An efficient iridium catalyst for reduction of carbon dioxide to methane with trialkylsilanes

Article abstract of DOI:10.1021/ja305318c

Cationic silane complexes of general structure (POCOP)Ir(H)(HSiR ₃) {POCOP = 2,6-[OP(tBu)₂]₂C₆H ₃} catalyze hydrosilylations of CO₂. Using bulky silanes results in formation of bis(silyl)acetals and methyl silyl ethers as well as siloxanes and CH₄. Using less bulky silanes such as Me ₂EtSiH or Me₂PhSiH results in rapid formation of CH ₄ and siloxane with no detection of bis(silyl)acetal and methyl silyl ether intermediates. The catalyst system is long-lived, and 8300 turnovers can be achieved using Me₂PhSiH with a 0.0077 mol % loading of iridium. The proposed mechanism for the conversion of CO₂ to CH₄ involves initial formation of the unobserved HCOOSiR₃. This formate ester is then reduced sequentially to R₃SiOCH₂OSiR ₃, then R₃SiOCH₃, and finally to R ₃SiOSiR₃ and CH₄.

Full text of DOI:10.1021/ja305318c

Communication
pubs.acs.org/JACS
An Efficient Iridium Catalyst for Reduction of Carbon Dioxide to
Methane with Trialkylsilanes
Sehoon Park, David Bezier, and Maurice Brookhart*
́
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
Recently we reported that the highly electrophilic η1-silane
ABSTRACT: Cationic silane complexes of general
structure (POCOP)Ir(H)(HSiR3) {POCOP = 2,6-[OP-
(tBu)2]2C6H3} catalyze hydrosilylations of CO2. Using
bulky silanes results in formation of bis(silyl)acetals and
methyl silyl ethers as well as siloxanes and CH4. Using less
bulky silanes such as Me2EtSiH or Me2PhSiH results in
rapid formation of CH4 and siloxane with no detection of
bis(silyl)acetal and methyl silyl ether intermediates. The
catalyst system is long-lived, and 8300 turnovers can be
achieved using Me2PhSiH with a 0.0077 mol % loading of
iridium. The proposed mechanism for the conversion of
CO2 to CH4 involves initial formation of the unobserved
HCOOSiR3. This formate ester is then reduced sequen-
tially to R3SiOCH2OSiR3, then R3SiOCH3, and finally to
R3SiOSiR3 and CH4.
complex 2, generated in situ from 1 and Et3SiH, is an efficient
catalyst for hydrosilylation of ketones and aldehydes,8a and for
reductive cleavage of C−O8b,c and C−X8d,e bonds (X =
halides). Complex 2 transfers Et3Si+ to oxygen or halogen
atoms of the substrate to form the corresponding cationic
species which are then reduced by reaction with the
nucleophilic dihydride, 3. The catalytic cycle is closed by
reaction of the resultant cationic monohydride with silane
(Scheme 1). Here we describe an efficient catalytic conversion
Scheme 1
here has been increasing interest in developing homoge-
T
neous catalysts for the reduction of CO2, an abundant and
sustainable C-1 source, under mild conditions to methanol or
methane.1−7 Sanford has recently reported a cascade system
which employs a series of three catalysts and hydrogen as the
reductant and involves initial production of formate, catalytic
esterification, and finally reduction of methyl formate to
methanol.2a A nickel-catalyzed hydroboration of CO2 with
catecholborane (HBcat) which yields CH3OBCat has been
described and proceeds through initial reaction of a nickel(II)
hydride complex with CO2 to yield a nickel formate complex.3
The strong Si−O bond has led to successful reduction of CO2
using hydrosilylation procedures.5−7 Ruthenium and copper
complexes have been reported as catalysts for the hydro-
silylation of CO2 to yield formoxysilanes.6 Ying reported the
first hydrosilylation of CO2 catalyzed by an N-heterocyclic
carbene which gives methoxysilanes with a high turnover
number and moderate turnover frequency.5b
of CO2 to CH4 via bis(silyl)acetal and methyl silyl ether
intermediates using 1 in combination with tertiary silanes.
Using PhMe2SiH, catalyst lifetimes are very long (days) and
turnover frequencies of ca. 190/h at 23 °C and 660/h at 60 °C
were observed.
Several silanes were screened by carrying out reductions in
C6D5Cl at 23 °C, 1 atm of CO2 and monitoring reactions by
NMR spectroscopy. A catalyst loading of 0.3 mol % relative to
silane was employed (eq 1).
