A new disilene with π-accepting groups from the reaction of disilyne RSi≡SiR (R = SiiPr[CH(SiMe3)2]) with isocyanides

Article abstract of DOI:10.1021/ja212065a

The reaction of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4- diisopropyltetrasila-2-yne (1) with tert-butylisocyanide or tert-octylisocyanide produced the corresponding disilyne-isocyanide adducts [RSiSiR(CNR′) ₂] (R = SiⁱPr[CH(SiMe₃)₂] ₂, R′ = ^tBu (2a) or CMe₂CH ₂^tBu (2b)), which are stable below -30 °C and were characterized by spectroscopic data and, in the case of 2a, X-ray crystallography. Upon warming to room temperature, 2 underwent thermal decomposition to produce 1,2-dicyanodisilene R(NC)Si=Si(CN)R (3) and 1,2-dicyanodisilane R(NC)HSiSiH(CN)R (4) via C-N bond cleavage and elimination of an alkane and an alkene. The 1,2-dicyanodisilene derivative 3 was characterized by X-ray crystallography.

Full text of DOI:10.1021/ja212065a

Communication
pubs.acs.org/JACS
A New Disilene with π-Accepting Groups from the Reaction of
Disilyne RSiSiR (R = SiiPr[CH(SiMe3)2]) with Isocyanides
Katsuhiko Takeuchi, Masaaki Ichinohe, and Akira Sekiguchi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
S
* Supporting Information
Recently, following reports of the isolation of alkyne
analogues of Ge, Sn, and Pb,10 silicon−silicon triply bonded
ABSTRACT: The reaction of 1,1,4,4-tetrakis[bis-
compounds (disilynes) were synthesized by our group11 and
(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne (1)
Wiberg’s group12 using bulky silyl groups, and by Tokitoh’s
with tert-butylisocyanide or tert-octylisocyanide produced
group using bulky aryl groups.13 To understand the nature of
the π-bonding of the silicon−silicon triple bond, we are
currently investigating its reactivity toward a variety of
the corresponding disilyne−isocyanide adducts [RSiSiR-
t
(CNR′)2] (R = SiiPr[CH(SiMe3)2]2, R′ = Bu (2a) or
t
CMe2CH2 Bu (2b)), which are stable below −30 °C and
t
were characterized by spectroscopic data and, in the case
of 2a, X-ray crystallography. Upon warming to room
temperature, 2 underwent thermal decomposition to
produce 1,2-dicyanodisilene R(NC)SiSi(CN)R (3)
and 1,2-dicyanodisilane R(NC)HSiSiH(CN)R (4) via
C−N bond cleavage and elimination of an alkane and an
alkene. The 1,2-dicyanodisilene derivative 3 was charac-
terized by X-ray crystallography.
reactants, such as alkenes, alkynes, RLi (R = Me, Bu), alkali
metals, nitriles, silylcyanides, amines, hydroboranes, 1,3,4,5-
tetramethylimidazol-2-ylidene (NHC), and 4-dimethylamino-
pyridine.8b,c,14 Herein, we report that the reaction of disilyne 1
with tert-butylisocyanide or tert-octylisocyanide produces
disilyne−isocyanide adducts [RSiSiR(CNR′)2] (R = SiiPr[CH-
t
t
(SiMe3)2]2, R′ = Bu (2a) or CMe2CH2 Bu (2b)), which are
stable at low temperature and were characterized by NMR
spectroscopy and, in the case of 2a, X-ray crystallography.
