Electrocatalytic syngas generation with a redox non-innocent cobalt 2-phosphinobenzenethiolate complex

Article abstract of DOI:10.1039/d0dt03270k

A cobalt complex supported by the 2-(diisopropylphosphaneyl)benzenethiol ligand was synthesized and its electronic structure and reactivity were explored. X-ray diffraction studies indicate a square planar geometry around the cobalt center with a trans arrangement of the phosphine ligands. Density functional theory calculations and electronic spectroscopy measurements suggest a mixed metal-ligand orbital character, in analogy to previously studied dithiolene and diselenolene systems. Electrochemical studies in the presence of 1 atm of CO₂and Br?nsted acid additives indicate that the cobalt complex generates syngas, a mixture of H₂and CO, with faradaic efficiencies up to >99%. The ratios of H₂?:?CO generated vary based on the additive. A H₂?:?CO ratio of ～3?:?1 is generated when H₂O is used as the Br?nsted acid additive. Chemical reduction of the complex indicates a distortion towards a tetrahedral geometry, which is rationalized with DFT predictions as attributable to the populations of orbitals with σ*(Co-S) character. A mechanistic scheme is proposed whereby competitive binding between a proton and CO₂dictates selectivity. This study provides insight into the development of a catalytic system incorporating non-innocent ligands with pendant base moieties for electrochemical syngas production.

Full text of DOI:10.1039/d0dt03270k

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Electrocatalytic syngas generation with a redox
non-innocent cobalt 2-phosphinobenzenethiolate
complex†
Cite this: Dalton Trans., 2021, 50,
10779
Nicholas M. Orchanian,
Smaranda C. Marinescu
Lorena E. Hong,‡ David A. Velazquez‡ and
*
A cobalt complex supported by the 2-(diisopropylphosphaneyl)benzenethiol ligand was synthesized and
its electronic structure and reactivity were explored. X-ray diffraction studies indicate a square planar geo-
metry around the cobalt center with a trans arrangement of the phosphine ligands. Density functional
theory calculations and electronic spectroscopy measurements suggest a mixed metal–ligand orbital
character, in analogy to previously studied dithiolene and diselenolene systems. Electrochemical studies
in the presence of 1 atm of CO
syngas, a mixture of H and CO, with faradaic efficiencies up to >99%. The ratios of H
vary based on the additive. A H : CO ratio of ∼3 : 1 is generated when H O is used as the Brønsted acid
2
and Brønsted acid additives indicate that the cobalt complex generates
2
2
: CO generated
2
2
additive. Chemical reduction of the complex indicates a distortion towards a tetrahedral geometry, which
Received 19th September 2020,
Accepted 13th May 2021
is rationalized with DFT predictions as attributable to the populations of orbitals with σ*(Co–S) character.
A mechanistic scheme is proposed whereby competitive binding between a proton and CO dictates
2
DOI: 10.1039/d0dt03270k
selectivity. This study provides insight into the development of a catalytic system incorporating non-inno-
cent ligands with pendant base moieties for electrochemical syngas production.
rsc.li/dalton
combination of CO and H
feedstock derived from coal or natural gas in widescale use for
2
in ratios of 1 : 1 to 1 : 3, is a valuable
Introduction
Displacing fossil fuels in the electricity generation, transpor- the synthesis of hydrocarbons for diesel fuel (through Fischer–
7–10
tation, and chemical production sectors with sustainable Tropsch chemistry) and the production of methanol.
A sus-
alternatives would allow for continued global advancement tainable energy future will require a non-fossil fuel method for
1
,2
2
while mitigating the resulting environmental impact. While the formation of syngas, and the electrolysis of CO using solar-
11
sunlight provides a renewable source of energy, its spatio-tem- derived electricity is a viable pathway to do so. In particular, it
poral variability demands distributed energy storage to meet is vital to develop solar-to-fuel technologies based on complexes
3
peak demands. An ideal solution is to redirect excess solar- containing only non-precious elements that catalyze water split-
11
2
derived electricity towards electrocatalytic cells at times of low ting and CO reduction with high selectivity. Fundamental
demand to convert abundant small molecules into liquid fuels studies into the electronic structure and reactivity of molecular
4
and industrial chemicals. Of particular interest for solar systems offers a promising path to develop mechanistic under-
energy conversion are the fundamental reactions of photosyn- standings of these transformations, which may ultimately lead
5
thesis: water splitting and CO reduction. Storing solar energy to new design principles.
