Deoxygenative reduction of carbon dioxide to methane, toluene, and diphenylmethane with [Et2Al]+ as catalyst

Article abstract of DOI:10.1002/anie.201201282

The strong Lewis acid [Et₂Al]⁺ catalyzes the reduction of carbon dioxide with hydrosilanes under mild conditions to methane. In benzene solution, the side products toluene and diphenylmethane are also obtained through Lewis acid catalyzed benzene alkylation by reaction intermediates. Copyright

Full text of DOI:10.1002/anie.201201282

Angewandte
Chemie
DOI: 10.1002/anie.201201282
Catalyzed CO2 Reduction
Deoxygenative Reduction of Carbon Dioxide to Methane, Toluene,
and Diphenylmethane with [Et2Al]+ as Catalyst**
Manish Khandelwal and Rudolf J. Wehmschulte*
Dedicated to Professor Josef Grobe on the occasion of his 80th birthday
Although nature uses CO2 as its main carbon source, the
employment of CO2 as a readily available inexpensive
feedstock for commodity chemicals is still in its infancy.[1]
Major industrial applications include the synthesis of urea,
salicylic acid, and cyclic and polymeric carbonates.[2] The
reduction of CO2 to formic acid, methanol, or methane on an
industrial scale is much less advanced. For example, there are
only few pilot plants that are investigating and optimizing the
heterogeneous catalytic reduction of CO2 with H2 to afford
methanol.[3] Homogeneous catalysis continues to rely on
expensive and rare late-transition-metal complexes,[2,4] as is
illustrated by a recent report describing a very active
homogeneous iridium-based system for the reduction of
CO2 with H2 to give formic acid.[5] Hydrosilanes have also
been applied as the reducing agent, mainly because the
amount of silylium ions with CO2 led to the formation of
benzoic acid, formic acid, and methanol after workup.[15]
Since we have observed during our investigations of the
chemistry of cationic low-coordinate organoaluminum[16] and
-zinc compounds[17] that these species catalyze the rapid
reduction of benzophenone to diphenylmethane with Et3SiH
at room temperature, we wondered whether CO2 (a “dike-
tone”) could also be reduced under these conditions. Herein,
we present the conversion of CO2 into methane, toluene, and
diphenylmethane with various hydrosilanes using [Et2Al]-
[CH6B11I6] (1)[18] as the Lewis acid catalyst.
A C6D6 solution of 1 and Et3SiH in an approximately 1:10
ratio was treated with CO2 (ca. 1.3 atm), and the progress of
the reaction was monitored by 1H and 13C NMR spectroscopy.
As no immediate reaction was observed at room temperature,
the mixture was heated to 808C. After 14 h, approximately
50% of the silane was consumed. Interestingly, not only the
expected signal due to methane, but also signals due to
À
slightly polar and weaker Si H bond (bond dissociation
energy (BDE) 384 kJmolÀ1 in SiH4)[6] is easier to activate than
À1 [7]
À
the strong non-polar H H bond (BDE 436 kJmol ).
Furthermore, the reaction products of the former are rather
inert siloxanes, whereas hydrogen will be oxidized to water,
which can be detrimental to certain catalysts. Some recent
reports describe the ruthenium-catalyzed hydrosilylation of
CO2 to formoxysilanes,[8] and a cationic zirconium phenoxide
complex catalyzed the reduction of CO2 to methane with
various hydrosilanes at room temperature.[9] Furthermore,
metal-free, basic organocatalytic systems involving N-hetero-
cyclic carbenes or amines were reported to catalyze the
reduction of CO2 to methoxysilanes[10] or formamides.[11]
There are also several examples in which frustrated Lewis
pairs (FLPs) have activated CO2,[12] and its catalytic reduction
to methane with Et3SiH has been achieved using the FLP
tetramethylpipiridine and B(C6F5)3.[13] Surprisingly, there are
no examples of CO2 reductions catalyzed by Lewis acids to
date. A few examples leading to aromatic and heteroaromatic
carboxylic acids under Friedel Crafts conditions use the Lewis
acid (usually an aluminum halide) as a stoichiometric
reagent.[14] Very recently, the reaction of a stoichiometric
[D5]toluene,
C6D5CH3,
and
[D10]diphenylmethane,
(C6D5)2CH2, were observed (Figure 1). These signals
increased in intensity until the hydrosilane was consumed
(60 h). Furthermore, the major silane product was Et4Si, not
the expected siloxane (Et3Si)2O. Some low-intensity broad
1H NMR signals indicate the formation of oligomeric or
polymeric siloxanes, such as (Et2SiO)n. The same reaction
with a catalyst loading of only 1% resulted in 58% Et3SiH
consumption after 216 h at 808C and the formation of
methane
(90%),
[D5]toluene
(8%),
and
[D10]diphenylmethane (2%) with a turnover number (TON)
of 14.
