Phosphazenes: Efficient organocatalysts for the catalytic hydrosilylation of carbon dioxide

Article abstract of DOI:10.1039/c5cc01282a

Phosphazene superbases are efficient organocatalysts for the metal-free catalytic hydrosilylation of carbon dioxide. They react with CO₂ to form the respective phosphine oxides, but in the presence of hydrosilanes, CO₂ can be selectively reduced to silyl formates, which can in turn be reduced to methoxysilanes by addition of an extra loading of silanes. Activities reach a TOF of 32 h^-1 with a TON of 759. It is also shown that unexpectedly, N,N-dimethylformamide can reduce CO₂ to a mixture of silyl formates, acetals and methoxides in the absence of any catalyst.

Full text of DOI:10.1039/c5cc01282a

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Phosphazenes: efficient organocatalysts for the
catalytic hydrosilylation of carbon dioxide†
Cite this:DOI: 10.1039/c5cc01282a
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Marc-Andre Courtemanche, Marc-Andre Legare, Etienne Rochette and
Frederic-Georges Fontaine*
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Received 11th February 2015,
Accepted 13th March 2015
DOI: 10.1039/c5cc01282a
www.rsc.org/chemcomm
Phosphazene superbases are efficient organocatalysts for the metal-free pairs (FLP) chemistry,8 notably for CO2 capture,9 has led to the
catalytic hydrosilylation of carbon dioxide. They react with CO2 to form development of efficient organocatalytic systems for the hydro-
the respective phosphine oxides, but in the presence of hydrosilanes, boration of carbon dioxide to methoxide derivatives that can be
CO2 can be selectively reduced to silyl formates, which can in turn be readily hydrolysed to methanol.10,11 It was later shown that
reduced to methoxysilanes by addition of an extra loading of silanes. phosphines,12 strong nitrogen bases,13 and even BH4 could act
À
Activities reach a TOF of 32 hÀ1 with a TON of 759. It is also shown that as catalysts for the hydroboration of CO2.14 In a seminal report,
unexpectedly, N,N-dimethylformamide can reduce CO2 to a mixture of Ying and co-workers have reported that N-heterocyclic carbenes
silyl formates, acetals and methoxides in the absence of any catalyst.
(NHCs) could act as catalysts for the hydrosilylation of carbon
dioxide to methoxysilanes with TOF and TON reaching 25.5 hÀ1
The alarming rate at which the concentration of carbon dioxide and 1840, respectively.15 Since then, a number of systems have
is increasing in the atmosphere is pushing the scientific com- emerged for the reduction of CO2 using hydrosilanes to generate
munity to find solutions in order to limit these emissions and value added products such as formamides,16a formamidines,16b
remove this greenhouse gas from the atmosphere.1 While many methylamines,17 acetals5c and methane.9b While base-activation
advances have been made in the capture and storage of CO2, of hydrosilanes has been used for the hydrosilylation of aldehydes,
the use of carbon dioxide as a feedstock for the synthesis of ketones and esters,18 to the best of our knowledge, Ying’s report of
valuable products could provide an important incentive for CO2 NHC-catalyzed system remains the only metal-free system for the
recycling.2 Of notorious importance is the large scale production hydrosilylation of CO2 to methanol or formic acid derivatives.
of energy vectors such as methanol from CO2, which would be one Phosphazene derivatives of the general structure (R2N)3PQN–R
very efficient way to sequester considerable amounts of carbon have been developed by Schwesinger as very strong organic bases.19
dioxide. This concept is promoted by Nobel laureate George A. They are well established as an alternative to NHC organocatalysts
Olah as the methanol economy.3 While some heterogeneous for the ring opening polymerization of cyclic esters.20 Surprisingly,
systems have been reported for the CO2 reduction to methanol, the use of those bases as catalysts for CO2 reduction was never
the use of transition metal catalyzed homogeneous systems have reported. In an effort to broaden the scope of organocatalyzed
also been reported for the hydroboration,4 the hydrosilylation5 reduction of CO2, we herein report that commercially available
and to a lesser extent the hydrogenation6 of carbon dioxide. phosphazene superbases can catalyze the reductive hydrosilylation
However, the search for new environmentally friendly and efficient of CO2.
systems remains a very active field of study.