However, these compounds undergo thermal decomposition
upon warming above −30 °C. In both cases, decomposition
yields the 1,2-dicyanodisilene derivative R(NC)SiSi(CN)R
(3), accompanied by elimination of isobutene and isobutane for
2a and 2,4,4-trimethylpentene and 2,2,4-trimethylpentane for
2b. Compound 3 is the first example of a disilene with π-
accepting groups.
uring the past 30 years, since West and co-workers
Dsynthesized and isolated the first stable disilene,
tetramesityldisilene (Mes2SiSiMes2, Mes = 2,4,6-trimethyl-
phenyl),1 many disilene derivatives have been reported.2
However, little is known experimentally about the effects of
heteroatom substituents on the properties of the SiSi double
bond. Apeloig and Karni investigated substituent effects on the
geometry and energy of H2SiSiHX systems by theoretical
calculations, and they concluded that the structures of the
substituted disilenes are strongly dependent on the substitu-
ents.3 The general tendencies of the influence of substituents
on the configuration of the SiSi bond are as follows:
electronegative substituents (X = NH2, OH, F) increase ΔEST
(the singlet-triplet separation of silylene, SiHX) and the degree
of π−σ* orbital mixing to cause remarkable trans-bending at the
doubly bonded Si atoms, stretching and weakening the SiSi
bond; electropositive groups (X = Li, BeH, BH2, SiH3) reduce
both ΔEST and the degree of π−σ* interaction, resulting in
significant flattening at the doubly bonded Si atoms.3,4 Indeed,
disilenes with electropositive substituents such as silyl groups,5
lithium,6,7 sodium,7 potassium,7 and boranes8 were exper-
imentally characterized by X-ray crystallography to show a
nearly planar SiSi double bond. In contrast, disilenes with
electronegative substituents such as amino groups have
pyramidal trans-bent geometry at the doubly bonded Si
atoms with elongation of the SiSi double bond.8b,9 However,
to our knowledge, there are no experimental or theoretical
studies of a disilene substituted by strong π-accepting groups
such as cyano, carbonyl, or nitro groups because of the
synthetic difficulty in preparing such molecules.
Disilyne 1 was allowed to react with 2 equiv of tert-
butylisocyanide or tert-octylisocyanide in toluene-d8 in a sealed
NMR tube at −50 °C to give disilyne−isocyanide adducts 2a,b
quantitatively (by NMR analysis), which were stable at
temperatures below −30 °C for over 1 month (Scheme 1).15
Scheme 1. Reaction of Disilyne 1 with tert-Butylisocyanide
or tert-Octylisocyanide, Giving Disilyne−Isocyanide Adduct
2
The low-temperature NMR spectra of deep-red crystals of
2a,b (260 K for 2a, 220 K for 2b) were recorded in toluene-d8.
The 29Si NMR spectra have broad signals at high field (−146.9
ppm for 2a, −142.5 ppm for 2b), which correspond to the Si
Received: December 26, 2011
Published: January 26, 2012
© 2012 American Chemical Society
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Communication
centers coordinated by isocyanide. The quaternary carbon is
observed in the 13C NMR spectrum at low field (184.3 ppm for
2a, 182.3 ppm for 2b). These results are very similar to those of
disilyne−trimethylsilylisocyanide adduct [RSiSiR(CNSiMe3)2]
(2c) (−172.0 ppm for skeletal Si atoms, 193.9 ppm for skeletal
C atoms), which we reported recently.14c
Scheme 2. Thermal Decomposition of 2, Giving 1,2-
Dicyanodisilene 3
As shown in Figure 1, the molecular structure of 2a was
definitively determined by X-ray crystallographic analysis.15 The
−10 °C in a sealed NMR tube also produced 3 (24%), meso-4
(16%), d,l-4 (20%), 2,4,4-trimethylpentene (68%), and 2,4,4-
trimethylpentane (29%). Compound 2b completely decom-
posed at −10 °C within 1 h; thus, 2b is more labile than 2a
(Scheme 2).
Unfortunately, 3 is insoluble in organic solvents, which
makes it difficult to measure NMR spectra. However, single
crystals of 3 were obtained from a THF suspension of 3, and
the molecular structure of 3 was determined by X-ray
diffraction analysis.15 According to the X-ray structure of 3,
shown in Figure 2, the skeletal Si1 atom is slightly
Figure 1. ORTEP drawing of 2a·C5H12 (30% thermal ellipsoids).
