NHC-containing manganese(I) electrocatalysts for the two-electron reduction of CO2

Article abstract of DOI:10.1002/anie.201311099

The synthesis and characterization of the first catalytic manganese N-heterocyclic carbene complexes are reported: MnBr(N-methyl-N'-2- pyridylbenzimidazol-2-ylidine)(CO)₃ and MnBr(N-methyl-N'-2- pyridylimidazol-2-ylidine)(CO)₃. Both new species mediate the reduction of CO₂ to CO following two-electron reduction of the Mn^I center, as observed with preparative scale electrolysis and verified with ¹³CO₂. The two-electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2'-bipyridine)(CO)₃. Catalytic current enhancement is observed at voltages similar to MnBr(2,2'-bipyridine)(CO)₃. Catalytic manganese N-heterocyclic carbene complexes have been synthesized and characterized: [MnBrL(CO)₃] (see scheme; L=N-methyl-N'-2- pyridylbenzimidazol-2-ylidine or N-methyl-N'-2-pyridylimidazol-2-ylidine). Both species mediate the reduction of CO₂ to CO following two-electron reduction of the Mn^I center at a single potential, as observed with preparative scale electrolysis and verified with ¹³CO₂.

Full text of DOI:10.1002/anie.201311099

Angewandte
Chemie
DOI: 10.1002/anie.201311099
Electrocatalysis
NHC-Containing Manganese(I) Electrocatalysts for the Two-Electron
Reduction of CO2**
Jay Agarwal,* Travis W. Shaw, Charles J. Stanton III, George F. Majetich, Andrew B. Bocarsly,
and Henry F. Schaefer III
Abstract: The synthesis and characterization of the first
catalytic manganese N-heterocyclic carbene complexes are
reported: MnBr(N-methyl-N’-2-pyridylbenzimidazol-2-ylidi-
ne)(CO)3 and MnBr(N-methyl-N’-2-pyridylimidazol-2-ylidi-
ne)(CO)3. Both new species mediate the reduction of CO2 to
CO following two-electron reduction of the MnI center, as
observed with preparative scale electrolysis and verified with
13CO2. The two-electron reduction of these species occurs at
a single potential, rather than in two sequential steps separated
by hundreds of millivolts, as is the case for previously reported
MnBr(2,2’-bipyridine)(CO)3. Catalytic current enhancement is
observed at voltages similar to MnBr(2,2’-bipyridine)(CO)3.
tions, such as the addition of alkyl substituents at the 4 and 4’
positions on bpy. Modification of the coordinating atom (that
is, replacing nitrogen) has received little attention, although
this should have a profound effect on the electronic structure.
Herein we report the replacement of one pyridine ring in
bpy with an N-heterocyclic carbene (NHC) moiety to afford
À
a fac-[MnBr(N C)(CO)3]-type catalyst. The advantage of this
approach lies in the versatility of NHC ligands, which can be
4, 5, or 6-membered and include substitutions for the nitrogen
atoms and for the coordinating carbon atom.[6] One disad-
vantage, however, is the enhanced s-donor character com-
pared to pyridine that may result in an increased HOMO–
LUMO gap and a cathodic shift of the one- and two-electron
reduction potentials. Notwithstanding that effect, we sought
to integrate NHC-pyridine ligands into the manganese
tricarbonyl framework and evaluate their efficacy for CO2
reduction.
LÀow-valent manganese(I) complexes of the form fac-[MnBr-
À
(N N)(CO)3], where N N is bare or modified 2,2’-bipyridine
(bpy), have recently been identified as electrocatalysts for the
two-electron reduction of CO2 to CO.[1,2] These catalysts offer
improvements over rhenium analogues, which have been
employed since the 1980s.[3–5] Namely, the one- and two-
electron reduction potential for manganese is shifted anodic,
yielding a decreased overpotential for CO2 conversion, and
the synthesis is less-expensive, which is due in part to the
abundance of manganese metal. Much improvement is still
needed, however, to optimize the rate of conversion and
further reduce the overpotential.
We chose N-methyl-N’-2-pyridylbenzimidazolium (L1)
and N-methyl-N’-2-pyridylimidazolium (L2) as model ligands
(Scheme 1) for testing the catalytic activity, and to assess the
Central to achieving those goals is investigating new
manganese(I) tricarbonyl-type complexes that also exhibit
catalytic activity. Several studies have aimed at optimizing the
catalytic process through functionalization of the pyridyl
ligand or replacement of the axial halogen. Investigations of
the former, however, are often limited to ancillary modifica-
Scheme 1. Ligands under study (top), and chelation strategy (bottom).
[*] Dr. J. Agarwal, Prof. Dr. H. F. Schaefer III
Center for Computational Quantum Chemistry
University of Georgia (USA)
structure–activity relationship of an extended p system.
