From carbon dioxide to methane: Homogeneous reduction of carbon dioxide with hydrosilanes catalyzed by zirconium-borane complexes

Article abstract of DOI:10.1021/ja0647250

A mixture of a zirconium benzyl phenoxide complex and tris(pentafluorophenyl)borane is reported that catalyzes the hydrosilation reaction of carbon dioxide to generate methane via a bis(silyl)acetal intermediate. Copyright

Full text of DOI:10.1021/ja0647250

Published on Web 08/31/2006
From Carbon Dioxide to Methane: Homogeneous Reduction of Carbon
Dioxide with Hydrosilanes Catalyzed by Zirconium-Borane Complexes
Tsukasa Matsuo* and Hiroyuki Kawaguchi*
Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
Received July 4, 2006; E-mail: tmatsu@ims.ac.jp; hkawa@ims.ac.jp
Chart 1
Carbon dioxide is the stable carbon end product of metabolism
and other combustions, and it is an abundant yet low-value carbon
source. This molecule would be very valuable as a renewable source
if it were to be effectively transformed into reduced organic
compounds under mild conditions.1 However, thermodynamic
stability of CO2 has prevented its utilization in industrial chemical
processes, and thus this represents a continuing scientific challenge.
Here we show that CO2 is catalytically converted into methane
Table 1. Reduction of CO2 with PhMe2SiH at Room Temperature
(CH4) and siloxanes via bis(silyl)acetals with a mixture of a
1
entry
catalyst (mol %)a
TOF (h-
)
TONb
yield (%)c
time (h)
zirconium benzyl phenoxide complex and tris(pentafluorophenyl)-
borane (B(C6F5)3). In addition, an appropriate choice of hydrosilane
may lead to selective formation of the initial acetal product and
isolation of polysiloxane materials.
1
2
1/B(C6F5)3 (0.45)
2/B(C6F5)3 (0.42)
3/B(C6F5)3 (0.44)
3/[Ph3C][B(C6F5)4] (1.8)
B(C6F5)3 (1.9)
23
31
150
34
46
225
15
19
98
0
1.5
1.5
1.5
1.5
e
3
4d
5
0
CO2 + 4Si-H f CH4 + 4Si-O
(1)
a The Zr/B ratio ) ca. 1. b TON and TOF are based on the zirconium
complex per Si-H bond. c Isolated yields of (PhMe2Si)2O. d Conversion
of PhMe2SiH into Ph2Me2Si and Me2SiH2 was observed. e 3 days.
The overall reaction stoichiometry described here proceeds as
illustrated in eq 1. We have studied early transition metal complexes
having phenoxide-based multidentate ligands to understand their
reactivity toward small molecules such as dinitrogen and carbon
monoxide.2 As part of these investigations, we have begun to study
activation of carbon dioxide. Synthetic systems capable of reducing
CO2 to CH4 have been elusive, despite the wealth for precedent of
metal complexes that undergo the conversion of CO2 into a variety
of organic products.3-5 Hydrosilation of the carbon-oxygen double
bond is a mild method for reduction of carbonyl functions of organic
compounds.6 Since the reaction is exothermic, the use of hydrosi-
lanes in the reduction of CO2 is very attractive. For example, the
reactions of CO2 with hydrosilanes mediated by late transition metal
complexes have produced silyl formate and methoxide.7,8
In a first set of experiments, we studied the catalytic activity of
cationic zirconium benzyl complexes bearing phenoxide ligands
(Chart 1) in the course of reducing CO2 with PhMe2SiH as the test
substrate. The catalysts used in this study were synthesized by
treatment of the dibenzyl complexes with B(C6F5)3 in toluene.2,9
Reaction of (L3)Zr(CH2Ph)2 with [Ph3C][B(C6F5)4] in toluene
solution produced Ph3CCH2Ph and [(L3)Zr(CH2Ph)][B(C6F5)4].
Since improved reactivity and yield were achieved through in situ
generation of cationic species, subsequent experiments were
performed by premixing the dibenzyl complexes and boranes prior
to the addition of hydrosilanes and CO2.
cationic species nor B(C6F5)3 alone provided an active catalyst. The
enhanced reactivity associated with the tridentate-phenoxide ligand
made (L3)Zr(CH2Ph)2 attractive for further study.
To obtain insight into the intervening processes in the reaction,
isotopically enriched 13CO2 (99 atom % 13C) was admitted into a
resealable NMR tube containing a solution of (L3)Zr(CH2Ph)2,
B(C6F5), and Et3SiH in benzene-d6 at room temperature. When Et3-
SiH is used as a hydrosilane, the reaction requires approximately
one week for completion and is easily monitored by NMR
spectroscopy (Figure 1). The reaction proceeded cleanly, during
which time 13CO2 and Et3SiH were fully consumed. Monitoring
the reaction by 13C{1H} NMR spectroscopy indicates the presence
of bis(silyl)acetal13CH2(OSiEt3)2 as a detectable intermediate. The
resonance at 84.5 ppm due to 13CH2(OSiEt3)2 grew to a maximum
relative intensity over a period of about 7 h and then decreased as
that at -4.4 ppm due to 13CH4. It then continued to grow until,
after about 1 week, it was the only resonance attributable to a 13C-
labeled product. Additionally, in the absence of proton decoupling,
the resonances due to 13CH2(OSiEt3)2 and 13CH4 split into a triplet
and a quintet with a coupling constant of 161.5 and 125.6 Hz,
respectively. This observation unambiguously confirms that the
carbon atom of CH4 originates from CO2, and the source of its
hydrogen atoms is added Et3SiH.
The results of the catalytic CO2 reduction with PhMe2SiH are
