Mild reduction of carbon dioxide to methane with tertiary silanes catalyzed by platinum and palladium silyl pincer complexes

Article abstract of DOI:10.1002/chem.201203226

Pincing CO₂: Group 10 (Pd, Pt) silyl pincer complexes in combination with B(C₆F₅)₃ can facilitate the efficient catalytic conversion of CO₂ to methane with a tertiary silane as the reductant under mild conditions. The reduction involves the formation of Pd and Pt formatoborate complexes. Copyright

Full text of DOI:10.1002/chem.201203226

DOI: 10.1002/chem.201203226
Mild Reduction of Carbon Dioxide to Methane with Tertiary Silanes
Catalyzed by Platinum and Palladium Silyl Pincer Complexes
Samuel J. Mitton and Laura Turculet*[a]
The large-scale combustion of fossil fuels (coal, petrole-
um, natural gas) has led to a significant and alarming rise in
levels of atmospheric CO2 over the past several decades,
and this trend is anticipated to continue.[1] As a greenhouse
gas, CO2 is a significant contributor to global warming, and
thus the current high level of anthropogenic CO2 in the at-
mosphere is of great concern.[1] Although efforts to capture
and sequester CO2 are being pursued, such efforts have so
Furthermore, although the Ir-catalyzed hydrosilylation of
CO2 to form methoxysilane species was initially reported
over twenty years ago, albeit with low efficiency,[13a] more
recently Ying and co-workers reported a metal-free N-het-
ACHUTNRGENeNUG roCAHTUNGTERNcNUGN yclic carbene catalyzed process for the reduction of CO2
with diphenylsilane to form the corresponding methoxysi-
lane with significantly improved turnover numbers and turn-
over frequencies.[13b,c]
Focusing on the complete reduction of CO2 to methane,
prior to our commencing work in this area, only three exam-
ples of such homogeneous catalyst systems had been report-
ed.[14,15] Matsuo and Kawaguchi disclosed the use of bis-
far proven relatively costly and remain in their infancy.[2]
A
complementary strategy for addressing both the high levels
of atmospheric CO2 and the dwindling supply of fossil fuels
available to meet our energy needs is the recycling of CO2
by conversion into a hydrocarbon fuel that is suitable for
use in our current energy infrastructure. As such, there is
significant interest in the development of efficient method-
ologies for the reduction of CO2 to methanol and/or meth-
ane, with the ultimate goal of achieving a carbon-neutral
catalytic process.[1c,3,4]
AHCTUNGTREG(NNNU phenoxo) Zr alkyl complexes, which in conjunction with
the highly Lewis acidic borane BACHTNUTRGNE(UNG C6F5)3, catalyzed the reac-
tion of CO2 with hydrosilanes to form methane.[14a] Piers
and co-workers subsequently disclosed a catalyst system
based on the frustrated Lewis pair (FLP) 2,2,6,6-tetra-
A
ACHTUNGNERTN(UNG C6F5)3, which reacts with CO2
In this context, the pursuit of homogeneous catalysts for
the conversion of CO2 to methanol and/or methane is an
area of growing interest,[4–8] and to date, only a handful of
such catalyst systems have been developed.[9–15] With respect
to methanol formation, Milstein and co-workers recently
demonstrated the facile hydrogenation of CO2-derived or-
ganic carbonates, carbamates, and formates to methanol uti-
lizing PNN pincer Ru complexes as catalysts.[10] Building on
this pioneering work, Sanford and co-workers reported a
three component cascade catalysis route for the hydrogena-
tion of CO2 to form methanol,[9d] and Leitner and co-work-
ers demonstrated that a single Ru phosphine catalyst is also
effective in this transformation.[9e] There has also been sig-
nificant interest in the development of efficient reduction
pathways that do not rely on hydrogen for the reduction of
CO2 to the methoxide level. Utilizing a POCOP pincer NiII
hydride catalyst, Guan and co-workers demonstrated the hy-
droboration of CO2 with catecholborane to give methoxy-
boryl species with a relatively high turnover frequency.[12]
AHCTUNGTRENNUNG
form methane.[14b] Lastly, Wehmschulte and co-workers re-
cently reported that the highly reactive Lewis acid Et2Al+
catalyzes the conversion of CO2 to methane as well as other
hydrocarbons upon reaction with hydrosilanes, albeit at ele-
vated temperatures and long reaction times.[14c]
In an effort to diversify the catalyst portfolio available for
the reduction of CO2 to methane, we became interested in
identifying a complementary late metal catalyst system for
this transformation. We envisioned that a platinum group
metal catalyst might offer increased stability and decreased
sensitivity to protic impurities relative to a d0 metal alkyl or
a cationic Group 13 alkyl species, while still achieving suit-
AHCTUNGTREGNUNaN ble activity in CO2 reduction chemistry. We were particu-
larly encouraged by the utility of such late metal species in
the reduction of CO2 to methanol, and considered that the
identification of an appropriately configured late metal cata-
lyst could provide an entry point towards complementary
proACTHUNRTGNEUNGcesses leading to formation of methane. Herein we
report our results detailing the utility of soluble, well-de-
fined Group 10 (Pd, Pt) silyl pincer complexes, in combina-
tion with BACHTUNRTGNEUNG(C6F5)3, for the efficient catalytic conversion of
CO2 to methane using tertiary silanes as the reductant.
