Reversible Silylene Insertion Reactions into Si?H and P?H σ-Bonds at Room Temperature

Article abstract of DOI:10.1002/anie.201606728

Phosphine-stabilized silylenes react with silanes and a phosphine by silylene insertion into E?H σ-bonds (E=Si,P) at room temperature to give the corresponding silanes. Of special interest, the process occurs reversibly at room temperature. These results demonstrate that both the oxidative addition (typical reaction for transient silylenes) and the reductive elimination processes can proceed at the silicon center under mild reaction conditions. DFT calculations provide insight into the importance of the coordination of the silicon center to achieve the reductive elimination step.

Full text of DOI:10.1002/anie.201606728

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International Edition: DOI: 10.1002/anie.201606728
German Edition: DOI: 10.1002/ange.201606728
Reactive Intermediates
ꢀ
ꢀ
Reversible Silylene Insertion Reactions into Si H and P H s-Bonds at
Room Temperature
Ricardo Rodriguez, Yohan Contie, Raphael Nouguꢀ, Antoine Baceiredo,* Nathalie Saffon-
Merceron, Jean-Marc Sotiropoulos, and Tsuyoshi Kato*
Abstract: Phosphine-stabilized silylenes react with silanes and
species.[10] However, such oxidative addition reactions are not
reversible since the reverse process (reductive elimination),
which generates the less stable divalent silylenes, usually
requires a thermal activation.[11] Nevertheless, Tobita et al.[12]
recently reported the reversible reaction of NHC-supported
(NHC = N-heterocyclic carbene) cationic metallogermylene
with silanes, and Aldridge, Jones, and co-workers reported
that diborylstannylene can undergo oxidative additions with
ꢀ
a phosphine by silylene insertion into E H s-bonds (E = Si,P)
at room temperature to give the corresponding silanes. Of
special interest, the process occurs reversibly at room temper-
ature. These results demonstrate that both the oxidative
addition (typical reaction for transient silylenes) and the
reductive elimination processes can proceed at the silicon
center under mild reaction conditions. DFT calculations
provide insight into the importance of the coordination of the
silicon center to achieve the reductive elimination step.
ꢀ
ꢀ
both N H and O H bonds, with subsequent reductive
eliminations.[9]
Recently, we reported that phosphine-supported SiII
complexes exhibit unusual stability, which is comparable to
that of SiIV derivatives, in spite of their enhanced silylenoid
character. This peculiarity of SiII complexes was highlighted
by the reversible [2+1] cycloaddition reaction with ethylene
at room temperature.[13] With stannyl/SiII and hydride/SiII
O
xidative addition and reductive elimination are exceed-
ingly important processes in organometallic chemistry. The
transition metal mediated bond activation is closely related to
the oxidative addition step, while the reductive elimination is
a crucial step for the formation of new bonds, and both
processes are essential in many catalytic reactions.
ꢀ
complexes, the activated alkene readily inserts into Si H
Although for many years it was believed that only
transition-metal centers could activate enthalpically stable
bonds, several nonmetallic systems are known to be capable
of performing these tasks.[1] Notably, frustrated Lewis pairs[2]
(-Sn) bonds, thus generating new SiII complexes.[14] Of
particular interest, the ethylene insertion reaction is perfectly
II
ꢀ
reversible at room temperature in the case of the Si Sn
complex.[15] Herein we report an unprecedented reversible
[3]
are able to reversibly cleave H2, and therefore are active
oxidative addition/reductive elimination of silanes and phos-
II
catalysts for hydrogenation reactions.[4] Stable divalent spe-
cies such as singlet carbenes and their heavier analogues can
also activate small molecules at the nonmetal metallylene
center by an oxidative addition.[1b,4,5] Indeed, stable silylenes
react with H2,[7] NH3,[8,9] and silicon hydrides under mild
reaction conditions, thus leading to the corresponding SiIV
phines at the silylene center of the Si H complexes 1.