Following these promising results, we screened two
phenylsilanes, PhSiH3 and Ph2SiH2, and the more bulky
tBuMe2SiH for their reducing/hydrosilylation capability
(Table 1). The two alkylsilanes are significantly more reactive
than the phenylsilanes. Application of tBuMe2SiH led to
a higher yield of [D10]diphenylmethane and thus a lower yield
of methane compared to Et3SiH. A possible reason for the
lower reactivity of the phenylsilanes is their tendency of
ligand scrambling in the presence of Lewis acids.[19] Within
15 minutes after addition of the phenylsilanes to a solution of
1 in C6D6, signals due to Ph4Si, Ph3SiH, Ph2SiH2, PhSiH3, and
SiH4 are observed. In an further experiment involving
Ph2SiH2 and a catalytic amount of 1, crystalline Ph4Si was
obtained after 24 h at room temperature, along with the other
silanes (Supporting Information, Figure S1).
[*] Dr. M. Khandelwal, Prof. Dr. R. J. Wehmschulte
Department of Chemistry, Florida Institute of Technology
150 West University Boulevard, Melbourne, FL 32901 (USA)
E-mail: rwehmsch@fit.edu
Homepage: http://cos.fit.edu/chemistry/faculty/wehmschulte/
[**] Financial support from the National Science Foundation (CHE
0718446) is gratefully acknowledged.
The lower activity of phenylsilanes with respect to
alkylsilanes is different from transition-metal- and amine-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201282.
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The silylium ions investi-
gated herein were synthe-
sized in situ through the
addition of 10% [Ph3C]-
[CH6B11I6] to the hydrosilane
solutions. The results show
that silylium ions catalyze the
CO2 reduction, but their
activity is significantly lower
than that of [AlEt2]+
(Table 2).
Interestingly,
[Et3Si]+ showed the lowest
activity, followed by the phe-
nylsilanes and tBuMe2SiH.
The phenysilanes underwent
ligand scrambling as before.
No
detected, indicating that the
putative intermediates
methane
was
R3SiOCH3 and (R3SiO)2CH2
(see below) alkylate the sol-
vent faster than being further
reduced to methane. Con-
trary to these results, a very
recent report showed that
silylium ions [Et3Si(arene)]
[B(C6F5)3] can activate CO2
Figure 1. Reduction of CO2 with Et3SiH catalyzed by 10% 1 at 808C. The expansion of the aliphatic region
shows the formation of Et4Si.
in
a stoichiometric reac-
tion.[15] The lower reactivity
observed in our systems
Table 1: Reaction of hydrosilanes with CO2 at 808C catalyzed by 1 with
10% loading.
involving catalytic amounts may be explained by the forma-
tion of less reactive hydride bridged species, such as
[Et3Si···H···SiEt3]+, in the presence of excess hydrosilane.[21]
The CO2 reduction with Et3SiH catalyzed by 1 was
investigated in more detail, beginning with the solvent
influence. CO2 was added to solutions of 1 and Et3SiH in
C6D6 and C6D5Br. After 40 h at room temperature, approx-
imately 10% of Et3SiH was consumed for the reaction in C6D6
and 25% for the reaction in C6D5Br. Small amounts of Et4Si
were detected in both reactions as initial products, along with
a trace of the formate ester HCOOSiEt3. Subsequent heating
at 808C for 24 h led to the consumption of the hydrosilane,
and the formation of [D10]diphenylmethane, [D5]toluene, and
methane for the reaction in C6D6 and mostly methane
(> 95%) and minor amounts of bromotoluene (< 1%) for
the reaction in C6D5Br. The significantly lower amount of
Silane
t [h]
Products [%][a]
C6D5CH3
(C6D5)2CH2
CH4
70[b]
40[b]
–
Et3SiH
60
26
32
19
9
4
tBuMe2SiH
Ph2SiH2
PhSiH3
60
28
25
12
148[c]
172[c]
–
1
[a] The yields were determined by integration of H NMR signals of
reactants and products versus an internal reference, and they are based
on the amount of hydrosilane used. [b] Estimated assuming that the
reactions were quantitative and that the remaining product is methane.
