An efficient nickel catalyst for the reduction of carbon dioxide with a borane

Article abstract of DOI:10.1021/ja103982t

Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO₂ with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH₃OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO₂. The reaction may also be catalyzed by an air-stable nickel formate.

Full text of DOI:10.1021/ja103982t

Published on Web 06/14/2010
An Efficient Nickel Catalyst for the Reduction of Carbon Dioxide with a
Borane
Sumit Chakraborty, Jie Zhang, Jeanette A. Krause, and Hairong Guan*
Department of Chemistry, UniVersity of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172
Received May 10, 2010; E-mail: hairong.guan@uc.edu
Scheme 1
The conversion of carbon dioxide to polymers, chemicals, and fuels
is an attractive means of making biodegradable materials, synthesizing
value-added products, and storing energy.1 In addition, it could help
reverse the negative impact of human activities on the earth’s
atmosphere. Of particular interest to us is the development of highly
efficient methods for the reduction of CO2. Transition-metal-catalyzed
homogeneous hydrogenation of CO2 often leads to either formic acid
that is thermodynamically stabilized by a base2 or CO via the reverse
process of the water-gas shift reaction.3 Transforming CO2 into
methane, the most reduced form of carbon, under homogeneous
conditions can be accomplished using silanes as the reducing reagents.4
Reducing CO2 to methanol would be even more desirable for the
advantages of transporting a liquid fuel rather than a gas. Catalytic
hydrosilylation of CO2 to methoxysilyl species is feasible with
Ir(CN)(CO)(dppe) [dppe ) 1,2-bis(diphenylphosphino)ethane] at 40
°C, albeit with limited turnover numbers.5 More efficient hydrosily-
lation reactions are catalyzed by N-heterocyclic carbenes (metal-free)
at ambient temperature with turnover frequencies (TOFs) as high as
25.5 h-1 (based on Si-H), and methanol is produced from the
hydrolysis of the initial reduction products.6 The recent development
of frustrated Lewis acid-base pair (FLP) chemistry7 has led to
alternative strategies for the reduction of CO2 to the methoxide level
given either H28 or H3NBH39 as a hydrogen source. These reactions
are presumably promoted by the formation of strong MeO-B and
MeO-Al bonds for the FLP systems involving 2,2,6,6-tetramethylpi-
peridine/B(C6F5)38 and PMes3/AlX3 (Mes ) 2,4,6-Me3C6H2; X ) Cl,
Br),9 respectively. On the other hand, cleavage of these robust chemical
bonds has proven to be difficult, and therefore, stoichiometric amounts
of FLPs are required. In this paper, we report a highly efficient nickel
system for the catalytic hydroboration of CO2 to methoxyboryl species
using a simple borane. The reactions operate at room temperature with
TOFs at least 1 order of magnitude higher than those of the related
reactions described above.
Insertion of CO2 into a metal-hydrogen bond constitutes a
critical step in many transition-metal-catalyzed reductions of CO2.2
Despite the high thermodynamic stability of CO2, we found that
