Nickel-catalyzed hydrosilylation of CO2 in the Presence of Et3B for the synthesis of formic acid and related formates

Article abstract of DOI:10.1021/om400876j

The reaction of CO₂ with Et₃SiH catalyzed by the nickel complex [(dippe)Ni(μ-H)]₂ (1) afforded the reduction products Et₃SiOCH₂OSiEt₃ (12%), Et ₃SiOCH₃ (3%), and CO, which were characterized by standard spectroscopic methods. Part of the generated CO was found as the complex [(dippe)Ni(CO)]₂ (2), which was characterized by single-crystal X-ray diffraction. When the same reaction was carried out in the presence of a Lewis acid, such as Et₃B, the hydrosilylation of CO₂ efficiently proceeded to give the silyl formate (Et₃SiOC(O)H) in high yields (85-89%), at 80 C for 1 h. Further reactivity of the silyl formate to yield formic acid, formamides, and alkyl formates was also investigated.

Full text of DOI:10.1021/om400876j

Article
pubs.acs.org/Organometallics
Nickel-Catalyzed Hydrosilylation of CO2 in the Presence of Et3B for
the Synthesis of Formic Acid and Related Formates
́
Lucero Gonzalez-Sebastian, Marcos Flores-Alamo, and Juventino J. Garcıa*
́
́
́
́ ́
Facultad de Quımica, Universidad Nacional Autonoma de Mexico, Mexico City 04510, Mexico
S
* Supporting Information
ABSTRACT: The reaction of CO2 with Et3SiH catalyzed by the nickel
complex [(dippe)Ni(μ-H)]2 (1) afforded the reduction products
Et3SiOCH2OSiEt3 (12%), Et3SiOCH3 (3%), and CO, which were characterized
by standard spectroscopic methods. Part of the generated CO was found as the
complex [(dippe)Ni(CO)]2 (2), which was characterized by single-crystal X-
ray diffraction. When the same reaction was carried out in the presence of a
Lewis acid, such as Et3B, the hydrosilylation of CO2 efficiently proceeded to
give the silyl formate (Et3SiOC(O)H) in high yields (85−89%), at 80 °C for 1
h. Further reactivity of the silyl formate to yield formic acid, formamides, and
alkyl formates was also investigated.
reported for the hydrosilylation of CO2. Nevertheless, to the
best of our knowledge, a nickel-catalyzed CO2 hydrosilylation
reaction has not yet been described in the literature.
INTRODUCTION
■
Even though carbon dioxide is a significant contributor to
global climate change, it also can be envisaged as a potential C1
building block for fine and commodity chemicals.1 However,
the activation of CO2 represents a challenge for chemists
because of its thermodynamic and kinetic stability. Hence,
research for new catalysts, particularly those that utilize cheap
and earth-abundant elements, to fix and transform CO2 into
useful products is a topic of growing interest. For example, a
number of transition-metal catalysts are known to be effective
in the direct hydrogenation of CO2 to formic acid.2
Nevertheless, this reaction is unfavorable thermodynamically
(ΔG = +33 kJ mol−1),2a and even under high H2 and CO2
pressures,3 it exhibits low activities.4 In order to face such
unfavorable thermodynamics, hydrosilanes, R3−xSiH1+x, can be
used to reduce and functionalize CO2 by means of the
formation of stable Si−O bonds. Subsequently, the reduction of
CO2 with hydrosilanes allows the further synthesis of silyl
formates,5 methoxysilanes,6 and methane.7 Several hydrosilanes
potentially useful as reducing agents are readily available, easy
to handle, nontoxic, and inexpensive.8 Among the mentioned
CO2 reduction products, silyl formates are some of the most
attractive compounds, since they can potentially be used as
valuable synthons to a wide range of value-added chemicals
derived from CO2.9 For instance, silyl formates can be easily
converted into formic acid by simple hydrolysis;10 this reaction
is a convenient approach toward formic acid production from
CO2, and a variety of carbonyl compounds can be synthesized
by reactions with the appropriate nucleophiles. Thus, the metal-
catalyzed hydrosilylation of CO2 is emerging as an alternative
methodology for catalytic CO2 fixation. To date, several
reaction systems using transition metals, such as Ir,5c,6a,11
Ru,5a Zr,5d,12 Rh, and Cu,10a and organocatalysts6b have been
Herein we report the first hydrosilylation of CO2 using the
nickel complex [(dippe)Ni(μ-H)]2 (1) as a catalytic precursor
in combination with Et3B, to give silyl formate in high yields.