Previously, these ligands were coordinated to ReI tricarbonyl
compounds in photochemical studies by Vaughan,[7] Li,[8] and
Casson.[9] Additionally, Chang and co-workers have reported
Ni-centered catalysts with related NHC-pyridine ligands for
CO2 reduction.[10,11] To our knowledge, L1 and L2 have not
been reported in an analogous manganese complex. In fact,
few NHC-containing MnI complexes are known, and none, of
any oxidation state, have been identified as catalysts.[12,13]
Our investigation began with the synthesis of ligands L1
and L2 from 2-bromopyridine and benzimidazole or imida-
zole, respectively.[14] The resulting compounds were N-methy-
lated with bromomethane, yielding a BrÀ salt. Both L1[15] and
E-mail: jagarwal@uga.edu
T. W. Shaw, Prof. Dr. A. B. Bocarsly
Department of Chemistry, Princeton University (USA)
C. J. Stanton III, Prof. Dr. G. F. Majetich
Department of Chemistry, University of Georgia (USA)
[**] Research at UGA was supported by the National Science Founda-
tion (NSF), Grant CHE-1054286. Research at Princeton was
supported by funding from the DOD-MURI program under AFOSR
Award FA9550-10-1-057 and the NSF under grant CHE-1308652.
T.W.S. acknowledges the NSF for a graduate research fellowship. We
thank Dr. Pingrong Wei for assistance in the acquisition and
refinement of X-ray structures, and Dr. Todd Harrop for the use of an
infrared spectrometer. NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311099.
1
L2[16] were verified with H NMR and compared to known
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values.[17] Our choice of the bromo salt stems from its
solubility in water, which allows for facile removal upon
aqueous workup after chelation. Each ligand was chelated to
pentacarbonylbromomanganese [MnBr(CO)5] at 608C in
THF for 12 h under argon with excess KOtBu.
[MnBr(L1)(CO)3] (1)[18] and [MnBr(L2)(CO)3] (2)[19] were
second reduction wave corresponds to a species formed at the
first reduction. No other reductive processes were observed
when the cyclic voltammetric window was expanded to the
onset of solvent reduction (roughly À3.0 V vs. SCE).
Under an atmosphere of CO2, the current doubles from
0.18 mAcmÀ2 to 0.36 mAcmÀ2 at the first reduction wave,
indicating catalytic turnover. Compared to [MnBr-
(bpy)(CO)3] under the same conditions, the first reduction
wave of 1 occurs at a similar voltage to the second one-
electron reduction wave of [MnBr(bpy)(CO)3]. As such, the
catalytic current enhancement for each species has a similar
onset. As noted previously, the presence of water is necessary
for catalytic activity.[1,2] Accordingly, when cyclic voltammetry
was performed in dry CH3CN there was no significant
difference in current between argon and CO2 saturated
electrolytes.
1
verified by FTIR, H NMR, and 13C NMR. Single crystals of
1 suitable for X-ray analysis were grown from slow diffusion
of hexanes into a solution of 1 in chloroform (Figure 1).
Detailed experimental procedures for all transformations are
given in the Supporting Information.
The cyclic voltammetry of 2 under argon in wet (5% H2O)
CH3CN is similar to that of 1. Two irreversible redox
processes occur at À1.46 V vs. SCE and À1.72 V vs. SCE, as
shown in Figure 3. These events are shifted cathodically by
110 mVand 80 mV, respectively, from 1. We attribute this shift
to the decreased p*-accepting ability of the coordinating
Figure 1. [MnBr(L1)(CO)3] (1) from crystal structure; ellipsoids are set
at 50% probability.[23] Selected bond lengths [ꢀ]: Mn1–C1 1.984(4),
Mn1–C14 1.794(4), Mn1–C15 1.795(4), Mn1–C16 1.828(5), Mn1–N3
2.060(3), N2–C8 1.399(4).
The cyclic voltammetry of 1 under argon in wet (5% H2O)
CH3CN shows two irreversible redox processes at À1.35 V vs.
the saturated calomel electrode (SCE) and À1.64 V vs. SCE,
as shown in Figure 2. The peak current of the first reduction
wave demonstrates a linear dependence on the square root of
the scan rate, indicating a diffusion-limited process (see the
Supporting Information). At successively faster scan rates, the
second redox process decreases in current until it can no
longer be seen at all (500 mVsÀ1 or greater). Therefore, the
Figure 3. Cyclic voltammagram of [MnBr(L2)(CO)3] (2) under Ar
(solid) and CO2 (dashed) in wet (5% H2O) CH3CN with 0.1m
tetrabutylammonium perchlorate (TBAP) supporting electrolyte at
10 mVsÀ1. Complex 1 was loaded at a concentration of 1 mm, and the
pH adjusted to 3.7 (HClO4) in the case of Ar.
carbon atom as a result of removing the annulated benzene
ring,[20] yielding increased net electron donation to the metal
center.[21] Comparing the symmetric CO stretching frequen-
cies[22] for 1[18] (2015 cmÀ1) and 2[19] (2012 cmÀ1) demonstrates
this effect; increased electron density at the metal center
results in additional carbonyl p*-backbonding that weakens
ꢀ
the C O bond and reduces the stretching frequency.