Firstly, as other strong Lewis bases such as NHCs (N-heterocyclic
During the past few years, metal-free catalysis has been shown carbenes) are known to bind carbon dioxide, the reaction between
to be a viable alternative to the transition-metal catalyzed various commercially available phosphazene bases (Scheme 1, 1–3)
reduction of CO2. In fact, in some cases organocatalysts have and carbon dioxide was investigated in order to verify if the
even been shown to exhibit catalytic activities surpassing that of isolation of a CO2 adduct was possible. When 0.03 mmol of 1
transition-metal based systems.7 The advent of frustrated Lewis was dissolved in benzene-d6 and was exposed to 1 atm of CO2, a
small amount of a new signal resonating at 21.3 ppm was observed
by 31P{1H} spectroscopy. Complete conversion to the new species
was achieved after 8 hours with only one notable change in the
1H NMR spectra. Indeed, the resonance attributed to the t-Bu
groups was shifted from 1.5 ppm to 0.9 ppm, with the loss of the
4JP–H coupling (1.2 Hz). Upon evacuation of the volatiles in vacuo
´
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Departement de Chimie and Centre de Catalyse et Chimie Verte (C3V), Universite
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Laval, 1045 Avenue de la Medecine, Quebec, Canada.
E-mail: frederic.fontaine@chm.ulaval.ca; Fax: +1-418-656-7916;
Tel: +1-418-656-5140
† Electronic supplementary information (ESI) available: Synthetic procedures and
solution NMR data. See DOI: 10.1039/c5cc01282a
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and coordinating solvents have been shown to accelerate the
reduction process.15 In a typical experiment, ca. 3 atm. of 13CO2
was condensed in a J-Young NMR tube containing a 19.0 mM
solution of 1 in presence of Ph2SiH2 (2.5 mol% based on Si–H) in
DMF-d7. After 30 minutes of reaction at room temperature, the
signals belonging to silyl formates (d = 8.6, 1JC–H = 231.8 Hz), silyl
acetals (d = 5.8, 1JC–H = 171.8 Hz) and silyl methoxides (d = 3.8, 1JC–H
=
144.2 Hz) could be identified using 1H NMR.23 The C–H correlations
were confirmed using HSQC experiments. Upon reaction com-
pletion, the sample was degassed by three subsequent freeze
pump thaw cycles to ensure complete removal of 13CO2 from
the reaction medium after which a second loading of Ph2SiH2
(2.5 mol% based on Si–H) was added. After 36 hours, the silyl
formates had completely converted to methoxysilane deriva-
1
tives with H chemical shifts ranging between 3.0–3.8 ppm.
Monitoring the same catalytic reaction using 31P{1H} NMR
spectroscopy revealed that after 30 minutes, the signal of 1 (d = À2.1)
completely disappeared to form one new species resonating at 22.9
Scheme 1 Rearrangement of phosphazenes in the presence of CO2 with
proposed intermediate.
and re-dissolution in benzene-d6, the 31P{1H} NMR spectra remained as well as 4 in a 6 : 1 ratio. It is important to note that both signals
unchanged showing a singlet at 21.3 ppm while the signal attri- remain unchanged throughout the entire catalytic process. Interest-
1
buted to the t-Bu protons was no longer present in the H NMR ingly, the signal at 22.9 ppm slowly reverted back to the signal of 1 at
spectrum.
À2.1 ppm upon additional loading of silane in the absence of 13CO2.
Based on these experimental evidences, it was hypothesized When the transformation of the silyl formates to methoxysilanes was
that the 21.3 ppm species was not a simple adduct between 1 complete, the signal at 22.9 ppm also completely reverted back to the
and CO2, but was the product of a transformation of the signal of 1 at À2.1 ppm. Based on the aforementioned spectroscopic
phosphazene with the elimination of the t-Bu group. Comparison observations, it seems that the signal at 22.9 ppm arises from an
1
of the H and 13C{1H} NMR signals for the new t-Bu containing interaction between 1 and silyl formates. The absence of coupling
species and that of a commercial sample confirmed that the with the 13C nucleus, points to an N–Si interaction rather than
solution contained t-Bu isocyanate. Repeating the experiment at a an N–C interaction. In a blank experiment, heating 1 at 80 1C
larger scale (see ESI†) allowed the isolation of the phosphine for 1 hour in the presence of an equimolar amount of Ph2SiH2
oxide 4 in quantitative yield. As noted in Scheme 1, a plausible in benzene-d6 yielded no change in the NMR spectra. Similarly,
mechanism would first involve the formation of an adduct exposing 4 to ca. 3 atm of CO2 in the presence of Ph2SiH2 in
between 1 and CO2, leading to the formation of a Lewis acidic benzene-d6 did not yield any reaction, even in the presence of
phosphonium center that can interact with the oxygen atom of t-Bu-isocyanate, suggesting that neither 4 nor the isocyanate
the activated CO2 molecule. tBuNCO and 4 are then formed in a has a role in the reduction of CO2.
concerted fashion, through a 2+2 rearrangement, reminiscent of
With a better understanding of the reaction, the reduction of
the Wittig reaction mechanism.21 A similar rearrangement has 3 atm of CO2 with a 13 mM solution of 1 in DMF using 40 equiv.