Furthermore, we also disclose the intermediacy of silyl formate
in the consecutive synthesis of several carbonyl compounds
from CO2 via a tandem process, which consists of a
hydrosilylation reaction followed by nucleophilic attack by
alcohols/HBF4, water, or amines.
RESULTS AND DISCUSSION
■
Reaction of CO2 with Et3SiH Catalyzed by [(dippe)Ni-
(μ-H)]2 (1). The reactivity of CO2 and Et3SiH in the presence
of catalytic amounts of [(dippe)Ni(μ-H)]2 was first assessed
using a variety of solvents, at 80 °C and an atmospheric
pressure of CO2; the main results for these experiments are
summarized in Table 1.
As shown in the above table, a moderate conversion of
Et3SiH was observed on using THF as solvent, yielding
Et3SiOSiEt3 (2s), Et3SiOCH2OSiEt3 (3s), Et3SiOCH3 (4s),
and CO. The reduction products were identified by their
characteristic signals in 1H NMR: 3s (O−CH2−O, 5.1 ppm, s)
and 4s (O−CH3, 3.5 ppm, s) and by direct comparison with
recently reported data.7b By determination of the correspond-
ing 13C{1H} NMR spectrum, the presence of 3s was confirmed
by a signature signal at 84.6 ppm. Compounds 3s and 4s are
interesting, since these species has been proposed as
intermediates toward the total reduction of CO2 to methane.7b
Received: September 1, 2013
Published: November 4, 2013
© 2013 American Chemical Society
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a
Table 1. Reaction of CO2 and Et3SiH Catalyzed by [(dippe)Ni(μ-H)]2
entry
solvent
conversn of Si−H (%)
2s (%)
3s (%)
4s (%)
1
2
3
4
THF
50
8
35
8
12
nd
nd
nd
3
toluene
nd
2
b
dioxane
MeCN
39
0
17
nd
nd
a
All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve using 5 mL of solvent. Reaction conditions: 0.01/1 of [(dippe)Ni(μ-
b
H)]2 and Et3SiH, respectively, and 1 atm of CO2 were used. Conversions and yields were determined by GC-MS analysis. nd = not detected. 20%
of Et3SiOCH2CH2OSiEt3 was observed.
Worthy of mention is the reactivity found in dioxane (entry 3);
here the ring opening of dioxane was involved to yield
Et3SiOCH2CH2OSiEt3. No reaction was observed in acetoni-
trile, probably due to the coordination and/or oxidative
addition of the MeCN to the nickel center, as previously
reported.13
In order to find the appropriate conditions for CO2
reduction with Et3SiH, further studies focused on the use of
THF, at 100 and 120 °C and reaction times of 22, 36, and 44 h;
however, all results were very similar to those of entry 1 (Table
1).
An increased load of [(dippe)Ni(μ-H)]2 (10% mol) was
used with results in yield similar to those in entry 1
(conversion, 65%; 2s, 39%; 3s, 11%; 4s, 5%), but this amount
of complex allowed us to monitor the reaction by 31P{1H}
NMR; only one major signal was observed at 73.9 ppm,
assigned to the stable complex [(dippe)Ni(CO)2] (2). Suitable
crystals for X-ray studies of this compound were obtained by
spontaneous crystallization from the reaction mixture. The
corresponding ORTEP representation for 2 is depicted in
Figure 1. In this case, CO2 was reduced to CO with Et3SiH, the
last acting both as reducing agent and oxygen acceptor. We
previously reported the CO2 reduction with [(dippe)Ni(μ-H)]2
to yield [(dippe)Ni(CO)2] as one of the reduction products
together with [(dippe)Ni(CO3)] and dippe oxides as the
oxidation products.14 The independent preparation of
[(dippe)Ni(CO)2] revealed it to be a very stable compound
and nonactive in the hydrosilylation reaction (see the
Experimental Section). Consequently, the low conversion in
the hydrosilylation process may be due to a deactivation of the
catalyst by the formation of the rather stable compound 2.