Under an atmosphere of CO2, 2 shows a current enhance-
ment of 1.9 ꢀ at the first reduction wave; slightly less than
found for 1, though both fall below the same enhancement in
[MnBr(bpy)(CO)3] (2.7 ꢀ). From previously published equa-
tions for the turnover frequency (TOF),[2] we calculated
a TOF for 1 and 2 as 0.08 sÀ1 and 0.07 sÀ1, respectively. For
reference, our calculated TOF for [MnBr(bpy)(CO)3] under
the exact same conditions is 0.14 sÀ1 (see the Supporting
Information). To further confirm the electrocatalytic con-
version of CO2 to CO in the compounds under study, we
Figure 2. Cyclic voltammagram of [MnBr(L1)(CO)3] (1) under Ar
(solid) and CO2 (dashed) in wet (5% H2O) CH3CN with 0.1m
tetrabutylammonium perchlorate (TBAP) supporting electrolyte at
10 mVsÀ1. Complex 1 was loaded at a concentration of 1 mm, and the
pH adjusted to 3.7 (HClO4) in the case of Ar.
2
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electrolyzed 1 at the potential of the first peak in the presence
of 13CO2. After passing nine coulombs, we evacuated the
headspace of the electrolysis cell and examined it with FTIR
spectroscopy. The presence of 13CO confirms successful
conversion (see the Supporting Information).
From these data, we postulated that 1 and 2 behave
similarly to [MnBr(bpy)(CO)3] where a two-electron reduc-
tion of the metal center yields a five-coordinate anion that
ligates and reduces CO2. [MnBr(bpy)(CO)3] displays two
sequential reductions, however, that occur several hundred
millivolts apart; the first reduction likely forms a neutral
radical after bromide loss, and the second reduction yields the
requisite anion. Only one reduction wave is observed for
1 and 2, yet they are able to mediate a 2eÀ process, consistent
with an ECE mechanism.
using gas chromatography (GC). Importantly, we did not
observe H2 production at any point, which illustrates the
excellent selectivity for reduction of CO2 to CO. Steady
production of CO was observed throughout the electrolysis,
with an average Faradaic efficiency of 34.6% at the four hour
mark. A summary of the electrolysis data is provided in the
Supporting Information.
In summary, we have reported the synthesis of two new
manganese complexes that contain N-heterocyclic carbene
ligands: [MnBr(N-methyl-N’-2-pyridylbenzimidazol-2-ylidi-
ne)(CO)3] and [MnBr(N-methyl-N’-2-pyridylimidazol-2-yli-
dine)(CO)3]. Subsequently, we have evaluated their promise
for CO2 reduction and find they are both capable of mediating
a two-electron reduction to CO. To our knowledge, this is the
first report of catalytic manganese organometallic species that
contain NHC ligands. Notably, the first- and second-electron
reduction processes for both catalysts occur at approximately
the same voltage. Thus, even though the strong s-donor
character of the NHC ligand pushes the reduction potentials
cathodic, both species exhibit catalytic current enhancement
at voltages comparable to [MnBr(2,2’-bipyridine)(CO)3]. Our
hope is that the numerous possibilities for functionalizing
NHC ligands will allow for rapid and significant optimization
of these promising catalysts.
To gather support for this hypothesis, we looked at the
reversibility of the redox process as a function of scan rate for
complex 2 (Figure 4).At scan rates below 100 mVsÀ1 the
process appears nonreversible. Assuming the reaction course
is an electrochemical step (reduction of 2), a chemical step
(loss of bromide), and a final electrochemical step (reduction
Received: December 20, 2013
Published online: && &&, &&&&
Keywords: carbon dioxide · catalysis · electrochemistry ·
.
manganese · N-heterocyclic carbenes
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m), 8.06 (1H, d, J = 8 Hz), 7.81–7.75 (2H, m), 7.72–7.69 (1H, m),
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m), 8.14 (1H, d, J = 10 Hz), 7.72 (1H, s), 7.49–7.46 (1H, m),
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117.37, 112.57, 38.06 ppm.
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10 Hz), 8.45 (1H, d, J = 5 Hz), 8.28–8.25 (1H, m), 7.95 (1H, d,
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4
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Electrocatalysis
Catalytic manganese N-heterocyclic car-
bene complexes have been synthesized
and characterized: [MnBrL(CO)3] (see
scheme; L=N-methyl-N’-2-pyridylbenzi-
midazol-2-ylidine or N-methyl-N’-2-pyri-
dylimidazol-2-ylidine). Both species
mediate the reduction of CO2 to CO
following two-electron reduction of the
MnI center at a single potential, as
observed with preparative scale electrol-
ysis and verified with 13CO2.
J. Agarwal,* T. W. Shaw, C. J. Stanton,
G. F. Majetich, A. B. Bocarsly,
H. F. Schaefer
&&&& — &&&&
NHC-Containing Manganese(I)
Electrocatalysts for the Two-Electron
Reduction of CO2
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