Hydrogen atoms and pentane molecule have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): Si1−Si2, 2.3575(17);
Si1−Si3, 2.4070(16); Si1−C1, 1.809(4); Si2−Si8, 2.4220(16); Si2−
C6, 1.803(5); N1−C1, 1.185(6); N1−C2, 1.476(5); N2−C6,
1.190(6); N2−C7, 1.485(6); Si2−Si1−Si3, 120.18(7); Si2−Si1−C1,
102.71(14); Si3−Si1−C1, 105.11(14); Si1−Si2−Si8, 114.65(7); Si1−
Si2−C6, 104.36(15); Si8−Si2−C6, 103.90(14); C1−N1−C2,
144.0(4); C6−N2−C7, 143.5(4); Si1−C1−N1, 166.0(4); Si2−C6−
N2, 176.5(4).
two isocyanide molecules bound to each of the skeletal silicon
atoms, Si1 and Si2, are oriented on the same side. The central
Si1 and Si2 atoms are significantly pyramidalized (sum of the
bond angles: 328.0° for Si1 and 322.9° for Si2), which suggests
substantial lone-pair character at both Si1 and Si2. The Si1−Si2
bond length [2.3575(17) Å] is in the typical range of Si−Si
single bonds.16 The Si1−C1 and Si2−C6 bond lengths are
1.809(4) and 1.803(5) Å, respectively, which are shorter than
the normal Si−C single bond (1.860 Å).16 The N1−C1
[1.185(6) Å] and N2−C6 [1.190(6) Å] bond lengths are
longer than that of the C−N triple bond of meso-4 [1.151(3)
Å],14c and the C1−N1−C2 [144.0(4)°] and C6−N2−C7
[143.5(4)°] angles are not linear. These structural parameters
are similar to those of disilyne−silylisocyanide adduct 2c; this
indicates that 2a, like 2c, has silaketenimine character with
contributions from the zwitterionic form, which has anionic
character on the central Si atoms and cationic character on the
nitrogen atoms.14c
Figure 2. ORTEP drawing of 3 (50% thermal ellipsoids). Hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (deg): Si1−Si1′, 2.213(2); Si1−Si2, 2.3989(15); Si1−C1,
1.860(4); N1−C1, 1.120(4); Si1′−Si1−Si2, 136.48(9); Si1′−Si1−C1,
107.96(13); Si2−Si1−C1, 111.93(12); Si1−C1−N1, 178.6(4).
pyramidalized (the sum of the bond angles around Si1 being
356.4°). The SiSi bond length of 2.213(2) Å is typical for
disilenes.2 This demonstrates that π-accepting cyano groups
have a negligible effect on the molecular structure of disilenes.
This is in contrast to π-donating groups, which induce
significant pyramidalization of the central Si atoms and
elongation of the SiSi double bond.8b,9,14h
The π-electron-withdrawing cyano groups on the central
silicon atoms are expected to influence the energy of the
HOMO and LUMO of 3, which directly affects the UV−vis
absorptions (π−π* transitions) of the SiSi double bond. The
UV−vis spectrum of 3 in THF shows an absorption maximum
assigned to the π−π* transition at 414 nm (ε = 3900 M−1
cm−1), occurring in the region for tetraaryldisilenes (400−430
nm).2a To understand the UV−vis of 3, computational studies
of the model compounds Me3Si(NC)SiSi(CN)SiMe3 (3′)
and Me3Si(Me)SiSi(Me)SiMe3 (3″) were performed at the
B3LYP/6-31G(d) level. Both the HOMO and LUMO levels of
3′ (−4.95 and −2.27 eV, respectively) are considerably lower
than those of 3″ (−3.96 and −0.94 eV, respectively). The
The disilyne−silylisocyanide adduct 2c is stable;14c however,
the corresponding 2a and 2b are thermally labile compounds.