While this manuscript was in preparation, Brookhart and
co-workers reported a related example of a cationic [(PO-
COP)IrIII] species that functions as a catalyst for the conver-
sion of CO2 to methane using hydrosilanes.[15]
[a] S. J. Mitton, Prof. Dr. L. Turculet
Department of Chemistry, Dalhousie University
6274 Coburg Road, P.O. Box 15000
Halifax, NS, B3H 4R2 (Canada)
Fax : (+1)902-494-1310
E-mail: laura.turculet@dal.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203226.
15258
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Chem. Eur. J. 2012, 18, 15258 – 15262

COMMUNICATION
We have previously described the synthesis, structural fea-
tures and bond activation reactivity of a variety of platinum
group metal pincer complexes supported by tridentate phos-
phinosilyl ligands of the type [k3-(2-R2PC6H4)2SiMe]À (R-
PSiP, R=alkyl, aryl),[16] including the synthesis of square
tion of the corresponding Pt and Pd formatoborate com-
plexes (Scheme 1; M=Pt, 5; M=Pd, 6). The H NMR spec-
1
tra of complexes 5 and 6 each contain a diagnostic formate
resonance at 8.65 and 8.49 ppm, respectively, whereas the
13C NMR spectra of these complexes feature a resonance at
170.8 and 171.7 ppm, respectively, corresponding to the for-
mate carbon. The formatoborate complexes 5 and 6 were
readily distinguished from the corresponding Pt and Pd for-
mate species (Scheme 1; M=Pt, 7; M=Pd, 8) on the basis
planar Group 10 (Ni, Pd, Pt) complexes that underwent
2
facile cleavage of Si H, Si Cl and Si C (sp and sp3)
bonds.[16c,e] Furthermore, [(Ph-PSiP)PdII] complexes have
been shown to be useful in the catalytic hydrocarboxylation
of allenes and 1,3-dienes with CO2 under mild conditions as
well as in the synthesis of diborylalkenes via dehydrogena-
tive borylation.[17] Efforts to prepare a PtII hydride complex
of the type [(Cy-PSiP)PtH] led to the formation of [(Cy-PSi-
À
À
À
1
of their H, 13C and 31P NMR spectroscopic features (e.g.,
the 1H NMR resonance corresponding to the formate
3
proton for 7, d=9.81 ppm, with Pt satellites, JHPt =40 Hz;
for 8, d=9.20 ppm). Furthermore, unlike complex 5, which
was isolated in 88% yield, the Pt formate complex 7 was
only observed under a CO2 atmosphere and reformed 1
upon removal of CO2 (Scheme 1). Although the Pd formate
complex 8 proved isolable, treatment of 8 with one equiva-
ACHTUNGTRENNUNG(m-H)P)Pt] (1), which was identified on the basis of NMR
and IR spectroscopic data as a bis(phosphino) Pt derivative
2
À
of (Cy-PSiP)H that features h -Si H coordination involving
the tethered silicon fragment (Scheme 1).[16c] The Pd ana-
lent of
BACTHNGUTERNNUG
(C6F5)3 resulted in quantitative (by 1H and
31P NMR spectroscopy) conversion to 6 (Scheme 1). Similar
À
insertion of CO2 into a hydroborate B H bond has previ-
ously been observed for main group fragments involving
FLPs.[11b,14b] Few related examples of formatoborate com-
plexes involving transition metal species have been report-
ed, and all previously reported examples appear to result ex-
clusively from the initial formation of a metal formate spe-
cies that subsequently forms a formate–borane adduct.[18]
Furthermore, Bercaw and Labinger have reported that, in
the case of [HNi
tate the formation of formate–borane adducts upon reaction
(dmpe)2]+, although trialkyl boranes facili-
ACHTUNGTRENNUNG
with CO2, BACHTUNTRGNEUNG(C6F5)3 exhibited only hydride transfer from Ni
to B and no CO2 reduction.[18] Thus, it appears that the
facile reduction of CO2 by 3 and 4 to form formatoborate
species is unprecedented.