2
through the chemical transformation of these substrates offers
The development of molecular catalysts allows for syntheti-
6
12,13
a strategy to recycle captured CO
2
. Synthesis gas (syngas), a cally-tunable active sites with rapid and selective reactivity.
Numerous molecular systems have been reported for the selec-
tive conversion of CO
protons to H .
2
2
to CO, as well as for the reduction of
However, fewer systems have been reported
which allow for tunable generation of syngas mixtures and
1
4,15
Department of Chemistry, University of Southern California, Los Angeles, CA, 90089,
USA. E-mail: smarinesc@usc.edu
1
6
†
Electronic supplementary information (ESI) available: Experimental pro-
fewer yet which use only non-precious metals. One literature
example describes photoelectrocatalytic syngas production
with a homogeneous rhenium bipyridine complex at a p-Si
1
cedures, H NMR spectroscopy, crystallographic tables, UV-Vis data, and electro-
−
chemical data (PDF). CCDC 1985377 (CoPS), and 2081397 ([CoPS] ). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0dt03270k
1
7
photoelectrode.
It is proposed that the homogeneous
‡
Equal contribution.
rhenium catalyst produces CO, while the surface of the p-Si
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2
photoelectrode facilitates H evolution. Single-component
systems have also been reported, such as nickel and cobalt
complexes, which generate tunable syngas mixtures by varying
1
8,19
the applied potential.
A ruthenium(II) polypyridyl carbene
complex was reported to facilitate both syngas production and
2
0
water oxidation in aqueous media. In both cases, the com-
+
2
petitive binding of CO or H as substrate in the catalytic cycle
was identified as a key step in determining the overall product Fig. 1 Left: Chemdraw illustration of CoPS. Right: Solid state structure
of CoPS, top view with the following color designations for atoms: Co =
pink, S = yellow, P = orange, C = gray, H = white.
selectivity. As such, we consider the need to include structural
motifs to facilitate this competitive binding.
Ligand non-innocence and hydrogen-bonding interactions
have been shown to lower the activation barrier of reactive
intermediates and to direct proton-coupled electron transfer arrangement of the phosphine ligands (Fig. 1, details tabu-
2
1
(
PCET) reactions. Many synthetic systems have been devised lated in Tables S1–S3†). The complex exhibits pseudo C2h sym-
to exploit these structural motifs, in analogy to CO-dehydro- metry with eclipsed isopropyl substituents, though the orien-
genase and [FeFe]-hydrogenase for the reversible conversion of tation of these substituents may be attributed to packing inter-
2
2–24
CO to CO and protons to H , respectively.
Thiolate and actions in the solid state. This geometry generates a sterically-
2
2
selenolate ligands in cobalt bis(dithiolene) and bis(diseleno- crowded cobalt center with an accessible binding pocket at
lene) complexes have been shown to serve as proton relays, cobalt and sterically-accessible thiolate moieties. The trans
proximal to a cobalt hydride, which facilitate rapid H
evolution.
arrangement of the phosphine ligands is consistent with a
These cobalt chalcogenide complexes have been recent report of a related complex of cobalt with a (thiolato)
primarily optimized for H evolution over CO
reduction. In phenylphosphine ligand derivative, as well as with that of ana-
order to develop a system with competitive CO binding, prior logous nickel and tin complexes.
2
2
5–28
2
2
3
3–35
To probe this structure
studies suggest that stronger donor ligands may be necessary further, DFT calculations were performed at the 6-31G(d)/PBE
to increase the nucleophilicity of the cobalt center.
2
2
9–31
level of theory (see ESI† for details) to ensure convergence. The
Hybrid phosphine–thiolate (PS) chelating systems offer resulting optimized geometry was confirmed as a stable
both a strong σ-donor phosphine for the stabilization of low- minimum with frequency calculations at the same level of
valent metal centers as well as a π-basic thiolate ligand, theory. The geometry of the optimized structure is consistent
which may serve to facilitate hydrogen-bond networks. It was with the solid-state structure observed for CoPS, and is similar
3
4
shown that ligand protonation in a cobalt complex with thio- to that of the analogous phenyl derivative. The measured
late and phosphine donors facilitates CO
reduction with Co–S and Co–P bond lengths of 2.1625(4) and 2.2163(5) Å,
), low overpotential respectively, are also consistent with the optimized DFT geo-
2
−
1
high turnover frequency (1559
s
3
2
(
70 mV), and high selectivity for CO (∼95%). Analogous metry. No counter-ions are observed in the outer-sphere, con-
systems based on a 2-phosphaneylbenzenethiol ligand have sistent with a neutral complex, with a single benzene solvent
shown promising activity towards H evolution and oxi- molecule per unit cell. Elemental analysis of the complex is
2
3
3
dation.