[c] The reaction was halted.
catalyzed CO2 reductions,[9,10] and is most likely a reflection of
the strong Lewis acidity of [AlEt2]+.
To better understand the role of [AlEt2]+ as catalyst or
precatalyst in the CO2 activation, the activity of silylium ions
[R3Si]+ toward CO2 activation was probed. Silylium ions are
known as potent catalysts in the reduction of ketones,[20] and
a small amount of silylium ions could be present in the
[AlEt2]+-catalyzed reaction mixtures, according to Equa-
tion (1). The existence of a weak adduct between B(C6F5)3
and Et3SiH has been demonstrated,[20] although NMR spectra
of mixtures of [AlEt2]+ and Et3SiH do not show any evidence
for the adduct formation.
Table 2: Reaction of hydrosilanes with CO2 at 808C catalyzed by 10%
silylium ion.
Silane
t [h]
Products [%][a]
C6D5CH3
(C6D5)2CH2
Et3SiH
187
187
187
187
0.2
15
9
0
tBuMe2SiH
Ph2SiH2
PhSiH3
66
20
39
10
1
[a] The yields were determined by integration of H NMR signals of
reactants and products versus Ph3CH as internal reference, and they are
based on the amount of hydrosilane used.
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alkylation products in C6D5Br is most likely due to the
deactivation property of the bromine susbtituent in this
aromatic compound. This also means that the product
distribution can be controlled by the choice of the solvent.
After the addition of a second charge of hydrosilane and CO2,
48 h of heating were required for the consumption of the
Et3SiH. The activity of the catalytic system slowly declined as
is indicated by the even slower conversion of the third charge:
After 55 h at 808C, 89% of the silane was consumed for the
reaction in C6D6 and only 35% for the reaction in C6D5Br.
Whereas the C6D5Br reaction mixture remained homoge-
neous, a small amount of a colorless oil separated from the
C6D6 reaction mixture. This oil possessed some catalytic
activity, as shown by the consumption of 86% Et3SiH during
104 h at 808C. The nature of this oil is subject to ongoing
investigations.
Next, the catalytic reduction of the silylethers CH3OSiEt3
and BnOSiEt3 (Bn = PhCH2) was investigated. The latter is
the assumed intermediate for the formation of diphenyl-
methane from CH2(OSiEt3)2 via electrophilic aromatic sub-
stitution according to Equation (3).
As expected, the reduction of CH3OSiEt3 with Et3SiH is
fast with 1 h at room temperature and 15 min at 808C. The
mixture remained active for at least one additional loading
with substrate. The products are mostly methane accompa-
nied by 12–14% [D5]toluene. The benzyl ether is even more
reactive, and the electrophilic alkylation of the solvent
competed successfully with the silane reduction. The benzyl
ether was consumed within 10–15 min at room temperature
(the time it takes to take the sample out of the dry box, go to
the NMR spectrometer, and collect a spectrum), and the
products are (C6D5)2CH2 (96%), C6D5CH3 (4%), unreacted
Et3SiH (73%), and (Et3Si)2O (72%).
Matsuo[9] and Piers[13] have shown that the reduction of
CO2 with silanes is a stepwise process (Scheme 1). As we have
The interaction of the silyl ethers in the absence of Et3SiH
with 1 in benzene solution was also probed. The benzyl ether
was consumed within 15 min at room temperature to give
exclusively the [D5]diphenylmethane Ph(C6D5)CH2 and
(Et3Si)2O. The nature of the colorless solid that precipitated
immediately after silyl ether addition is likely a hydrated
cationic aluminum oxide/hydroxide/silyloxide based on the
idealized balanced reaction in Equation (4).
Scheme 1. Proposed pathway of the CO2 reduction.
not observed any intermediates during the 808C reactions, we
suspected that those will be reduced much faster than CO2.
We have prepared the silyl ester HCOOSiEt3 and the silyl
ethers H3COSiEt3 and BnOSiEt3 (Bn = PhCH2) and moni-
tored their reduction with Et3SiH in C6D6 solution catalyzed
by compound 1. As expected, the silyl ester was reduced
rapidly at 808C, and the silyl ethers alkylated the solvent at
room temperature even in the absence of Et3SiH. The
reaction of HCOOSiEt3 with 5% catalyst loading and
3 equiv of Et3SiH per ester was complete within 1 h at
808C. The intermediate CH3OSiEt3 was observed during the
reaction, whereas CH2(OSiEt3)2 was not detected. The
ultimate products were CH4, C6D5CH3, and (Et3Si)2O (Sup-
porting Information, Table S1). This result is also consistent
with the CO2 reduction by Et3SiH where only 4% diphenyl-
methane was formed [Equation (2)].
