Reversible binding of ethylene to silylene-phosphine complexes at room temperature

Article abstract of DOI:10.1002/anie.201105097

On and off: The concerted [2+1] cycloaddition reaction of phosphine-silylene complexes with ethylene affords the corresponding pentacoordinate siliranes. The conversion is strongly related to the ethylene pressure, and the reaction is reversible at room t

Full text of DOI:10.1002/anie.201105097

Communications
DOI: 10.1002/anie.201105097
Siliranes
Reversible Binding of Ethylene to Silylene–Phosphine Complexes at
Room Temperature**
Ricardo Rodriguez, David Gau, Tsuyoshi Kato,* Nathalie Saffon-Merceron, Abel De Cꢀzar,
Fernando P. Cossꢁo, and Antoine Baceiredo*
2
rare.[12] The only known example concerns a stannirane
derivative which is in rapid equilibrium with its stannylene
precursor.[13] Not surprisingly, in the case of siliranes the retro-
cycloaddition requires a higher thermal activation.[12] Fur-
thermore, in contrast to the extremely reactive transient
silylenes,[14] only few examples of cycloaddition reactions
involving a stable silylene, photochemically activated or non-
activated, with an alkene have been reported.[15]
Recently we described the synthesis of the first stable
phosphonium sila-ylides 1[16] with an enhanced silylenoid
character. In particular, they react with carbonyl compounds
in a barrier-free concerted [2+1] cycloaddition reaction.[17]
Extrapolating this result, we report herein on the interaction
of 1a,b with the much less reactive ethylene gas (Scheme 1).
=
Since the discovery of Zeiseꢀs salt [Cl3Pt(h -CH2 CH2)]K in
1825,[1] the binding in p-ethylene transition-metal complexes
rationalized by Dewar–Chatt model[2] has been recognized as
one of the most fundamental coordination modes;[3] its labile
character allows the reversible process (dissociation–associ-
ation)[4] and is essential for various catalytic reactions. The
p backdonation (dmetal!p*ethylene) associated with the Lewis
acidic character (pethylene!dmetal) is one of the key features of
transition metals,[2b,c,5] which further distinguish their coordi-
nation properties and reactivity from those of non-metal
Lewis acids.
A few types of main-group-element species, presenting
small energy splitting of the valence orbitals, efficiently
interact with nonpolarized poorly reactive molecules[6] such as
ethylene. The bulky borane–phosphine combination known
as a “frustrated Lewis pair”[7] irreversively forms a zwitter-
ionic adduct with ethylene, in which the interaction of boron-
centered Lewis acid is assisted by the Lewis base compo-
nent.[8] The heavier Group 14 element analogues of alkynes
with two nondegenerate p bonds, owing to their trans bent
structure, also react with ethylene by means of a [2+1]
cycloaddition reaction.[9] Remarkably, the stable distannyne
reacts with two ethylene molecules, resulting in the formation
of a bicyclic ethylene bis-adduct and the reactions are
Scheme 1. The reversible reactions of silylene–phosphine complexes
1a,b with ethylene.
reversible.[10] The thermal lability of Sn C s bond, a major
ꢀ
factor for the reversibility, was elegantly demonstrated by the
much higher thermal stability of the germanium analogue. In
addition to these new systems, the concerted [2+1] cyclo-
addition is a typical reaction for divalent species.[11] In the case
of singlet species, the filled and unfilled orbitals, with s and p
symmetry respectively, interact with the valence orbitals of
ethylene in the cycloaddition process. It is interesting to note
that retro-cycloadditions, at room temperature, are extremely
Phosphonium sila-ylides 1a,b react with ethylene at room
temperature to afford the corresponding pentacoordinate
siliranes 2a,b. The conversion of the reaction is strongly
dependent on the ethylene pressure (80% conversion with
10 bar; Table 1a). The formation of silirane 2a was clearly
indicated, in its 29Si NMR spectrum, by a high-field doublet
1
(d = ꢀ68.6 ppm, JPSi = 24.0 Hz), and inequivalent signals in
1
13C and H NMR spectra due to the adjacent chiral silicon
center. Particularly, the two doublets in the 13C NMR
spectrum at ꢀ0.06 and ꢀ1.68 ppm (2JPC = 14.3 and 30.6 Hz)
are in agreement with a silirane structure rather than a h2-
ethylene complex. The 31P NMR signal at 75 ppm, which is in
the region between the signals of the free phosphine (d =
116 ppm) and the previously reported sila-oxirane analogue
[*] Dr. R. Rodriguez, Dr. D. Gau, Dr. T. Kato, Dr. A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS, LHFA UMR5069
31062 Toulouse Cedex 9 (France)
E-mail: baceired@chimie.ups-tlse.fr
Homepage: http://hfa.ups-tlse.fr
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse (France)
ꢀ
(d = 40 ppm), suggests a weaker Si P interaction. More
interestingly, the reaction is reversible. This was clearly
Dr. A. De Cꢁzar, Prof. F. P. Cossꢂo
Universidad del Paꢂs Vasco/Euskal Herriko Unibertsitatea
San Sebastian–Donostia (Spain)
Table 1: a) Proportions of silylene–phosphine complex 1a and silirane
2a at different ethylene pressures. b) Equilibrium constants (K2-1) at
room temperature; NA=not available.
[**] We are grateful to the CNRS, the ANR (NOPROBLEM), and Ingenio-
Consolider (CSD2007-00006) for financial support of this work. We
also thank the SGI/IZO-SGIker UPV/EHU for allocation of compu-
tational resources.
a) C2H4 [bar]
1
3
8
10 b) Cmpd. 1a
1b
1c
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105097.
1a/2a
25:1 4:1 1:1 1:4 K2-1 1.147 0.142 NA
10414
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10414 –10416

