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Metal-Free Reduction of CO2
Frédéric-Georges Fontainea and Douglas W. Stephanb
a Département de Chimie, Centre de Catalyse et Chimie Verte (C3V), Université Laval,
1045 Avenue de la Médecine, Québec (Québec), Canada, G1V 0A6
b Department of Chemistry University of Toronto, 80 St. George St. Toronto, Ontario,
Canada M5S 3H6
Email : [email protected]
This is the peer reviewed version of the following article: [Metal-Free Reduction of CO2, Current
Opinion in Green and Sustainable Chemistry, 2017, 3, 28- 32], which has been published in final form
at [http://dx.doi.org/10.1016/j.cogsc.2016.11.004].
Even if carbon dioxide is now considered a major pollutant and the main source of global
warming, it remains the feedstock used by Nature to generate organic compounds. There
is considerable interest to find chemical processes that could replace fossil fuels by CO2 to
generate industrially relevant carbon containing molecules. Mostly relying on frustrated
Lewis pair technology and on the nucleophilic activation of CO2 or reducing agents by
organic Lewis bases, novel catalytic opportunities have emerged to affect the metal-free
catalytic reduction of carbon dioxide.
Introduction
Efforts to mitigate the effects of climate change have prompted a surge of interest in the
functionalization of the most abundant of the green-house gases, CO2. Current industrial
use of CO2 is limited, largely a result of its thermodynamic stability. While CO2 has no
dipolar moment, the carbon centre is electrophilic and indeed it is this feature that has been
exploited to effect reactions with a range of nucleophiles. This feature is central to current
protocols for CO2 scrubbing technologies1 and current industrial applications of CO2
chemistry for the generation of urea, dyes and cyclic carbonates.2
The possibility of using CO2 as a practical C1 source is an attractive notion as this
mimics Nature and offers a renewable alternative to fossil fuels for carbon-based industrial
feedstocks and fuels. This potential has prompted a flood of studies targeting reduction
products such as methane or methanol as such conversions could in principle, be done on
a large scale, offering a strategy to reduce the CO2 output and generate value from these
emissions. To overcome the stability of CO2, catalysts are required. Indeed, various
transition metal based homogenous and heterogeneous catalysts have been explored to
effect either thermal, electrochemical or photochemical CO2 reductions.3-15 In the context
of sustainable and green chemistry, the possibility of using metal-free catalysts in lieu of
transition metal-based catalysts is another enticing approach. This avenue offers low cost,
earth-abundant, non-toxic reagents, with a significantly reduced carbon-footprint in the
overall process.16-18 This article focuses on the recent advancements in such metal-free
reduction of CO2 using main group element-based catalyst technologies.
Frustrated Lewis Pairs and Reactions of CO2
Figure 1. Frustrated Lewis complexes of CO2
CO2 Capture and Stoichiometric Reduction
Frustrated Lewis pairs (FLPs) are combinations of a Lewis acid and base that are not
mutually quenched as a result of steric or geometric constraints.19-22 These reagents are
well suited to the activation of carbon dioxide as FLPs are ambiphilic allowing nucleophilic
activation at the carbon and the subsequent electrophilic capture of the oxygen atoms in
the resulting species (Figure 1). Such a bifunctional interaction has been shown to be key
features for both the active site of carbon monoxide dehydrogenase that converts CO to
CO2 and for highly efficient transition metal CO2 reduction catalysts.23-25 In a 2009
contribution, Stephan, Erker and coworkers26 reported that FLPs derived from phosphines
and boranes can bind CO2 affording zwitterionic products in which phosphines bind to
carbon and the borane binds one of the oxygen of CO2. Analogous binding by various other
FLPs derived from donors (N-heterocyclic carbenes, phosphines, amines, phosphinimines,
pyrazoles) and acceptors (B, Al, Ti, Zr, Hf and Si electrophiles) has been reviewed.27-28
Figure 2. Stoichiometric reductive transformations of FLP CO2 adducts
Some of the products of such trapping of CO2 have been shown to undergo further
stoichiometric transformations. For example, once the FLP P(SiMe3)3/B(p-C6F5H)3 traps
CO2, it undergoes silyl migration to generate a silyl ester, which in turn traps another
equivalent of CO2 generating an unusual phosphaalkene species (Figure 2A).29 It was also
shown that the CO2 adduct of ambiphilic Me2PCH2AlMe2 undergoes methylene migration
to generate an aluminum carboxylate species (Figure 2B).30 Menard et al. described the
complex of CO2 by phosphine/aluminum FLPs such as PMes3/AlX3 leading to the
stoichiometric reduction to CO31 while subsequent reaction with ammonia borane effects
the stoichiometric reduction to methoxy-aluminium species which liberates methanol on
hydrolysis (Figure 2C).32
Catalytic Hydrosilylation of CO2
To advance metal-free strategies towards catalysis a number of studies have
exploited silane reductants. In 2009 Ying and co-workers33 described the use N-
heterocyclic carbenes (NHCs) to activate silanes leading to the catalytic reduction of CO2
to methanol and methoxysilanes with turn-over frequencies up to 25 h-1 (Figure 3, I) The
authors proposed that this reduction was a result of the increased hydridicity of the Si-H
bonds and Lewis acidity at the hypercoordinate silicon, resulting from coordination of the
NHC to silicon.
