Generation of stannabenzenes and their properties

Article abstract of DOI:10.1021/om100382n

Stannabenzenes 3, neutral stannaaromatic compounds having the simplest aromatic unit, were successfully generated by the reaction of the corresponding bromostannanes 5-Br with lithium diisopropylamide in hexane at -40 °C. The generation of 3 was indicated

Full text of DOI:10.1021/om100382n

Organometallics 2010, 29, 4781–4784 4781
DOI: 10.1021/om100382n
Generation of Stannabenzenes and Their Properties†
Yoshiyuki Mizuhata, Naoya Noda, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Received April 30, 2010
Summary: Stannabenzenes 3, neutral stannaaromatic compounds
development, among which was our success in the synthesis
and isolation of the first stable examples for neutral sila and
germa aromatic compounds,6,7 including sila-8 and germa-
benzenes,9 by taking advantage of the kinetic stabilization
afforded by an efficient steric protection group, 2,4,6-tris-
[bis(trimethylsilyl)methyl]phenyl (Tbt). Quite recently, our
group and Sekiguchi’s have reported the synthesis and
isolation of 1,2-disilabenzenes by the reaction of stable
disilynes (-SitSi-) with acetylenes.10 In view of the recent
progress in the chemistry of sila and germa aromatic com-
pounds, the synthesis of stannaaromatic compounds is of
great interest from the standpoint of systematic elucidation
of the properties of heteraaromatic systems of heavier group
14 elements.
In comparison with sila and germa aromatic compounds,
stanna aromatic compounds are still elusive and their prop-
erties have not been disclosed so far. Saito et al. have reported
the synthesis of a series of stannole anions and dianions, i.e.,
ionic stanna aromatic compounds, as stable compounds along
with their full characterization.11 However, stable and neutral
stanna aromatic compounds are limited to our 2-stanna-
naphthalene 1,12 which is kinetically stabilized by the com-
bination of a Tbt group on the tin atom and a t-Bu group
on the adjacent carbon atom. On the other hand, 9-Tbt-9-
stannaphenanthrene (2) was found to undergo ready [2 þ 2]
dimerization at ambient temperature, most likely due to
insufficient steric protection.13 In the case of 2-stanna-
naphthalene, two Sn-C bonds in the ring were nonequivalent,
reflecting the bond-alternating nature of the parent naphtha-
lene skeleton. In order to investigate the effect of the tin-
carbon bond in the aromatic ring, the synthesis of a stanna-
benzene essentially having no bond alternation is of great
having the simplest aromatic unit, were successfully generated
by the reaction of the corresponding bromostannanes 5-Br with
lithium diisopropylamide in hexane at -40 °C. The generation
of 3 was indicated by the formation of the [4 þ 2] dimer 6 at
room temperature. The thermal stability of 3 was completely
different from that of the sila- and germabenzenes bearing the
same substituent. The structural and electronic properties of
stannabenzene were estimated by theoretical calculations.
Benzene is representative of aromatic compounds, which
are important from the viewpoints of not only fundamental
chemistry but also application toward organic synthesis
and material science.1 Its chemical behavior, structure, and
energetic and magnetic features have been widely studied
from both experimental and theoretical standpoints, and
such findings have contributed to understand the concept
“aromaticity”.1,2 Although a variety of heterabenzene, in
which one carbon atom of benzene is replaced with a
heteroatom, have been synthesized and focused for a long
time to discuss “aromaticity”, the benzene analogues of
heavier group 14 elements (Si, Ge, Sn, and Pb) have been
recognized as highly reactive species.3 For example, the
parent silabenzene (SiC5H6) has been generated by flash
thermolysis and studied in the gas phase and in low-tem-
perature matrices, but it was found to oligomerize above
80 K.4 This result is in sharp contrast with the high thermal
stability of phosphabenzene, containing a phosphorus atom,
which is an element in the same row as silicon.5
Recently, the chemistry of heteraaromatic compounds
of heavier group 14 elements has experienced remarkable
† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar
Seyferth on the occasion of his retirement as Editor-in-Chief of Organometallics.
*To whom correspondence should be addressed. E-mail: tokitoh@
boc.kuicr.kyoto-u.ac.jp.