In an effort to determine if less Lewis acidic boranes
would facilitate similar stoichiometric CO2 reduction chem-
istry, complexes 1 and 2 were each treated with one equiva-
lent of BPh3. Although 1 did not appear to undergo a reac-
tion with BPh3, complex 2 reacted readily at room tempera-
ture to quantitatively (by 31P NMR spectroscopy) afford 4’,
the BPh3 analogue of 4, which was isolated in 95% yield
(Scheme 1). Treatment of 4’ with CO2 (1 atm) resulted in
the formation of the desired formatoborate complex 6’,
Scheme 1. Synthesis of (Cy-PSiP)Pt and Pd Complexes (only one reso-
nance structure is shown for formatoborate complexes 5–6’).
logue (2) of 1 was prepared by a similar route involving
treatment of [(Cy-PSiP)PdCl] with LiEt3BH (Scheme 1).
In an effort to further explore the reactivity of these un-
2
À
usual h -Si H complexes, we investigated their ability to un-
dergo hydride abstraction. Interestingly, treatment of com-
1
plexes 1 and 2, respectively, with the strong Lewis acid B-
which features a characteristic formate H NMR resonance
A
at d=8.85 ppm ([D6]benzene), as well as a 13C NMR reso-
nance at d=173.3 ppm corresponding to the formate carbon
(Scheme 1). Complex 6’ was also accessed by the treatment
of the Pd formate species 8 with one equivalent of BPh3. Al-
though a BPh3 analogue of 3 was not available, we were
able to access a related platinum formatoborate complex
(5’) by treating a mixture of 1 and BPh3 with CO2
(Scheme 1). Complex 5’ was isolated in 90% yield and, like
6’, features both 1H and 13C NMR resonances consistent
with a formate group. We postulate that 5’ forms via the in
situ formation of 7, which can subsequently react with BPh3
to form the observed formatoborate complex.
(Scheme 1), as evidenced by the appearance of a sharp
11B NMR resonance in each case (for 3: d=À25.4 ppm, d,
1JBH =92 Hz, [D8]THF; for 4: d=À23.3 ppm, d, JBH =77 Hz,
[D6]benzene) that featured B H coupling. We considered
that complexes 3 and 4 offered an intriguing entry point for
studying CO2 fixation due to parallels between these com-
plexes and the FLP-derived salt [TMPH][HBACHTUNRGTNEUNG(C6F5)3], which
reacts with CO2 to form a formatoborate species that has
been implicated in the stoichiometric hydrogenation of CO2
to methanol as well as the catalytic formation of methane
upon reaction with excess BACHTNUGRTNEUNG
(C6F5)3 and Et3SiH.[14b]
Treatment of a benzene solution of either complex 3 or 4
with CO2 gas (ca. 1 atm) resulted in the immediate forma-
Treatment of a [D6]benzene solution of either 5 or 6 with
four equivalents of Me2PhSiH resulted in the quantitative
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www.chemeurj.org
15259

L. Turculet and S. J. Mitton
(by 31P NMR spectroscopy) formation of 3 or 4, respectively,
with the concomitant evolution of methane and
(Me2PhSi)2O (Scheme 1), which were observed in the
1H NMR spectra of the reaction mixtures at d=0.14 and
0.32 ppm, respectively. Having thus observed the stoichio-
metric reduction of CO2 to methane, we pursued a catalytic
variant of this reaction, using in situ generated 3 or 4 (from
of reintroducing CO2 (16 h total heating at 658C), a TON of
2156 was achieved, indicating that catalyst activity does di-
minish with time (entry 3). The bulkier silane Et3SiH led to
significantly decreased activity (entry 4). Control experi-
ments carried out in the absence of CO2 confirmed that the
observed silyl ether formation cannot be attributed to side