been observed previously in a titanium complex leading to the of Ph2SiH2 as a reductant was monitored using 1H NMR
generation of carbodiimides.22 The less bulky phosphazene 2 spectroscopy. After only 60 minutes, 60% of the silane was
undergoes the same rearrangement in the presence of carbon consumed (TOF = 41 hÀ1, TON = 48) while 90% was converted
dioxide within 15 minutes, while the more bulky 3 also undergoes after 5 hours (TOF = 14.4 hÀ1, TON = 72). Interestingly, it
a similar rearrangement, releasing ethylisocyanate after 12 hours was observed that under these conditions, silyl formates were
under 1 atm of CO2 at 60 1C. It is interesting to note that in the the predominant reduction products, contrasting with the
case of 3, one phosphinimine moiety remains intact.
reported reactivity of NHC catalysts which formed a majority
Since the rearrangement with phosphazenes 1 and 3 is slow, of methoxysilanes (see Fig. S6A, ESI†).15
we were curious to see if they could be active catalysts in the
The preferential formation of silylformates over methoxy
hydrosilylation of carbon dioxide. In an initial experiment, 1 was species suggests that the first reduction of CO2 to generate
dissolved in benzene-d6 in the presence of 5 equiv. of Ph2SiH2. formates occurs faster than the subsequent reduction steps
The sealed J-Young NMR tube was then exposed to ca. 3 atm of which will generate acetal and methoxysilane derivatives. Per-
CO2 and heated at 80 1C for 4 hours. Interestingly, 1H NMR forming the catalytic reduction (1.25 mol% 1, Ph2SiH2) under
spectroscopy revealed that the hydrosilane was completely con- 5 atm of CO2 led almost exclusively to the formation of
sumed, giving rise to set of resonances in the d B 3.8, B 5.8, and silylformates in a 97% ratio when compared to silylacetals
B 8.6 regions, indicative of CO2 reduction to methoxy-, acetal- (2%) and methoxysilanes (1%) after only two hours (see ESI†).
and formylsilanes. In order to ascertain that the reduction The remaining CO2 was then removed in vacuo and an addi-
products did originate from CO2 reduction, the reaction was tional loading of Ph2SiH2 was added, leading to complete
carried out using 13CO2 in N,N-dimethylformamide since polar conversion of the silyl formates into methoxysilane derivatives
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of 3 to 6, the activity of 6 was also investigated, but no
significant reaction was observed (entry 11).
In order to mimic the activity presented in the reduction of
CO2 with NHC catalysts, the use of very low catalytic loading
was carried out. With a catalytic loading of 0.01 mol%, complete
conversion of the silanes was achieved after 48 hours. These
results would represent a TOF of 206 hÀ1 and a TON of 9900,
which is quite unexpected since one would not expect such an
increase in turn-over frequency upon dilution (entry 13). To our
Scheme 2 Product ratio after the catalytic reduction under 5 atm of CO2
and after addition of an extra loading of silane in the absence of CO2. Yield
based on Si–H.
after 36 h (Scheme 2). Such selectivity in the formation of either surprise, repeating the experiment with the same parameters
silylformates or silylmethoxydes is of notable interest. while omitting the catalyst showed that 21% of the silane was
The scope and efficiency of various silanes, solvents and consumed after 4 h and as much as 82% was consumed after 24 h
phosphazene catalysts were probed using a variety of conditions (entries 14–15). It is important to note that no silane was
(Table 1). As can be observed in entry 3, changing the silane from consumed when the experiment was repeated without CO2,
Ph2SiH2 to the less bulky PhSiH3 resulted in the reaction sustaining confirming that the products do not originate from DMF
41 turnovers over 4 h. On the other hand, no catalytic activity was reduction. While it was previously observed that the choice of solvent
observed using the bulkier Et3SiH, even after 24 hours under 3 atm was an important factor in silane mediated CO2 reductions,15,25 it is
of CO2 (entry 4). The alkoxysilane (EtO)3SiH did show modest surprising that such an important reduction of CO2 by the solvent
activity, completing 41 turnovers after 24 h under 3 atm of CO2 alone was not reported.26 When comparing the conversion with and
(entry 5), while PMHS (polymethylhydrosiloxane) decomposed without catalyst (entries 1 and 14), it can be shown that the
readily in the presence of 1 in DMF to generate a gas which was phosphazenes are nevertheless efficient catalysts for the hydrosily-
assigned to be MeSiH3 as well as an insoluble white solid indicative lation of CO2. In acetonitrile or THF, the presence of a catalyst
of the formation of polysiloxanes. Indeed, base catalyzed decom- was required to induce the reduction of CO2. In acetonitrile, the
position of PMHS is well documented.24 Changing the solvent to reduction catalyzed by 1 reached a TON of 759 after 24 h,
another polar, coordinating solvent such as acetonitrile gave representing a TOF of 32 hÀ1. The latter result shows that
activities comparable to those obtained in DMF, as can be seen phosphazene remains robust catalyst over a long period of time.
in entry 6 with TOF of 16 hÀ1. On the other hand, the use of A large scale reaction was carried out using 4.0 g of Ph2SiH2 in the
non-coordinating solvents such as toluene or DCM resulted in a presence of 1 (1.25 mol%) in CH3CN and from which methanol
dramatic decrease in reactivity (entries 7–8).
was generated with 69% yield upon hydrolysis.
