Electroreduction of Benzaldehyde with a Metal-Ligand Bifunctional Hydroxycyclopentadienyl Molybdenum(II) Hydride

Article abstract of DOI:10.1021/acs.organomet.0c00630

An electrocatalytic system for the hydrogenation of benzaldehyde to benzyl alcohol and dibenzyl ether is described. The molybdenum hydride (C5Ph4OH)Mo(CO)3(H) (3) is shown to be the actively hydrogenating species. This hydride is demonstrated to be remarkably acid-stable, leading to a ca. 67% Faradaic efficiency for aldehyde reduction over hydrogen evolution despite strongly acidic and reducing conditions. Hydride 3 is prepared via 1-proton-2-electron reduction of cation [(C5Ph4OH)Mo(CO)3(CH3CN)][OTf] ([2][OTf]), which is generated by protonation of the Mo(0) precursor [(C5Ph4O)Mo(CO)3(L)](1).

Full text of DOI:10.1021/acs.organomet.0c00630

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Electroreduction of Benzaldehyde with a Metal−Ligand Bifunctional
Hydroxycyclopentadienyl Molybdenum(II) Hydride
Keith C. Armstrong and Robert M. Waymouth*
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ABSTRACT: An electrocatalytic system for the hydrogenation of
benzaldehyde to benzyl alcohol and dibenzyl ether is described.
The molybdenum hydride (C5Ph4OH)Mo(CO)3(H) (3) is shown
to be the actively hydrogenating species. This hydride is
demonstrated to be remarkably acid-stable, leading to a ca. 67%
Faradaic efficiency for aldehyde reduction over hydrogen evolution
despite strongly acidic and reducing conditions. Hydride 3 is
prepared via 1-proton-2-electron reduction of cation [(C5Ph4OH)-
Mo(CO)3(CH3CN)][OTf] ([2][OTf]), which is generated by
protonation of the Mo(0) precursor [(C5Ph4O)Mo(CO)3(L)](1).
might lead to high selectivities for electroreduction vs
hydrogen evolution. To this end, we became interested in
testing whether bifunctional molecular catalysts,19 which have
proven effective for transfer hydrogenation, might prove
effective as selective electrohydrogenation catalysts (Figure
1). Metal hydrides are common intermediates in molecular
electrohydrogenation catalytic cycles; competitive protonation
of the metal hydride can lead to hydrogen evolution,
overwhelming the desired reduction of the organic carbonyl.
The potential advantages of bifunctional catalysts for
electrohydrogenation are based on the hypothesis that
hydrogen evolution from the reduced, substrate-reactive form
should be disfavored due to the electronic and geometric
separation of the hydrogen equivalents LX-H and M-H. In
addition, to the extent that bifunctional M-H/LX-H
activation19 might facilitate reduction of the carbonyl, the
selectivity for reduction of the substrate over hydrogen
evolution (by protonation of the metal hydride) might be
enhanced.
INTRODUCTION
■
Electrocatalytic hydrogenation is a promising alternative to
thermal catalytic hydrogenation for the upgrading and refining
of biofuels1−9 as well as a strategy for the storage of energy10,11
in liquid organic hydrogen carriers.12,13 Despite considerable
effort, advances in electrocatalytic hydrogenation have been
limited by challenges in the development of stable catalysts
that exhibit a high selectivity for hydrogenation over hydrogen
evolution.3 While several electroreduction systems have been
reported that exhibit high selectivities for hydrogenation over
hydrogen evolution,4,8,9,14 a mechanistic understanding of the
factors that lead to high Faradaic efficiencies are limited.4,5,8,15
The thermodynamic challenges for selective electrocatalytic
reduction are formidable; the thermodynamic potential for the
reduction of acetone or other organic substrates is so similar to
that of the reduction of protons to hydrogen (ΔE = 0.13 V, eq
1) that any selectivity based purely on thermodynamics would
To test this hypothesis, we targeted the Shvo type20−23 Mo
complex (C5Ph4OH)Mo(CO)3(H) 324 as a candidate for an
acetone electroreduction catalyst. The Mo complex 3 had been
previously shown to reduce acetone to isopropanol stoichio-
metrically and to function as a transfer hydrogenation catalyst
for the reduction of ketones and aldehydes with isopropanol as
a hydrogen donor.24
be extremely challenging. The kinetics are critical; for any
selective electrocatalytic hydrogenation catalyst, the rate of
substrate hydrogenation must be higher than that of hydrogen
evolution even when both are thermodynamically favor-
able.16−,18 For heterogeneous electrode catalysts, recent
insights suggest that particular step edges or surface sites can
exhibit selectivities for electrocatalytic hydrogenation,4,5
particularly for situations where the substrate binds to the
surface sites more avidly than hydrogen atoms (Habs), leading
to high surface coverages of the substrate.8
Received: September 21, 2020
Molecular electrocatalysts provide an opportunity to
investigate the molecular details of individual proton and
electron transfer steps and to test different hypotheses that
https://dx.doi.org/10.1021/acs.organomet.0c00630
© XXXX American Chemical Society
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Figure 1. Competitive hydrogenation of carbonyl compounds vs hydrogen evolution with bifunctional catalysts (L = dative atom in backbone, X =
O, NH).