Concerning the solid-state structure of complex 2, this has a
nickel center in a distorted -tetrahedral geometry, coordinated
to two P atoms from dippe and two terminal CO ligands. The
bite angle for the chelating dippe ligand of 90.72(4)° is similar
to those for other dippeNi0 complexes,14a while the OC−Ni−
CO angle of 113.22(16)° (average) is larger than that reported
for [(dtbpe)Ni(CO)2] (108.1(3)° average).14b The P−Ni−CO
angle 112.75° (average) is close to that reported for the
complex [Ni(CO)(np3)] (112.7.0(9)° average)15 (np3 = tris(2-
diphenylphosphinoethyl)amine) and slightly larger than that
reported for [(dtbpe)Ni(CO)2] (114.03 average).14b On the
other hand, the Ni−CO bond distance in 2 (1.762 Å average) is
close to the Ni−CO bond distance in [Ni(CO)(np3)] (1.74
Å),15 [(dtbpe)Ni(CO)2] (1.787 Å average),14b and {[(dippe)-
Ni(CO)]2(μ-dippe)} (1.759 (3) Å)14a but shorter than those
reported for Ni(CO)4 (1.84 Å) and [(CO)3Ni(PPh2)2Ni-
(CO)3)] (1.8003(8) Å).16 The Ni−P (2.206 Å average) bond
Figure 1. ORTEP drawing (50% probability ellipsoids) for complex 2.
Selected distances (Å) and angles (deg): C(15)−O(1) = 1.145(4),
C(16)−O(2) = 1.153(4), C(16)−Ni(1) = 1.761(4), C(15)−Ni(1) =
1.764(4), Ni(1)−P(1) = 2.2042(10), Ni(1)−P(2) = 2.2079(10);
O(1)−C(15)−Ni(1) = 178.7(3), O(2)−C(16)−Ni(1) = 177.9(3),
C(16)−Ni(1)−C(15) = 113.22(16), C(16)−Ni(1)−P(1) =
113.33(12), C(15)−Ni(1)−P(1) = 112.52(12), C(16)−Ni(1)−P(2)
= 112.19(12), C(15)−Ni(1)−P(2) = 112.96(13), P(1)−Ni(1)−P(2)
= 90.72(4).
distances are in the range of closely related tetrahedral d10
nickel complexes such as [Ni(P(OCH2)3CCH3)3NO]BF4
(2.186 Å average),17 Ni(CO)(np3)] (2.215 Å average),15
[(dtbpe)Ni(CO)2] (2.23 Å average),14b and {[(dippe)Ni-
(CO)]2(μ-dippe)} (2.23 Å).14a
Reaction of CO2, Et3SiH, and Et3B Catalyzed by 1. In
order to improve the CO2 hydrosilylation, some experiments
were carried out using a Lewis acid (Et3B) as an additive,
aiming to polarize the Si−H and/or CO bonds and thus
drive the reaction.7a,b,18 The reactivity of 1 toward the
hydrosilylation reaction using CO2, Et3SiH, and Et3B was first
studied in THF. In a typical reaction, [(dippe)Ni(μ-H)]2,
Et3SiH, and Et3B were dissolved in 5 mL of THF and then a
CO2 stream was bubbled at room temperature for 10 min.
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Scheme 1
a
Table 2. Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2 in Various Solvents
entry
solvent
conversn of Si−H (%)
2s (%)
5s (%)
6s (%)
1
2
3
4
5
THF
96
21
87
0
1.1
nd
3.0
nd
nd
90
11
76
nd
nd
6
toluene
dioxane
MeCN
neat
10
8
nd
nd
0
a
All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve using 5 mL of solvent. Reaction conditions: 0.01/0.1/1 of
[(dippe)Ni(μ-H)]2, Et3B, and Et3SiH, respectively, and 1 atm of CO2 were used. Conversions and yields were determined by GC-MS analysis. nd =
not detected.
a
Table 3. Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2 at Different Temperatures
entry
temp (°C)
conversn (%)
2s (%)
5s (%)
6s (%)
1
2
3
25
80
80
96
95
nd
1
80
90
75
nd
5.7
19
100
1
a
Reactions were carried out in a Schlenk flask equipped with a Rotaflo valve using 5 mL of solvent. Reaction conditions: 0.01/0.1/1 of
[(dippe)Ni(μ-H)]2, Et3B, and Et3SiH, respectively, and 1 atm of CO2 were used. Conversions and yields were determined by GC-MS analysis. nd =
not detected.
Scheme 2. Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2 using 5 mol % of Et3B
Then the reaction was monitored by GC-MS. It was found that
Et3SiH was almost consumed, 97%, yielding the products 2s
(1%), 6s (6%), and the silyl formate 5s (90%) (Scheme 1).
