Efficient homogeneous catalysis in the reduction of CO2 to CO

Article abstract of DOI:10.1021/ja0566679

The well-defined copper(I) boryl complex [(IPr)Cu(Bpin)] [where IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, and pin = pinacolate: 2,3-dimethyl-2,3-butanediolate] deoxygenates CO₂ rapidly and quantitatively, affording CO and the borate complex [(IPr)Cu(OBpin)]. The boryl may be regenerated by treatment with the diboron compound pinB-Bpin, giving the stable byproduct pinB-O-Bpin. The use of a copper(I) alkoxide precatalyst and stoichiometric diboron reagent results in catalytic reduction of CO₂, with high turnover numbers (1000 per Cu) and frequencies (100 per Cu in 1 h) depending on supporting ligand and reaction conditions. Copyright

Full text of DOI:10.1021/ja0566679

Published on Web 11/18/2005
2
Efficient Homogeneous Catalysis in the Reduction of CO to CO
David S. Laitar, Peter M u¨ ller, and Joseph P. Sadighi*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge Massachusetts 02139
Received September 28, 2005; E-mail: jsadighi@mit.edu
Scheme 1 a
Nature uses carbon dioxide, on a massive scale, as a one-carbon
1
building block for the synthesis of organic molecules. An important
pathway for the consumption of CO is its reduction to CO by the
enzyme acetyl-CoA synthase/CO dehydrogenase (ACS-CODH).
Due to the large energy input required to generate it from CO
2
2
2
,
3
CO is produced industrially from fossil fuels. Even with strong
reducing agents, however, overcoming the OdCO bond enthalpy
4
5,6
of 532 kJ/mol often presents kinetic difficulties.
Certain metal complexes abstract oxygen readily from CO
,7 but
2
the resulting metal-oxygen bonds are necessarily strong, and
catalytic turnover is rare.8 Photolytic9 and photocatalytic ap-
proaches show promise, and synthetic electrocatalysts have achieved
10
a
Isolated yield, contains some 2 (5 mol % by 11B NMR); (b) reaction is
impressive yields and selectivities in the reduction of CO
2
to CO.11
complete in <10 min at ambient temp; (c) L ) IPr or ICy.
However, the chemical processes involved are obscure, making it
difficult to improve these systems by design, and CODH remains
notably the most efficient catalyst for this reduction.12 We report
herein that a new carbene-supported copper(I) boryl complex
abstracts oxygen from CO and undergoes subsequent turnover
2
readily. Using an easily handled diboron reagent as the net oxygen
acceptor,13 these key steps permit unprecedented turnover numbers
and frequencies for the chemical reduction of CO
homogeneous system.
2
to CO in a
While exploring the chemistry of organocopper(I) complexes
supported by N-heterocyclic carbene (NHC) ligands,14 we sought
to synthesize a copper(I) boryl complex and explore its reactivity
Figure 1. X-ray crystal structures, shown as 50% thermal ellipsoids, of
15
6 14 7 8
boryl complex 1‚C H (a), and borate 2‚C H (b). Hydrogen atoms (calcd)
toward CO
2
. Metal boryls often display distinctive reactivity,
catalyzing a number of remarkable transformations.1 Although
6
and solvent are omitted for clarity. Selected bond lengths (Å) and angles
deg), (a): Cu(1)-B(1) 2.002(3), Cu(1)-C(1) 1.937(2), C(1)-N(1) 1.363(3),
(
C-B bond-forming reactions have been achieved using diboron
compounds with catalytic17a or stoichiometric copper(I), well-
defined copper boryl complexes have not been described.
The known (IPr)Cu(Ot-Bu) reacts rapidly with bis(pinacolato)-
diboron (pinB-Bpin), forming a product identified as (IPr)Cu(Bpin)
C(1)-N(2) 1.363(3), C(1)-Cu(1)-B(1) 168.07(16), N(1)-C(1)-N(2)
102.97(18); (b): Cu(1)-O(1) 1.8096(16), O(1)-B(1) 1.306(3), Cu(1)-
C(1) 1.857(2), C(1)-N(1) 1.355(3), C(1)-N(2) 1.364(3), C(1)-Cu(1)-
O(1) 174.85(10), B(1)-O(1)-Cu(1) 133.61(16), N(1)-C(1)-N(2) 103.09(18).
17b
conversion of 1 to 2. The sole labeled products visible in the 13C
1
11
(
1, Scheme 1) by H and B NMR spectroscopy. Diffusion of
13
NMR spectrum (Figure 2b) are CO (δ 184 ppm) and an adduct
hexane vapor into a concentrated solution of 1 in toluene, carried
out at -40 °C to avoid thermal decomposition,18 produces single
crystals suitable for analysis by X-ray diffraction. The resulting
structure (Figure 1a) shows a monomeric, nearly linear coordination
geometry with a Cu-B distance of 2.002(3) Å.
δ 164 ppm) formed reversibly from CO and borate 2.20
(
Treatment of 2 in C D solution with pinB-Bpin smoothly
6
6