Interestingly, more recent computational studies by Zhou and Li34 demonstrated that the
likely mechanism involves the cooperative action of NHC-CO2 adduct and CO2 on the
silane. In this case the NHC-CO2 acts a Lewis base, while CO2 acts as an electrophile or
Lewis acidic hydride acceptor. This mechanism is conceptually reminiscent of what is now
referred to as the frustrated Lewis pair (FLP) mechanism35 for the hydrosilylation of
ketones, first described by Piers and coworkers.36
A more recent study37 showed that phosphazenes also mediate the reaction of CO2
with hydrosilanes, generating selectively formylsilanes or methoxysilanes, the distribution
of which is altered by reaction conditions (Figure 3, II). In addition, it was demonstrated
that DMF as the solvent impacts on the hydrosilylation of CO2, effecting reduction without
catalyst.
The stoichiometric reduction of CO2 to methane using hydrosilanes has also been
observed using Lewis acids. For example, the highly electrophilic species [AlEt2]+ (Figure
3, III),38-39 gave a mixture of reduced products when the reaction was carried out in
benzene (methane, toluene, diphenylmethane), although selectivity has been shown to be
limited at best. In a similar fashion, Piers and coworkers40 also reported that the FLP
derived from TMP/B(C6F5)3 promotes the hydrosilylation of CO2 to methane, albeit with
low yields(Figure 3, IV). In these systems, the Lewis acids are used to generate silylium
ions that promote such chemistry.
More recently, Chen and coworkers41 have optimized this catalytic reduction of
CO2 in the presence of silanes, demonstrating that high yields of methane could be obtained
when Al(C6F5)3 and B(C6F5)3 are used synergistically as catalysts (Figure 3, V). In this
system, the highly Lewis acidic Al(C6F5)3 is proposed to effect the initial reduction of CO2,
while B(C6F5)3 catalyzes the reduction of formate and methylene-diolate-intermediates in
a mechanism that is reminiscent of that described by Piers for the hydrosilylation of
carbonyls.36 Interestingly, it was elegantly demonstrated by Okuda et al.42 that weaker
Lewis acids, such as BPh3, could also promote the hydrosilylation of CO2 (Figure 3, VI).
However, in the latter case, the milder conditions allow for a selective reduction to silyl
formates.
Catalytic Hydroboration of CO2
Another alternative for metal-free reduction of CO2 has been the use of B-H bonds
as the reducing agent. Lewis bases including phosphines,43 guanidines,44
diazafluorenides,45 carbenes,46 and proton sponge47 (Figure 3, VII-XI) have been used in
concert with boranes or borohydrides such as 9-BBN, BH4- and Ph3BH- to effect the
reduction of CO2. It should be noted that in most of these systems, the catalysts are needed
for the first reduction of CO2 to HC(O)OBR2 since the latter species can get reduced to
methoxyboranes by several boranes without catalysis.48 The insertion of CO2 into a B-H
bond of BH4- readily generates formatoborates49 and indeed, NaBH4 can be used as a CO2
reduction catalyst in presence of excess boranes.50
In 2013, Fontaine and coworkers51 described the FLP catalyst 1-Bcat-2-PPh2-C6H4
and its use in the hydroboration of CO2 using BH3·SMe2, HBpin or HBcat to produce
methoxyboranes, with a turn-over frequencies of up to 853 h-1 (Figure 3, XII).