the reaction of PCP-pincer nickel hydride (1)10 with CO2 proceeds
rapidly at room temperature to give a nickel formate complex (2)
as the sole product (Scheme 1). The related complexes trans-
(Cy3P)2Ni(H)(Ph)11 and [2,6-(tBu2PCH2)2C6H3]PdH12 show similar
reaction patterns with CO2. In contrast, the interaction between CO2
and [(tBu2PCH2SiMe2)2N]NiH, a PNP-pincer nickel hydride, gives
rise to a hydridonickel cyanate complex through an N/O transposi-
tion.13 The CO2 insertion in our system is reversible, as evidenced
by the following observations: (1) mixing 13C-labeled nickel formate
(2-13C) with CO2 (∼1 atm) at room temperature resulted in the
formation of nonlabeled 2, and (2) heating a solid sample of 2 under
vacuum at 60 °C for 5 h generated 1 in a nearly quantitative yield.
Compound 2 was also independently synthesized by rapid protonation
of 1 with 1 equiv of HCO2H. X-ray analysis of single crystals of 2
(Figure 1) revealed a distorted square-planar geometry for Ni, with
the O3 atom displaced 0.41 Å out of the least-squares plane defined
by the P1, C1, Ni, and P2 atoms;14 however, the long through-space
Ni · · ·O4 distance [3.271(3) Å] would argue against a potential η2
coordination mode for the formato group. The bond lengths of
C23-O3 [1.250(4) Å] and C23-O4 [1.236(4) Å] are fairly close as
a result of a significant degree of electron density delocalization within
the formate moiety;15 however, these bonds are slightly shorter than
the C-O bond lengths in HCO2Na (1.27 Å).16
Regenerating 1 from 2 would complete a potential catalytic cycle
for the reduction of CO2. To test this hypothesis, we investigated
the reactions between 2 and various reducing reagents at room
temperature. When catecholborane (HBcat) was used at 1 equiv
with respect to Ni, 1 was observed along with a new boron species
that was assigned as HCOOBcat. Interestingly, when HBcat was
employed in large excess, a new resonance appeared at 3.34 ppm
1
(singlet) in the H NMR spectrum of the reaction, consistent with
the formation of a methanol derivative. To further understand this
process, we carried out a series of NMR experiments involving
13CO2. When 2-13C (generated in situ from 1 and 13CO2) was treated
with 4 equiv of HBcat, the corresponding 1H NMR spectrum
exhibited a doublet (1JC-H ) 145.2 Hz) at 3.34 ppm (Figure 2).
The same reaction monitored by 13C NMR spectroscopy revealed
a singlet at 53.6 ppm in the 1H-decoupled spectrum (Figure 3) and
a quartet (1JC-H ) 145.2 Hz) in the 1H-coupled spectrum,
confirming the presence of a methoxy group. Thus, the reduction
product was identified as 13CH3OBcat, the spectra of which are
consistent with those reported in the literature17 as well as those
Figure 1. ORTEP view of complex 2 (50% probability level). Selected
bond lengths (Å) and angles (deg): Ni-C1, 1.886(2); Ni-P1, 2.2040(7);
Ni-P2, 2.1817(8); Ni-O3, 1.920(2); C23-O3, 1.250(4); C23-O4, 1.236(4);
P1-Ni-P2, 164.02(3); O3-C23-O4, 128.0(3). Through-space Ni · · ·O4
distance: 3.271(3) Å.
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8872 J. AM. CHEM. SOC. 2010, 132, 8872–8873
10.1021/ja103982t  2010 American Chemical Society