It is worth mentioning that in a blank experiment
hydrosilylation did not occur in the absence of [(dippe)Ni(μ-
H)]2. To confirm the reaction products shown in Scheme 1, 1H
and 13C NMR analyses were conducted after the reaction. The
1H NMR spectrum for 5s showed a signal assigned to Et3Si−
OCH(O)H, located at 8.05 ppm. In addition, the 13C{1H}
NMR spectrum of Et3Si−OC(O)H showed a key resonance at
161.4 ppm and the corresponding 13C NMR yielded a doublet
1
for that signal with JC−H = 223 Hz. Compound 6s was also
1
identified by H NMR with characteristic resonances at 2.3
ppm as a quadruplet and 1.05 ppm as a triplet for the
OC(O)CH2CH3 moiety. In addition, the 29Si NMR spectrum
for 5s presented a resonance at 25 ppm. The formation of 6s
can be explained by considering the known reactivity of Et3B,
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Scheme 3. Optimized Conditions for the Hydrosilylation of CO2 Catalyzed by [(dippe)Ni(μ-H)]2
Scheme 4. Mechanistic Proposal for the CO2 Hydrosilylation Reaction
since its use as an alkylating agent has long been reported in the
literature.19 Additionally, the reaction mixture was studied by
11B NMR spectroscopy, displaying two resonances at 76.6 and
56.1 ppm assigned to Et3B and Et2B−O−BEt2, respectively.
The main results for the catalytic hydrosilylation of CO2 with
different solvents in the presence of Et3B are shown in Table 2.
As shown before, again, the best yield, according to Table 2,
is for THF as solvent with a high conversion and selectivity
toward the formation of silyl formate 5s. Both THF and
dioxane were found to be good solvents for this reaction. The
effect of temperature for this reaction was also assessed; a
summary of the results is presented in Table 3. Of note, the
reaction takes place at room temperature (25 °C) with good
conversion and selectivity, just slightly lower than that at 80 °C.
At 100 °C the conversion is higher (entry 3); however, the
selectivity to 5s is lower than at 80 °C (entry 2, Table 3) with
an increase in the yield of the alkylation product 6s.
The use of lower amounts of Et3B (5 mol %) results in lower
conversion and longer reaction time, as depicted in Scheme 2.
Using the conditions from entry 2 of Table 3 as a starting
point, the reaction time was improved to 1 h at 80 °C, resulting
in 95% conversion of Et3SiH to yield 88% of silyl formate; this
yields involves a TOF of 87.7 h−1 (Scheme 3).
A mechanistic proposal for the CO2 hydrosilylation is
depicted in Scheme 4, on the basis of the following
considerations: first, Lewis acids are generally envisaged as
activators of CO bonds through the formation of CO B
adducts, thus polarizing the CO bond7b,18,20 and making the
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a
Scheme 5. Synthesis of Formic Acid from Et3SiH, CO2, and H2O
a
Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol) Et3SiH (1 mmol), CO2 (Patm= 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) H2O (0.2 mL).
Scheme 6. Synthesis of Potassium Formate
a
Table 4. Consecutive Synthesis of Formamides from CO2 and Et3SiH with Primary Amines
a
All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve. Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol), Et3SiH (1
mmol), CO2 (Patm = 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) amine (1 mmol). Conversions and yields were determined by GC-MS analysis.
carbon atom even more electrophilic. Second, the ability of R3B
species to abstract an alkide21 and hydride7a,22 fragment from
neutral organometallic precursors is well-known.23 Further-
more, it has been shown that the tributylstannyl cation can be
generated via hydride abstraction from Bu3SnH using B-
(C6F5)3.12,24 Therefore, it seems reasonable to propose the
hydride abstraction from Et3SiH by Et3B. In this regard, the
formation of silyl formate can be achieved by one of the two
pathways depicted in Scheme 4. In one of them (center of
Scheme 4) an initial step, involving the CO2 coordination to
the nickel center, is carried out to give I; this step is followed by
the coordination of Et3B to CO2 and the silane, to favor a
nucleophlic attack of the hydride over CO2 yielding species II,
which undergoes a reductive elimination and then releases the
product 5s and regenerates the nickel catalyst. Alternatively, an
oxidative addition of Et3SiH over nickel(0) can occur to yield
I′, followed by insertion of the adduct CO2·BEt3 into the Ni−H
bond, generating the nickel formate II; finally, a reductive
elimination of intermediate II releases 5s.
Further reactivity showed the silyl formate hydrolysis by
addition of H2O, yielding formic acid (Scheme 5) and silanol as
a byproduct.
1
This reaction was monitored by H and 13C NMR and GC-
MS. The conversion of 5s after the addition of water was
complete, as indicated by the absence of signals for silyl formate
(1H 8.18 ppm, 13C 161.5 ppm, JC−H = 223.63 Hz) and the
appearance of signals due to formic acid (1H 8.05, 13C 163.15
ppm, JC−H = 213.7 Hz). Additionally, the addition of potassium
fluoride into the solution containing 5s yielded the potassium
formate in quantitative amounts and Et3SiF as byproduct
(Scheme 6).