Electronic structure calculations of cobalt and also consistent with the predicted chemical formula (see
rhenium complexes with 2-(diphenylphosphaneyl)benze- Experimental methods).
1
nethiol ligand derivatives have indicated that this ligand plat-
H NMR spectroscopy studies of CoPS in benzene-d
6
form facilitates similar orbital interactions as those facili- (selected for increased solubility) reveal four broad paramag-
3
3,34
tated by dithiolenes.
Catalytic studies of nickel and netic resonances at δ −22.5, −16.1, 11.2, and 24.9 ppm, which
rhenium complexes supported by PS ligands have provided each integrate to two protons and are attributed to the aro-
insight into the behavior of this ligand class, suggesting both matic protons of the benzene backbone. Additional resonances
3
3
metal-centered and ligand-centered reactivity. Herein, we at δ −0.5, 2.5, and 4.8 ppm are observed and integrate to a
describe a cobalt complex supported by the 2-(diisopropyl- total of 28 protons in a 3 : 1 : 3 ratio. These resonances are
II
phosphaneyl)benzenethiol
ligand,
[Co (P(iPr)
2
SC
6
H
4
)
2
], assigned to the distinct isopropyl environments (Fig. S1†). The
designated CoPS. Along with structural characterization, molecular nature of this complex was also confirmed in aceto-
density functional theory (DFT) and time-dependent DFT cal- nitrile through NMR spectroscopy studies, which show analo-
culations are explored to model the electronic structure of gous resonances in the paramagnetic spectra (Fig. S1†).
this system. The reactivity of CoPS is studied in the presence Paramagnetic susceptibility measurements were performed in
of CO and various Brønsted acid additives, including water.
benzene-d
effective magnetic moment of CoPS (see ESI†). Based on
these studies, a μeff of 1.77μ was determined, which corres-
ponds to one unpaired electron and is consistent with the
value of 2.07μ reported for the (thiolato)phenylphosphine
These results are supported by unrestricted
6
according to Evan’s method to determine the
2
3
6
B
Results and discussion
B
3
4
Single crystal X-ray diffraction studies of CoPS indicate a derivative.
square planar geometry around the cobalt center with a trans single-point energy calculations at the def2-TZVP/PBE level of
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Fig. 2 Electronic spectroscopy study of 0.2 mM CoPS in a benzene
solution. Inset highlights a low-energy transition at ∼830 nm. Also
shown is the calculated spin density for the ground state, indicating
∼
20% thiyl-radical character.
Fig. 3 Molecular orbital diagram based on TD-DFT calculations at the
def2-TZVP/B3LYP level of theory. Donor orbitals shown with negative
theory, which predict the neutral complex to exhibit ground regions colored red, and positive regions colored blue, acceptor orbitals
state doublet character with minor spin density localized on are shown with negative regions colored green, and positive regions
colored pink. Vertical lines indicate major contributions (solid) and
the thiolate ligands (ρ = 0.2), suggesting some degree of thiyl-
radical character as illustrated in Fig. 2 and S2.†
The electronic structure of this complex was probed experi-
mentally through UV-Vis spectroscopy in a benzene solution.
additional contributions (dashed), with transitions identified by color (λ
red, λ = orange, λ = green, λ = blue, and λ = violet).
1
=
2
3
4
5
Upon dissolution in benzene, the intense orange crystals ESI†). The predicted UV-Vis spectrum parallels the experi-
produce a vivid, yellow solution. As seen in Fig. 2, four domi- mental spectra with five dominant transitions, though the
nant electronic transitions were observed with one additional absolute energies of these predictions vary due to the gas-
shoulder. Dilution studies were also performed to determine phase nature of these calculations (Table S8†). As such, these
the molar extinction coefficients for these transitions. The calculations offer qualitative predictions for the character of
−1
results of these studies reveal transitions at λ (ε) [nm (M
the electronic transitions. Based on these calculations, the
cm )] = 834 (3.8), 480 (2862.8), 388 (3975.3), 358 (2491.0, ground state electronic configuration of the complex is pre-
shoulder), and 288 (15 871.0) (see Fig. S3 and Table S7†). dicted to have a singly occupied 3b orbital, which displays π*
−1
g
Similar spectra were recently reported for a related complex, (Co–S) character with significant contribution from the S(p)
II
Co [P(Ph) (C H -3-SiMe -2-S)] , which was shown to rapidly and Co(d ) orbitals. This qualitative SOMO character parallels
2
6
3
3
2
xz
react with oxygen resulting in a change in the electronic spec- that predicted for both the cobalt bis(dithiolene) and bis(dise-
34
21
trum. In contrast to this prior report, the measured UV-Vis lenolene) complexes. The lowest-lying unoccupied one-elec-
spectra for the complex reported here, CoPS, does not display a tron orbital corresponds to the 3b β orbital, which exhibits
g
change with air exposure and all UV-Vis measurements were similar character as the α SOMO with more substantial metal
performed under ambient atmosphere with no color change contribution.