demonstrated by the regeneration of the phosphonium sila-
ylide 1a after a decrease in ethylene pressure. The ethylene
adduct 2a is stable up to ꢀ308C in solution, and in the solid
state at room temperature for weeks. Silirane 2c (R = Ph) was
obtained by generating the unstable phosphonium sila-ylide
1c in the presence of ethylene gas (Scheme 2). In contrast to
2a,b, the ethylene adduct 2c is perfectly stable at room
temperature and no back reaction was observed even at
elevated temperatures (in toluene at 1008C).
A vanꢀt Hoff analysis of the results of variable-temper-
ature 31P NMR spectroscopy affords a small value of Gibbs
free energy for the addition reaction of 1a to ethylene
(DG208C = (ꢀ0.717 ꢁ 0.452) kcalmolꢀ1) showing the thermo-
neutrality of the reaction; this is in strong contrast to the
generally highly exothermic cycloaddition reactions of sily-
lenes with an olefin. This result is consistent with the
reversible nature of the reaction. The value is also in good
agreement with that theoretically predicted (DG =
ꢀ2.04 kcalmolꢀ1). The equilibrium constant for the reversible
addition of ethylene to sila-ylides (1a–c) is strongly depen-
dent on the type of phosphine ligand (Table 1b). Particularly
in the case of phosphonium sila-ylide 1c, which has a weaker
nucleophilic diphenylphosphine ligand, the reaction results in
the formation of a perfectly stable silirane 2c. Indeed, the fact
ꢀ
that Si P bond in 2c (3.323 ꢁ) is slightly longer than that in
[18]
Scheme 2. The synthesis of silirane 2c.
ꢀ
2a suggests a weaker P Si interaction (Figure 1b).
The
calculations for the formation of 2c reveal a more exothermic
reaction (DG = ꢀ6.63 kcalmolꢀ1). These results are consistent
with the previously reported ligand-dependent thermal sta-
bility of the neutral pentacoordinated silicon species, in their
ligand coupling reactions.[20]
We have calculated the transition structures (TSa, TSc)
associated with the formation of adducts 2a,c. Both saddle
points were fully optimized and characterized at the M06-
(PCM)/6-31G*//BHandH(PCM)/6-31G* level of theory in
THF solution[21–23] (see the Supporting Information for addi-
tional details). The main features of these structures are
Colorless crystals of silirane 2a were obtained from a cold
pentane solution (ꢀ608C), and the structure was unambigu-
ously confirmed by X-ray analysis (Figure 1a).[18] The mole-
cule shows a strongly distorted trigonal–bipyramidal (TBP)
geometry around the pentacoordinate silicon center with the
phosphorus atom and one methylene group at the apical
positions (P-Si-C5 = 147.08). The sum of the equatorial bond
angles around the silicon atom (358.58) is close to the ideal
ꢀ
value for the TBP geometry. The apical and equatorial Si C
ꢀ
ring bonds are almost identical with values typical for s bonds
in a three-membered ring.[12b,19] This is probably a result of the
weak phosphorus–silicon interaction and the steric conges-
summarized in Figure 2. In the case of TSa, the P Si bond
index is found to be 0.54, whereas in the case of TSc the
corresponding value is 0.36. This is probably due to the lower