These compounds readily underwent thermal decomposition
upon warming above −30 °C. At room temperature, the
toluene solution of 2a changed from dark red to yellow over 5
h, accompanied by the formation of 1,2-dicyanodisilene 3 as a
yellow powder, which was isolated in 21% yield.15,17
Monitoring of the reaction in toluene-d8 by NMR spectroscopy
showed the disappearance of 2a and the formation of 3 as well
as meso-4 (21%), d,l-4 (7%), isobutene (22%), and isobutane
(10%) (Scheme 2). Similarly, thermal decomposition of 2b at
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Communication
HOMO energy level is lowered by the electronegative cyano
groups, and the LUMO energy level also is greatly lowered by
the π-conjugation between the cyano groups and the SiSi π-
bond. This energy difference clearly corresponds to the
observed red shift in the π−π* transition of 3. This π-
conjugation is also supported by the decrease of the Wiberg
bond index of the SiSi double bond in 3′ (1.883) compared
to that in 3″ (1.944).
thermal parameters for 2a and 3 (PDF/CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
sekiguch@chem.tsukuba.ac.jp
■
Notes
The authors declare no competing financial interest.
To understand the C−N bond cleavage of 2a and 2b at low
temperature, the bond dissociation energies (BDEs) relevant
for the cleavage of the nitrogen substituent were estimated (see
the Supporting Information).18 The BDE of the nitrogen−tert-
butyl N−C bond of 2a is smaller than that of a typical C−N
single bond (35.9 kcal/mol for 2a and 77.4 kcal for Me2N−
CMe3 at the B3LYP/6-31G(d) level). The low BDE for the
bis(silaketenimine) system in 2a can be understood on the
basis of resonance stabilization of the radical intermediate, in
which the radical spin density is delocalized over the Si−C−N
π-system (Chart 1, Figure 3).
ACKNOWLEDGMENTS
■
We thank Prof. Y. Apeloig for discussions and suggestions for
the computational studies. This work was financially supported
by Grants-in-Aid for Scientific Research program (Nos.
19105001, 21350023, 23108701, 23655027) from the Ministry
of Education, Science, Sports, and Culture of Japan, JSPS
Research Fellowship for Young Scientist (K.T.).
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Figure 3. SOMO of [(Dsi2 PrSi)SiSi(SiiPrDsi2)(CNtBu)(CN)]• at
HF/6-31G(d).
In summary, we report a new synthetic strategy for the 1,2-
dicyanodisilene derivative 3, the first example of a disilene with
a π-accepting group, by the reaction of disilyne 1 with tert-
butylisocyanide or tert-octylisocyanide. This reaction involved
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stable below −30 °C. Upon warming to room temperature, 2
underwent thermal decomposition to give the unexpected 1,2-
dicyanodisilene R(NC)SiSi(CN)R 3 via C−N bond cleavage
and elimination of an alkane and an alkene. The 1,2-
dicyanodisilene derivative 3 was characterized by X-ray
crystallography, revealing that the doubly bonded Si atoms
have a slightly pyramidalized geometry with the SiSi bond
length of 2.213(2) Å, in contrast to the highly pyramidal
structure of disilenes with electronegative substituents such as
amino groups.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures for 2a, 2b, and 3; computational
results on model compounds 3′ and 3″; calculated BDEs of 2a,
2c, Me2N−CMe3, Me2N−SiMe3, and model compound 2d;
and crystallographic data including atomic positional and
■
S
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2008, 130, 13856. (b) Han, J. S.; Sasamori, T.; Mizuhata, Y.; Tokitoh,
N. J. Am. Chem. Soc. 2010, 132, 2546. (c) Han, J. S.; Sasamori, T.;
Mizuhata, Y.; Tokitoh, N. Dalton Trans. 2010, 39, 9238.
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(15) For experimental procedures and spectral data for 2a, 2b, and 3
as well as crystal data for 2a and 3, see the Supporting Information.
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ArGeGeAr (Ar = C6H3-2,6(C6H3-2,6-iPr2) with tert-butylisocyanide
yielded the adduct [ArGeGeAr(CNtBu)]; however, no cleavage
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more likely than heterolytic cleavage because the reaction proceeds in
nonpolar solvents such as toluene, benzene, hexane, and pentane.
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