Synthesis and structure of a heavier congener of biphenyl: 1,1′-disila-4,4′-biphenyl

Article abstract of DOI:10.1021/om901095w

The 1,1′-disila-4,4′-biphenyl species 1, the first molecule bearing directly connected two silaaromatic rings, was synthesized and characterized by its spectroscopic and X-ray crystallographic data. The UV-vis spectrum of 1 showed not only a red shift but also a 6-fold increase in absorbance of the longest absorption maximum in comparison with those of Tbt-substituted silabenzene, indicating that the concept of conjugation through the single bond connecting two aromatic rings is applicable even in the silaaromatic systems.

Full text of DOI:10.1021/om901095w

Organometallics 2010, 29, 721–723 721
DOI: 10.1021/om901095w
Synthesis and Structure of a Heavier Congener of Biphenyl:
1,10-Disila-4,40-biphenyl
Yusuke Tanabe, Yoshiyuki Mizuhata, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Received December 21, 2009
Summary: The 1,10-disila-4,40-biphenyl species 1, the first
tions indicated that these silaaromatic compounds have
much smaller energy gaps between their frontier orbitals
than do those of the parent aromatic hydrocarbons, in
addition to their high aromaticity. These results clearly show
that silaaromatic frameworks are promising for the creation
of novel π-conjugated systems containing heavier main-
group elements.6 That is, the connection or fusing of
“ready-made” π-conjugated systems containing a heavier
group 14 element(s) can be a new synthetic methodology to
create novel extended π-conjugated systems. In order to
understand the conjugation effect of a silaaromatic ring with
the other π-unit, the biphenyl skeleton can be viewed as one of
the simplest models. Herein, we wish to report the synthesis and
structure of the 1,10-disila-4,40-biphenyl species 1, the first com-
pound bearing directly connected two silaaromatic rings.
molecule bearing directly connected two silaaromatic rings,
was synthesized and characterized by its spectroscopic and
X-ray crystallographic data. The UV-vis spectrum of 1
showed not only a red shift but also a 6-fold increase in
absorbance of the longest absorption maximum in comparison
with those of Tbt-substituted silabenzene, indicating that the
concept of conjugation through the single bond connecting two
aromatic rings is applicable even in the silaaromatic systems.
Biphenyl can be regarded as the simplest model compound
for polyphenylenes, which have been widely used as attrac-
tive materials in many areas, and has been one of the most
controversial molecules for a long time.1 The important
factor in the geometry of biphenyl is the torsion angle, which
depends on the balance between π-electron conjugation and
the steric repulsion of ortho hydrogen atoms. It is well-
known that two phenyl rings of biphenyl in the crystalline
state (at a temperature above 40 K) are coplanar dominantly
due to crystal-packing forces2 but are twisted across the
central C-C bond in the vapor phase and in solution.3
Detailed structural studies from such kinds of viewpoints
are important to understand the conjugation effect for
various types of aromatic compounds containing biphenyl
skeletons.
As part of our ongoing research projects in the area of
novel aromatic compounds, we have already succeeded in
the incorporation of a silicon atom into various aromatic
ring skeletons by the application of an efficient protecting
group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), to
afford heavier analogues of benzene, naphthalene, anthra-
cene, and phenanthrene.4,5 Studies on their structural and
spectroscopic properties together with theoretical calcula-
As illustrated in Scheme 1, the bis(hydrosilane) 3, bearing
two SiC5 rings, was obtained as a mixture of isomers by the
coupling reaction of the corresponding silacyclohexadi-
enide,7 which was generated by the reaction of hydrosilane
28 with n-BuLi.9 Bis(chlorosilane) 5, the potential precursor
for the synthesis of disilabiphenyl, was prepared via bis-
(hydroxysilane) 4 according to the procedure for the synthe-
sis of Tbt-substituted silabenzene.8 The dehydrochlorina-
tion reaction of 5 with LDA afforded the title compound, the
1,10-disila-4,40-biphenyl species 1, in 14% yield as colorless
1
crystals. In the H NMR spectrum of 1 in C6D6, signals
assignable to the protons at the R- and β-positions of the
silicon atoms (Si1) in the SiC5 rings were observed at 7.12 and
8.22 ppm as doublet signals (J=12.8 Hz), which were similar
to those in the case of silabenzene. In the 29Si NMR spectrum
of 1, the signal of Si1 was observed at 87.2 ppm, the value
of which is shifted slightly upfield from that of silabenzene
(93.6 ppm).8 Due to its very low solubility, it was difficult to
measure the 13C NMR spectrum of 1 in C6D6, and hence, full
characterization of 1 in solution was completed by using
THF-d8 as a solvent. In the 29Si NMR spectrum in THF-d8, a
signal assignable to the Si1 atoms was observed at 91.0 ppm.
