Frustrated lewis pair inspired carbon dioxide reduction by a ruthenium tris(aminophosphine) complex

Article abstract of DOI:10.1002/anie.201205741

Frustrating ruthenium: The ruthenium complex 1 is shown to bind carbon dioxide or aldehyde in a manner similar to a frustrated Lewis pair. Compound 2 catalyzes the reduction of CO₂ in the presence of pinacolborane (HBpin), yielding MeOBpin and O(Bpin)₂ (see picture; Ru red, P orange, N green, O light red, C black). Copyright

Full text of DOI:10.1002/anie.201205741

Angewandte
Chemie
DOI: 10.1002/anie.201205741
CO Reduction
2
Frustrated Lewis Pair Inspired Carbon Dioxide Reduction by
a Ruthenium Tris(aminophosphine) Complex**
Michael J. Sgro and Douglas W. Stephan*
The global concentration of carbon dioxide continues to rise,
contributing to global warming and climate change. While
numerous creative strategies are being developed for CO2
capture, there is an increasing interest in using carbon dioxide
as a C carbon source. The hydrogenation of CO to formic
FLPs to stoichiometrically capture CO . The oxophilicity of
2
early metal centers in these systems precludes catalytic
activity. In the present work, we have developed a Lewis
acidic ruthenium species with a pendant phosphine donor.
This FLP-like system is shown to capture and activate CO for
1
2
2
acid and its derivatives is one such well-developed strategy
catalytic reduction with boranes, yielding methoxyboranes
and O(BR ) as products. Aspects of the mechanism of this
[
1–4]
[5]
[6,7]
[8,9]
based on Ru,
Ir, and Fe
catalysis.
2
2
Another particularly appealing approach is the idea of the
unique approach are considered.
[
10,11]
methanol economy proposed by Olah.
In this scenario,
Targeting a Ru complex of the tripodal ligand N-
((CH ) NHPiPr ) (1), the reaction of 1 with [Ru(PPh )Cl ]
CO is reduced to the liquid fuel methanol by exploiting the
2
2
2
2
3
3
2
[
10,11]
existing distribution infrastructure.
It is this latter pros-
in CH Cl2 ultimately gave 2 in 94% yield. Compound 2
2
pect that has prompted considerable recent attention in new
exhibited two singlets at 121 and 41 ppm in a 2:1 ratio in the
P{ H} NMR spectrum, which is consistent with two dramat-
[
12–17]
31
1
strategies for the reduction of CO2.
Inherent in the
reduction of CO to methanol is the cleavage of one of the CÀ ically different phosphorus environments. This inequivalence
2
1
O bonds. This can been achieved by employing metal-based
processes using silanes, boranes, or aldehydes as oxygen
scavengers. For example, the groups of Sadighi and Guan
was also mirrored in the H NMR spectrum, as the methylene
groups exhibit four different multiplets between 2.68 and
3.33 ppm. Furthermore, a single proton gave rise to a doublet
[
17]
[12]
1
have achieved catalytic reduction of CO using borane to CO
of triplets (JPH = 521, 5 Hz) at 7.16 ppm in the H NMR
2
and methoxide, respectively. Most recently, Sabo-Etienne and
spectrum, consistent with a diisopropylphosphonium group.
A single-crystal X-ray diffraction study of 2 (Figure 1)
[
13]
[28]
co-workers have illuminated the mechanism of reduction of
CO by [RuH (H ) (PCy ) ] and pinacolborane (HBpin).
2
2
2
2
3 2
In a seemingly separate approach, several studies have
exploited the notion of frustrated Lewis pairs (FLPs) for CO
2
[
18–26]
activation and reduction.
For example, Ashley, Thomp-
son, and OꢀHare demonstrated that amine–borane-based
FLP-CO species could be reduced to methanol by heating at
2
[
18]
1
608C for 6 days.
Subsequently, we showed that AlX3–
phosphine-based FLPs could be employed to effect the
stoichiometric reduction of CO to methanol or CO, depend-
2
ing on the conditions. The bis(borane) C H (BCl ) , in
6
4
2 2
combination with PtBu , affords similar stoichiometric reduc-
Figure 1. POV-ray depiction of the cation of 2. Hydrogen atoms except
P
ÀH omitted for clarity.
3
[26]
tions of CO to methanol, while Piers et al. have utilized
2
FLP systems in the catalytic deoxygenative hydrosilylation of
[
20]
CO , generating methane.
2
Herein we adopt a hybrid approach to CO reduction by
revealed a five-coordinate distorted trigonal-bipyramidal
geometry about Ru. Two phosphine fragments of 1 and an
amido nitrogen from the remaining arm occupy the pseudo
trigonal plane, while the axial positions are completed by the
central tertiary amine and a chloride atom. In the latter case,
the N4-Ru-Cl angle is 166.00(4)8 and the computed ClÀH
2
exploiting both transition-metal systems and the concept of
FLPs for activation. In this regard, we note the insightful work
[
27]
of Wass et al., who have described early transition-metal
cations with pendant phosphine donors. These systems act as
distance of 2.72(1) ꢁ is consistent with a hydrogen bonding
interaction between the phosphonium proton and the bound
chloride atom.
[
*] M. J. Sgro, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
The phosphonium proton and metal bound halide are
removed in the reaction of compound 2 with KN(SiMe ) in
E-mail: dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
3
2
THF followed by anion metathesis with NaBPh . This affords
4
[
**] D.W.S. gratefully acknowledges the financial support of NSERC of
Canada and the award of a Canada Research Chair. M.J.S. is grateful
for the support of an NSERC scholarship.
3
in 78% yield; this species gives rise to three inequivalent
3
1
1
doublets of doublets in the P{ H} NMR spectrum. The
coupling constants of 7, 15, and 91 Hz are consistent with both
cis and trans phosphine dispositions. The H NMR spectrum
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205741.
1
Angew. Chem. Int. Ed. 2012, 51, 11343 –11345
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11343