The reaction with the methyl ether MeOSiEt3 in the
absence of Et3SiH requires three days for completion at room
temperature. The products are C6D5CH3 and (Et3Si)2O.
Intermittently, methanol is also formed, but is consumed
during the reaction. The Lewis acid mediated alkylation of
aromatics with silyl ethers has been observed previously, but
compound 1 is significantly more reactive.[22]
In many of the reactions discussed above, especially those
involving long reaction times or heating, Et4Si was the
ultimate product rather than (Et3Si)2O. As it is known that
siloxanes and silyl ethers can undergo ligand exchange in the
presence of a Lewis acid,[23] we have reacted (Et3Si)2O with 1.
Et4Si formation was observed after 30 min at 808C, and the
siloxane was consumed after 38 h. Broad signals in the
1H NMR spectrum (Supporting Information, Figure S6) indi-
cated the formation of oligo- and polysiloxanes similar to the
side products in the CO2 reductions. An idealized balanced
reaction is given in Equation (5).
An additional charge of HCOOSiEt3 was reduced within
45 min at 808C and a third charge within 1 h, thus indicating
that the catalytic system remains active.
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Mass spectrometric analyses of those compounds identified
(Et2SiO)4 and various larger linear fragments possibly deriv-
ing from Et3Si(OSiEt2)nOSiEt3.
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It should also be mentioned that CO2 did not react with 1,
even after 30 h at 808C. This is contrary to the facile insertion
[24]
À
of CO2 into the Al Et bond in AlEt3, but it is consistent
À
with the observed lower activity of Al C bonds in cationic
aluminum species.[25]
For comparison, we have also tested the catalytic activity
of AlBr3 for the CO2 reduction. After 20 h at room temper-
ature, approximately 24% of Et3SiH was consumed, and the
1H NMR spectrum showed peaks around d = 8 ppm, indica-
tive of coordinated HCOOSiEt3, and small amounts of
Et3SiOCH3 were also detected. Additional Et3SiH was
consumed during heating at 808C for two days, after which
time no significant changes were observed. The silyl ester and
silyl ether signals disappeared, and the formation of methane
and Et3SiBr was detected. Overall, approximately 1.7 moles
of Et3SiH were consumed per AlBr3, leading at best to
0.4 equiv of methane. An idealized equation for a stoichio-
metric reaction is given below [Equation (6)], but the lower
amount of actually consumed hydrosilane suggests other
pathways, including the formation of aluminum formates.
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A
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These results show that a strong Lewis acid, such as
[Et2Al]+ and even [R3Si]+, catalyzes the CO2 reduction by
hydrosilanes. Based on the faster catalytic reduction of the
intermediates HCOOSiEt3 and MeOSiEt3, the rate-limiting
step is most likely the reduction of CO2 to HCOOSiEt3. The
strong Lewis acidity of [Et2Al]+ manifests itself in solvent
alkylation reactions and the conversion of the primary
siloxane product into Et4Si and polysiloxanes (Et2SiO)n.
These side reactions may contribute to the observed activity
decrease after three cycles. Current activities focus on the
development of more reactive catalytic systems based on the
potentially more robust [(RO)2Al]+ platform. Finally, pre-
liminary results have shown that [EtZn][CHB11Cl11] also
catalyzes the CO2 reduction with Et3SiH, albeit a slightly
more slowly than [Et2Al][CH6B11I6].
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Received: February 15, 2012
Revised: March 22, 2012
Published online: && &&, &&&&
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Keywords: CO2 reduction · diethylaluminum ·
homogeneous catalysis · hydrosilanes · Lewis acids
.
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Catalyzed CO2 Reduction
The strong Lewis acid [Et2Al]+ catalyzes
the reduction of carbon dioxide with
hydrosilanes under mild conditions to
methane. In benzene solution, the side
products toluene and diphenylmethane
are also obtained through Lewis acid
catalyzed benzene alkylation by reaction
intermediates.
M. Khandelwal,
R. J. Wehmschulte*
&&&& — &&&&
Deoxygenative Reduction of Carbon
Dioxide to Methane, Toluene, and
Diphenylmethane with [Et2Al]+ as Catalyst
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