It is known that the complexation of silenes (SidC) with
THF results in the remarkable upfield shift of 29Si NMR
*To whom correspondence should be addressed. Tel: þ81-774-38-
3200. Fax: þ81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp.
€
(1) For reviews, see: (a) Grimsdale, A. C.; Mullen, K. Adv. Polym.
ꢀ
ꢀ
ꢀ
ꢀ
Sci. 2006, 199, 1. (b) Ivanovic, N.; Radisavljevic, I.; Marjanovic, D.; Bojanic,
S.; Carreras, C. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1783.
(c) Berresheim, A. J.; M€uller, M.; M€ullen, K. Chem. Rev. 1999, 99, 1747.
(2) (a) Trotter, J. Acta Crystallogr. 1961, 14, 1135. (b) Charbonneau,
G.-P.; Delugeard, Y. Acta Crystallogr. 1976, B32, 1420. (c) Cailleau, H.;
Baudour, J. L.; Zeyen, M. E. Acta Crystallogr. 1979, B35, 426.
(3) (a) Charbonnier, S.; Beguemsi, S. T.; N’Guessan, Y. T.; Legoff, D.;
Proutiere, A.; Viani, R. J. Mol. Struct. 1987, 158, 109. (b) Almenningen, A.;
Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin, S. J.; Samdal, S. J. Mol.
Struct. 1985, 128, 59.
(6) Recently, the syntheses of novel π-conjugated molecules bearing
two SidSi or SidP units were reported; see: (a) Bejan, I.; Scheschkewitz,
D. Angew. Chem., Int. Ed. 2007, 46, 5783. (b) Fukazawa, A.; Li, Y.;
Yamaguchi, S.; Tsuji, H.; Tamao, K. J. Am. Chem. Soc. 2007, 129, 14164.
(c) Li, B.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K.
J. Am. Chem. Soc. 2009, 131, 13222.
(7) Tanabe, Y.; Mizuhata, Y.; Tokitoh, N. Pure Appl. Chem., in press.
DOI: 10.1351/PAC-CON-09-10-08.
(8) (a) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S. Angew.
Chem., Int. Ed. 2000, 39, 634. (b) Wakita, K.; Tokitoh, N.; Okazaki, R.;
Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 5648.
(4) For a review, see: (a) Tokitoh, N. Acc. Chem. Res. 2004, 37, 86 and
references cited therein. For selected recent examples: (b) Tokitoh, N.;
Shinohara, A.; Matsumoto, T.; Sasamori, T.; Takeda, N.; Furukawa, Y.
Organometallics 2007, 26, 4048. (c) Tanabe, Y.; Mizuhata, Y.; Tokitoh, N.
Chem. Lett. 2008, 37, 720. (d) Mizuhata, Y.; Shinohara, A.; Tanabe, Y.;
Tokitoh, N. Chem. Eur. J. 2009, 15, 8405 and references cited therein.
(5) Sekiguchi reported the synthesis of 1,2-disilabenzene; see: Kinjo,
R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto, M.; Nagase, S.
J. Am. Chem. Soc. 2007, 129, 7766.
(9) Experimental procedures and chemical data for the newly obta-
ined compounds are given in the Supporting Information.
r
2010 American Chemical Society
Published on Web 01/26/2010
pubs.acs.org/Organometallics

722 Organometallics, Vol. 29, No. 4, 2010
Tanabe et al.
groups prevent intermolecular interactions between the di-
silabiphenyl units, resulting in the retention of a distorted
structure even in the solid state. The length of the central
˚
C-C bond connecting the two SiC5 rings is 1.505(7) A, which
is almost similar to that of the parent biphenyl.2 The planar
geometries of the two SiC5 rings were clearly revealed, and
the lengths of the two Si-C bonds and the four C-C bonds
in the SiC5 ring were found to be almost equal to each other,
respectively, indicating the delocalization of π-electrons in
each ring. These structural parameters, including the internal
angles, are quite similar to those of Tbt-substituted silaben-
zene,8 in spite of the connection of the two silaaromatic rings.
