Synthesis of a stable disilyne bisphosphine adduct and its non-metal-mediated CO2 reduction to co

Article abstract of DOI:10.1002/anie.201006265

Two P or not two P: The synthesis of the first stable disilyne bisphosphine adduct is reported. Its X-ray structure, showing a short Si-Si bond with a certain multiple-bond character, illustrates the peculiar ligand effect of phosphine towards Si^I

Full text of DOI:10.1002/anie.201006265

Communications
DOI: 10.1002/anie.201006265
Silicon Chemistry
Synthesis of a Stable Disilyne Bisphosphine Adduct and Its Non-Metal-
Mediated CO2 Reduction to CO**
David Gau, Ricardo Rodriguez, Tsuyoshi Kato,* Nathalie Saffon-Merceron, Abel de Cꢀzar,
Fernando P. Cossꢁo, and Antoine Baceiredo*
Since the first synthesis of stable disilynes I, with a silicon–
silicon triple bond, independently by the groups of Sekiguchi
and Wiberg,[1] the unique chemical behavior of these species
has been attracting much attention. Structural determinations
revealed that I presents a trans-bent structure resulting in two
addressed. However, compared to transition-metal deriva-
tives, the coordination chemistry of main-group-element
species such as disilynes[2–4] or silylenes[5] is not very well
developed, although their transition-metal-like behavior is of
great interest.[6] Thus, although phosphines are ubiquitous
ligands in organometallic chemistry, their use in main-group-
element chemistry is not so frequent. Recently we reported
that phosphine-stabilized silylenes (phosphonium silaylides)
react either as sila-Wittig reagents[7] or as nucleophilic silylene
complexes.[8] These results highlight the efficiency of com-
plexation to stabilize low-coordinate species without perturb-
ing their high reactivity. Herein we present the synthesis of an
original disilyne bisphosphine adduct 2 (Scheme 1) and its
reaction with carbon dioxide, thus demonstrating a powerful
function as an efficient CO2-reducing agent.
À
nondegenerate Si Si p molecular orbitals and particularly a
weak in-plane p bond with a lowest unoccupied molecular
orbital that is low-lying in energy.[1a] As a consequence, the
formation of bis(Lewis base) adducts II by the addition of two
donating ligands is strongly exothermic.[2]
Only a few stable disilyne bis(Lewis base) adducts IIa,b
with different ligands have been synthesized to date. Strongly
s-donating ligands with a weak p-accepting character such as
N-heterocyclic carbenes[2] or imines[3] induce highly polarized
Scheme 1. Synthesis of disilyne bisphosphine adduct 2.
We have already described the synthesis of stable cyclic
phosphonium silaylides by reduction of dichlorophenylsilane
derivatives bearing a 2-phosphinoenamine fragment using
magnesium metal.[7] By analogy, we now investigated the
À
1,2-diylidic structures with long Si Si single bonds. As a
consequence, each silicon center in these adducts II exhibits a
lone pair of electrons.[4] In fact, the chemical properties of the
Lewis base stabilized disilynes should be strongly affected by
the complexation, and this important issue needs to be
reduction of trichlorosilane derivative
1 using lithium
(3 equiv) in THF at room temperature. The reaction readily
affords the disilyne bisphosphine adduct 2, which can be
isolated as red crystals (13% yield). In the 29Si NMR
spectrum, derivative 2 displays a doublet of doublets at d =
[*] D. Gau, Dr. R. Rodriguez, Dr. T. Kato, Dr. A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS, LHFA UMR 5069
31062 Toulouse (France)
2
À18.5 ppm (1JSiP = 192.1 Hz, JSiP = 60.1 Hz), thus indicating
the presence of two phosphorus atoms in the molecule. Large
silicon–phosphorus coupling constants have been observed
for monophosphonium silaylides (1JSiP = 141–157 Hz).[7] In the
31P NMR spectrum, only one singlet at d = 34.7 ppm was
detected in the typical region for phosphonium derivatives,
which is downfield from that of starting trichlorosilane
derivative 1 (d = 2.2 ppm). These data are in good agreement
with a symmetrical arrangement of the molecule.
