Synthesis of a stable stibabismuthene; the first compound with an antimony-bismuth double bond

Article abstract of DOI:10.1039/b001900n

The condensation reaction of an overcrowded dihydrostibine with dibromobismuthine using 1,8-diazabicyclo[5.4.0]undec-7-ene as a base afforded the first stable stibabismuthene, the formation of which was evidenced by UV-VIS and Raman spectra and its chemical reactivity.

Full text of DOI:10.1039/b001900n

Synthesis of a stable stibabismuthene; the first compound with an
antimony–bismuth double bond
Takahiro Sasamori,a Nobuhiro Takeda†b and Norihiro Tokitoh*†b
a Department of Chemistry and Physics of Condensed Matter, Graduate School of Science, Kyushu University, Japan
b Institute for Fundamental Research of Organic Chemistry, Kyushu University, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
Received (in Cambridge, UK) 8th March 2000, Accepted 14th June 2000
Published on the Web 30th June 2000
The condensation reaction of an overcrowded dihydrosti-
bine with dibromobismuthine using 1,8-diazabicy-
clo[5.4.0]undec-7-ene as a base afforded the first stable
stibabismuthene, the formation of which was evidenced by
UV–VIS and Raman spectra and its chemical reactivity.
The condensation reaction of BbtBiBr2 with BbtSbH2, which
was prepared by the reaction of BbtSbBr2 with LiAlH4, in the
presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in THF
at 295 °C afforded stibabismuthene 1‡ as red–purple crystals
quantitatively (Scheme 1). Stibabismuthene 1 showed sat-
isfactory spectral data, as discussed below.
In recent years there has been much interest in compounds with
a double bond between heavier group 15 elements. Since the
first isolation of a stable diphosphene (Mes*PNPMes*; Mes* =
2,4,6-tri-tert-butylphenyl) in 1981,1 a number of examples of
kinetically stabilized diphosphenes (RPNPR)2 and diarsenes
(RAsNAsR)2c,3 have been isolated and fully characterized.
Recently, we have succeeded in the synthesis and character-
ization of the first stable distibene (TbtSbNSbTbt)4 and
dibismuthene (TbtBiNBiTbt),5 even heavier congeners of azo
compounds, by taking advantage of an efficient steric protection
group, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl.6 Very
recently, Power and coworkers also synthesized another type of
stable distibene and dibismuthene substituted by bulky
2,6-Ar2C6H3 groups (Ar = mesityl or 2,4,6-triisopropylphe-
nyl).7 As for the case of heteronuclear double-bond compounds
between heavier group 15 elements, several phosphaarsenes3a,8
and phosphastibenes8,9 have been synthesized as stable com-
pounds. However, there are no examples of a heteronuclear
doubly bonded system between antimony and bismuth, i.e.
stibabismuthene. Although the successful results on the kinetic
stabilization of distibene and dibismuthene (TbtENETbt; E =
Sb, Bi) naturally prompted us to apply the Tbt group to the
synthesis of stable stibabismuthene, we were apprehensive that
the extremely low solubility of the Tbt-substituted doubly
bonded system of heavier group 15 elements may prevent us
from utilising the possible synthetic approaches or spectroscop-
ically detecting the reaction products. On the other hand, during
the course of our investigation on the kinetic stabilization of
low-coordinated highly reactive species we have developed
another bulky aromatic substituent, 2,6-bis[bis(trimethylsi-
lyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt),10 which
is expected to be a potentially more useful steric protecting
Scheme 1
In the Raman spectrum (in the solid state) of 1, a strong line
attributable to the Sb–Bi stretching was observed at 169 cm21
.
This wavenumber lies between the value of the Sb–Sb
stretching vibration in TbtSbNSbTbt (207 cm21)4 and that of the
Bi–Bi stretching vibration in TbtBiNBiTbt (135 cm21),5 and is
higher than the Sb–Sb and Bi–Bi stretching frequencies for
Ph2E–EPh2 (E = Sb, Bi).12 The UV–VIS spectrum of 1 in
hexane shows two absorption maxima at 709 nm (e 200 dm3
mol21 cm21) and 516 (7500), which are most likely assignable
to the forbidden n ? p* and the allowed p? p* transitions of
the SbNBi chromophore, respectively.
These results are consistent with the characteristic red-shifts
in the electronic spectra of previously reported heavier
congeners of azo compounds, and the lmax value for the p ? p*
transition of 1 lies between those of BbtSbNSbBbt 2 [lmax 490
nm (e 6000 dm3 mol21 cm21)]11 and BbtBiNBiBbt 3 [lmax 537
nm (e 6000 dm3 mol21 cm21)].11 These spectral data suggest
that 1 features a double bond between antimony and bismuth in
solution as well as in the solid state. The reason for the
additional red-shift for the n ? p* transition of 1, which is 39
nm longer than that of 3 [lmax 670 nm (sh, e 20)], is not clear at
present.
The molecular structure of stibabismuthene 1 was also
supported by X-ray crystallographic analysis, but definite
structural parameters for 1 have not been obtained yet owing to
the inevitable disorder of the antimony and bismuth atoms,
which cannot be solved by data collection with a number of
different single crystals of 1 even at low temperature
(2180 °C).§
Stibabismuthene 1 is stable at ambient temperature in
hydrocarbon solvents in the absence of air and light. When a
solution of 1 in benzene-d6 was heated at 70 °C, 2 and 3 were
formed very slowly as judged by 1H NMR spectroscopy
group than TbT. In fact, a new distibene and dibismuthene
substituted by Bbt groups, which have relatively high solubility
compared with TbtENETbt (E = Sb, Bi), have been successfully
synthesized and characterized.11 We now report the successful
application of the Bbt group to the synthesis of the first stable
stibabismuthene, BbtSbNBiBbt 1.
† Present address: Institute for Chemical Research, Kyoto University,
Gokasho, Uji, Kyoto 611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-
u.ac.jp
Scheme 2
DOI: 10.1039/b001900n
Chem. Commun., 2000, 1353–1354
This journal is © The Royal Society of Chemistry 2000
1353