Two groups have reported catalyst systems employing silanes
which reduce CO2 to methane.7a,b Matsuo and Kawaguchi7a
showed that a tandem catalytic system using Zr(IV) bis-benzyl
complexes in combination with B(C6F5)3 reduces CO2 to
methane with various silanes. Using PhMe2SiH, a TOF of 150/
h was observed and a total turnover number of 225 was
achieved. Piers et al.7b showed that B(C6F5)3 and an
ammonium borate work in tandem to achieve catalytic
reduction of CO2 to methane with Et3SiH at modest rates
but with high catalyst stability. Sequential reduction products
Et3SiO2CH, Et3SiOCH2OSiEt3, and Et3SiOCH3 could be
observed as intermediates.
Received: June 1, 2012
Published: July 5, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
a
Table 1. Reduction of CO2 with 1 and Trialkylsilanes
b
Product
c
c
entry
silanes
Et3SiH
time (h)
(R3SiO)2CH2:
R3SiOMe:
(R3Si)2O
TON
TOF (h−1
)
d
1
2.5
22
7
1
4
48
19
6
2
Et3SiH
1
2
5
138
36
de
,
3
4
5
6
7
Ph3SiH
22.7
1.7
1.7
20
100
0
<1
0
0
2
Me2EtSiH
Me2PhSiH
Me2iPrSiH
Et2MeSiH
1
149
119
105
112
88
70
5
0
0
1
d
d
∼0
2
∼0
1
1
3.8
178
29
a
b
1
Reaction conditions: 0.0025 mmol of 1, solvent = C6D5Cl, 300 equiv of silane, 23 °C. Determined by H NMR with toluene as an internal
c
d
standard. Determined by 1H NMR based on mol of Si−H bond reacted per mol Ir. [Ir](CO) complex observed in addition to [Ir]H(H2)+ and/or
2 during the reaction. White precipitates observed in the NMR tube.
e
Methyl formate undergoes reductive hydrosilylation with
Me2EtSiH to give Me2EtSiOSiEtMe2 and CH4 in 0.3 h; no
intermediates were observed. In contrast, the reaction with
Et3SiH produces CH3OSiEt3 (>99%) and traces of the
silylmethylacetal (<1%) in 0.2 h. These results are consistent
with the proposal that CO2 is transformed to CH4 through the
formoxysilane and that the C−O bonds of the bis(silyl)acetal
and the silyl methyl ether are cleaved more slowly with Et3SiH
than with the less bulky Me2EtSiH. Monitoring the reaction by
31P NMR spectroscopy shows major peaks due to 2 and/or
[Ir]H(H2)+ and a minor peak due to the [Ir](CO) complex, 5,
when using bulky silanes Et3SiH, Et2MeSiH, iPrMe2SiH, or
Ph3SiH as reductants. In order to identify a possible pathway
for the formation of 5, we performed reactions of neutral
dihydride 3 or [Ir](H)(SiEt3) 4 with H13COOMe (20 equiv)
(eqs 4, 5).
Results of the screening reactions are summarized in Table 1.
A turnover number (TON) of 1 is defined as the consumption
of 1 equiv of silane. Using Et3SiH as a reductant, 48 turnovers
are achieved in 2.5 h resulting in a mixture of CH2(OSiEt3)2,
CH3OSiEt3, and (Et3Si)2O in a ratio of 7:1:4; after 22 h, 138
turnovers are observed with a product ratio of 1:2:5 (Table 1,
entries 1, 2).9 The decrease in turnover frequency (TOF) over
reaction time is attributed to the reduced concentration of CO2
in the NMR tube. The 1H NMR spectrum shows characteristic
resonances at δ5.09 and δ3.39 due to the CH2− of
CH2(OSiEt3)2 and the CH3− of CH3OSiEt3, respectively.7b
In addition, the 13C{1H} NMR spectrum clearly exhibits signals
due to triethylsilyl groups of the products (see Supporting
Information). Triphenylsilane reacts slowly with CO2 to give
CH2(OSiPh3)2 and trace amounts of triphenylsilylmethyl ether
(entry 3). The GC trace of the gas in the head space of this
reaction mixture shows no CH4.
Use of less bulkier silanes leads to higher activity and
selectivity for formation of CH4 (entries 4−6). CO2 is readily
reduced with Me2EtSiH and Me2PhSiH to afford the
corresponding siloxanes and CH4 with TON = 149 and
TON = 119 in 1.7 h, respectively. No silyl alkyl ether
intermediates are observed by NMR spectroscopy throughout
the reaction. Me2iPrSiH reacts selectively to give (Me2iPrSi)2O
and CH4 with trace amounts of silyl alkyl ethers, although 20 h
are required to achieve a TON of 105. Similar to Et3SiH, the
reaction of Et2MeSiH with CO2 over 3.8 h affords a mixture of
CH2(OSiMeEt2)2, MeOSiMeEt2, and (Et2MeSi)2O in a ratio of
2:1:178 (entry 7). Observation of CH2(OSiR3)2 and MeOSiR3
intermediates implies that the reduction initially proceeds via
hydrosilylation of CO2 to yield a formoxysilane followed by
conversion to the bis(silyl)acetal and then sequential reductive
hydrosilylation of the two C−O bonds to form MeOSiR3 and
then the siloxane and CH4.