ꢀ
II
ꢀ
The Si H complex 1A readily reacts with phenylsilane
(a) at room temperature to give the corresponding disilane
2A-a, which was isolated as colorless crystals in 49% yield
(Scheme 1). The presence of a disilane fragment is clearly
[*] Dr. R. Rodriguez, Dr. Y. Contie, R. Nouguꢀ, Dr. A. Baceiredo,
Dr. T. Kato
Universitꢀ de Toulouse, UPS, and CNRS, LHFA UMR 5069
188 route de Narbonne, 31062 Toulouse (France)
E-mail: baceired@chimie.ups-tlse.fr
kato@chimie.ups-tlse.fr
Homepage: http://hfa.ups-tlse.fr
Scheme 1. The synthesis of the SiIV hydrides 2.
Dr. R. Rodriguez
CSIC-Universidad de Zaragoza
Departamento de Quımica Inorgꢁnica
Instituto de Sꢂntesis Quꢂmica y Catꢁlisis Homogꢀnea (ISQCH)
50009 Zaragoza (Spain)
indicated by resonances at d = ꢀ34.1 (JSi-P = 7.3 Hz, NSiH2)
and d = ꢀ60.4 ppm (JSi-P = 44.8 Hz, PhSiH2) in the 29Si NMR
spectrum, and they are within the typical shift range for
amino-[16] and phenyl-substituted[17] disilanes. These chemical
shifts, as well as, the moderate JSiP coupling constants suggest
a weak phosphine–silane interaction in solution. However,
the two hydrogen atoms of the NSiH2 fragment are not
magnetically equivalent. Indeed, the 1H NMR spectrum
displays two signals with large JHP coupling constants [d =
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, and CNRS, ICT FR2599
31062 Toulouse (France)
Dr. J.-M. Sotiropoulos
UNIV PAU & PAYS ADOUR, Institut des Sciences Analytiques
et de Physico-Chimie pour l’Environnement et les Matꢀriaux
IPREM, ECP, UMR 5254, 64053 Pau (France)
5.23 (d-pseudo-t,
5.74 ppm (ddd, JPH = 22.1 Hz, JHH = 5.2 and 4.1 Hz)], thus
JPH = 39.3 Hz, JHH = 4.1 Hz) and d =
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606728.
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Table 2: Silylene (1)/silane (2) molar ratio in the reaction of 1 with
phenylsilane (1 equiv) in C6D6 at different temperatures.
suggesting a chiral pentacoordinate geometry around the
silicon center resulting from the coordination of the phos-
phine. The single-crystal X-ray diffraction analysis of
2A-a reveals a trigonal bipyramidal geometry around the
Si1 center with H1 and P1 atoms in the apical positions (H1-
Si1-P1 176.58; Figure 1).[18] This arrangement was clearly
T [8C]
25
40
50
60
70
19:81
65
80
25:75
75
1A/2A-a[a]
3:97
15
6:94
10:90
14:86
T [8C]
25
35
55
1B/2B-a[a]
61:39
71:29
80:20
90:10
94:6
96:4
[a] Determined by 31P and 1H NMR spectroscopy.
temperature (Table 2). These results taken together clearly
indicate that the reaction is an equilibrium process.
The steric congestion around the silicon center has
a dramatic effect on the ratio between 1 and 2. Indeed, an
increase of the steric hindrance (1B) shifts the equilibrium
toward the starting, less-hindered 1B compared to the
pentacoordinate disilane 2B-a. The same trend is observed
in the reaction of 1Awith the more hindered b, which gives an
increased 1A/2A-b molar ratio. Furthermore, in the case of
the more congested 1B and the more crowded b no traces of
the disilane adduct 2B-b could be detected. However, we can
unambiguously state that 1B and b do interact since the use of
deuterated diphenylsilane (D2SiPh2) led to the formation of
Figure 1. Molecular structure of 2A-a. Thermal ellipsoids shown at
30% probability. Disordered and H atoms (except for that on Si1 and
Si2) are omitted for clarity. Selected bond lengths [ꢃ] and angles [8]:
Si1-P1 3.344(1), Si2-P1 3.396(1), Si1-Si2 2.344(1), Si1-N1 1.753(2), Si2-
C3 1.880(3), P1-C1 1.793(4), C1-C2 1.361(6), N1-C2 1.388(5); N1-Si1-
Si2 121.7(1), Si1-Si2-C3 110.5(1), Si1-N1-C2 126.4(3), N1-C2-C1 131.6-
(5), C2-C1-P1 127.8(4).