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(Scheme 2). Heating a solution of 1 at 80 °C for 20 days led to
the formation of a mixture of 1, 2 and 3 with the ratio of
1 1.4 1.1, respectively. On the other hand, when a solution of
1 in benzene-d6 was irradiated with a medium pressure mercury
lamp (100 W) in a sealed Pyrex NMR tube at room temperature,
the disproportionation reaction was complete in 4 h to give a
1 1 mixture of 2 and 3. The results of the thermal and
photochemical disproportionation reactions of 1 into the
homonuclear double-bond species 2 and 3 can be regarded as
chemical evidence for the formation of stibabismuthene 1.
Taking the previous reports on the reactivities of diphos-
phenes2,13 into consideration, two different pathways can be
postulated for the disproportionation reactions of stibabismu-
thene 1 (Scheme 3). The first is the dimerization of 1 by heating
or irradiation followed by decomposition of the resulting four-
membered dimer 4 into the homonuclear double-bond species
2 and 3 (path A), while the other is based on the dissociation of
1 giving the corresponding monovalent species, i.e. stibinidene
5 and bismuthinidene 6, both of which might undergo ready
dimerization leading to the formation of 2 and 3, respectively
(path B).
11166250) from the Ministry of Education, Science, Sports and
Culture of Japan. T. S. thanks Research Fellowships of the
Japan Society for the Promotion of Science for Young
Scientists. We acknowledge Professor Y. Furukawa of Waseda
University for measuring the Raman spectra. We are also
grateful to Shin-Etsu Chemical Co. Ltd., for the generous gifts
of chlorosilanes.
Notes and references
‡ Spectral data for 1: red–purple crystals, mp 259–260 °C (decomp.); H
1
NMR (400 MHz, C6D6) d 0.31 (s, 36H), 0.34 (s, 36H), 0.39 (s, 27H), 0.40
(s, 27H), 1.98 (s, 2H), 2.07 (s, 2H), 7.19 (s, 2H), 7.53 (s, 2H); 13C NMR (100
MHz, C6D6) d 1.37 (q), 2.42 (q), 2.82 (q), 5.76 (q), 21.42 (s), 22.15 (s),
41.35 (d), 43.64 (d), 124.23 (d), 128.95 (d), 145.48 (s), 148.54 (s), 151.81
(s), 152.20 (s), 153.70 (s), 195.34 (brs). FT-Raman (Nd YAG laser 1064
nm) 169 cm21 (nSbNBi). UV–VIS (hexane) lmax/nm (e/dm3 mol21 cm21
)
709 (200), 516 (7500). FAB-MS: m/z 880 ([BbtBiSb 2 Si(CH3)3]+), 832
([BbtBi]+), 745 ([BbtSb + H]+).
§ Crystal data for 1: C60H134Si14BiSb, M = 1579.64, triclinic, space group
¯
P1 (no. 2), a = 12.574(2), b = 18.056(2), c = 9.318(1) Å, a = 92.756(3),
b = 98.623(4), g = 88.678(8)°, V = 2088.9(5) Å3, Z = 1, Dc = 1.256 g
cm23, Mo-Ka (l = 0.71069 Å) radiation, m(Mo-Ka) = 26.54 cm21, T =
2180 °C, 13387 reflections measured, 8279 unique (Rint = 0.048).
CCDC 182/1683. See http://www.rsc.org/suppdata/cc/b0/b001900n/ for
crystallographic files in .cif format
1 M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and T. Higuchi, J. Am.
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Scheme 3
Although we have examined the thermolysis and photolysis
of 1 in the presence of 2,3-dimethylbuta-1,3-diene in expecta-
tion of trapping the intermediary monovalent species 5 and 6, no
[4 + 1] cycloadducts of 5 and 6, but only distibene 2 and
dibismuthene 3, were obtained in high yields. Since we have
already found that the stibinidene 5 generated by thermal
cycloreversion of the corresponding overcrowded stibolene
derivative readily undergoes [4 + 1] cycloaddition with
2,3-dimethylbuta-1,3-diene to give the stable stibinidene ad-
duct,10,11 the disproportionation reaction of stibabismuthene is
not rationalized by the mechanism via stibinidene and bismuthi-
nidene intermediates but most likely interpreted in terms of the
association–dissociation mechanism via the head-to-head di-
merization of 1.
In summary, we have succeeded in the synthesis of the first
stable stibabismuthene 1 by taking advantage of kinetic
stabilization afforded by a new and effective steric protecting
group, Bbt. Further investigations on the physical and chemical
properties of stibabismuthene and syntheses of other variations
of heteronuclear doubly bonded systems between heavier main
group elements are currently in progress.
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This work was partially supported by Grants-in-Aid for
Scientific Research on Priority Areas (No. 09239101 and
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Chem. Commun., 2000, 1353–1354