These results suggested the absence of significant electronic
correlations between the two silaaromatic rings in the crys-
talline state. In comparison with the parent biphenyl, the
internal angle at the C3 position of 1 is larger by ca. 4°, owing
to the introduction of a silicon atom, which has an atomic
radius larger than that of a carbon atom.
Figure 1. Thermal ellipsoid plot of [1 2toluene] drawn at the
3
50% probability level. Hydrogen atoms and toluene molecules are
˚
omitted for clarity. Selected bond lengths (A): Si1-C2=1.737(4),
C2-C3 = 1.395(5), C3-C4 = 1.393(6), C4-C5 = 1.396(5),
C5-C6=1.401(5), C6-Si1 = 1.762(4), C4-C4*= 1.505(7).
Scheme 1. Synthesis of the 1,10-Disila-4,40-biphenyl Species 1a
In order to clarify the properties of 1,10-disila-4,40-biphenyl
in detail, theoretical calculations for several model com-
pounds of 1 bearing a hydrogen atom or some substi-
tuted-phenyl groups instead of the Tbt group were carried
out at the B3LYP level with 6-31G(d) basis sets (see the
Supporting Information). In the optimized structures, the
structural parameters of the disilabiphenyl units were almost
similar regardless of the substituents, although the twist
angles between the benzene ring of the substituents and the
core silabenzene ring varied depending on the bulkiness of
the substituents. The twist angles and the lengths of the
central C-C bonds between the two silaaromatic rings were
a Conditions (i) n-BuLi (1 equiv), THF, -60 °C; (ii) BrCF2CF2Br
(1 equiv), THF, -60 °C then room temperature; (iii) NBS (2 equiv),
CCl4, 4 °C; (iv) NaHCO3 (excess), THF/H2O, 50 °C; (v) PCl5 (excess)
and NEt3 (excess), Et2O, reflux; (vi) LDA (1 equiv), hexane/THF, room
temperature. b Mixture of isomers. c The isomer syn,anti-4b was ob-
tained in 10% yield.
˚
optimized to be 44.8-45.6° and 1.492-1.493 A, respectively,
both of which well reproduced those observed for 1 in the
crystalline state. Understanding the rotational barrier of the
parent 1,10-disila-4,40-biphenyl is a matter of vital impor-
tance to elucidate the π-conjugation effect in solution. The
relative energy is the lowest when the twist angle is 45.5°.
Meanwhile, a coplanar conformation is the most unstable
geometry, and the difference between the energies of the
most stable and unstable geometries is 5.4 kcal/mol.
Although the rotational barrier of disilabiphenyl is estimated
to be somewhat larger than that of the parent biphenyl
(2.4 kcal/mol, calculated at the same level as that used for
the aforementioned model compounds)12 because of the
inevitably greater steric repulsion between the hydrogen
atoms on the β-carbons, the small energy barrier indicates
that the two silaaromatic rings in the 1,10-disila-4,40-biphenyl
skeleton can freely rotate around the central bond at room
temperature.
resonances (for example, from 144.2 to 52.4 ppm in the case
of Me2SidC(SiMe3){SiMe(t-Bu)2}).10 Therefore, there is al-
most no interaction between the oxygen atom of THF and
the ring silicon atoms of the disilabiphenyl unit because of
the effective protection of the Tbt group around the sp2
silicon atoms. 13C NMR signals of the carbon atoms in the
SiC5 rings were observed at 144.2 (C3 and C5), 132.9 (C4),
and 122.4 (C2 and C6) ppm, the values of which are quite
similar to those of Tbt-substituted silabenzene (143.6 ppm
for C3 and C5; 122.2 ppm for C2 and C6; 116.1 ppm for C4),
except for C4.8 The chemical shift of the signal for the C4
atom is shifted more downfield than that of Tbt-substituted
silabenzene, as observed in the case of parent aromatic
hydrocarbon systems: i.e., biphenyl and benzene.
Crystals of 1 suitable for X-ray diffraction analysis were
obtained by slow evaporation from its toluene solution.
Structural analysis has been completed satisfactorily on the
basis of space group C2/c, and the two TbtSiC5H4- moieties
are crystallographically equivalent (Figure 1). The two SiC5
rings are twisted with respect to each other with a dihedral
angle of ca. 41°. The parent biphenyl has a coplanar structure
due to packing forces in the crystalline state,2 and its dihedral
angles in solution or in the gas phase are ca. 45°.3 On the
other hand, it is known that bulky substituents at the 4- and
40-positions in the parent biphenyl minimize the influence of
crystal packing.11 In the case of 1, extremely bulky Tbt
In the UV-vis spectrum of 1 in hexane at room tempera-
ture, the longest absorption maximum was observed at 357
nm (ε=2.9 ꢀ 104) as a broad peak, as illustrated in Figure 2.