C O M M U N I C A T I O N S
obtained for CH3OBcat that was independently synthesized from
CH3ONa and ClBcat.18 The byproduct of the CO2 reduction process
was catBOBcat, which resulted in a downfield shift of the
resonances of 2-13C (Figures 2 and 3), presumably through a Lewis
acid-base interaction. 13CH3OBcat proved to be hygroscopic; it
absorbed moisture quickly from the air and generated 13CH3OH,
as confirmed by mass spectroscopy.18
2·catBOBcat, and further addition of HBcat reinitiated the reaction
with no loss of catalytic activity. In fact, the independently synthesized
2 exhibited a catalytic efficiency comparable to that of 1, an advantage
in our catalytic system as compound 2 is air-stable.
Scheme 3
In summary, we have shown the first catalytic hydroboration of
CO2, with the highest TOF (495 h-1 based on B-H) reported to
date for the reduction of CO2 to the methoxide level. Further studies
to elucidate the mechanistic details and improve the catalytic
efficiencies are in progress.
Figure 2. 1H NMR spectra of 2-13C in C6D6 (bottom) and 2-13C with added
HBcat in C6D6 [top;
[ * denotes
denotes H13COOBcat and
H13COONi(tBuPCP)·catBOBcat].
Acknowledgment. We thank the National Science Foundation
(CAREER Award CHE-0952083), the donors of the American
Chemical Society Petroleum Research Fund (49646-DNI3), and
Cambridge Isotope Laboratories, Inc., for support of this research.
We are also grateful to Dr. Keyang Ding and Dr. Larry Sallans for
their assistance with NMR and mass spectral analysis. J.Z. thanks
the University of Cincinnati University Research Council for a
postdoctoral research fellowship. X-ray data were collected on a
Bruker SMART6000 diffractometer that was purchased with
funding provided by an NSF-MRI Grant (CHE-0215950).
Supporting Information Available: Experimental details, complete
ref 1a, and crystallographic data for 2 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 3. 13C{1H} NMR spectra of 2-13C (with residual 13CO2) in C6D6
(bottom) and 2-13C with added HBcat in C6D6 (top).
References
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9, 605–623. (c) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. ReV. 2007,
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We also discovered that the reduction of CO2 with HBcat was
catalytic in nickel. The mixture of HBcat and 1 in a ratio of 500:1
under 1 atm CO2 generated CH3OBcat with 495 turnovers (based
on B-H) in 1 h, while the other boron product (catBOBcat)
precipitated from the C6D6 solution (Scheme 2). A control experi-
ment showed no reaction in the absence of 1, confirming that the
reduction was indeed catalyzed by the nickel hydride. The initial
reduction products were further hydrolyzed in a large excess of
water, and vacuum distillation of the resulting mixture yielded 0.28
M methanol in C6D6 (i.e., 61% yield based on HBcat).
(3) For discussions of homogeneously catalyzed water-gas shift reactions, see:
(a) Ford, P. C.; Trabuco, E.; Mdleleni, M. M. Water Gas Shift
ReactionsHomogeneous. In Encyclopedia of Catalysis, 1st ed; Horva´th,
I. T., Ed.; Wiley Interscience: Hoboken, NJ, 2003; Vol. 6, pp 651-658.
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3322–3325.
Scheme 2
(7) Stephan, D. W. Dalton Trans. 2009, 3129–3136.
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(14) The deviation from planarity for the P1, C1, Ni, and P2 atoms was calculated
to be 0.0148 Å.
Catalytic cycles consistent with our observations are outlined in
Scheme 3. The catalytic reduction of CO2 begins with a reversible
insertion of CO2 into a Ni-H bond. The subsequent cleavage of the
Ni-O bond with HBcat regenerates 1 and releases HCOOBcat, which
is reduced to formaldehyde by another HBcat. A second cycle involves
the hydroboration of formaldehyde, which is analogous to the nickel-
catalyzed hydrosilylation of aldehydes reported earlier.10 In accord with
this mechanistic hypothesis, the reduction of paraformaldehyde with
HBcat in the presence of 1 also gave CH3OBcat. Two possible
equilibria outside the catalytic loops, the formation of the adducts
1·HBcat and 2·catBOBcat (not shown), were separately tested.18
When HBcat was depleted in the solution, the nickel species rested as
(15) In the related complex [2,6-(Me2NCH2)2C6H3]NiOCHO, the C-O bond
lengths are 1.270(3) and 1.209(3) Å for the Ni-bonded and nonbonded C-O
bonds, respectively. See: Grove, D. M.; van Koten, G.; Ubbels, H. J. C.;
Zoet, R.; Spek, A. L. Organometallics 1984, 3, 1003–1009.
(16) Zachariasen, W. H. J. Am. Chem. Soc. 1940, 62, 1011–1013.
(17) For 1H and 13C NMR spectra of nonlabeled CH3OBcat (prepared from HBcat
and CH3OH), see: Povie, G.; Villa, G.; Ford, L.; Pozzi, D.; Schiesser, C. H.;
Renaud, P. Chem. Commun. 2010, 46, 803–805.
(18) See the Supporting Information.
JA103982T
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