observed in the solution. The observed electronic spectrum of
the CoPS complex is also analogous to that reported for a cobalt observed at 834 nm (λ
bis(dithiolene) complex, suggesting similar electronic structure singlet-to-singlet excitation of a β spin in an orbital of a
As illustrated in Fig. 3, the lowest-energy transition
) is predicted to correspond to the
sym-
1
g
and a mixed metal–ligand ground state. As such, assignment of metry with Co(d ) and S(p ) orbital contribution to the mixed
xy
y
the oxidation state of cobalt is ambiguous, although related metal–ligand 3b
g
orbital. The next lowest-energy transition (λ
2
3
4
studies suggests a Co(II) ground state. Analogous transitions = 480 nm) is predicted to correspond to a ligand-to-metal
were observed in acetonitrile, suggesting coordination charge transfer (LMCT) excitation of an α electron from the
environment analogous to that in benzene (Fig. S17†). 3b mixed metal–ligand character SOMO with predominant
To further probe the electronic structure of CoPS, unrest- ligand-character to the 2a LUMO, which exhibits σ*(Co–S)
ricted time-dependent density functional theory (TD-DFT) cal- character with contributions from S(p ) and Co(d ) orbitals.
a
g
g
x2−y2
x
3
culations were performed at the def2-TZVP/B3LYP level of The electronic transition observed at 388 nm (λ ) is predicted
theory in the gas phase. The B3LYP functional was selected to exhibit metal-to-ligand charge transfer (MLCT) character
based on literature precedent, as this functional provides an with both α and β contribution. An additional shoulder
appropriate description of a broad range of molecular tran- appearing at 358 nm (λ
4
) is attributed to excitation of an elec-
sition metal complexes. The resulting Kohn–Sham molecular tron from occupied orbitals with S(p ) and Co(d ) contribution
orbital picture is depicted in Fig. 3 (details tabulated in the to the 2ag LUMO. Finally, the highest-energy observed tran-
3
7
z
z
2
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sition (λ
5
= 288 nm) is predicted to correspond to a ligand-to- the occupation of an α orbital with σ*(Co–S) character. The
metal charge transfer (LMCT) transition from the 1bu orbital population of this orbital can be expected to result in a change
exhibiting S(p
character.
y
) contributions to the 2a
g
LUMO of Co(d
x
2
−y
2)
in geometry at the cobalt center, possibly leading to metal–
ligand bond elongation, bond cleavage, or torsion away from a
The electrochemical behavior of CoPS was analyzed by square planar coordination environment. In a related example,
cyclic voltammetry (CV) studies in an acetonitrile solution with a square planar Co(II) complex with a (thiolato)phenylpho-
0
.1 M [nBu
4 6
N][PF ] electrolyte using a glassy carbon working sphine ligand derivative was shown to result in a tetrahedral
3
8
electrode (3 mm diameter). Acetonitrile was selected as the coordination environment around the Co(II) center following
electrochemical solvent in analogy to the studies performed on oxidation in air. The cathodic feature at −2.36 V can be tenta-
3
4
the cobalt bis(dithiolene) and bis(diselenolene) complexes. tively assigned to the one-electron reduction of the species
Under N atmosphere, the complex displays a reversible one- resulting from this reduction-initiated structural change.