Finally, we turned out our attention to the reactivity of the
silyl formate 5s, produced during the hydrosilylation of CO2,
with a variety of nucleophilic reagents. The overall conversion
of CO2 to formic acid derivatives was accomplished by simple
addition of the selected nucleophilic reagent to the reaction
solution once the CO2 hydrosilylation with 1/Et3B was
complete (vide infra).
In the case of primary and secondary amines the silyl formate
could act as a “formyl synthon” to produce the formamides as
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a
Table 5. Consecutive Synthesis of Formamides from CO2 and Et3SiH with Secondary Amines
a
All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve. Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol), Et3SiH (1
mmol), CO2 (Patm = 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) amine (1 mmol). Conversions and yields were determined by GC-MS analysis.
a
Table 6. Consecutive Synthesis of Alkyl Formates from CO2 and Et3SiH with Alcohol/HBF4
a
All reactions were carried out in a Schlenk flask equipped with a Rotaflo valve. Reaction conditions: (1) 1 (0.01 mol), Et3B (0.1 mmol), Et3SiH (1
mmol), CO2 (Patm = 1 atm CO2), THF (5 mL), 80 °C, 1 h; (2) alcohol (1 mmol), HBF4 (1 mmol), NaSO4 (2 mmol). Conversions and yields were
determined by GC-MS analysis.
shown on Tables 4 and 5. With primary amines, 5s gave the N-
alkyl formamides with high conversions and good yields as
expected, and silanol was obtained as a coproduct (Table 4).
Likewise, on using secondary amines, N,N-disubstituted
formamide products were produced in moderate yields along
with the byproducts 2s and the protected Et3Si carbamate 8s
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General Procedure for the Reaction of CO2 with Et3SiH
Catalyzed by [(dippe)Ni(μ-H)]2 with Different Solvents. A typical
methodology was used as follows. A 25 mL Schlenk flask, equipped
with a Rotaflo valve and an inner magnetic stirring bar, was loaded into
the glovebox with a THF solution (5 mL) of [(dippe)Ni(μ-H)]2 (6.4
mg, 0.01 mmol) and Et3SiH (116 mg, 1 mmol). A color change from
wine red to brown was immediately observed. The solution was stirred
for 10 min, and then a CO2 stream was bubbled at room temperature
for 10 min. Then the flask was closed and heated in an oil bath at 80
°C for 22 h. After this time, the heating was stopped and the flask was
vented into the hood. An aliquot of the reaction mixture was
immediately analyzed by GC-MS. This methodology was followed
using toluene, dioxane, and acetonitrile instead of THF. [(dippe)Ni-
(CO)2] was isolated from the reaction using 10 mol % of
[(dippe)Ni(μ-H)]2 (32.2 mg, 0.05 mmol) and Et3SiH (58 mg, 0.5
mmol) in 5 mL of THF. The reaction mixture was concentrated to
dryness, giving a yellow residue, which was dried under vacuum for 6 h
and then redissolved in CD3OD (0.8 mL). This residue was identified
as [(dippe)Ni(CO)2] (2). Yield for complex 2: 99%. Anal. Calcd for
C16H32NiO2P2 (2): C, 50.97; H, 8.55. Found: C, 50.77; H, 8.52.
31P{1H} NMR (22 °C, 300 MHz, CD3OD): δ 73.9 (s, (i-
(Table 5). The product 8s was fully identified by GC-MS, and
additionally the 13C{1H} NMR spectrum showed a key signal at
154.7 ppm.
Since the formylation of alcohols is of relevance in organic
synthesis,26 we also explored the reactivity of 5s with a wide
variety of alcohols and HBF4. This reaction is driven by the
formation of the highly energetic Si−F bond (565 kJmol−1)25
and thus gave the alkyl formates in good yields. The main
results of this transformation are summarized in Table 6, all
with high conversions.
CONCLUSIONS
■
In the current study we report the first nickel-catalyzed CO2
hydrosilylation promoted by Et3B to selectively give silyl
formate. An important aspect of this transformation is the
strong dependence on the use of Et3B as an additive to enhance
the desired reactivity. Also, this work provided relevant findings
in the utilization of silyl formate as a building block in the
synthesis of value-added products. The simple reaction of silyl
formate with water produced formic acid and silanol in
excellent yields. The hydrosilylation of CO2 was successfully
applied to the reaction with either amines or alcohols/HBF4,
which can be considered as a potentially useful method to
afford a great variety of amides and alkyl formates in good
yields. Last but not least, we have also described the reduction
of CO2 by 1 generating CO and some intermediates proposed
toward the reduction of CO2 to methane.
