The isolable cation radical of disilene: Synthesis, characterization, and a reversible one-electron redox system

Article abstract of DOI:10.1021/ja801761w

The highly twisted tetrakis(di-tert-butylmethylsilyl)disilene 1 was treated with Ph₃C⁺·BAr₄^- (BAr₄^-: TPFPB = tetrakis(pentafluorophenyl)borate) in toluene, producing disilene cation radical 3 upon one-electron oxidation. Cation radical 3 was isolated in the form of its borate salt as extremely air- and moisture-sensitive red-brown crystals. The molecular structure of 3 was established by X-ray crystallography, which showed a highly twisted structure (twisting angle of 64.9°) along the central Si-Si bond with a bond length of 2.307(2) A, which is 2.1% elongated relative to that of 1. The cation radical is stabilized by σ-π hyperconjugation by the four tBu2MeSi groups attached to the two central sp²-Si atoms. An electron paramagnetic resonance (EPR) study of the hyperfine coupling constants (hfcc) of the ²⁹Si nuclei indicates delocalization of the spin over the central two Si atoms. A reversible one-electron redox system between disilene, cation radical, and anion radical is also reported. Copyright

Full text of DOI:10.1021/ja801761w

Published on Web 04/19/2008
The Isolable Cation Radical of Disilene: Synthesis,
Characterization, and a Reversible One-Electron Redox
System
Shigeyoshi Inoue, Masaaki Ichinohe, and Akira Sekiguchi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of
Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received March 10, 2008; E-mail: sekiguch@chem.tsukuba.ac.jp
Chart 1
Alkenes with a CdC double bond are the simplest π-system
compounds, and the cation radical or anion radical formed by one-
electron oxidation or reduction are highly reactive intermediates
in various organic reactions.1 The π-electron redox system provides
a significant contribution to various applications, for example, in
electrochemistry, material sciences, and polymer chemistry. In
general, the alkene cation radical is stabilized by electron-donating
substituents,2 whereas the alkene anion radical is stabilized by
electron-withdrawing substituents.3 Therefore, this hampers the
preparation of both cation radical and anion radical from the same
parent in alkene systems.
In contrast to alkenes, disilenes with a SidSi double bond are
known to readily undergo oxidation or reduction because of their
high-lying HOMOs and low-lying LUMOs compared with those
of the corresponding alkene analogues,4 which makes possible the
construction of a one-electron reversible redox system. There are
some examples of the one- or two-electron reduction of disilenes.5,6
However, there is no precedent for the one- or two-electron
oxidation of a disilene to form the cation radical or dication,
respectively.7 The disilene cation radical is very interesting, being
not only a silyl cation8 but also a silyl radical.9 We report here the
isolation and structural characterization of the isolable cation radical
of a disilene, representing the first isolable example of a heavy
alkene analogue. In addition, an interesting reversible one-electron
redox system is also reported.
Scheme 1
Scheme 2
We recently reported the synthesis of a highly twisted tetrakis(di-
tert-butylmethylsilyl)disilene (1),5b (tBu2MeSi)2SidSi(SiMetBu2)2,
which showed a characteristic intense blue color as the result of
the small HOMO-LUMO energy gap.10 Indeed, disilene 1 readily
undergoes one-electron reduction by tBuLi to give an isolable anion
radical 2 (for the synthesis of 2, see Scheme 2),5b,10 which has a
highly twisted central Si-Si bond (twisting angle 88°). We have
also used disilene 1 for the synthesis of the cation radical because
Single crystals of 3 were grown from concentrated benzene
solution. The crystal structure of 3 was determined by X-ray
crystallography (Figure 1).11 The shortest distance between the
borate ion and the central silicon atoms of 3 is greater than 5.9 Å,
indicating no interactions between them. The most peculiar
structural features are the central Si-Si bond and its twisting angle.
Thus, upon oxidation, this bond became highly twisted (64.9°) and
2.1% stretched (2.307(2) Å), compared with that of the starting
disilene 15b (54.5°, 2.2598(18) Å). This phenomenon should
certainly be ascribed to a decrease in the bond order after the one-
electron oxidation. The geometry around the Si1 and Si1#1 atoms
is nearly planar. This is in marked contrast to the disilene anion
radical 2 (twisting angle 88°, the central Si-Si bond 2.341(5) Å);5b
the unpaired electron and negative charge in 2 are in the
diagnostically planar geometry of one of the central Si atoms
(radical center) and distinct pyramidality of the other central Si
atom (anionic center).
t