Interestingly, one of the criteria for the relatively high activity in this system appears to be
the very weak interaction between CO2 and the FLP precatalyst, destabilizing the adduct
and thus precluding it from being a thermodynamic sink.48 Indeed, in a subsequent
collaboration, Bourissou and Fontaine52 showed that the catalytically active species were
actually formaldehyde adducts of 1-Bcat-2-PPh2-C6H4, emphasizing that a weak Lewis
acid seems to be a key factor to effect catalytic conversions. This kind of synergistic
relation has been exploited by Cantat et al53 in ferrocene-based phosphino-boranes for the
catalytic hydroboration of CO2 to methanol, with TON up to 250 h-1 using 9-BBN as
reducing agent.
In a 2015 study, the work of Wegner et al54 showed that the diborane 1,2-(BH2)2-
C6H4 catalyzes the hydrosilylation of CO2 to give methane while reduction with HBpin
afforded methanol (Figure 3, XIII). In either reaction pathway, the intermediate is
proposed to be the aromatic heterocycle derived from by the reaction of 1,2-(BH2)2-C6H4
and CO2.
Catalytic Hydrogenation of CO2
Although academic interest in hydrosilylation and hydroboration of CO2 to
methanol and methane has garnered considerable attention, these approaches have little
potential for industrial applications as a result of the relatively high cost of boranes and
silanes. It is only catalytic reductions that are exploiting molecular hydrogen that have the
potential for commercialization. Ideally, such reduction would exploit hydrogen generated
by electrolysis driven by an alternative energy source, rather than hydrogen derived from
fossil fuels. This strategy offer a green approach to CO2 utilization. Indeed this principle
has been demonstrated at the George Olah Renewable Methanol Plant in Iceland which
uses geothermal energy to generate the reductant, H2.
In what was a seminal finding, O’Hare and Ashley55 reported that the FLP derived
from TMP/B(C6F5)3 on exposure to CO2/H2 at 160 °C for 6 days afforded the generation
of methanol albeit in 17-25% yield (Figure 3, XIV). Despite the low yield and forcing
conditions, this 2009 finding has continues to inspire much of the work discussed above as
well as continuing studies of FLPs and main group species in CO2 reduction. Despite this
early finding, the only other study that describes metal-free catalytic reduction of CO2
using H2 was recently reported by the groups of Fontaine and Stephan.56 In this work, the
FLP, 1-BMes2-2-NMe2-C6H4 was shown to mediate the conversion of CO2 and H2 to
generate formyl, diacetal and methoxyboranes (Figure 3, XV).
Figure 3. Reported catalysts for the reduction of CO2.
Catalytic Amination of CO2
An alternative avenue for the utilization of CO2 is its reduction via amination. This
provides a strategy of the production of compounds that might have applications as
precursors for pharmaceutical and agrochemical processes. In a break though finding
Cantat et al57 reported a three-component reaction between secondary or primary amines,
hydrosilanes and CO2 could be catalyzed by Lewis base 1,5,7-triazabicyclo[4.4.0]dec-5-
ene (TBD) to generate the corresponding formamides under mild reaction conditions. This
metal-free catalyst was shown to function by the hydrosilylation of the carbamate
generated by the reaction of CO2 and the amine. Interestingly, it was subsequently shown
that N-heterocyclic carbenes could catalyze the related reaction using the industrial silane
polymethylhydrosiloxane (PMHS).58 Since these studies, other catalysts including
thiozolium carbenes,59 diazaphospholene60 and imidazolium-based ionic liquids61 have
been shown to promote the catalytic synthesis of formamides from CO2 and amines. In an
interesting perturbation, using superbases such as N-heterocyclic carbenes and Verkade’s
base, Cantat and coworkers62 observed reduction of CO2 in the presence of 9-BBN to
methylamines. Further variations of this theme afforded routes to aminals,
benzothiazoles,63 benzimidazole, formamidine and quinazolinone.64
Conclusion
The use of metal-free catalysts, including most notably FLPs, for the reduction of
CO2 has been the subject of study for less than a decade old. Some of these systems have
been shown to compete in activity and selectivity with the best transition metal catalysts.
It is clear that such metal-free catalyst systems will continue to be the subject of study and
that the correlations of the structure-reactivity relationship will facilitate the careful design
of ambiphilic FLP catalyst systems by judicious modification of the acidity and basicity
accommodating the intrinsic polarity of carbon dioxide. The low financial and
environmental cost together with high catalytic efficiencies of the envisioned metal-free
catalysts offer significant industrial potential and a green alternative to transition metal
catalysts.
Acknowledgements
The authors would like to thank NSERC for funding.
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