(1) Minkin, V. J.; Glukhovtsev, M. N.; Simkin, Y. B. Aromaticity and
Antiaromaticity; Electronic and Structural Aspects; Wiley: New York,
1994.
(2) Schleyer, P. v. R.; Jiao, H. Pure Appl. Chem. 1996, 68, 209.
(3) For reviews, see: (a) Haaf, M.; Schmedake, T. A.; West, R. Acc.
Chem. Res. 2000, 33, 704. (b) Lee, V. Y.; Sekiguchi, A.; Ichinohe, M.;
Fukaya, N. J. Organomet. Chem. 2000, 611, 228. (c) Sekiguchi, A.; Lee,
V. Y. Chem. Rev. 2003, 103, 1429. (d) Lee, V. Y.; Sekiguchi, A. Angew.
Chem., Int. Ed. 2007, 46, 6596.
(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. (c) Tokitoh, N.;
Wakita, K.; Matsumoto, T.; Sasamori, T.; Okazaki, R.; Takagi, N.; Kimura,
M.; Nagase, S. J. Chin. Chem. Soc. 2008, 55, 487.
(9) (a) Nakata, N.; Takeda, N.; Tokitoh, N. J. Am. Chem. Soc. 2002,
124, 6914.
(10) (a) Kinjo, R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto,
M.; Nagase, S. J. Am. Chem. Soc. 2007, 129, 7766. (b)Han, J. S.;Sasamori, T.;
Mizuhata, Y.; Tokitoh, N. Dalton Trans. 2010, 39, 9238.
(4) (a) Maier, G.; Mihm, G.; Reisenauer, H. P. Angew. Chem., Int. Ed.
Engl. 1980, 19, 52. (b) Solouki, B.; Rosmus, P.; Bock, H.; Maier, G. Angew.
Chem., Int. Ed. Engl. 1980, 19, 51. (c) Maier, G.; Mihm, G.; Reisenauer,
H. P. Chem. Ber. 1982, 115, 801.
(11) (a) Saito, M.; Haga, R.; Yoshioka, M. Chem. Commun. 2002,
1002. (b) Saito, M.; Haga, R.; Yoshioka, M. Chem. Lett. 2003, 32, 912.
(c) Saito, M.; Haga, R.; Yoshioka, M. Eur. J. Inorg. Chem. 2005, 3750.
(d) Saito, M.; Haga, R.; Yoshioka, M.; Ishimura, K.; Nagase, S. Angew.
Chem., Int. Ed. 2005, 44, 6553. (e) Haga, R.; Saito, M.; Yoshioka, M. J. Am.
Chem. Soc. 2006, 128, 4934. (f) Haga, R.; Saito, M.; Yoshioka, M. Eur.
J. Inorg. Chem. 2007, 1297. (g) Haga, R.; Saito, M.; Yoshioka, M. Chem.
Eur. J. 2008, 14, 4068.
(12) (a) Mizuhata, Y.; Sasamori, T.; Takeda, N.; Tokitoh, N. J. Am.
Chem. Soc. 2006, 128, 1050. (b) Mizuhata, Y.; Sasamori, T.; Nagahora, N.;
Watanabe, Y.; Furukawa, Y.; Tokitoh, N. Dalton Trans. 2008, 4409.
(13) Mizuhata, Y.; Takeda, N.; Sasamori, T.; Tokitoh, N. Chem.
Lett. 2005, 34, 1088.
€
(5) For a review on phosphabenzenes, see: Markl, G. In Multiple
Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M.,
Scherer, O. J., Eds.; Thieme: Stuttgart, Germany, 1990; Chapter D.5.
(6) For a review, see: Tokitoh, N. Acc. Chem. Res. 2004, 37, 86.
(7) Recent examples: (a) Sasamori, T.; Inamura, K.; Hoshino, W.;
Nakata, N.; Mizuhata, Y.; Watanabe, Y.; Furukawa, Y.; Tokitoh, N.
Organometallics 2006, 25, 3533. (b) Tokitoh, N.; Shinohara, A.; Matsumoto,
T.; Sasamori, T.; Takeda, N.; Furukawa, Y. Organometallics 2007, 26, 4048.