.
Angewandte
Communications
[
18–26]
reveals an apparent doublet of triplets in the hydride region at
complexes.
The RuÀO bond length of 2.207(3) ꢁ is
2
1
À6.34 ppm ( J = 20, 33 Hz). Furthermore, a DEPT H/
considerably longer than the ZrÀO bond distance in
HP
1
3
+
C NMR experiment confirms the presence of a new CH
[Cp Zr(OC H PtBu )(CO )] , which is consistent with the
2
6
4
2
2
[
27]
fragment. While repeated attempts yielded crystals of poor
reduced oxophilicity at Ru. Nonetheless, 4 proved to be
thermally robust, remaining intact when heated to 808 C for
over a week. The present reactivity is reminiscent of the
[
28]
quality, a preliminary X-ray study
a structure of 3 in which the Ru was pseudo octahedral
Scheme 1), with the amido nitrogen and the adjacent
was consistent with
[
19]
(
reactions of the Erker intramolecular FLPs with CO in
2
phosphorus atom binding to Ru affording a NPRu three-
which ring strain prompts PÀB dissociation and subsequent
reaction.
Five equivalents of HBpin were added to a solution of 4 in
CD Cl in an attempt to reduce the bound CO . Monitoring
2
2
2
the reaction by NMR spectroscopy for 24 h revealed the
stoichiometric reduction of CO2 to MeOBpin with the
concomitant formation of O(Bpin) . This reaction could also
2
be achieved in a catalytic fashion. One equivalent of 4 and 18
equivalents of HBpin were heated to 508C under an
1
atmosphere of CO2. H NMR integration relative to an
internal standard showed the slow but steady consumption
of the borane and conversion into MeOBpin and O(Bpin)
2
[
28]
with the consumption of 75% of the borane after 96 h. Use
of a higher excess of borane had little effect on the rate,
although use of 4:HBpin in a ratio of 1:100 resulted in a small
increase in the TON to nine equivalents of MeOBpin after
Scheme 1. Synthesis of 2–5.
9
6 h. Altering the CO pressure from 1 to 5 atm had no effect
2
[28]
on the product distribution. Use of catecholborane (HBcat)
or 9-borabicyclo[3.3.1]nonane (9-BBN) as reducing agents
membered ring. The hydride in 3 arises from metalation of
[
28]
a carbon atom alpha to the central nitrogen atom. Similar
resulted in similar rates of reduction of CO to the corre-
2
[29,30]
b-hydride abstractions have previously been observed.
sponding products MeOBcat, O(Bcat) and MeOBBN, O-
2
The strain in the NPRu ring in 3 suggests the ring-opened
form, which links the Lewis acidic Ru and Lewis basic P, is
readily accessible and prompted us to probe the reactivity
(BBN) , respectively.
2
The mechanism of these reductions is thought to involve
successive hydroborations of the CO fragment of 4. Employ-
2
1
3
with CO . Exposure of a solution of 3 to an atmosphere of
ing CO , the reduction by 4 using HBpin was monitored by
2
2
1
3
CO resulted in the immediate precipitation of the orange
C NMR spectroscopy. After 4 h, the presence of the mono-
2
3
1
1
solid 4 in 81% yield. This species exhibits a P{ H} NMR