reactions involving, for example, adventitious water or O2
(entry 5). Further control experiments confirmed the key
treatment of 1 or 2 with one equivalent of BACHTUNRGTNEUNG(C6F5)3) as the
catalyst (Table 1). Using Me2PhSiH as the reductant, catalyt-
role of both the platinum group metal complex and BACHTUNGTRENNUNG(C6F5)3
in achieving the efficient catalytic reduction of CO2
(entries 7–10). Notably, the catalytic productivity
(TON) and rates (TOF) exhibited by 3 are compa-
rable in magnitude to those observed by Brookhart
and co-workers in their recently reported cationic
Table 1. Reduction of CO2 with trialkylsilanes.[a]
Entry Catalyst
Catalyst
[mol%]
Silane
Time
[h]
G
TON[c] [(POCOP)IrIII] system.[15]
[mmol][b]
A proposed catalytic cycle for the reduction of
CO2 utilizing complexes such as 3 and 4 and tertiary
silanes is shown in Scheme 2. Based on the ob-
served stoichiometric reactivity, we postulate that
the abstracted hydride species 3 and 4 initially react
with CO2 to form the corresponding formatoborate
complexes 5 and 6, respectively, which subsequently
react with two equivalents of silane to reform 3 and
1
3
3
3
3
3
4
0.065
0.016
0.016
0.065
0.065
0.065
0.4
Me2PhSiH
Me2PhSiH
Me2PhSiH 16
4
8
1063[d]
1781
2156
22
0
469
57
2[e]
3[f]
4
Et3SiH
4
4
4
4
5[g]
6[h]
7[i]
Me2PhSiH
Me2PhSiH
Me2PhSiH
NaH/B
15-c-5
ACHTUNGTRENNUNG(C6F5)3/
8
LiEt3BH/B-
(C6F5)3
0.1
Me2PhSiH
4
16
4 and generate one equivalent of CH
dence for the intermediacy of the bis
species was observed from the treatment of com-
plexes and with one equivalent of either
(OSiR3)2. Evi-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
9
10
1
2
0.0065
0.0065
Me2PhSiH
Me2PhSiH
4
4
0
0
5
6
[a] Reaction conditions: 2 mL C6H5F, 1 atm CO2, 9.78 mmol silane, 658C. [b] deter-
mined on the basis of calibrated GC data. [c] calculated based on mol of Si H reacted
À
Me2PhSiH or Et3SiH. In both cases, this reaction re-
sulted in the complete consumption of silane and
afforded about 50% conversion (by 31P NMR spec-
troscopy) to the abstracted hydride species 3 and 4.
per mol of catalyst. [d] TOF of 956 hÀ1 (0.5 h) and 616 hÀ1 (1 h), and 266 hÀ1 (4 h).
[e] reaction was carried out by using 39.16 mmol of silane; CO2 (1 atm) was reintro-
duced to the reaction mixture after 4 h at 658C. [f] reaction was carried out by using
39.16 mmol of silane; CO2 (1 atm) was reintroduced to the reaction mixture after 4, 8
and 12 h at 658C. [g] reaction was conducted in the absence of CO2. [h] reaction was
carried out at 858C. [i] 15-c-5=[15]Crown-5.
The corresponding bis
served in the reaction mixture by H NMR spectro-
ACHTUNGERTN(NUNG silyl)acetal was also ob-
1
ic reactions were carried in fluorobenzene at 658C, 1 atm of
CO2, and a catalyst loading of 0.065 mol% relative to silane.
The amount of (Me2PhSi)2O produced was determined on
the basis of calibrated GC data and turnover numbers
(TON) were determined based on mol of silane reacted per
mol of catalyst. Using 3 as the catalyst afforded 1063 turn-
overs after 4 h, whereas the Pd analogue at the same loading
resulted in 469 turnovers with heating at 858C (Table 1, en-
tries 1 and 6). In the case of the more active Pt-based cata-
lyst 3, the turnover frequency (TOF) after 0.5 h at 658C was
determined to be 956 hÀ1. As expected, the TOF was ob-
served to decrease over the course of the reaction (TOF=
616 hÀ1 after 1 h and 266 hÀ1 after 4 h). Notably, the use of
BPh3 in place of B(C6F5)3 under comparable conditions af-
Scheme 2. Proposed catalytic cycle for the reduction of CO2 to CH4.