(c) Tanabe, Y.; Mizuhata, Y.; Tokitoh, N. Organometallics 2010, 29, 721.
r
2010 American Chemical Society
Published on Web 10/11/2010
pubs.acs.org/Organometallics

4782 Organometallics, Vol. 29, No. 21, 2010
Mizuhata et al.
Chart 1
Scheme 1. Synthesis of Bromostannanes 5-Bra
a Conditions: (a) ArSnX3 (X = Cl and Br), Et2O, -50 °C, 1 h;
(b) LiAlH4, THF, 0 °C, 1 h, 20% (5a-H from 4), 18% (5b-H from 4);
(c) NBS, benzene, room temperature, 1 h, 100% (5a-Br), 88% (5b-Br).
importance. Here, we report the generation and properties of
stannabenzenes 3, having the simplest aromatic unit: i.e., a
benzene skeleton (Chart 1).
At first, the Tbt-substituted 1-bromo-1-stannacyclohexa-
2,5-diene 5a-Br, a suitable precursor of 3a, was prepared by
the synthetic route shown in Scheme 1. (1Z,4Z)-1,5-Dilithio-
penta-1,4-diene (4) was prepared according to the reported
procedure14 and isolated by filtration of its hexane suspen-
sion under an inert atmosphere. Since the initial starting
material (TbtSnCl3) contained a considerable amount of
bromostannanes due to the ready halogen-exchange reac-
tions with LiBr inevitably generated in the preparation of
TbtLi (TbtBr þ 2 t-BuLi f TbtLi þ LiBr þ isobutane þ
isobutylene), the initially generated halostannanes were sub-
jected to LiAlH4 reduction followed by rebromination with
NBS in order to prepare the corresponding pure bromo-
stannanes. Even with the pure TbtSnBr3 thus prepared,12c
the reaction of TbtSnBr3 with 4 did not afford the desired
product 5a-Br exclusively but a complicated mixture con-
taining 5a-Br. After chromatographic separation, the molec-
ular structure of 5a-Br was confirmed by its spectral data
and finally established by X-ray crystallographic analysis
(Figure 1).
Figure 1. Thermal ellipsoid plot of 5a-Br (50% probability).
Hydrogen atoms, except for those on the SnC5 ring, were omitted
for clarity. Selected bond lengths (A) and angles (deg): Sn1-C1 =
2.115(3), Sn1-C5=2.124(3), Sn1-C6=2.150(2), Sn1-Br1=
2.5329(3), C1-C2=1.330(4), C2-C3=1.506(4), C3-C4=
1.498(4), C4-C5=1.330(4); C1-Sn1-C5=96.58(11), C1-
Sn1-C6=121.99(9), C5-Sn1-C6=118.80(9), C1-Sn1-Br1=
105.60(7), C5-Sn1-Br1=106.89(8), C6-Sn1-Br1=105.63(6),
C2-C1-Sn1=119.9(2), C1-C2-C3=130.1(3), C4-C3-C2=
121.8(2), C5-C4-C3=130.8(3), C4-C5-Sn1=119.2(2).
˚
Scheme 2. Generation of Stannabenzenes 3
The synthesis of 3a was attempted by the dehydrobromi-
nation of 5a-Br with lithium diisopropylamide (LDA) at -40 °C
and then at room temperature, resulting in the formation of
6a, the [4 þ 2] dimer of stannabenzene 3a, almost quantita-
tively (the isolated yield was 53% due to decomposition
during the purification) (Scheme 2). The molecular structure
of 6a was fully established by the results of spectroscopic
analysis. Although its dimeric structure was unambiguously
confirmed by X-ray crystallographic analysis, the detailed
structural parameters of 6a could not be discussed due to the
low quality of the crystals and severe disorder in the central
part (see Figure S1, Supporting Information). In any event,
the formation and isolation of 6a strongly suggested the
generation of 3a as a transient species.
under the same conditions. This result prompted us to
change the steric protection group from Tbt to Bbt in order
to improve the stability of stannabenzene. The correspond-
ing Bbt-substituted precursor 5b-Br could be synthesized
by a method similar to that for the synthesis of 5a-Br
(Scheme 1). Contrary to our expectation, the reaction of
5b-Br with LDA also readily afforded the [4 þ 2] dimer 6b at
room temperature, similarly to the case for the Tbt-substi-
tuted system (Scheme 2). When several trapping reactions of
3b by 2,3-dimethyl-1,3-butadiene at-78 °C were tried, dimer
6b had already formed. Although further investigations are
needed, the stability of stannabenzenes is considered to be
quite low.