2
electron oxidation feature with E1/2 at −0.21 V, and two quasi-
To further probe the electrochemical behavior of this
reversible reductive features at −2.08 V, and −2.36 V, respect- system, cyclic voltammetry experiments were performed at
ively (Fig. 4 and Fig. S4†). All potentials are referenced versus various scan rates (Fig. 5). The feature at −0.21 V appears
0
/+
Fc and the reversibility and one-electron nature of these fea- reversible at all scan rates, and the Randles–Sevcik analysis is
0
/+
tures is based on comparison to the internal Fc reference. consistent with that of a freely diffusing molecular species (see
The features at −2.08 V and −2.36 V appear as quasi-reversible Fig. S5†). More complex behavior is observed for the sub-
and one-electron waves, and no change in the reduction event sequent two events at −2.08 V and −2.36 V. At low scan rates,
at −2.08 V is observed if the potential is reversed before reach- both reduction events are irreversible (see Fig. 5). Increasing
ing the second reduction (Fig. S4†). While the features at the scan rate causes the first feature (at −2.08 V) to exhibit
−
−
0.21 V and −2.08 V are similar in magnitude, the feature at reversibility, while diminishing the second redox event. As this
2.36 V is diminished indicating less charge passed (Fig. S4†). behavior is consistent scan-to-scan, it is unlikely to be attribu-
This observation suggests the formation of a minor species fol- table to deposition of a heterogeneous species onto the elec-
lowing the reduction event at −2.08 V, which is subsequently trode surface and instead suggests that a structural change
further reduced at −2.36 V. Based on the predicted molecular follows the one-electron reduction of CoPS. If this structural
orbital diagram (Fig. 3), the anodic feature at −0.21 V is change occurs on a slower timescale than that of the CV
+
assigned to the one-electron oxidation of CoPS to [CoPS] , experiment, the appearance of the peak at −2.36 V would be
which results in the depopulation of the mixed metal–ligand expected to diminish. Scan-rate dependence studies reveal that
SOMO. Following this event, the cathodic feature at −2.08 V is the feature at −2.36 V approaches the magnitude of the fea-
−
assigned to the one-electron reduction of CoPS to [CoPS] , tures at −0.21 V and −2.08 V as the scan rate is decreased, con-
resulting in a diamagnetic complex with the occupation of the sistent with a structural change on a timescale competitive
mixed metal–ligand β LUMO level. The complex behavior with that of the CV sweep. As the scan rate is decreased, more
observed for the feature at −2.36 V can be rationalized by the time is allowed for the generation of a new species and for the
predicted character of the next-lowest lying orbital for the subsequent reduction of this species at −2.36 V to be detected
square planar configuration of CoPS. As shown in Fig. 3, a two- as a feature in the CV profile. An additional oxidation feature
2
−
electron reduction of CoPS to [CoPS] is predicted to result in appears at −0.56 V following reduction, further indicating the
formation of a new species following reduction of the complex.
Fig. 4 Cyclic voltammograms of
solution under N atmosphere with 0.1 M [nBu
00 mV s , where a represents the initial scan, and b corresponds to
the subsequent scan.
1
mM CoPS in an acetonitrile
2
4
N][PF ] with scan rate =
6
Fig. 5 Cyclic voltammograms of 1 mM CoPS in an acetonitrile solution
under N atmosphere with 0.1 M [nBu N][PF ] and 1 mM ferrocene at
various scan rates.
−1
1
2
4
6
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To understand the observed electrochemical behavior, gas-
phase geometry optimizations were performed at the 6-31G(d)/
PBE level of theory for the one- and two-electron-reduced
forms of the complex (see ESI† for details). The one-electron
−
reduced form, [CoPS] , is predicted to be diamagnetic with a
singlet ground state and distorts towards a tetrahedral geome-
try upon relaxation, as shown in Fig. 6. This result indicates
that a change in coordination geometry of the complex is pre-
dicted to be more energetically favorable than metal–ligand
bond elongation in a square planar geometry or metal–ligand
bond cleavage. Recent computational studies have shown that
the related (thiolato)phenylphosphine derivative similarly
undergoes a change in coordination geometry from square
planar to tetrahedral upon addition of O , providing some Fig. 7 Solid state structure of [CoPS]− with the following color designa-
2
additional support for a geometry change towards a tetra- tions for atoms: Co = pink, S = yellow, P = orange, C = gray, H = white.
