Elgrishi, M. B. Chambers, X. Wang,
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2
Cp*Co(CO)I (50.0 mg, 0.105 mmol) were mixed to it under argon
atmosphere. After 5-10 min, a yellow-colored precipitate appeared. At the
same time, the solution changed to red. The reaction mixture was kept
for stirring at room temperature for next 24 h. After 24 h, the reaction
mixture was filtered through Celite, and all volatiles were removed under
reduced pressure. The complex was obtained as a brick-red solid by
1
precipitation from CH
MHz, CD
(
3
CN and Et
2
O (yield 49.7 mg, 65%). H NMR (400
2436. j) M. Aresta, A. Dibenedetto, A. Angelini, Chem. Rev. 2014, 114,
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3
CN) δ: 9.38 (d, J = 5.5 Hz, 1H), 8.55 (d, J = 7.9 Hz, 1H), 8.15
TD, J = 7.8, 1.2 Hz, 1H), 7.93 (d, J = 8.2 Hz, 1H), 7.84 (d, J = 8.1 Hz,
1H), 7.78 (dd, J = 9.6, 3.6 Hz, 1H), 7.43 (t, J = 7.6 Hz, 1H), 7.34 (t, J =
13
1
Smieja, Chem. Soc. Rev. 2009, 38, 89−99. m) A. M. Appel, J. E.
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3
7.6 Hz, 1H), 1.96 (s, 3H), 1.30 (s, 15H). C{ H} NMR (101 MHz, CD CN):
δ = 206.43, 154.88, 153.10, 152.80, 145.37, 143.07, 140.86, 126.57,
22.98, 122.13, 119.69, 118.54, 115.84, 97.97, 29.74, 9.13.HRMS (ESI,
1
+
24 3 3 3
positive ion): 538.08 (calcd 538.0817 for [C23H F N O SCo] .
Synthesis of Complex 2-Me:
Step-1: Synthesis of ligand precursor: The ligand precursor was
synthesized by following the reported procedure.[13] 1H NMR (500 MHz,
DMSO-d6) δ: 8.76 (d, J = 4.28 Hz 1H), 8.31 (d, J = 7.90 Hz 1H), 8.01 (td,
J = 1.77, 7.78 Hz 1H), 7.73 (d, J = 7.92 Hz 1H), 7.65 (d, J= 8.07 Hz 1H),
7
.52 (ddd, J = 1.04, 4.81, 7.50 Hz 1H), 7.34 (m, 1H), 7.28 (m, 1H), 4.24
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1
synthesized by following the previously described procedure. H NMR
500 MHz, CD CN) δ:9.59 (d, J= 5.45 Hz 1H), 8.52 (d, J= 8.08 Hz 1H),
.41 (td, J= 1.32, 7.92 Hz 1H), 8.10 (d, J= 9.21 Hz 1H), 8.00 (m, 1H),
(
3
8
7
13
1
.89 (d, J= 9.34 Hz 1H), 7.69 (m, 2H), 4.29 (s, 3H), 1.32 (s, 15H). C{ H}
CN) δ = 157.18, 151.73, 149.55, 142.47, 139.91,
39.29, 129.28, 127.32, 126.48, 126.27, 118.89, 114.09, 100.06, 34.04,
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NMR (101 MHz, CD
1
1
3
+
24 26 3 3 3
[C H F N O SCo] .
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All the electrochemical experiments were performed using CHI 620E
Electrochemical Analyzer. The electrochemical setup contained
customized four-neck electrochemical cell, glassy carbon electrode (3
mm diameter) as working electrode, Pt wire as counter electrode and
Ag/AgCl (saturated KCl) as reference electrode. The electrochemical cell
a
was evacuated and filled with Ar or CO
experiments. A 0.1 M Bu NPF
4 6
2
gas before respective
solution was used as supporting
electrolyte in dry acetonitrile (or water/acetonitrile as mentioned). The
solutions were saturated with CO gas (or Ar gas) for 90 min prior to each
n
2
experiment. The glassy carbon electrode was polished before each CV
run. For controlled potential electrolysis (CPE) experiment, a customized
four-neck two-compartment electrochemical cell was used. The gas
evolved in bulk electrolysis experiment was detected by using a gas
chromatograph (GC) instrument (7890B) equipped with thermal
conductivity detector (TCD).
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This work was financially supported by DST-SERB (grant nos.
EMR/2016/003002 and SERB/EMR-II/080063) and IISER
Bhopal. I.K.P. thanks DST-SERB for NPDF and A.K. thanks
IISER Bhopal for IPhD fellowship. The authors sincerely thank
Professor Abhishek Dey (Indian Association for the Cultivation of
Science, Kolkata, India) and his group for help in conducting a
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This article is protected by copyright. All rights reserved.

Chemistry - An Asian Journal
10.1002/asia.201901805
FULL PAPER
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Chemistry - An Asian Journal
10.1002/asia.201901805
FULL PAPER
Entry for the Table of Contents
FULL PAPER
A Cp*Co(III) half-sandwich catalyst
system was demonstrated for
Indresh Kumar Pandey, Abhishek
Kumar, and Joyanta Choudhury*
2
electrocatalytic CO reduction to
produce CO in aqueous acetonitrile
solution selectively over the other
potential 2e/2H reduced product
HCOOH, in contrast to the only known
similar half-sandwich CpCo(III)-based
Page No. – Page No.
2
Electrocatalytic CO Reduction with
Half-Sandwich Cobalt Catalyst:
Selectivity towards CO
+
2
CO -reduction catalysts which
produced HCOOH selectively.
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