depicted in Table 1. The reaction proceeded exothermically to form
(PhMe2Si)2O. Performing the analogous reaction in benzene-d6 in
a NMR tube revealed the release of CH4 as the byproduct. The
resonance due to CH4 was observed as singlet at 0.15 ppm in the
1H NMR spectrum. Mono- and bisphenoxide ligands L1 and L2
gave zirconium complexes that preformed with low activity (entries
1 and 2) relative to the tridentate ligand L3. It is obvious from
entries 4 and 5 that the combination of a zirconium complex with
B(C6F5)3 is responsible for this result. Thus, neither zirconium
The scope of reduction of CO2 by various hydrosilanes was
examined with 3/B(C6F5)3 as the catalyst (Table 2). The six
hydrosilanes examined were shown to be effective for reduction
of CO2 to CH4, and the 3/B(C6F5)3 catalyst is sensitive to steric
hindrance around the substrate Si-H bond. For example, triethyl-
silane reacted more slowly than diethylmethylsilane (entries 1 and
2), while bulkier triphenylsilane did react but required more catalyst
and prolonged reaction time to give a mixture of (Ph3SiO)2CH2
and (Ph3Si)2O (entry 4). Primary and secondary silanes can also
be employed. The reaction with diethylsilane produced a mixture
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10.1021/ja0647250 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
4 in Table 1). The [PhCH2B(C6F5)3]- counterion seems to play
important roles in the course of transformation of CO2 and R3SiH
into CH2(OSiR3). The resulting bis(silyl)acetal is subsequently
reduced by R3SiH to CH4 and (R3Si)2O. This final step was found
to be facilitated by a catalytic amount of B(C6F5)3 alone, and
B(C6F5)3 is known to act as an effective catalyst for hydrosilane
reduction of a variety of aldehydes, ketones, esters, ethers, and
alcohols.11 The [PhCH2B(C6F5)3]- counterion slowly decomposed
during the reaction to generate B(C6F5)3, as monitored by 19F NMR
spectroscopy. It is noteworthy that the yield of bis(silyl)actal was
increased with a (L3)Zr(CH2Ph)2/B(C6F5)3 ratio equal to 1.96 to
prevent the release of free B(C6F5)3, whereas the use of excess
B(C6F5)3 promoted conversion of CH2(OSiEt3)2 into CH4 and (Et3-
Si)2O (entries 2 and 3 in Table 2).
The method for catalytic reduction of CO2 presented here offers
some significant advantages, since it proceeds under mild conditions
and permits complete reduction of CO2 to CH4. Another curious
aspect of this system is the formation of polysiloxane from CO2
and hydrosilane in chemical CO2 fixation. The present results are
promising, but we note that catalytic activity will need to be
improved and the long-term stability and performance of the catalyst
demonstrated.
Figure 1. (A) 13C{1H} NMR spectra showing conversion of 13CO2 (4) to
13CH2(OSiEt3)2 (b) to 13CH4 (0) using 1.5 mol % catalyst (L3)Zr(CH2-
Ph)2/ B(C6F5)3 at room temperature; (B) 13C NMR spectrum of the reaction
mixture after 8 h at room temperature.
Acknowledgment. The authors acknowledge Institute for Mo-
lecular Science and the Ministry of Education, Culture, Sports,
Science and Technology, Japan, for financial support for this
research.
Table 2. Reduction of CO2 with Hydrosilanes by 3/B(C6F5)3 at
Room Temperaturea
1
entry
substrate
product
TOF (h-
)
TONb
yield (%)c
time (h)
1
2
3
Et2MeSiH (Et2MeSi)2O
7.3
1.1
1.2
211
180
203
93
93
82
11
64
28
45
46
74
29
162
165
Et3SiH
Et3SiH
(Et3Si)2O
(Et3SiO)2CH2
(Et3Si)2O
Supporting Information Available: Experimental procedures. This
material is available free of charge via the Internet at http://pubs.acs.org.
4
Ph3SiH
(Ph3SiO)2CH2
(Ph3Si)2O
0.13
50
384
References
5
6
7
Et2SiH2
Ph2SiH2
PhSiH3
(Et2SiO)n
(Ph2HSi)2O
(PhSiO1.5
0.57
0.29
1.1
108
49
162
189
168
145
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)
n
a The Zr/B ratio ) ca. 1, except for entries 2 (0.69) and 3 (1.96) (0.41
to ∼0.52 mol % (L3)Zr(CH2Ph)2/B(C6F5)3 except for entry 4 (1.8 mol %)).
b TON and TOF are based on the zirconium complex per Si-H bond.
c Isolated yields of siloxane.
Scheme 1
of cyclic and linear siolxane oligomers (Et2SiO)n (n ) 3-11, entry
5), and phenylsilane was transformed into a silsequioxane polymer
(PhSiO1.5)n (Mw ) 4220, Mw/Mn ) 2.72, entry 7). This result implies
the possibility to prepare polysiloxane materials from hydrosilanes
and CO2. However, when diphenylsilane was used, the formation
of (Ph2HSi)2O remarkably reduced the reactivity of its remaining
Si-H bonds (entry 6).
A detailed mechanism for the overall catalytic process cannot
yet be deduced, but an outline of a potential mechanism is provided
in Scheme 1. First, the zirconium cationic complex forms the adduct
with CO2, because of the highly electrophilic character of the
phenoxide-supported zirconium(IV) cationic species.10 In the
absence of zirconium complexes, reduction of CO2 is not observed,
suggesting coordination of CO2 to the Zr center prior to the actual
reduction. This coordination might render the CO2 reactive toward
hydrosilation to yield the initial product CH2(OSiR3)2. The nature
of the counteranion is critical, because replacing [PhCH2B(C6F5)3]-
with [B(C6F5)4]- resulted in disproportionation of hydrosilane (entry
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