Fax: (+33)5-6155-8204
E-mail: kato@chimie.ups-tlse.fr
baceired@chimie.ups-tlse.fr
Homepage: http://hfa.ups-tlse.fr
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, and CNRS, SFTCM FR 2599
31062 Toulouse (France)
Dr. A. de Cꢁzar, Prof. F. P. Cossꢂo
Departamento de Quꢂmica Organica, Universidad del Paꢂs Vasco-
Euskal Herriko Unibertsitatea Facultad de Quꢂmica
P. K. 1072, San Sebastian-Donostia (Spain)
The molecular structure of 2 was unambiguously deter-
mined by X-ray crystallography (Figure 1).[9] The N-Si1-Si1’-
N1’ skeleton presents a cis-bent geometry (torsion angle 4.98).
Each silicon atom interacts with the phosphorus center on
both sides of the Si2N2 plane (P1-Si1-Si1’-P1’ 159.108),
forming two five-membered cyclic fragments. A value of
[**] We are grateful to the CNRS (ERA-Chemistry) and Ingenio-
Consolider (CSD2007-00006) for financial support of this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006265.
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Angew. Chem. Int. Ed. 2011, 50, 1092 –1096

ylide 3 (0.85).[7] These results show a clear contrast in ligand
effect between phosphine and the p-acidic isocyanide ligands.
À
In the latter case, a smaller Si Si Wiberg bond index (0.839)
À
and larger Si C Wiberg bond index (1.049) were calculated
(4’: R = H, L = CNSiH3), which is expected for a disilaketen-
imine character. The strongly pyramidalized silicon center
À
and the relatively small Si Si Wiberg bond index in 2
illustrate the major contribution of the highly polarized
À
electronic structure 2a (Scheme 2), with a Si Si single bond
Figure 1. Molecular structure of 2. Thermal ellipsoids represent 30%
probability. H atoms are omitted for clarity. Selected bond lengths [ꢃ],
angles, and torsion angles [8]: Si1–Si1’ 2.3306(12), Si1–P1 2.3274(9),
P1–C1 1.760(2), C1–C2 1.375(3), C2–N1 1.360(3), Si1–N1 1.8805(18);
P1-Si1-Si1’ 103.64(4), N1-Si1-Si1’ 137.60(6), N1-Si1-P1 88.41(6),
Si1-P1-C1 93.03(8), P1-C1-C2 115.52(17), C1-C2-N1 124.97(19),
C2-N1-Si1 113.65(14), C2-N1-C5 118.57(17), C5-N1-Si1 117.03(14),
N1-Si1-Si1’-N1 4.86, P1-Si1-Si1’-P1’ 159.10.
Scheme 2. nSi–s*Si negative hyperconjugative p interactions in 2.
À
2.331 ꢀ for the Si Si bond is at first glance in the range of
[10]
À
ordinary Si Si single bonds (2.34 ꢀ).
However, it is
and a lone pair on each silicon center. However, the increased
À
substantially shorter than those observed for the other
reported disilyne bis(Lewis base) adducts IIa,b (2.393–
2.413 ꢀ)[2,3] and is even shorter than in the similar adduct 4
(2.369 ꢀ) with 1,2-disilaketenimine character[11] (Table 1).
Si Si Wiberg bond index in 2 compared to 4 also agrees with a
certain degree of multibonding character (resonance struc-
ture 2b), probably owing to enhanced negative hyperconju-
gative p interactions between the two silicon centers (nSi–s*Si)
rather than between phosphorus and silicon centers (nSi–s*P),
as indicated by the weak phosphine–silicon p interaction
(resonance structure 2c in Scheme 2). A similar ligand effect
was observed in the recently synthesized phosphine-stabilized
À
À
Table 1: Si Si and Si L bond lengths of different disilyne adducts. Data
for 3 are given for comparison.
À
silyne showing a short Si C bond corresponding to a triple
bond, despite the coordination of phosphine ligand.[13]
Although this new species 2 is stable at room temperature
under inert atmosphere for weeks, it shows a high reactivity
with carbon dioxide. Indeed, the reaction of 2 with CO2 is
complete upon mixing to afford the aminosilicate 5 with two
pentacoordinate silicon atoms (78% yield of isolated product,
Scheme 3). During the reaction clear gas evolution was
observed, suggesting the formation of CO gas.