regenerates 1, forming the stable byproduct pinB-O-Bpin,21 over
a reaction time of about 20 min. The success of this turnover step
closes a catalytic cycle for the deoxygenation of CO . Addition of
2
Complex 1 reacts with CO
2
under atmospheric pressure in C
6
D
6
a THF solution of (IPr)Cu(Ot-Bu) to a 100-fold excess of pinB-
solution, quantitatively forming a new complex within minutes. The
resonance for 1 in the 11B NMR spectrum, a broad singlet at 41.7
ppm, is replaced by a singlet at 21.8 ppm, indicative of boron bound
to three oxygen atoms.19 Single crystals of this new copper complex
are grown by diffusion of hexane vapor into a concentrated toluene
solution. The X-ray crystal structure (Figure 1b) reveals the product
to be (IPr)Cu(OBpin) (2): The copper boryl complex abstracts
Bpin under an atmosphere of CO results in the complete conversion
2
of pinB-Bpin to pinB-O-Bpin within 20 h at ambient temper-
ature, as judged by 11B NMR analysis of an aliquot from the reaction
mixture (Figure 3a). When labeled CO is used as the limiting
2
1
3
reagent, in the presence of ca. 2 mol % of precatalyst 1, the
C
NMR spectrum indicates complete consumption of CO , with CO
2
representing the sole significant product (Figure 3b).
oxygen from CO
To confirm the formation of CO, 13C-labeled CO
to a resealable NMR tube containing a solution of 1 in THF-d
80 °C. After 30 min of gradual warming, analysis by 11B (Figure
a) and 1H NMR spectroscopy at -40 °C indicates complete
2
, implying the release of CO as the byproduct.
is introduced
at
In the absence of copper catalyst, under otherwise identical
conditions, no pinB-O-Bpin is detected, demonstrating that the
2
8
diboron compound by itself is kinetically unable to reduce CO
any observable extent. Control reactions run using copper precata-
lyst and pinB-Bpin in the absence of CO (under an atmosphere
2
to
-
2
2
17196
9
J. AM. CHEM. SOC. 2005, 127, 17196-17197
10.1021/ja0566679 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
to CO in homogeneous solution, with high turnover numbers and
frequencies, depending on the reaction conditions and supporting
ligand. Both the oxygen abstraction and the catalyst turnover involve
well-defined reactants and products, facilitating further study and
2
pointing the way to future advances in catalytic reactions of CO .
Acknowledgment. We thank the National Science Foundation
(Grant No. CHE-0349204), Corning Inc., and the MIT Department
of Chemistry for funding. We are indebted to Profs. C. C. Cummins
and D. G. Nocera for helpful discussions.
Supporting Information Available: All experimental procedures;
complete ref 3; structural parameters for 1 and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 2. (a) 11B NMR spectra before and after reaction with CO2: 1
1
3
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13
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13
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2
deoxygenation of CO .
Considerably higher turnover numbers are achieved at higher
reaction temperatures. Turnover of pinacolborate 2 presumably
occurs much more rapidly; the boryl complex 1, generated in situ,
is sufficiently stable toward decomposition to react productively
(
(
2 2
with CO . The reduction of CO at 100 °C, using 0.1 mol % (IPr)-
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Bpin to pinB-O-Bpin after 20 h, corresponding to 1000 catalytic
turnovers per copper.
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(
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(
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CO
2
with pinB-Bpin using 1 mol % (ICy)Cu(Ot-Bu) display only
81% conversion at ambient temperature, with visible precipitation
6 6
(18) Solutions of 1 in C D slowly deposit metallic copper at room temperature;
of copper metal after less than 1 h. However, when the reaction is
run at 0 °C for 30 min and then at ambient temperature for 30
min, complete conversion of pinB-Bpin to pinB-O-Bpin is
observed. This turnover frequency, corresponding to 100 turnovers
within 1 h, is dramatically higher than that achieved using the IPr
supporting ligand.
decomposition to a mixture of unidentified byproducts is ca. 25% complete
1
1
after 1 day as judged by B NMR spectroscopy.
(
(
(
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In summary, we have achieved the catalytic reduction of CO
2
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