Isolation of a stable, acyclic, two-coordinate silylene

Article abstract of DOI:10.1021/ja301091v

The synthesis and characterization of a stable, acyclic two-coordinate silylene, Si(SArMe₆)₂ [ArMe₆ = C ₆H₃-2,6(C₆H₂-2,4,6-Me ₃)₂], by reduction of Br₂

Full text of DOI:10.1021/ja301091v

Communication
pubs.acs.org/JACS
Isolation of a Stable, Acyclic, Two-Coordinate Silylene
Brian D. Rekken,† Thomas M. Brown,† James C. Fettinger,† Heikki M. Tuononen,*,‡
and Philip P. Power*,†
†Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
‡Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland
̈
̈
̈
̈
S
* Supporting Information
Scheme 1. Synthesis of Silylene 2 by Reduction of 1
ABSTRACT: The synthesis and characterization of a
Me6
stable, acyclic two-coordinate silylene, Si(SArMe )2 [Ar
=
6
C6H3-2,6(C6H2-2,4,6-Me3)2], by reduction of Br2Si-
(SArMe )2 with a magnesium(I) reductant is described. It
6
features a V-shaped silicon coordination with a S−Si−S
angle of 90.52(2)° and an average Si−S distance of
2.158(3) Å. Although it reacts readily with an alkyl halide,
it does not react with hydrogen under ambient conditions,
probably as a result of the ca. 4.3 eV energy difference
between the frontier silicon lone pair and 3p orbitals.
The precursor, 1, was obtained by reaction of SiBr4 with
14,15
(LiSArMe )2 in diethyl ether,
and 2 was synthesized by
6
reduction with Jones’s complex, (IMesMg)2 (IMes = [(2,4,6-
trimethylphenyl)NC(CH3)]2CH), in toluene.16,17 The yellow
solution of Br2Si(SArMe )2 became darker, with concomitant
6
or several decades, silylenes, the silicon analogues of
Fcarbenes, had been known only as transient species either
in the gas phase, in solution, or trapped in frozen matrices.1−3
In 1986, however, Jutzi and co-workers reported the isolation of
decamethylsilicocene, which was the first monomeric, divalent
silicon (II) compound that was stable at room temperature.4
The formally 10-coordinate Si(η5-C5Me5)2 exists as two
conformers: a centrosymmetric species with parallel C5Me5
rings or a bent form with a 25.3° interplanar angle. In 1994, the
synthesis and structure of a two-coordinate, N-heterocyclic
silylene, in which silicon is single bonded to two nitrogens, was
reported by West and co-workers.5 Currently, numerous stable
divalent silicon species are known,2,6−8 but strictly two-
coordinate species invariably involve silicon as part of a ring,9
with the most common being the aforementioned N-
heterocyclic silylenes.10 A cyclic alkyl silylene, which is stable
at 0 °C, was reported by Kira, but it isomerizes in solution via a
1,2-migration of an adjacent trimethylsilyl group to give a
silene.11 Other examples include the bisamido derivative
Si(NPri2)2, which exists in a monomer/dimer equilibrium
with the SiSi double-bonded disilene {Si(NPri2)2}2,12 as well as
Si{N(SiMe3)2}2, which persists for more than 12 h at −20 °C
but decomposes into a complex mixture of products at
precipitation of IMesMgBr, upon stirring for 2 days at ca. 25
°C. Workup afforded colorless crystals of 2 in moderate yield
(51%). The silylene, 2, was found to be stable up to 146 °C.
The X-ray crystal structure of 2 (Figure 1) showed that the
Figure 1. Thermal ellipsoid (30%) plot of 2 without H atoms. Selected
bond lengths (Å) and angles (°): Si(1)−S(1) 2.1607(5), Si(1)−S(2)
2.1560(5), S(1)−C(1) 1.7916(13), S(2)−C(25) 1.7902(13), Si(1)−
centroid(1) 3.453, Si(1)−centroid(2) 3.419, S(1)−Si(1)−S(2)
90.519(19), C(1)−S(1)−Si(1) 100.85(5), C(25)−S(2)−Si(1)
105.01(4).
increased temperatures.13 We now report that the reduction
Me6
of the silicon(IV) precursor, Br2Si(SArMe )2 (1) [Ar = C6H3-
6
silicon bonds to two thiolate sulfurs with a S−Si−S angle of
90.519(19)°. The Si−S distances are 2.1607(5) and 2.1560(5)
Å, and the Si−S−C angles are 100.85(5) and 105.01(4)°. The
closest other approaches to silicon involve C(7) and C(12) at
3.004(1) and 3.232(1) Å as well as C(31) and C(32) at
2,6(C6H2-2,4,6-Me3)2), affords the silicon dithiolate, Si-
(SArMe
)
(2), which has a monomeric, two-coordinate
6
2
structure and is an example of a stable acyclic two-coordinate
silylene. In addition, the Si(IV) bisthiolatosilane 3 was prepared
in order to compare its structural and spectroscopic parameters
with those of 2. Furthermore, 2 was characterized by its
derivatization with MeI to afford Si(Me)(I)(SArMe )2 (4). An
6
Received: February 2, 2012
Published: March 23, 2012
overview of the synthesis is given in Scheme 1.
© 2012 American Chemical Society
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Table 1. Selected Bond Lengths (Å) and Angles (°) for the Experimental and Calculated Structural Data for 1−3
1
2
3
exptl
exptl
calcd
exptl
calcd
a
a
a
a
Si−S
S−C
Si−centroid
S−Si−S
C−S−Si
2.113(1)
1.791(2)
2.158(3)