IIa
IIb
2
3
4[a]
À
Si Si [ꢃ]
2.413
1.874
2.393
1.939
2.331
2.327
2.369
1.826
À
Si L [ꢃ]
2.319
Two singlets at d = À6.5 and À7.5 ppm in the 31P NMR
spectrum suggest the formation 5 as a pair of two diastereo-
mers. The pentacoordination of silicon centers[4b] is clearly
indicated in the 29Si NMR spectrum by significant upfield
shifts of signals (d = À100.7 and À101.6 ppm) appearing as
two doublets of doublets (JSiP = 7.7 and 121.9 Hz and 7.6 and
120.2 Hz). The large phosphorus–silicon coupling constants
[a] TMS=Me3Si.
À
The P Si bond lengths (2.327 ꢀ) are typical for single bonds
and are slightly greater than that observed for phosphonium
silaylide 3 (2.319 ꢀ),thus indicating the absence of P Si
p interaction. The silicon centers in 2 are less pyramidalized
(ꢀ8Si = 3308) than that in monophosphonium ylide 3 (ꢀ8Si =
2998)[7] and even less than those in 4 with diketenimine
character (ꢀ8Si = 3198).[11]
À
À
are in good agreement with strong P Si interactions. Owing
To gain more insight into the properties of the molecule,
DFT calculations were performed on the real compound 2.
The computation at the bHandH/6-311 + G* level reasonably
reproduces the experimental geometry and structural param-
À
À
À
eters (Si Si 2.323 ꢀ, Si P 2.314 ꢀ). As expected, the Si Si
bond order (1.262)[12] is larger than that expected for Si Si
À
À
single bonds. In contrast, the Wiberg bond index of the Si P
bond is practically unchanged compared with that in mono-
Scheme 3. The reaction of disilyne derivative 2 with CO2. Product 5 is
isolated in 78% yield.
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1093

Communications
to the presence of a mixture of the two diastereomers (50:50),
the X-ray structure of 5 is disordered. Each diastereomer
shows a quasi-C2-symmetric structure, which is consistent
with the observed two singlet signals in the 31P NMR
À
spectrum. The structure reveals the cleavage of the Si Si
À
bond (Si1 Si2 2.4151 ꢀ) and the presence of a central tricyclic
structure (Figure 2). Both silicon centers arrange in a some-
Scheme 4. The reaction of disilyne derivative 2 with CO2 in the
presence of {RhI(cod)} complex 6.
Figure 2. Molecular structure of 5. Thermal ellipsoids represent 30%
probability. H atoms are omitted for clarity. Selected interatomic
distances [ꢃ] and angles [8]: Si1–O1 1.7419(12), Si1–O2 1.6651(11),
Si1–O3 1.7170(12), Si1–P1 2.4978(6), Si1–N1 1.7824(14), P1–C2
1.764(3), C2–C3 1.358(3), C3–N1 1.395(3), C1–O3 1.346(2), C1–O4
1.340(2), C1–O5 1.201(2), Si1–Si2 2.4151(6); Si1-O1-Si2 90.38(6), Si1-
O2-Si2 90.44(5), O1-Si1-O2 84.56(6), O1-Si1-O3 96.16(6), O1-Si1-N1
95.56(6), O1-Si1-P1 173.27(5), O2-Si1-O3 103.78(6), O2-Si1-N1
142.39(6), O2-Si1-P1 91.36(4), O3-Si1-N1 113.55(6), O3-Si1-P1
90.01(4), N1-Si1-P1 84.39(5), C1-O3-Si1 123.15(11), C1-O4-Si2
122.67(11), O3-C1-O4 117.20(14), O3-C1-O5 121.42(16), O4-C1-O5
121.38(16).
Scheme 5. Proposed reaction mechanism.
derivatives are known to be labile molecules,[17,18] the
pentacoordinate silaketene 8 could readily release CO to
give the transient oxadisilirene derivative 9. After a similar
process with a second equivalent of CO2, the 2,4-dioxabicyclo-
[1.1.0]tetrasilane derivative 10 could react with a third
equivalent of CO2, leading to 11.[19] Since intermediate 11
cannot be involved in a Peterson type reaction, the last CO-
releasing reaction is probably assisted by the nucleophilic
attack of one of the phosphine ligands, leading to 12 and then
13 after CO elimination. Finally, the highly reactive tricyclic
silicate 13[20] would be trapped by a fourth equivalent of CO2
to give the isolated aminosilicate 5. Compound 5 is highly
stable, and no decomposition was observed even after heating
at 1808C for 12 h.