RESULTS AND DISCUSSION
■
A sample of the Mo(0) compound [(C5Ph4O)Mo(CO)3(L)]
was prepared following the literature procedure25 and was used
without further purification. Rigorous analysis of the crude
product by multiple characterization methods revealed it to be
a mixture of the acetonitrile adduct (C5Ph4O)Mo-
(CO)3(CH3CN) 1a and the related, known25 Mo0 dimer
and monitored by 1H NMR spectroscopy. In the presence of 2
[(C5Ph4O)(CO)3Mo]2 1b (see Supporting Information (SI)).
equiv of Cp2Co, protonation of [2][OTf] with either
[DMFH][OTf] (1.1 equiv) or HBF4·Et2O (1.5 equiv) in
C6D6 afforded (C5Ph4OH)Mo(CO)3(H) 3 in yields of 48% or
65%, respectively. The 3 so generated is capable of reducing
benzaldehyde to benzyl alcohol (SI, Figure S11). Reductive
protonation of [2][OTf] with excess acid (5.1 equiv. HBF4·
Et2O, 2 equiv of Cp2Co) afforded a lower yield of the Mo−H 3
Treatment of a freshly prepared sample of [(C5Ph4O)Mo-
(CO)3(L)] with [DMFH][OTf] in toluene resulted in the
formation of a precipitate. Washing of this precipitate with
toluene and recrystallization from a mixture of dichloro-
methane/hexanes afforded red-orange crystals. Notably, treat-
ment of a sample of pure dimer [(C5Ph4O)(CO)3Mo]2 1b
1
with [DMFH][OTf] shows no protonation activity by H
1
(22%), but when this solution was monitored by H NMR
NMR (SI, Figure S12), suggesting that exclusively (C5Ph4O)-
Mo(CO)3(CH3CN) 1a undergoes productive protonation.
Microanalysis and high-resolution mass spectrometry (HRMS)
of these crystals are consistent with the formation of the
cationic [(C5Ph4OH)Mo(CO)3(CH3CN)][OTf], ([2]-
[OTf]), with coordinated acetonitrile and outer-sphere triflate
(eq 2).24 The complex [2][OTf] is soluble in chlorinated
over the course of 3 h, the amount of 3 in solution did not
decrease appreciably (final yield 19%). This latter result
suggests that the Mo−H 3 is relatively stable in the presence of
acid. This conclusion is corroborated by the fact that 3,
independently prepared from 1 and excess isopropanol in
benzene, is unreactive toward excess [DMFH][OTf] (SI,
Table S2). The incomplete conversion of [2][OTf] to 3 is
likely a consequence of competitive protonation of Cp2Co and
hydrogen evolution under these conditions.26,27
The cyclic voltammetry of 1 (∼1 mM) in ortho-
difluorobenzene (ODFB) reveals an irreversible reductive
feature at ca. −1700 mV vs Fc+/0 on a cathodic sweep at 100
mV/s (SI, Figure S13). Addition of 1.0 mM [DMFH][OTf] to
this solution results in the appearance of a new reductive wave
at −1000 mV vs Fc+/0, which we tentatively assign as the two-
electron-one-proton reduction of [2][OTf] (formed in situ
after the addition of acid) to 3.