Thus, the extremely high reactivity of stannabenzenes is
in sharp contrast with the considerable thermal stability of
Tbt-substituted sila- and germabenzenes (stable at 100 °C in
C6D6).8,9 To evaluate the differences in thermal stability
among sila-, germa-, and stannabenzenes, theoretical calcu-
lations were performed. In Table 1, selected parameters for
the optimized structures of 3a,b are shown together with the
related parameters of Tbt-substituted sila- and germa-
benzenes experimentally observed. The SnC5 rings of 3a,b
have perfectly planar geometries (sum of the internal angles
of SnC5 rings 720°) similar to those of Tbt-substituted sila- and
In the case of 1-silanaphthalene,15 the Tbt-substituted species
was marginally stable enough in solution to make observa-
tion of its NMR spectra possible and underwent gradual
dimerization at room temperature. On the other hand, the
Bbt-substituted 1-silanaphthalene was found not to dimerize
€
(14) Jutzi, P.; Baumgartner, J.; Schraut, W. J. Organomet. Chem.
1977, 132, 333.
(15) (a) Takeda, N.; Shinohara, A.; Tokitoh, N. Organometallics
2002, 21, 4024. (b) Shinohara, A.; Takeda, N.; Sasamori, T.; Tokitoh, N.
Bull. Chem. Soc. Jpn. 2005, 78, 977.

Communication
Organometallics, Vol. 29, No. 21, 2010 4783
Table 1. Selected Structural Parameters of Metallabenzenesa
˚
a/A
˚
b/A
˚
c/A
compd
R/deg
β/deg
γ/deg
δ/deg
TbtSiC5H5 (obsd)b
1.765(4)
1.770(4)
1.829(2)
1.827(2)
2.026
2.026
2.018
2.019
1.391(6)
1.394(7)
1.389(3)
1.396(3)
1.396
1.397
1.396
1.389
1.399(6)
1.381(6)
1.389(3)
1.385(3)
1.407
1.407
1.406
1.402
107.8(2)
117.7(3)
118.0(3)
117.88(15)
117.82(16)
117.12
116.67
116.42
115.87
126.3(4)
126.1(4)
126.55(19)
126.55(19)
128.49
126.81
128.47
128.73
124.1(4)
TbtGeC5H5 (obsd)c
105.81(9)
125.48(18)
3a (calcd)d
100.97
103.89
102.01
102.39
127.87
129.14
128.20
128.42
3b (calcd)d
HSnC5H5 (calcd)d
HSnC5H5 (calcd)e
a
b Reference 8. c Reference 9. d Calculated at the B3LYP/6-31G(d) (LANL2DZ on Sn) level. e Calculated at the B3LYP/6-311þG(2d,p) (TZ(2d) on Sn)
level.
Figure 3. Energy levels of HOMOs and LUMOs of HEC5H5
(E = Si, Ge, Sn) calculated at the B3LYP/6-311þG(2d,p) (TZ(2d)
on Si, Ge, Sn) level.
Figure 2. LUMOs (a) and HOMOs (b) of HEC5H5 (E=Si,Ge,Sn)
calculated at the B3LYP/6-311þG(2d,p) (TZ(2d) on Si, Ge, Sn)
of benzene (-10.0). In Figures 2 and 3, the energy levels and
shapes of the HOMOs and LUMOs are summarized for the
level.
parent metallabenzenes. While the shapes of their HOMOs
germabenzenes, implying efficient conjugation in the SnC5
rings. The lengths of the two Sn-C bonds in each stanna-
benzene ring of structurally optimized 3a,bwere found to be equal
and LUMOs resemble each other, the coefficients on the
E atoms slightly get larger, following the order of Si to Sn.