The stable phosphonium silaylide 3 also rapidly reduces
CO2 at room temperature, producing an original P-chiral
tricyclic phosphine 14 with an oxygen atom bridging silicon
and phosphorus atoms, which was isolated in 32% yield
(Scheme 6). Only one singlet signal was observed in the
31P NMR spectrum (d = 103 ppm), thus indicating the diaste-
reoselective formation of 14 starting from a mixture of
diastereomers of 3 (85:15). A similar observation was made
for the reaction of phosphonium silaylide 3 with acetylenes,
which also leads, by kinetic resolution, to the diastereoselec-
tive formation of P-chiral tricyclic phosphines.[8a] The struc-
ture of 14 was unambiguously confirmed by an X-ray
diffraction analysis (see the Supporting Information).
what distorted trigonal bipyramid with the phosphorus and
oxygen atoms in the apical positions (O1-Si1-P1 173.278,
À
ꢀq8equatorial = 359.78). The long apical P1 Si1 bond (2.498 ꢀ) is
of the same order as that observed for the previously reported
pentacoordinate oxasilirane (2.491 ꢀ).[8b] The apical Si1 O1
À
À
bond (1.742 ꢀ) is much longer than the equatorial ones (Si1
À
O2 1.665 ꢀ, Si1 O3 1.717 ꢀ).
In fact, disilyne 2 reacts with four equivalents of CO2,
formally extracting three oxygen atoms and capturing one
CO2 molecule. To quantify and characterize the released
carbon monoxide (CO), the reaction was performed in the
presence of a slight excess of {RhI(cod)} (cod = cycloocta-l,5-
diene) complex 6. A quantitative ligand-exchange reaction[14]
was detected by 1H NMR spectroscopy, leading to the
corresponding RhI carbonyl complex 7 (Scheme 4).[15] Alter-
natively, the obtained gas was diffused into another Schlenk
tube connected by a metallic bridge and filled with a THF
solution of 6. In this case, the same result was obtained,
although the ligand exchange reaction (6 to 7) takes much
longer (24 h).[15]
Taking into account that the phosphorus center was not
directly involved, we conclude that the reaction probably
proceeds by a Peterson olefination type reaction[16] instead of
a Wittig type reaction (Scheme 5). The addition of the first
equivalent of CO2 probably gives the intermediate 8 featuring
a silaketene and a silaketone function. Since silaketene
In contrast to the case of 2, in which the phosphine ligand
only assists the CO elimination, the reaction of 3 with CO2
involves the phosphine center, probably by a sila-Wittig type
reaction (Scheme 6).[8b] Although the reaction pathway is
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In summary, we have succeeded in the synthesis of the first
stable disilyne bis(phosphine) adduct 2, which has been
isolated and fully characterized. Structural and theoretical
investigations highlight a peculiar ligand effect of phosphine
towards the SiI center compared to other ligands. Indeed, the
phosphine ligands behave as weakly s-donating ligands
without p-accepting character inducing a strong p donor–
acceptor character of the SiI centers. As a consequence, the
À
Si Si fragment in 2 presents a certain multiple-bond charac-
ter. Furthermore, we also described the first non-metal-
mediated direct CO2 reduction to CO at room temperature
using disilyne derivative 2 as well as silaylide 3. Taking into
account the growing interest in the development of new
processes for the efficient utilization of the abundant and
renewable CO2 resource,[22] especially mediated by metal-free
systems,[23,24] the two reactions presented herein are of great
importance. Particularly, the reduction of CO2 to the more
reactive CO by oxygen abstraction is known to be attractive
but difficult in practice owing to the large thermodynamic
input required for the process (532 kcalmolÀ1).[25] Although
several reductions of CO2 to CO have been reported,[26–29] the
direct reduction using nonmetallic reagents at room temper-
ature is still an elusive goal.[30] The research on other
applications of the new disilyne adduct 2 as well as on the
CO production by CO2 reduction using simplified systems are
under active investigation.
À
[17] The weak bond energy of the silylene (H2Si) CO complex (21–
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[18] The reaction between stable silylene (C5Me5)2Si and CO is
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[20] Only a few theoretical investigations have been reported for this
type of molecule: a) W. Schmidt, K. A. Nguyen, M. S. Gordon,
J. A. Montgomery, Jr., J. Am. Chem. Soc. 1991, 113, 5998; b) S.
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Received: October 6, 2010
Published online: December 22, 2010
Keywords: carbon dioxide · phosphorus · silicon · silylenes ·
.
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