On the other hand, Tbt-substituted silabenzene shows some
vibrational fine structures and the longest absorption max-
imum at 331 nm (ε=5.0 ꢀ 103). It should be noted that the
longest absorption maximum of 1,10-disila-4,40-biphenyl 1 is
red-shifted 26 nm compared with that of the corresponding
silabenzene, together with an increase in absorption inten-
sity. These drastic differences may indicate that effective
π-conjugation is achieved in solution between the two
silaaromatic rings of 1 through the central C-C bond. The
broadened absorption of 1 is most likely interpreted in terms
(10) Wiberg, N.; Wagner, G. Chem. Ber. 1986, 119, 1467.
(11) Nather, C.; Jess, I.; Havias, Z.; Bolte, M.; Nagel, N.; Nick, S.
Solid State Sci. 2002, 4, 859.
€
(12) Karpfen, A.; Choi, C. H.; Kertesz, M. J. Phys. Chem. A 1997,
101, 7426.

Communication
Organometallics, Vol. 29, No. 4, 2010 723
Figure 3. Frontier orbitals of a model compound bearing Dip
groups: (a) HOMO; (b) LUMO.
also support the existence of π-conjugation between two
silaaromatic rings.
In summary, we succeeded in the synthesis of the first
compound bearing two silaaromatic rings in a molecule, i.e.,
the 1,10-disila-4,40-biphenyl species 1. In the UV-vis spec-
trum of 1 in hexane, the longest absorption maximum is
significantly red-shifted compared with that of Tbt-substi-
tuted silabenzene, indicating that silaaromatic rings can also
conjugate with each other as well as the corresponding
aromatic hydrocarbons. Construction of further extended
π-conjugated systems containing more silaaromatic ring
skeletons is ongoing.
Figure 2. UV-vis spectra of the 1,10-disila-4,40-biphenyl spe-
cies 1 and silabenzene.
of the free rotation between the two silaaromatic rings
around the central C-C bond, as reported in the case of
the parent biphenyl.13 In order to assign the absorptions,
TD-DFT calculations for the model compound bearing Dip
(2,4,6-triisopropylphenyl) groups were performed with a
conformation similar to that of the observed one, including
the orientation of the aryl substituents (TD-B3LYP/6-311þ
G(2d,p)//B3LYP/6-31G(d)). The calculated value of the
longest absorption maximum was 373 nm (f=0.5535), which
is in good agreement with the observed one. Therefore,
the absorption at 357 nm was found to be assignable to the
electron transition from the π orbital (HOMO) of the
disilabiphenyl unit to the π* orbital (LUMO) of the disila-
biphenyl unit together with the aryl substituents on the
silicon atoms, as shown in Figure 3,14 the results of which
Acknowledgment. This work was partially suppor-
ted by Grants-in-Aid for Creative Scientific Research
(No. 17GS0207), Science Research on Priority Areas
(No. 20036024, “Synergy of Elements”), Young Scien-
tist (B) (No. 21750042), and the Global COE Program
(B09, “Integrated Materials Science”) from the Ministry
of Education, Culture, Sports, Science and Techno-
logy, of Japan. Y.M. thanks Mitsubishi Chemical Corp.
and Synthetic Organic Chemistry, Japan, for financial
support.
(13) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: New York, 1970.
(14) The shape of the LUMO indicated that 1 would possess the
character of a disilaquaterphenyl derivative in no small part. Actually,
the longest absorption maximum calculated for the hydrogen-substi-
tuted disilabiphenyl was 343 nm (f=0.6066), which was blue-shifted by
30 nm compared to that of the Dip-substituted one. This correlation
resembles that between the longest absorption maxima of biphenyl (248
nm) and p-quaterphenyl (294 nm) in cyclohexane: Ozasa, S.; Hatada, N.;
Fujioka, Y.; Ibuki, E. Bull. Chem. Soc. Jpn. 1980, 53, 2610.
Supporting Information Available: Text, figures, and tables
giving experimental procedures and spectral data for new com-
pounds and a CIF file giving crystallographic data for 1. This
material is available free of charge via the Internet at http://
pubs.acs.org.