nucleophilicity of the diphenylphosphino moiety, leaving the
silylene moiety of 1c more reactive toward the [2+1] process.
Indeed, the activation energy associated with the 1c!2c
process is roughly 5.0 kcalmolꢀ1 lower than that associated
with the formation of 2a from 1a. In addition, the large
negative volumes of activation (ꢀ180.3 bohr3 molꢀ1 and
ꢀ229.4 bohr3 molꢀ1 for TSa and TSc, respectively) as well as
the relatively high computed activation energies are in
qualitative agreement with the requirement of high ethylene
ꢀ
tion, which is consistent with a substantially long P Si
ꢀ
distance (3.273 ꢁ). This P Si bond is much longer than that
observed in the related pentacoordinate sila-oxirane
(2.58 ꢁ).[17]
Figure 1. Molecular structures of a) 2a and b) 2c. Thermal ellipsoids
represent 30% probability. H atoms are omitted for clarity. Selected
bond lengths [ꢃ] and angles [8]: 2a: P–Si 3.2733(11), Si–C4 1.8341(19),
Si–C5 1.8354(18), C4–C5 1.570(3), Si–C3 1.8595(18), Si–N1
1.7475(13), N1–C2 1.4075(19), C1–C2 1.364(2), C1–P 1.8336(17); N1-
Si-C4 121.85(9), N1-Si-C5 113.61(8), C4-Si-C5 50.66(9), N1-Si-C3
115.83(7), C4-Si-C3 119.48(9), C5-Si-C3 118.44(9); 2c: P–Si 3.3234(8),
Si–C4 1.825(2), C4–C5 1.576(3), Si–C5 1.828(2), Si–C3 1.8559(19),
N1–Si 1.7400(15), P–C1 1.8031(19), C1–C2 1.348(2), C2–N1 1.407(2);
N1-Si-C3 116.36(8), C4-Si-C5 51.13(11), N1-Si-C4 122.85(9), C4-C5-Si
64.34(12), N1-Si-C5 114.68(10), C4-Si-C3 117.89(10), C5-Si-C3
116.80(10).
Figure 2. Fully optimized (M06(PCM)/6-31G*//BHandH(PCM)/6-31G*
level of theory, THF solution) transition structures TSa and TSc leading
to adducts 2a and 2c, respectively. Hydrogen atoms have been
omitted for clarity. Activation Gibbs energies have been computed at
298 K and are given in kcalmolꢀ1. Bond lengths are given in ꢃ.
Numbers in parentheses correspond to the NBO bond indices.
Angew. Chem. Int. Ed. 2011, 50, 10414 –10416
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www.angewandte.org 10415

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In conclusion, we successfully demonstrated the reversible
reaction of the stable phosphonium sila-ylides with ethylene
at room temperature. Indeed, phosphine–silylene complexes
1, featuring nucleophilic silylenoid character, react with
ethylene under mild conditions to give the corresponding
pentacoordinate siliranes 2. The stability of the resulting
pentacoordinate sila-cyclopropanes was found to be strongly
related to the nucleophilic character of the phosphine ligand.
Further studies on the application of this unique reversible
reaction are underway.
[18] For crystal data for 2a and 2c see the Supporting Information.
CCDC 833279 (2a), 833280 (2c) contains the supplementary
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free of charge from The Cambridge Crystallographic Data
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Received: July 20, 2011
Published online: September 14, 2011
Keywords: cycloaddition · phosphorus · silicon · silylenes ·
.
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Angew. Chem. Int. Ed. 2011, 50, 10414 –10416