This same feature appears in the cyclic voltammogram of
independently prepared [2][OTf] with excess acid (SI, Figure
S14A). In the absence of acid, [2][OTf] displays a reductive
feature at a slightly more anodic potential, approximately −900
mV vs Fc+/0 (SI, Figure S14A). Voltammetric scan rate studies
under acid-free conditions reveal that this feature only
becomes quasi-reversible at high (1+ V/s) rates Figures 2,
SI, S15), possibly indicative of a fast, reduction-induced
rearrangement of the [2]+ fragment, such as ligand
dissociation. These reduction events are independent from
that for [DMFH][OTf] alone (SI, Figure S14B).
solvents and acetonitrile, sparingly soluble in aromatic solvents
and is air-stable over several hours as a solid or in
dichloromethane solution, with noticeable darkening of the
solid or its solutions after a period of several days. To assess
the feasibility of generating a Mo(II) hydride from the
protonation and reduction of Mo(0) carbonyls, a sample of
[(C5Ph4O)Mo(CO)3(L)] was dissolved in C6D6 and treated
with excess [DMFH][OTf], then two equivalents of
cobaltocene (Cp2Co) as a chemical reductant. 1H NMR
analysis of the resulting solution revealed a new resonance at δ
−3.83 ppm, consistent with the formation of a Mo−H (eq
3).24 High-resolution mass spectrometry (electrospray ioniza-
tion, acetonitrile, negative detection mode, −3.0 kV) of an
aliquot of this C6D6 solution corroborates our successful
generation of (C5Ph4OH)Mo(CO)3(H) 3 (SI, Figure S7; M −
[H+], m/z calcd 567.0488, obsd 567.0483) from 1, acid, and
cobaltocene. The reductive protonation of [(C5Ph4OH)Mo-
(CO)3(CH3CN)][OTf], ([2][OTf])] was also investigated
Controlled-potential electrolysis of a 5 mM ODFB solution
of [2][OTf] and 6 mM [DMFH][OTf] at −700 mV (vs
Fc+/0) in the presence of 100 mM TBAPF6 was carried out for
1 h. An aliquot of the resulting solution was diluted into C6D6
1
and analyzed by H NMR with solvent suppression of the
ODFB resonances (see SI). A resonance at δ −4.04 ppm was
B
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Figure 2. A selection of traces from a CV scan rate study of [2][OTf] in 5 mL of 100 mM nBu4NPF6 in ODFB. The complete study is available in
SI, Figure S15.
Figure 3. Proposed electrocatalytic mechanism for the reduction of benzaldehyde by [2][OTf] (S = ODFB).
observed, consistent with the formation of (C5Ph4OH)Mo-
(CO)3(H) 3 (SI, Figure S17). These results indicate that the
reductive protonation of 1 to give 3 can be carried out both
chemically and electrochemically. Analysis of the charge passed
during the controlled-potential electrolysis reveals that 1.7 mol
electrons/initial mol [2][OTf] were transferred over the
C
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Communication
course of the 1 h electrolysis. The total charge passed is
approximately 15% less than that predicted for the 2-electron
1-proton reduction of [2][OTf], which might be attributed to
the partial decomposition of [2][OTf] under the reaction
conditions.
In summary, we report the electrohydrogenation of
benzaldehyde with a Shvo-type Mo complex (C5Ph4OH)Mo-
(CO)3(H) 3. Controlled potential electrolysis of benzaldehyde
at −0.90 V (vs Fc+/0) in ortho-difluorobenzene with the acid
[DMFH][OTf] affords a mixture of the reduction products
benzyl alcohol (BnOH) and dibenzyl ether (Bn2O) with a
Faradaic efficiency of approximately 67%. Stoichiometric
experiments reveal that the reductive protonation of either
the Mo(0) complex [(C5Ph4O)Mo(CO)3(L)] or the cationic
Mo(II) complex [(C5Ph4OH)Mo(CO)3(CH3CN)][OTf] af-
fords the hydride (C5Ph4OH)Mo(CO)3(H) 3, which was
previously shown to be competent to reduce aldehydes. These
studies provide support for an electrocatalytic mechanism
involving the rate-limiting reduction of the aldehyde by the
Mo−H 3 and subsequent reductive protonation of [(C5Ph4O)-
Mo(CO)3(L)] to regenerate the Mo−H 3. The high Faradaic
efficiencies observed with these metal−ligand bifunctional
catalysts provide new design motifs for the design of selective
electrocatalysts for electrohydrogenation relative to hydrogen
evolution.