hydroboration product of CO (from intermediate A) was
2
spectrum that is comprised of two singlets at 119 and 29 ppm.
evidenced by the appearance of a signal at 158 ppm. This
signal is similar to that recently reported by Sabo-Etienne and
1
3
31
1
The analogous reaction with CO resulted in the P{ H}
2
[13]
resonance at 29 ppm becoming a doublet while a doublet was
co-workers.
Structure A appears initially but is fully
1
3
1
1
also seen in the C{ H} NMR spectrum at 174 ppm with a J
consumed after 24 h, suggesting its intermediate nature. A
CP
of 127 Hz. These data are consistent with the formation of
subsequent reaction of A with HBpin is proposed to liberate
À1
a new PÀC bond. The observed IR absorption at 1651 cm
O(Bpin) and generate the intermediate B. While this species
2
provides further evidence of CO activation. Structural data
was not observed spectroscopically, an analogue was prepared
by the reaction of 3 with benzaldehyde. The product 5 is
isolated in high yield (Scheme 1) and was shown to exhibit
2
for 4 confirmed the capture of CO by the concurrent action
2
of the pendant phosphine base and the Lewis acidic Ru center
[
31]
1
2
(
1
Figure 2).
The PÀC, CÀO, and C=O bond lengths are
a doublet in the H NMR spectrum at 5.53 ppm with a J of
HP
1
3
1
.908(6), 1.239(6), and 1.244(6) ꢁ respectively, which are
6 Hz, while the C{ H} NMR spectrum showed a doublet at
1
similar to the bond lengths seen in main group FLP–CO2
88 ppm with J of 69 Hz. These data are consistent with
CP
formation of a PÀC bond, a fact further confirmed by
[
28]
crystallography. The aldehyde is captured between the Ru
center and the pendant phosphorus center (Figure 3) similar
to that proposed for intermediate B. Compound 5 was shown
to react with HBpin upon heating to 508C for 24 h to generate
PhCH OBpin. Subsequent hydrolysis with D O afforded the
2
2
isolation of [D ]benzylalcohol (PhCH OD) in 80% yield.
1
2
Moreover, compound 5 in the presence of excess HBpin and
under a CO2 atmosphere was shown to be catalytically
competent in the reduction of CO2 consistent with the
proposed intermediate B (Scheme 2).
In summary, the species 3 activates CO for reduction by
Figure 2. POV-ray depiction of the cation of 4. Hydrogen atoms
2
omitted for clarity.
HBpin to yield MeOBpin and O(Bpin) . Analogous products
2
1
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Angew. Chem. Int. Ed. 2012, 51, 11343 –11345

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for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
9
7
2
Received: July 18, 2012
Revised: August 23, 2012
Published online: October 4, 2012
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Keywords: boranes · CO reduction · frustrated Lewis pairs ·
homogeneous catalysis · ruthenium
2
.
31] Related ruthenium complex capture of CO has been previously
2
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www.angewandte.org 11345