+
3
4
The coordinating cation (K ) and hydrogen atoms are omitted for clarity.
hedral structure. Following this reduction, an additional
one-electron reduction generates the doubly-anionic species,
[
2
−
CoPS] , which is predicted to be paramagnetic with a
doublet ground state. This complex distorts further towards a around the cobalt center with a S(1)–Co(1)–S(2) angle of
tetrahedral geometry, and the spin density for this species is 115.93° and a P(1)–Co(1)–P(2) angle of 149.82° (Tables S4–S6†).
predicted to localize predominantly on the metal center The measured Co–S and Co–P bond lengths of 2.3161(6) and
(
Fig. S6†). Based on these calculations, the quasi-reversibility 2.2203(7) Å, respectively, indicate elongation of these bonds
of the observed reduction features is attributed to this struc- upon reduction (Table S5†). In particular, the Co–S bond
tural change from a square planar geometry towards a tetra- length displays a larger elongation upon reduction, from
hedral geometry at the cobalt center. Further, the localization 2.1625(4) to 2.3161(6) Å, whereas the Co–P bond length
of spin density predicted by DFT for the reduced species changes only from 2.2163(5) to 2.2203(7) Å. Additionally, the
suggests that the first reduction (E1/2 = −2.08 V) has mixed measured S(1)–Co(1)–S(2) and P(1)–Co(1)–P(2) angles deviate
metal- and ligand-based character, while the second reduction from the ideal 109.5° for tetrahedral complexes, attributed to
event (E1/2 = −2.36 V) appears to predominantly have metal- steric crowding of the close-approaching isopropyl groups
based character.
(Table S6†). The structural parameter τ[CoPS] = 0.67, indicates
−
To confirm the DFT predictions, chemical reduction of that the one-electron reduced complex displays a cobalt coordi-
CoPS with excess potassium graphite (KC ) was performed and nation environment that is closer to a tetrahedral configur-
8
crystals suitable for single crystal X-ray diffraction studies were ation rather than square planar (see ESI† for details). The
prepared though vapor diffusion of pentane into a THF solu- structural parameter for CoPS is much closer to zero (τCoPS
tion of the reduced complex (see ESI† for details). The result- 0.03), as expected for a square planar geometry. These results
ing red needles were mounted under N
atmosphere with care are consistent with the DFT predictions, and suggest that the
to prevent exposure to O
. As shown in Fig. 7, single crystal reduction of CoPS indeed results in a distortion away from the
=
2
2
diffraction studies indicate a pseudo-tetrahedral geometry square planar geometry of the neutral complex due to popu-
lation of orbitals with σ*(Co–S) character.
The catalytic behavior of the CoPS complex was studied via
CV experiments in the presence and absence of CO2 and
various proton donors. Upon switching the atmosphere from
N
2 2
to CO , an irreversible increase in current density is
observed at a potential corresponding to the first reduction
event for CoPS (E1/2 = −2.08 V), as shown in Fig. 8 and Fig. S7.†
This behavior suggests reduction-induced reactivity with CO
2
rather than a pre-association of the complex with CO as no
2
2
shift in the onset potential is observed upon addition of CO .
Scanning to more negative potentials reveals a pseudo-plateau
shape at −2.18 V. On the return scan, a small return oxidation
feature is present at −1.58 V. This return feature does not
appear under N
oxidation of an intermediate in the CO2 reduction cycle. As
generation of CO from CO requires two equivalents of
2
alone, which suggests that it may indicate the
0
−
2−
2
Fig. 6 Calculated geometries for [CoPS] , [CoPS] , and [CoPS] at the
-31G(d)/PBE level of theory. All geometries were confirmed as stable
protons, CV experiments were performed in the presence of
6
minima with frequency calculations at the same level of theory.
acid additives under CO atmosphere. Larger current densities
2
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Fig. 8 Cyclic voltammograms of 1 mM CoPS in acetonitrile solutions
Fig. 9 Summary of CPE results illustrating faradaic efficiencies for H
2
] and scan rate = 100 mV s− under N
1
(black),
with 0.1 M [nBu
CO (red), and under CO
orange), 0.5 M TFE (green), or 0.5 M H
4
N][PF
6
2
(blue) and CO (red) production.
2
2
in the presence of either 0.5 M PhOH
O (blue).
(
2
absence of an acid additive, a stable current density of 1.1 mA
are measured in the presence of Brønsted acid additives,
−2
cm
Neither H
common CO
was measured for the duration of the experiment.