The HOMO-LUMO energy gaps, correlating with the
reactivity of the Diels-Alder type dimerization, change
dramatically (Figure 3), and the difference between the cases
of Si and Sn is about 1 eV. These results reasonably explain
the high reactivity of stannabenzenes.
to each other and similar to those of the reported all-carbon-
substituted Sn-C double bonds (2.003(5),16 2.016(5) A )
17
˚
and the shorter Sn-C bond of the isolated 2-stannanaphthalene
12
˚
(2.029(6) A ). Furthermore, the C-C bond lengths in the
stannabenzene rings of structurally optimized 3a,b are almost
Furthermore, the energy differences between the mono-
mer and dimer of Tbt-substituted sila- and stannabenzenes
were calculated at the B3LYP/6-31G(d) (LANL2DZ on Sn)
level. Although a detailed investigation of the transition
states in the dimerization reaction of 3a has not yet been
achieved, the formation of 6a was found to be exothermic
(-11.7 kcal/mol), in contrast to the endothermic dimeriza-
tion of Tbt-substituted silabenzene (þ10.5 kcal/mol).8c
In the case of the silabenzene dimer, the steric repulsion
between the two Tbt groups is not negligible. In the case of
6a, on the other hand, elongation of the bond lengths around
the tin atoms was considered to allow more facile formation
of its dimer. As a result of the combination of structural
demand and electronic features, stannabenzene might under-
go ready dimerization, showing a striking difference from
sila- and germabenzenes.
In summary, we have succeeded in the generation of
stannabenzenes 3 for the first time and revealed their high
reactivity. With the hope of isolating 3 as a stable compound,
further investigation into the introduction of additional
substituent(s) to the stannabenzene skeleton and the co-
operative stabilization method (the contribution of kinetic
and thermodynamic stabilization) is currently in progress.
equal to each other and are also almost similar to the C-C
1
˚
length of the parent benzene (1.39-1.40 A). Thus, it has
been theoretically demonstrated that the stannabenzene
skeleton should have a delocalized six-π-electron ring system
similar to that of benzene. On the other hand, the difference
between the Sn-C and Ge-C bond lengths was found to be
˚
0.198 A, which is much longer than those between the Ge-C
˚
and Si-C bond lengths (0.06 A). This marked elongation
may result in the insufficient protection of the reactive Sn
center of the stannabenzene skeleton.
The aromaticity of stannabenzene was evaluated by NICS
calculations (GIAO-B3LYP/6-311þG(2d,p) (TZV on Sn)//
B3LYP/6-31G(d) (LANL2DZ on Sn)).18 The NICS(1) value
of the parent stannabenzene is -8.1, the absolute value of
which is slightly small as compared with those for sila- and
germabenzenes (-8.5) and is estimated as about 80% of that
(16) Fatah, A.; El Ayoubi, R.; Gornitzka, H.; Ranaivonjatovo, H.;
ꢀ
Escudie, J. Eur. J. Inorg. Chem. 2008, 2007.
(17) Mizuhata, Y.; Takeda, N.; Sasamori, T.; Tokitoh, N. Chem.
Commun. 2005, 5876.
(18) Schleyer, P. v. R.; Manoharan, M.; Wang, Z.-X.; Kiran, B.; Jiao,
H.; Puchta, R.; Hommes, N. J. R. v. E. Org. Lett. 2001, 3, 2465.

4784 Organometallics, Vol. 29, No. 21, 2010
Mizuhata et al.
Acknowledgment. This work was supported by
Grants-in-Aid for Creative Scientific Research (No.
17GS0207), Scientific Research (B) (No. 22350017),
Science Research on Priority Areas (No. 20036024,
“Synergy of Elements”), Young Scientist (B) (No. 21750042),
and the Global COE Program (B09, “Integrated Materials
Science”) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan. Y.M. thanks
Mitsubishi Chemical Corp. and the Society of Synthetic
Organic Chemistry of Japan for financial support.
Supporting Information Available: Text, figures, and tables
giving details of preparation procedures, characterization data,
X-ray analyses of 5a-Br and 6a, and theoretical calculations and
CIF files giving the X-ray crystallographic data of 5a-Br and 6a.
This material is available free of charge via the Internet at http://
pubs.acs.org.