1.791(2)
2.153
1.775
3.606
90.04
104.36
2.139(1)
1.787(3)
2.143
1.778
3.649
95.74
107.05
a
a
a
a
4.452
3.431
3.546
a
100.05(2)
110.44(8)
90.52(2)
93.5(0.9)
a
a
a
102.9(2.1)
107.8(1.5)
a
Average.
3.052(1) and 3.293(1) Å, respectively. The Si−centroid
distances to the rings are 3.453 and 3.419 Å.
The Si−S bonds are mainly single in character, as indicated
by the calculated Wiberg bond index of 0.95 for the Si−S
bonds,18 although weak Si−S π-bonding may exist, as indicated
by computations (vide infra) and the low average torsion angle
(13.4(2.1)°) between the coordination plane of silicon and
those at the sulfurs.7a,19,20 The Si−S distances in 2 are longer
than those reported for the bisthiolatosilylene platinum
complex, trans-(Cy3P)2Pt(H)Si(SEt)2OTf, by ca. 0.07 Å,7a
but are comparable to those predicted for 1,2-ethanedisulfide
silylene.20 DFT calculations,21 using the hybrid PBE1PBE
exchange-correlation functional22 in combination with the def2-
TZVP basis set for the whole molecule of 2 and 3,23 afforded
structural parameters very close to those experimentally
measured (Table 1).
The 29Si NMR spectrum of 2 revealed a downfield signal at δ
= 285.5 (cf. δ = −23.1 for 1). The signal is farther downfield
than those of cyclic amidosilylenes (δ = 78−119),5,10 Driess’s
ylide-stabilized silylenes (δ = 212.4, 213.3),17 and the thermally
unstable silylene, Si{N(SiMe3)2}2 (δ = 223.9);13 however, it is
well upfield of that reported for the dialkylsilylene (δ = 567).11
The downfield shift of 2 is consistent with a two-coordinate
silicon since an increased coordination number produces a
significant upfield shift, as observed in (C6H3-2,6(C6H2-2,4,6-
Pri3)2)Si(η5-C5Me5) (δ = 51.6) (four-coordinate silicon)24 and
decamethylsilicocene (δ = −577) (10-coordinate silicon).4,25
The 1H and 13C NMR spectra suggest free rotation around the
C−S bond due to the observation of only two signals for the o-
and p-methyl groups of the flanking arene rings.
The electronic transitions were calculated by the TD-DFT
approach using the same functional−basis set combination as
employed in the geometry optimization.21−23 The Kohn−Sham
orbitals for 2 are shown in Figure 2. The calculations reveal
several excitations at wavelengths between 250 and 400 nm,
although only the five strongest predicted absorptions are
discussed here. The calculated values may be compared to the
four transitions observed in the electronic spectrum. The
HOMO−LUMO (silicon n→3p) absorption appears as a
shoulder at 382 nm (ε = 8300 M−1 cm−1).26 A more intense
absorption at 318 nm (ε = 23 000 M−1 cm−1) (HOMO−1→
LUMO) corresponds to a transition from a sulfur lone pair to
the silicon 3p orbital. The calculated absorption at 296 nm
corresponds to the HOMO→LUMO+1 transition but is partly
obscured by other absorptions that are close in energy. It arises
from transitions between the silicon lone pair and the arene π*
orbitals. The absorptions at 291 (ε = 20 000 M−1 cm−1) and
269 nm (ε = 25 000 M−1 cm−1) are due to an arene π-to-silicon
p transition and transitions from both silicon and sulfur lone
pairs to arene π* orbitals, respectively. A series of very intense
absorptions, centered at ca. 220 nm, correspond to arene π→π*
transitions that partially mask the absorptions at 291 and 269
Figure 2. Kohn−Sham molecular orbitals for 2 (orbital energies are
given in eV).
nm. Overall, the experimental and computational values for the
spectral data are in good agreement (Supporting Information).
The bisthiolatosilane, 3 (Figure 3), was obtained from
H2SiCl2 and (LiSArMe )2. The structural data reveal a relatively
6
close resemblance between the structural parameters for the
Si[S(C-ipso)]2 moiety and those of silylene 2, with very similar
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Journal of the American Chemical Society
Communication
calculated electronic spectra of 2; computational results on the
models of 2 and 3; tables of crystallographic data for 1−3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
power@chem.ucdavis.edu, heikki.m.tuononen@jyu.fi
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 3. Thermal ellipsoid (30%) drawing of one of the
crystallographically independent molecules of 3. Selected bond lengths
(Å) and angles (°): Si(1)−S(1) 2.1372(6), Si(1)−S(2) 2.1403(6),
S(1)−C(1) 1.7859(15), S(2)−C(25) 1.7892(16), Si(1)−centroid(1)
3.500, Si(1)−centroid(2) 3.591, S(1)−Si(1)−S(2) 94.41(2), C(1)−
S(1)−Si(1) 104.97(5), C(25)−S(2)−Si(1) 108.79(5).
We are grateful to the U.S. National Science Foundation
(CHE-09848417), the Academy of Finland, University of
Jyvaskyla, and the Technology Industries of Finland Centennial
̈
̈
Foundation Fund for financial support.
REFERENCES
■
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