ACHTUNGTRENNUNG
forded no catalytic turnover.
In an effort to assess the lifetime of the catalyst
(0.016 mol% 3 relative to the starting amount of silane), we
carried out experiments in which a fresh charge of CO2
(1 atm) was reintroduced to the reaction vessel after an ini-
tial run of 4 h at 658C. Subsequent heating for an additional
4 h at the same temperature resulted in a TON of 1781 after
a total of 8 h heating (entry 2). After two additional rounds
scopy [d=5.0 ppm, s, CH
quent reduction of the bis
the corresponding bis(silyl)ether is achieved by the previ-
ously reported route involving B(C6F5)3 mediated hydrosily-
lation.[14a,b,19,20] Support for this latter series of steps is drawn
from the observation that the replacement of B(C6F5)3 with
the less Lewis acidic borane BPh3 does not lead to the for-
(OSiR3)]. We propose that subse-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Chem. Eur. J. 2012, 18, 15258 – 15262

CO2 Reduction
COMMUNICATION
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only the formation of CH2A(OSiPhMe2)2 was observed.
In summary, we have demonstrated that Group 10 (Pd,
Pt) silyl pincer complexes in combination with B(C6F5)3 can
CTHUNGTRENNUNG
AHCTUNGTRENNUNG
facilitate the efficient catalytic conversion of CO2 to meth-
ane with a tertiary silane as the reductant under mild condi-
tions. The reduction involves the formation of Pd and Pt for-
matoborate complexes, which have been isolated and char-
acterized. This reactivity represents a rare example of transi-
tion metal catalyzed CO2 reduction to methane. Encouraged
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catalysts for this transformation.
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Experimental Section
General information: All reactions were performed under nitrogen in an
MBraun glove-box or by using standard Schlenk techniques. Gas chroma-
tography was performed on a Shimadzu GC-2014 equipped with a SGE
BP-5 30 m, 0.25 mm I.D. column. For conversions and yields given on the
basis of GC experiments, the data were corrected by calibration with do-
decane as an internal standard, and product identity was confirmed by
comparison with authentic samples. Chemicals were purchased from Al-
drich, Gelest, and Strem and used as received. Dry, oxygen-free solvents
were used unless otherwise indicated. The compounds [(Cy-PSiACTHNUTRGNE(UNG m-
H)P)Pt] (1)[16c] and [(Cy-PSiP)PdCl][16e] were prepared according to liter-
ature procedures.
Typical procedure for the catalytic reduction of CO2 with hydrosilane:
All catalytic runs were conducted under a nitrogen atmosphere in reseal-
able glass reaction cells (50 mL) containing a magnetic stir-bar and fitted
with a gas-tight PTFE stopcock. Dodecane (0.50 mL) was added to a sol-
ution of 1 (0.005 g, 0.0064 mmol) and BACHTNUTRGNE(UNG C6F5)3 (0.003 g, 0.0064 mmol) in
C6H5F (2 mL). The solution was transferred to a reaction cell and was de-
gassed by two freeze-pump-thaw cycles. CO2 (1 atm) was introduced.
Under a purge of CO2, neat Me2PhSiH (1.50 mL, 9.78 mmol) was added
to the reaction mixture via syringe. The sealed reaction cell was subse-
quently heated for 4 h at 658C. Once the reaction was complete the cell
was submerged in a room temperature water bath and quickly exposed
to vacuum and refilled with N2 gas. In the glove-box, the reaction mixture
was filtered through a silica plug and was subsequently analyzed by use
of gas chromatographic methods.
Acknowledgements
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We are grateful to the NSERC of Canada (including a Discovery Grant
for L.T.) and Dalhousie University for their generous support of this
work. We also thank Dr. Michael Lumsden (NMR3, Dalhousie) for his
assistance.
Keywords: catalysis
· platinum · silanes
· carbon dioxide · pincer ligands
[10] E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon, D. Mil-
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[20] A sufficient amount of free B
possibly as a result of its dissociation from complexes 3/4 and/or 5/6.
Upon addition of excess B(C6F5)3 (10%) to a catalytic run involving
ACHTNUGTERN(NGNU C6F5)3 is likely present in solution,
AHCTUNGTRENNUNG
3, the yield of (R3Si)2O produced decreased relative to the reaction
run in the absence of excess borane; this phenomenon is currently
being investigated further.
Received: September 10, 2012
Published online: November 5, 2012
15262
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