II
II
ꢀ
a deuterated Si D complex. Similarly, the Si complexes
1 undergo reversible oxidative additions with diphenylphos-
phine (c) at room temperature.
II
ꢀ
The Si Sn complex 3A reacts with 1 equivalent of
phenylsilane at 408C to give a mixture of disilanes (2A-a,
indicated by the presence of an AIM bond critical point
between P1 and Si1 atoms (for more details, see the
supporting information). As expected, the phosphine
ligand only weakly coordinates to the silicon center as
indicated by the long Si1-P1 distance (3.344 ꢀ) as
compared to other pentacoordinate phosphine-silicon
complexes (2.407–2.987 ꢀ),[19] even though it is shorter
than the sum of van der Waals radii (3.90 ꢀ). The Si1-Si2
bond length (2.344 ꢀ) is slightly longer than those
observed for other disilanes (2.329–2.339 ꢀ).[20]
Scheme 2. Substituent exchange between the SiII-Sn complex 3A and phenyl-
silane by a reversible oxidative addition/reductive elimination process.
Of particular interest, monitoring the reaction by
31P NMR spectroscopy shows that the conversion of
1A-a stops at 97% and an extended reaction time does
not change the ratios (Table 1). In fact, the completion of the
reaction needs the addition of an excess of silane. In contrast,
removal of phenylsilane under vacuum drives the reaction
towards the starting 1A-a. Also, the 1A/2A-a ratio is temper-
ature dependent, and the proportion of 1A increases with
4A, and 5A) and 3A (Scheme 2). This result can be related to
the readily reversible oxidative addition/reductive elimina-
tion reactions (Scheme 3). Although 1A was not detected, by
extrapolating the already obtained results it is reasonable to
suggest its transient formation from 4A by the reductive
elimination of trimethylstannyl(phenyl)silane. The silylene
1A can subsequently react either with Ph3SiH3 or with
ꢀ
ꢀ
ꢀ
Me3Sn SiH2Ph (Si H or Si Sn activation) to give a mixture
of SiIV derivatives (2A-a, 4A,and 5A). The reversible nature
of the reactions is clearly demonstrated by the exclusive
ꢀ
Table 1: Reaction of 1 with different E H reagents (1 equiv, 298 K, C6D6):
silylene (1)/silane (2) ratio, and equilibrium constants (keq2-1
)
[b]
[b]
[b]
ꢀ
1
H E 1/2 (298 K)[a] Keq2-1(298K) DG298K
DH298K
DS298K
ꢀ
formation of 2A-a and Me3Sn SiH2Ph after the addition of an
excess amount of phenylsilane (15 equiv) to this complex
reaction mixture (Scheme 2). In this case, the excess amount
of PhSiH3, relative to Me3Sn SiH2Ph, displaces the equilib-
rium to the formation of 2A-a. These results also demonstrate
that not only E H s-bonds (E = Si, P) but also Si Sn s-bonds
can be reversibly activated by phosphine-stabilized silylene
complexes.
1A
1B
1A
1B
1A
1B
a
a
b
b
c
c
3:97
61:39
18:82
100:n.d.
6:94
5561.3
3.0
27.1
ꢀ5.2
ꢀ0.8
ꢀ1.9
ꢀ11.4
ꢀ10.2
ꢀ12.3
ꢀ21.5
ꢀ32.0
ꢀ35.3
ꢀ
ꢀ
ꢀ
33:67
53.3
ꢀ2.6
ꢀ12.8
ꢀ35.1
[a] Determined by 31P and 1H NMR spectroscopy. [b] In kcalmolꢀ1
.