nor CO were detected. Detection analyses of other
-reduction products, such as formate, oxalate,
including phenol (PhOH), 2,2,2-trifluoroethanol (TFE), or H O
2
2
(
2
Fig. 8). While experiments in the presence of H O exhibit a
2
−
1
pseudo-plateau shape at 100 mV s , peaks are observed prior
to a pseudo-plateau region in the presence of 0.5 M TFE or
PhOH. This pseudo-plateau region has previously been attribu-
methane, methanol, carbonate, and other organics (acids or
aldehydes) were performed in the post-electrolysis solution
and working compartment headspace, but none were detected
by either NMR spectroscopy or gas chromatography measure-
ments. Previous studies of an analogous nickel complex with
both thiolate and phosphine donors suggest reduction-
initiated decomposition may proceed in the absence of sub-
3
9
ted to entering a kinetic regime. To further probe the behav-
ior of this complex in the presence of CO , variable scan rate
2
experiments were conducted in the absence of an acid addi-
tive. These studies indicate a change in waveshape from a
plateau shape to a peak shape (see Fig. S8†) at faster scan
rates. This behavior suggests an “EC”-type mechanism, where
a chemical step (C) follows an electrochemical reduction step
40
strate. Additionally, no products were detected for the CoPS
2
complex under N in the absence of an acid additive.
In the presence of Brønsted acid additives (PhOH, TFE, or
2
O), increased electrolysis currents were measured with a
(
E) and has insufficient time to proceed as the scan rate
increases. This result further suggests that pre-association of
CO to the neutral CoPS complex (a “CE”-type mechanism) is
unlikely, as a reduction event precedes a chemical step in the
presence of CO
H
concurrent increase in the quantity of the gaseous products
and relatively stable total FE. The addition of 0.5 M PhOH gen-
2
−2
erates a stable current density of 4.8 mA cm (Fig. S9† and
2
.
2
Table 1). A H : CO ratio of 4.1 ± 0.5 was measured in the pres-
Following CV studies, controlled potential electrolysis (CPE)
experiments were performed with CoPS under N2 and CO2
atmosphere to determine product selectivity. Results for these
studies are summarized in Table 1 and Fig. 9 (full tabulations
provided in Tables S9–S12†). In a typical experiment, a glassy
carbon working electrode was held at a potential of −2.18 V for
one hour in a 1 mM solution of CoPS. A sample of the head-
space was collected via syringe and analyzed by gas chromato-
ence of phenol, with a total FE of >99% (Table 1 and Fig. 9). In
the presence of 0.5 M TFE, an average current density of
−2
2
2
.9 mA cm was observed, with a H : CO ratio of 10.2 ± 1 and
a total FE of 91 ± 9%. Upon addition of 0.5 M H O, a current
2
−2
density of 1.6 mA cm was measured with a total FE of >99%
and a H
It was observed that the choice in Brønsted acid impacts
both the measured TON and the H : CO ratio of the resulting
syngas mixture, with the highest preference for CO production
measured in the presence of H O (Fig. and S10†).
: CO ratio measured
with water was maintained for a 3-hour CPE experiment
Tables S9 and S10†). This time-dependent product quantifi-
cation indicates negligible change in the H : CO ratio, as both
2
: CO ratio of 2.7 ± 0.3.
2
graphy for gaseous products. Under CO
2
atmosphere in the
9
2
Additionally, the high FE and favorable H
2
Table 1 Summary of results from CPE studies
(
a
Acid
Total FE (%)
H
2
: CO
i
CPE (mA cm−2
)
2
the 1 hour and 3 hours measurements in the presence of water
None
PhOH
TFE
—
—
1.1
4.8
2.9
1.6
>99
91 ± 9
>99
4.1 ± 0.5
10.2 ± 1
2.7 ± 0.3
generate H : CO ratios that are within the experimental error.
2
2
Neither H nor CO was detected in the absence of catalyst, and
no formate or oxalate or additional products (including
methane, methanol, carbonate, or other organics) were
2
H O
a
Average current density measured over 1 hour.
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detected under any of the conditions studied (see ESI†). As the
total FE is near unity in the CPE experiments performed in the
presence of CoPS complex, CO
PhOH, TFE, or H O), the absence of additional side products
is consistent with the quantities of H and CO detected. No H
2
and Brønsted acid additives
(
2
2
2
or CO was detected in the CPE experiments performed in the
presence of CoPS complex under N atmosphere, and upon the
addition of acid only H was detected in all cases (Fig. S11 and
2
2
2
Tables S11, S12†). Cyclic voltammetry under N in the presence
and absence of acid confirms that a sharp onset is observed at
the CoPS− couple, consistent with H evolution at these large
/0
2
overpotentials (Fig. S12†). Additionally, higher current den-
sities were generated in the presence of CoPS complex and an
acid additive under N relative to the currents measured under
2
CO
This suggests a competition exists between CO
and protonation, which hinders the hydrogen evolution reac-
tion (HER) in the presence of CO . A decrease was observed in
2
atmosphere, as indicted in Fig. S12 and S13,† respectively.