Complex 3 quantitatively converts to the [Ir](13CO)
complex with formation of MeOH in 0.3 h by reaction with
excess H13COOMe.10 Similarly, the neutral complex 4 in the
presence of H13COOMe converts to the [Ir](13CO) complex in
0.3 h with formation of MeOH. Based on these results,
carbonyl complex 5 is likely formed through the reaction of
complex 3 and/or 4 with the formoxysilane, the first formed
intermediate in the catalytic cycle (Scheme 2).11
To assess the lifetime of the catalyst system, we first carried
out reduction of CO2 using Me2PhSiH as a reductant under
To obtain further insight into the mechanism for formation
of the silyl alkyl ether intermediates, we examined the
hydrosilylation of methyl formate, a model compound for the
presumed first-formed formoxysilane intermediate, using both
Me2EtSiH and Et3SiH at 23 °C (eqs 2, 3).
Scheme 2
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Journal of the American Chemical Society
Communication
normal conditions (Table 1) and observed a TON of 76 and
TOF of 127/h in 0.6 h. This reaction mixture was then allowed
to stand at 23 °C for 22.5 h which led to depletion of the CO2
in the NMR tube. Following the 22.5 h reaction, a fresh charge
of CO2 (1 atm) was introduced into the reaction mixture,
whereupon formation of the siloxane (Me2PhSiOSiPhMe2) and
CH4 resumes with a TON of 70 and a TOF of 132/h in 0.53 h.
This activity is essentially the same as that in the first catalytic
cycle and indicates a very long catalyst lifetime.
CH4 with less crowded silanes, whereas bulky silanes slowly
cleave C−O bonds of the acetal resulting in the observation of a
mixture of products.
In summary, we have described a well-defined single iridium
pincer catalyst for the reduction of CO2 to CH4 with
trialkylsilanes under mild conditions. Using Me2PhSiH as the
reductant, more than 8200 turnovers were observed, thus
showing this catalyst system is long-lived. The highly
electrophilic nature of the iridium silane complex suggests
that catalysis is initiated by transfer of R3Si+ to CO2 to generate
a silyloxy carbenium ion.
In view of the high stability of the catalytic system, we
examined a large scale reduction of CO2 with Me2PhSiH.
Results are summarized in Table 2.
ASSOCIATED CONTENT
* Supporting Information
■
S
Table 2. Results of Large Scale Reduction of CO2 with
a
1H and 13C NMR spectroscopic data of reaction mixtures
resulted from the reduction of CO2 with trialkylsilanes, GC
traces of the gas in the head space of the reaction mixture, and
full ref 2b. This material is available free of charge via the
Internet at http://pubs.acs.org.
Me2PhSiH
time
(h)
TOF
isolated yield
(g)
b
b
entry
1
TON
(h−1
)
product
24
48
72
3
4635
7092
8293
1982
193
148
115
661
Me2PhSiOSiPhMe2 +
CH4
1.66
2.54
2.97
0.71
2
3
Me2PhSiOSiPhMe2 +
CH4
AUTHOR INFORMATION
Corresponding Author
mbrookhart@unc.edu
■
Me2PhSiOSiPhMe2 +
CH4
c
4
Me2PhSiOSiPhMe2 +
CH4
Notes
a
The authors declare no competing financial interest.
Reaction conditions: 0.0025 mmol of 1, solvent = C6H5Cl (3 mL),
b
Me2PhSiH (5 mL), CO2 (1 atm), 23 °C. Based on mol of Si−H bond
reacted per mol Ir. Reaction at 60 °C.
c
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of the National
Science Foundation as part of the Center for Enabling New
Technologies through Catalysis (CENTC, CHE-0650456). We
thank Dr. Peng Kang for help with the GC measurement.
Complex 1 (0.0077 mol %) together with Me2PhSiH initiates
the reductive hydrosilylation of CO2 to yield
Me2PhSiOSiPhMe2 and CH4 with up to a TON of ∼8300
after 72 h. The turnover frequency falls over this period of time
due to the consumption of silane. The reaction at 60 °C
proceeds more rapidly to achieve TON = 1982 in only 3 h
(entry 4), suggesting good thermal stability of the catalytic
system.
REFERENCES
■
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Journal of the American Chemical Society
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