it has four Bu2MeSi groups on the sp2-Si atoms, and hence the
resulting cation radical could be stabilized by σ-π hyperconju-
gation of the four silyl groups, considering either perpendicular
structure A or twisted structure B (Chart 1). Moreover, the central
SidSi bond in 15b is highly twisted (54.5°) as a result of steric
reasons, which may facilitate such hyperconjugation.
As expected, disilene 1 readily undergoes chemical oxidation
-
with Ph3C+ ·BAr4 (BAr4-: TPFPB ) tetrakis(pentafluorophe-
-
nyl)borate). When a mixture of 1 and 1.1 equiv of Ph3C+ ·BAr4
in toluene was stirred at room temperature over 30 min, it formed
two liquid phases, accompanied with a color change from the dark
blue of 1 to dark brown.11 The lower layer was separated and
washed with benzene/hexane to remove neutral materials. The
disilene cation radical 3 was isolated as its borate salt in 65% yield
as air- and moisture-sensitive red-brown crystals (Scheme 1). The
structure of 3 was unequivocally characterized by X-ray crystal-
lographic analysis and EPR spectroscopic data.11
The structure of the ethylene cation radical is predicted to be
twisted by 30° because of the contributions from both the planar
structure (π-bonding) and the perpendicular structure (σ-π hy-
perconjugation of C-H bonds).12 Experimentally, Kochi et al.
reported the twisted structures of cation radicals of sesquihomoada-
9
6078 J. AM. CHEM. SOC. 2008, 130, 6078–6079
10.1021/ja801761w CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
material 1 by reduction and oxidation processes, respectively,
making a fully reversible redox system. Thus, the one-electron
reduction of 3 with KC8 in toluene proceeds rapidly to form 1
(Scheme 2). On the other hand, the one-electron oxidation of 2
with Ph3C+ ·BAr4- in toluene also cleanly formed 1.
Acknowledgment. We are grateful for Grants-in-Aid for
Scientific Research (Nos. 18064004, 19105001, 19020012,
19022004, 20038006) from the Ministry of Education, Science,
Sports, and Culture of Japan.
Supporting Information Available: Experimental procedures
and EPR spectrum of 3, table of crystallographic data including
atomic positional and thermal parameters for 3 (PDF/CIF), com-
putational results of 4 and 5, and molecular orbitals (SOMO) of 3
and 5. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 1. ORTEP drawing of 3•2(C6H6) (30% probability level). #1
indicates the following symmetry transformations: #1 ) -x, -y + 1/2, z.
Hydrogen atoms and benzene molecules as crystal solvent are omitted for
t
clarity. Bu2MeSi groups on the Si1 atom were disordered with 50:50
probability and one of them is shown. Selected bond lengths (Å): Si1-Si1#1
) 2.307(2), Si1-Si2 ) 2.463(2), Si1-Si3 ) 2.466(2). Selected bond angles
(deg):Si1#1-Si1-Si2)128.25(5),Si1#1-Si1-Si3)111.09(6),Si2-Si1-Si3
) 120.10(10).
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2
compared with that of 4 (2.159 Å). The Si(sp )-SiMe3 bond lengths
of 5 are also stretched by 1.1% relative to 4 due to the hypercon-
jugation.
The EPR spectrum of 3 measured at 298 to 200 K in
fluorobenzene showed a strong signal with a g value of 2.0049,
accompanied by a pair of satellite signals (2.30 mT) due to coupling
of the unpaired electron with the central 29Si nuclei. The magnitude
of hfcc(R-29Si) of 2.30 mT is less than half that of the similar
per(silyl)silyl radical (tBu2MeSi)3Si• (5.80 mT),14 implying delo-
calization of the unpaired electron over both Si1 and Si1#1 atoms
in 3. The behavior of the cation radical 3 in solution contrasts with
that of anion radical 2; the latter showed a rapid spin exchange
between the two central Si atoms on the EPR time scale.5b These
differences assumed that cation radical 3 possesses a delocalized
system, whereas anion radical 2 features an sp3-silyl anion and an
sp2-silyl radical.
(10) The cyclic voltammogram of disilene 1 in THF shows an irreversible
oxidation wave at +0.41 V and a reversible reduction wave at -1.47 V,
corresponding to the formation of disilene cation radical and disilene anion
radical, respectively.
(11) For the experimental procedures, EPR spectrum, and crystal data, see the
Supporting Information.
(12) (a) Mulliken, R. S.; Roothaan, C. C. J. Chem. ReV 1947, 41, 219. (b)
Mulliken, R. S. Tetrahedron 1959, 5, 253.
(13) Optimization of model compounds 4 and 5 was performed at the (U)B3LYP/
6-31G(d) level; see the Supporting Information.
(14) Sekiguchi, A.; Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Ichinohe, M. J. Am.
Chem. Soc. 2002, 124, 9865.
Finally, it is particularly interesting that cation radical 3 and anion
radical 2 could be quantitatively converted back to the starting
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J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008 6079