Although 2 was shown to rapidly rearrange to 5 in the
The reduction mechanism for this system is most likely
presence of carbon dioxide, it was found to exhibit catalytic similar to the NHC catalyzed CO2 hydrosilylation mechanism
activity in the presence of Ph2SiH2, reaching 31 turnovers after which was studied computationally and was proposed to occur
4 hours of reaction under 3 atm of CO2 (entry 9). The bulkier via activation of the hydrosilane rather than through activation
phosphazene 3 did not surpass the activity of 1 but still of CO2.27 While an experimental/theoretical report by Ying
managed 93 TON in 4 h for a TOF of 23 hÀ1 (entry 10). Since contested the study,28 the important solvent effects were not
a phosphinimine moiety remains intact in the transformation studied in detail in any of the two reports. Another plausible
a
Table 1 Catalytic activity of phosphazene bases for the hydrosilylation of CO2
Catalyst
Silanec ([ ] mmol
À1; equiv.)
Pressure
(atm)
Time
(h)
Conversion
(% Si–H)
HCO2
(%)
OCH2Ob
(%)
CH3Ob
(%)
TOFc
b
Entry
([ ] mmol LÀ1
)
L
Solvent
(hÀ1
)
TONc
1
2
3
4
5
6
7
8
1 (12.5)
1 (12.5)
1 (12.5)
1 (25)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 80)
PhSiH3 (1; 80)
Et3SiH (1; 40)
DMF
DMF
DMF
DMF
1
5
1
3
3
1
1
1
3
3
1
1
1
1
1
1
4
2
4
24
24
4
4
4
4
4
96
95
42
0
41
79
14
1
39
93
1
93
98
21
82
76
84
97
85
0
100
66
33
Trace
81
84
Trace
72
13
2
15
0
3
1
0
0
19
38
8
0
1
16
3
0
8
19
0
N/A
N/A
N/A
N/A
32
77
76
33
0
16
63
11
0
31
74
0
N/A
N/A
N/A
N/A
759
1 (25)
(EtO)3SiH (1; 40)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 80)
Ph2SiH2 (1; 2000)
Ph2SiH2 (1; 10 000)
Ph2SiH2 (1; —)
Ph2SiH2 (1; —)
Ph2SiH2 (1; 1000)
DMF
0
0
10
17
Trace
0
4
Trace
5
9
0
3
1 (12.5)
1 (12.5)
1 (12.5)
2 (12.5)
3 (12.5)
6 (12.5)
1 (0.5)
1 (0.1)
—
CH3CN
Toluene
DCM
DMF
DMF
DMF
DMF
DMF
DMF
DMF
24
50
Trace
19
12
Trace
23
24
38
29
29
9
10
11
12d
13d
14
15
16
1
24
48
4
24
24
66
62
68
62
—
1 (0.1)
CH3CN
10
a
The reactions were carried out at ambient temperature and the yields were determined using 1H NMR integration with cyclohexane as an internal
b
c
d
standard. Distribution of products observed. Based on the number of Si–H transferred. Under these conditions, the DMF will be responsible
for the reduction.
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It was even shown by Kobayashi that the use of DMF could
promote the formation of hypervalent silicon species.30 Further
investigation of the reaction mechanism is currently ongoing
in our research group.
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¨
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In summary, it was shown that although phosphazene
superbases can rearrange to their respective oxide in an unusual
way, they represent an efficient new class of organocatalysts for
the reduction of carbon dioxide. One can easily control selectivity
to methoxysilanes or silyl formates by controlling the reaction
conditions. Using a very simple protocol TOF reaching 32 hÀ1 and
TON reaching 759 were observed. Furthermore, it was shown that
the role of DMF in such a reduction process is of critical importance
and its role as a catalyst should be considered when evaluating the
catalytic efficiency of organocatalysts for CO2 reduction. Current
studies focus on the use of phosphazene bases for other transforma-
tions involving the formation of value added products from carbon
dioxide reductions and will be reported in due course.
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The authors would like to acknowledge National Sciences
and Engineering Research Council of Canada (NSERC, Canada)
and the Centre de Catalyse et Chimie Verte (Quebec) for
financial support. M.-A. C., M.-A. L., and E. R. would like to
thank NSERC and FQRNT for scholarships.
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