As stoichiometric experiments had shown that (C5Ph4OH)-
Mo(CO)3(H) 3 can be generated electrochemically from
[(C5Ph4OH)Mo(CO)3(CH3CN)][OTf] ([2][OTf]), and
previous studies had shown that 3 can reduce benzaldehyde,24
the electrocatalytic reduction of aldehydes was investigated
with [2][OTf]. Initial experiments by cyclic voltammetry were
uninformative, as titration of benzaldehyde into a [2][OTf]/
[DMFH][OTf] solution, even at very low scan rates (i.e., 25
mV/s), did not show any evidence of an enhanced current at
−1000 mV. This was not unexpected, as previous studies24 had
shown that catalytic reduction of aldehydes by (C5Ph4OH)-
Mo(CO)3(H) 3 only occurs over the course of hours at 65 °C
(99% conversion after 12 h, 65 °C).
To investigate the electrocatalytic reduction of aldehydes,
controlled-potential electrolysis of an ODFB solution of 1 mM
[2][OTf], 100 mM [DMFH][OTf], and 100 mM benzalde-
hyde at −900 mV (vs Fc0/+) were carried out for 2 h. Analysis
of the resulting solution by 1H NMR (see SI) after 2 h revealed
the formation of benzyl alcohol (BnOH) and dibenzyl ether
(Bn2O), with turnover number of TON = 4.4 and a Faradaic
efficiency of approximately 67% (averaged from two runs).
The results of this experiment are summarized in the SI, Table
S3. A control experiment with no Mo (SI, Figures S22, S23)
showed very low yield (0.3%) and Faradaic efficiency (1.7%),
confirming the catalytic role of Mo. Similarly, an experiment
with no applied potential showed no conversion (SI, Figure
S24), indicating the requirement of an electrochemical step in
the catalytic cycle.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00630.
■
sı
General experimental methods, syntheses, additional
cyclic voltammograms, quantification procedures and
calculations, procedures for controlled potential elec-
trolysis studies, control experiments, and additional
nuclear magnetic resonance studies (PDF)
AUTHOR INFORMATION
Corresponding Author
■
The stoichiometric and catalytic experiments are consistent
with the mechanism outlined in Figure 3. In the presence of
[DMFH][OTf], [2][OTf] is converted to the Mo−H 3 by a
two-electron-one-proton reduction at the working electrode.
The slow reduction of benzaldehyde by Mo−H 3 affords
benzyl alcohol and the Mo(0) tetraphenylcyclopentadienone
complex.24 Competitive acid-catalyzed dehydration of benzyl
alcohol generates dibenzyl ether. Protonation of the Mo(0)
tetraphenylcyclopentadienone regenerates [2][OTf].
Robert M. Waymouth − Department of Chemistry, Stanford
University, Stanford, California 94305, United States;
orcid.org/0000-0001-9862-9509; Email: waymouth@
stanford.edu
Author
Keith C. Armstrong − Department of Chemistry, Stanford
University, Stanford, California 94305, United States;
orcid.org/0000-0002-2585-7276
In light of previous studies showing that the rate of aldehyde
reduction is slow at room temperatures, it is likely that the
chemical step between benzaldehyde and 3 is turnover-
limiting. The ca. 67% Faradaic efficiencies observed are
noteworthy in that they imply that the Mo−H 3, generated
electrochemically in the presence of benzaldehyde, exhibits a
selectivity for reduction of the aldehyde over protonation to
release H2 (hydrogen evolution reaction, HER). The origin of
this selectivity is not clear, but has some precedent in the
chemistry of ionic hydrogenation28 with related Mo piano
stool complexes. The related Cp’Mo(H)(CO)3 (Cp′ =
cyclopentadiene, pentamethylcyclopentadiene, pentabenzylcy-
clopentadiene29) complexes28−31 were observed to react with
triflic acid in CD2Cl2 to generate H2 and Cp’Mo(OTf)-
(CO)3,30 but when protonation of either CpMo(CO)2(PPh3)
H (CH3CN) or CpMo(CO)2(PPh3)H (CH2Cl2) was carried
out in the presence of a ketone (acetone or 3-pentanone),
competitive reduction to the alcohol and hydrogen evolution
were observed.29,31 The bifunctional nature of the hydrox-
ycyclopentadienyl ligand might also contribute to the observed
selectivity for electrohydrogenation (Figure 1).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00630
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based on work supported by the National
Science Foundation (CHE-1565947, initial studies) and the
Department of Energy (DE-SC0018168). K.C.A. is grateful to
Dr. Stephen Lynch at the Stanford University NMR Facility for
valuable technical discussions and expertise. K.C.A. thanks
Daniel Marron, Dr. Katherine Walker, and the research group
of Prof. Richard N. Zare for assistance with mass spectrometry
instrumentation.
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