2
coordination
2
the electrolysis current density in the presence of 0.5 M PhOH
under N , which had not been detected under CO atmosphere
2
2
(
Fig. S11†). Cyclic voltammetry before and after electrolysis Scheme 1 Possible mechanistic paths for observed reactivity.
under these conditions indicates no change in the CoPS
0
/+
couple at −0.21 V, suggesting that the current drop is likely
associated with depletion of PhOH at these high catalytic −56.8 ppm, attributed to the formation of protonated species,
−
2
current densities (>18 mA cm ) (Fig. S14†). Similarly, no along with the resonances of the CoPS complex. These results
0
/+
change in the CoPS couple at −0.21 V was observed under indicate that the CoPS complex and the proposed protonated
catalytic conditions in the presence of CO
, further suggesting species are in equilibrium under these conditions. As these
that the current drop observed under N
is not associated with resonances are not observed in the presence of phenol or H O,
2
2
2
a change in the catalyst (Fig. S13†). Rinse tests were performed the results of this study suggest that only TFE is capable of pro-
following each experiment to confirm that the solubilized tonating the neutral complex prior to reduction. These results
complex is responsible for the observed activity (Fig. S15, are consistent with the measured H
2
: CO ratios as TFE is
further details provided in ESI†). Additional control experi- observed to generate the highest H : CO ratio of 10.2 ± 1. Due
2
ment studies were performed in the absence of catalyst, such to the literature precedent established for cobalt bis(dithio-
as CVs conducted with a glassy carbon electrode and acid addi- lene) and bis(diselenolene) catalysts, we propose that this pro-
tives under N . These studies display low current densities for tonation event takes place at the thiolate ligand. This is
2
H
2
O and TFE, or a sharp onset for PhOH, consistent with low further predicted by our DFT results, which indicate non-negli-
background activity for hydrogen evolution by the glassy gible thiyl-radical character. The presence of stronger acids is
carbon working electrode at the operating potential of catalysis expected to follow this protonation-first pathway (“CE”-type
(Fig. S16†).
mechanism), while weaker acids may follow a reduction-first
Based on the experimental observations, mechanistic path- pathway (“EC”-type mechanism). As such, multiple competing
ways can be constructed to model the measured reactivity, as mechanistic pathways may be available. As CPE studies are
shown in Scheme 1. Based on the electrochemical studies, no conducted at a lower potential than the second reduction of
reactivity is observed between CO
2
and CoPS prior to the complex, a second reduction step (E) prior to chemical
reduction, suggesting that an initial CO -binding step to the step (C) is ruled out, further suggesting EC- rather than
2
neutral species is unfavorable. In contrast, a color change from EE-type activation. Following reduction of the neutral complex,
yellow to green was observed upon treatment of an acetonitrile a competition exists between CO
solution of CoPS with trifluoroacetic acid (TFA), with a corres- protonation. This step is expected to determine the overall
ponding return to the original UV-Vis spectrum upon the selectivity of the catalyst, as binding of either substrate (CO or
addition of triethylamine base (Fig. S17†). This result suggests protons) allows the catalyst to enter either the CO -reduction
that the neutral complex may undergo reversible protonation reaction (CO RR) or HER cycles. Following liberation of CO
prior to reduction. As TFA is a reasonably strong acid, with a or H to regenerate the neutral species, the complex can
of 12.8 in acetonitrile, H-NMR spectra were collected in either receive another electron to regenerate the reduced
acetronitrile-d in the presence of 0.5 M phenol, TFE, and H O complex or react with protons to regenerate the protonated
2
-adduct formation and
2
2
2
2
1
pK
a
3
2
to test whether protonation is facilitated by these acids species.
Fig. S18†). In the presence of 0.5 M TFE, additional paramag-
The overpotential for the two-electron reduction of CO
netic resonances are observed at
−18.5, −29.9, and CO in acetonitrile is determined based on our applied poten-
(
2
to
δ
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tial of −2.18 V and the reported thermodynamic potential for M. Dawlaty and Prof. Ralf Haiges for discussions, and Thomas
CO reduction in the presence of various acid additives with Saal for assistance with single crystal X-ray diffraction studies.
2
4
1
the reported pK
formed by the reaction of CO
a
values (Table S13†). As carbonic acid is
with water, the overpotential (η)
2
for CO reduction in the presence of carbonic acid is also pro-
2
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