2
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In conclusion, we have observed the oxidative
ꢀ
ꢀ
addition of Si H and P H s-bonds to the silicon
center of phosphine-stabilized SiII complexes (1)
under very mild reaction conditions. Of particular
interest, the reaction is reversible at room temper-
ature, thus demonstrating that a reductive elimina-
tion can take place at a nonmetallic silicon center
under extremely mild reaction conditions. DFT
calculations indicate that the coordination of a nucle-
ophilic ligand to the silicon center is crucial for the
reductive elimination step. Detailed mechanistic
studies and applications of the reaction are currently
under active investigation.
Scheme 3. Possible reductive elimination reactions.
Usually reductive eliminations to produce low-valent
silicon species from tetravalent silicon derivatives are strongly
endothermic processes. For instance, the reductive elimina-
tion of hydrosilanes forms disilanes to generate transient
silylenes, and requires high temperatures (225–4008C).[11]
Thus, in the case of the phosphine-stabilized silylene com-
plexes 1, the coordination of the silicon center is crucial to
realizing readily reversible oxidative addition/reductive elim-
ination reactions. Furthermore, it is well known that the
disproportionation of chlorodisilanes to polychlorosilanes is
a base-catalyzed reaction in which the nucleophilic catalyst
coordinates to the silicon center, thus inducing the elimina-
tion of chlorosilanes and the formation of transient chloro-
silylenes.[21] To better understand the role played by the
ligand, we have calculated the Gibbs energies of the reactions
between phenylsilane and several silylene complexes (1A and
6A) featuring different ligands.
Acknowledgments
We are grateful to the CNRS and the European Research
Council (ERC Starting grant agreement no. 306658) for
financial support of this work. MCIA (Mꢁsocentre de Calcul
Intensif Aquitain) and IPREM are gratefully acknowledged
for calculation facilities. R.R. acknowledges Ministerio de
Economꢂa y Competitividad of Spain for a Ramꢃn y Cajal
(RYC-2013-13800) grant.
Keywords: density functional calculations ·
reactive intermediates · silylenes · structure elucidation ·
X-ray crystallography
As expected, the results clearly show that the nucleophilic
character of the ligand strongly influences on the energy
balance of the reaction (Figure 2). Indeed, a very strong
nucleophilic ligand, such as an NHC, clearly disfavors the
silylene insertion reaction (6A!7A, DG =+ 6.3 kcalmolꢀ1)
but favors the reductive elimination step (7A!6A). For the
phosphine-silylene complex 1A, the oxidative addition is
predicted to be slightly exothermic (DG = ꢀ6.5 kcalmolꢀ1),
which is in good agreement with the experimental observa-
tion (DG298K = ꢀ5.2 kcalmolꢀ1). In the absence of silicon
coordination (L = H), the oxidative addition is strongly
exergonic (DG298K = ꢀ28.1 kcalmolꢀ1). From these calcula-
tions it is clear that the reductive elimination step is favored
upon increasing the nucleophilicity of the ligand.
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Received: July 11, 2016
Revised: September 1, 2016
Published online: && &&, &&&&
4
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Communications
Reactive Intermediates
New inserts: Phosphine-stabilized sily-
lenes react with silanes (or phosphine) by
ꢀ
R. Rodriguez, Y. Contie, R. Nouguꢁ,
A. Baceiredo,* N. Saffon-Merceron,
J.-M. Sotiropoulos,
silylene insertion into E H s-bonds (E=
Si,P) at room temperature to give the
corresponding silanes. Of special inter-
est, the process occurs reversibly at room
temperature. These results demonstrate
that both the oxidative addition and the
reductive elimination processes can pro-
ceed at the silicon center under mild
reaction conditions.
T. Kato*
&&&& — &&&&
Reversible Silylene Insertion Reactions
ꢀ
ꢀ
into Si H and P H s-Bonds at Room
Temperature
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
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