Pr2)2PCH2CH2P(i-Pr2)2). 13C{1H} NMR (22 °C, 300 MHz,
CD3OD): δ 204.3 (t, CO, JC−P = 4.1 Hz), 25.8 (t, (CH(CH3)2),
JC−P = 9.8 Hz), 23.8 (dd, (CH(CH3)2), JC−P = 8.6, 6.9 Hz), 19.6 (m,
1
(CH2CH2)). H NMR (22 °C, 300 MHz, CD3OD): δ 2.0−1.82 (m,
CH, 4H), 1.56−1.48 (m, CH2, 4H), 1.1−0.9 (m, CH2, 24H). Crystals
of [(dippe)Ni(CO)2] suitable for X-ray diffraction studies were
obtained by cooling the CD3OD solution to −30 °C and isolating the
formed crystals by decantation.
Reaction of CO2 with Et3SiH Catalyzed by [(dippe)Ni(μ-H)]2
in THF-d8. A NMR tube equipped with a J. Young valve was loaded in
a glovebox with a THF-d8 solution (0.8 mL) of [(dippe)Ni(μ-H)]2
(3.2 mg, 0.005 mmol) and Et3SiH (58 mg, 0.5 mmol). This solution
was bubbled with a CO2 stream at room temperature for 5 min. Then,
the tube was closed and heated in an oil bath at 80 °C for 22 h. The
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all operations
were carried out in a MBraun glovebox (<1 ppm H2O and O2) or by
using high-vacuum and standard Schlenk techniques under an argon
atmosphere. Toluene was dried and distilled over sodium. Dioxane
and THF were dried and distilled from dark purple solutions of
sodium/ketyl benzophenone. Deuterated solvents for NMR experi-
ments were purchased from Cambridge Isotope Laboratories and
stored over 3 Å molecular sieves in the glovebox. Celite and silica gel
were dried by heating at 200 °C under vacuum for 20 h and were also
stored in the glovebox. [(dippe)Ni(μ-H)]2 (1) was prepared according
to the reported procedure.27 The bisphosphine ligand dippe (1,2-
bis(diisopropylphosphino)ethane) was synthesized according to the
reported methods.28 All other chemicals and filter aids were reagent
grade and were used as received. The purification of the new
compounds was carried out by crystallization or by column
chromatography. Carbon dioxide was supplied by Air Products and
Chemicals Inc. (purity >99.99%) and was used without further
purification. All reagents for the catalytic reactions were loaded in the
glovebox using Schlenk flasks equipped with Rotaflo high-vacuum
stopcocks and further loaded with CO2. The crude reaction mixtures
for each catalytic run were immediately analyzed by GC-MS. GC-MS
determinations were performed using an Agilent 5975C system
equipped with a 30 m DB-5MS capillary (0−32 mm i.d.) column. 1H,
13C{1H}, and 31P{1H} NMR spectra were recorded at room
temperature on a 300 MHz Varian Unity spectrometer in THF-d8,
CD3OD, or CD2Cl2, unless otherwise stated. 1H and 13C{1H}
chemical shifts (δ, ppm) are reported relative to the residual proton
resonance in the corresponding deuterated solvent. 31P{1H} NMR
spectra were referenced to an external 85% H3PO4 solution. 29Si and
11B NMR spectra were recorded at room temperature on a 400 MHz
Varian Unity spectrometer in THF-d8, unless otherwise stated. 29Si
NMR spectra were referenced to an external 85% Si(CH3)4 solution,
and 11B NMR spectra were referenced to external 85% BF3. All air-
sensitive NMR samples were handled under an inert atmosphere using
thin-wall (0.38 mm) Wilmad NMR tubes equipped with J. Young
valves. Single-crystal X-ray diffraction determinations were performed
on an Oxford Diffraction Gemini-Atlas diffractometer.
1
sample was monitored by H and 13C{1H} NMR.
Using multinuclear NMR the following assignations were made.
1
3
Et3SiH: H NMR (22 °C, 300 MHz, THF-d8): δ 3.7 (septet, JH−H
=
3.0 Hz, SiH), 0.99 (t, 3JH−H = 7.5 Hz, 9H, CH3CH2−), 0.61 (d quartet,
3JH−H = 7.5 Hz, 3JH‑SiH = 3.3 Hz, 6H, CH3CH2−); 13C{1H} NMR (22
°C, 300 MHz, THF-d8) δ 8.6 (s, CH3CH2−); 3.3 (s, CH3CH2−); 29Si
1
NMR (22 °C, 400 MHz, THF-d8) δ 0.99 (s, Et3SiH). (Et3Si)2O: H
3
NMR (22 °C, 300 MHz, THF-d8) δ 0.8 (t, JH−H = 7.8 Hz,
CH3CH2−); 0.4 (q, JH−H = 7.8 Hz, CH3CH2−); 13C{1H} NMR (22
3
°C, 300 MHz, THF-d8) δ 7.25 (s, CH3CH2−); 6.8 (s, CH3CH2−); 29Si
NMR (22 °C, 400 MHz, THF-d8) δ 7.7 (s, (Et3Si)2O). (Et3Si)2CH2:
1H NMR (22 °C, 300 MHz, THF-d8) δ 5.06 (t, JH−H = 7.8 Hz,
3
(Et3Si)2CH2); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 84.6 (s,
(Et3Si)2CH2) 6.9 (s, CH3CH2−), 5.3 (s, CH3CH2−).
Reaction of CO2, Et3SiH, and Et3B Catalyzed by [(dippe)Ni(μ-
H)]2. A 25 mL Schlenk flask equipped with a Rotaflo valve and a
magnetic stirrer was charged in the glovebox with a THF (5 mL)
solution of [(dippe)Ni(μ-H)]2 (6.4 mg, 0.01 mmol), Et3SiH (116 mg,
1 mmol), and Et3B (9.8 mg, 0.1 mmol). A color change from wine red
to brown and effervescence were observed. The reaction mixture was
stirred for 10 min, and then a CO2 stream was bubbled at room
temperature for 10 min. After that the flask was closed, followed by
heating in an oil bath at 80 °C for 22 h. Then, the flask was vented in a
hood and analyzed by GC-MS to give the products (Et3Si)2O (2s),
Et3Si−OCH(O)H (5s), and Et3Si−OCH(O)CH2CH3 (6s). The very
same procedure was followed using toluene, dioxane, and acetonitrile
instead of THF. Neat conditions: [(dippe)Ni(μ-H)]2 (19.2 mg, 0.03
mmol), Et3SiH (348 mg, 3 mmol), and Et3B (29.4 mg, 0.3 mmol)
were used and, on mixing, a brown-orange solution was obtained. The
solution was transferred to a Schlenk flask, and then a CO2 stream was
bubbled at room temperature for 10 min, followed by heating in an oil
bath to 80 °C for 22 h.
Reaction of CO2, Et3SiH, and Et3B Catalyzed by [(dippe)Ni(μ-
H)]2 in THF-d8. [(dippe)Ni(μ-H)]2 (3.2 mg, 0.005 mmol), Et3SiH (58
mg, 0.5 mmol), and Et3B (4.9 mg, 0.05 mmol) were mixed in 0.8 mL
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Organometallics
Article
of THF-d8. This mixture was transferred to a NMR tube equipped
with a J. Young valve, and a CO2 stream was bubbled at room
temperature for 5 min, followed by heating in an oil bath at 80 °C. The
formation of Et3Si−OCH(O)H (5s) was followed up by 1H, 13C{1H},
13C, 29Si.
Characteristic signals for the silyl formate Et3Si−OCH(O)H (5s):
1H NMR (22 °C, 300 MHz, THF-d8) δ 8.05 (s, Et3Si−OCH(O)H),
0.99 (t, 9H, CH3CH2−), 0.61 (d quartet, 6H, CH3CH2−); 13C{1H}
NMR (22 °C, 300 MHz, THF-d8) δ 161.4 (s, Et3Si−OCH(O)H), 6.8
(s, CH3CH2−), 5.4 (s, CH3CH2−); 13C NMR (22 °C, 300 MHz,
THF-d8) δ 161.4 (d, JC−H = 223.6 Hz, Et3Si−OCH(O)H); 29Si NMR
(22 °C, 400 MHz, THF-d8) δ 24.97 4 (s, Et3Si−OCH(O)H).
Procedure for the Synthesis of Formic Acid. Once the
hydrosilylation reaction of CO2 with 1/Et3B was completed (vide
supra), water (0.2 mL, 11 mmol) was added and the reaction mixture
was vigorously stirred at room temperature for 20 min. The
conversions and yields of the products were determined by GC-MS
data collection and data integration. Data sets consisted of frames of
intensity data collected with a frame width of 1° in ω, a counting time
of 5.6 s/frame, and a crystal-to-detector distance of 55.00 mm. The
double-pass method of scanning was used to exclude any noise. The
collected frames were integrated by using an orientation matrix
determined from the narrow frame scans. Final cell constants were
determined by a global refinement; collected data were corrected for
absorbance by using analytical numeric absorption correction30 with a
multifaceted crystal model based on expressions upon the Laue
symmetry using equivalent reflections.
Structure solution and refinement were carried out with the
program(s) SHELXS9731 and SHELXL97; ORTEP-3 for Windows32
was used for molecular graphics, and WinGX33 was used to prepare
the material for publication.
Full-matrix least-squares refinement was carried out by minimizing
2
(Fo − Fc2)2. All non-hydrogen atoms were refined anisotropically. H
atoms attached to C atoms were placed in geometrically idealized
positions and refined as riding on their parent atoms, with C−H =
0.98−0.99 Å and Uiso(H) = 1.2[Ueq(C)] and Uiso(H) = 1.5[Ueq(C)]
for methylene and methyl groups, respectively.
1
1
and H, 13C, and 13C{1H} NMR analyses. HOC(O)H: H NMR (22
°C, 300 MHz, THF-d8) δ 11.23 (s, HOC(O)H), 8.05 (s, HOC(O)H);
13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 163.1 (s, HOC(O)H);
13C NMR (22 °C, 300 MHz, THF-d8) δ 163.1 (d, JC−H = 213.7 Hz,
1
ASSOCIATED CONTENT
* Supporting Information
HOC(O)H). Et3SiOH: H NMR (22 °C, 300 MHz, THF-d8) δ 3.8
■
(m, Et3SiOH), 0.95 (t, 3JH−H = 7.5 Hz, 9H, CH3CH2−), 0.54 (q, 3JH−H
= 7.5 Hz, CH3CH2−); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ
6.3 (s, CH3CH2−), 6.2 (s, CH3CH2−).
S
Figures, a table, and a CIF file giving selected multinuclear
NMR data and crystallographic data for complex 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Procedure for the Synthesis of Potassium Formate. After
completion of the hydrosilylation of CO2 with 1/Et3B described
above, potassium fluoride (95 mg, 1 mmol) was added to the reaction
solution of 5s and this mixture was vigorously stirred at room
temperature for 15 min; then the solution was evaporated to dryness,
yielding a yellow solid. The potassium formate was separated with
methanol, and this was removed by evaporation to yield KOC(O)H as
a white solid. KOC(O)H: 1H NMR (22 °C, 300 MHz, THF-d8) δ 8.05
(s, HOC(O)H); 13C{1H} NMR (22 °C, 300 MHz, THF-d8) δ 163.1
(s, HOC(O)H); IR 1609, 1388, 1358, 1218, 1136, 1060 cm−1.
Typical Procedure for the Formylation of Amines. Following
the hydrosilylation of CO2, the corresponding amine (1 mmol) was
added to the reaction solution of 5s produced in situ. The Schlenk
tube was sealed, and the reaction mixture was vigorously stirred at 80
°C. The conversions and yields of the products were determined by
GC-MS.
Procedure for the Synthesis of Alkyl Formates. After the
hydrosilylation of CO2 described above, NaSO4 (284 mg, 2 mmol)
and a mixture of the corresponding alcohol (1 mmol) with HBF4 (1
mmol) were added to the reaction solution of 5s. Then, the Schlenk
tube was sealed and the reaction mixture was vigorously stirred at 80
°C. The conversions and yields of the products were determined by
CG-MS.
Attempts of the Hydrosilylation of CO2 with [(dippe)Ni-
(CO)2]. The complex [(dippe)Ni(CO)2] was independently prepared
and tested as a potential catalyst precursor in the hydrosilylation
reaction: a 25 mL Schlenk flask, equipped with a Rotaflo valve and a
magnetic stirring bar, was charged in a glovebox with a THF solution
(5 mL) of [(dippe)Ni(CO)2] (0.19 mg, 0.05 mmol) and Et3SiH (58
mg, 0.5 mmol). The reaction mixture was stirred for 10 min, and then
a CO2 stream was bubbled at room temperature for 10 min. Then the
Schlenk was closed and heated in an oil bath to 80 °C for 22 h. After
this time the reaction mixture was cooled to room temperature and an
aliquot was analyzed by GC-MS. No hydrosilylation products were
observed. The 31P{1H} NMR spectrum of this sample confirmed the
presence of unreacted [(dippe)Ni(CO)2] by a key signal at 73.9 ppm,
confirming the high stability of the complex.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.J.G.: juvent@unam.mx.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank PAPIIT-DGAPA-UNAM (IN-210613) and CON-
ACYT (0178265) for the financial support for this work. L.G.-
S. also thanks CONACYT for a Ph.D. grant. We also thank Dr.
́
Alma Arevalo for her technical assistance.
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