A stable neutral stannaaromatic compound: Synthesis, structure and complexation of a kinetically stabilized 2-stannanaphthalene

Article abstract of DOI:10.1021/ja057531d

A kinetically stabilized 2-stannanaphthalene, the first example of a stable, neutral aromatic compound containing a tin atom, has been synthesized and fully characterized. The results of spectroscopic and crystallographic structural analyses of the compou

Full text of DOI:10.1021/ja057531d

Published on Web 01/06/2006
A Stable Neutral Stannaaromatic Compound: Synthesis, Structure and
Complexation of a Kinetically Stabilized 2-Stannanaphthalene
Yoshiyuki Mizuhata, Takahiro Sasamori, Nobuhiro Takeda, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan
Received November 3, 2005; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
Scheme 1. Synthesis of 2-Stannanaphthalene 1aa
Aromatic hydrocarbons such as benzene and naphthalene are
among the most fundamental class of organic compounds and play
very important roles in organic chemistry. By contrast, the chemistry
of metallaaromatic compounds of heavier group 14 elements, i.e.,
heavier congeners of the cyclic conjugated systems with [4n + 2]-π-
electrons, showed a marked development only in the past decade,
and the examples for the isolation of stable systems are limited to
those containing Si or Ge atom.1 Recently, we have succeeded in
the synthesis and isolation of the first stable examples for neutral
a Conditions (a) n-BuLi (2.0 equiv), THF, -78 °C; (b) TbtSnX3 (X )
sila- and germaaromatic compounds by taking advantage of kinetic
stabilization afforded by an efficient steric protection group, 2,4,6-
tris[bis(trimethylsilyl)methyl]phenyl (Tbt).2 In view of the recent
Cl or Br, ca. 1 equiv), THF, -78 °C; (c) LiAlH4 (excess), THF, 0 °C, 37%
(from 2); (d) NBS (1.1 equiv), benzene, rt, 91%; (e) LDA (1.1 equiv),
hexane, -40 °C to room temperature, 63%.
progress in the chemistry of sila- and germaaromatic compounds,
the synthesis of stannaaromatic compounds is of great interest from
the standpoint of systematic elucidation of the properties of
metallaaromatic systems of heavier group 14 elements. Although
some stannole anions and dianions, i.e., ionic stannaaromatic com-
pounds, have been synthesized as stable compounds and fully
characterized,3 neutral stannaaromatic compounds are still elusive,
and their properties have not been disclosed yet so far. The main
reason for the lack of neutral stannaaromatic compounds is probably
due to the difficulty in their synthesis and isolation as a result of
the extremely high reactivity of Sn-C double bonds.4 In fact, 9-Tbt-
9-stannaphenanthrene was found to undergo ready dimerization at
ambient temperature most likely due to the insufficient steric
Figure 1. ORTEP drawing (50% probability) of 1a with nonhydrogen
atoms. Selective bond lengths (Å) and angles (deg): Sn1-C1 2.029(6),
protection.5 Here, we report the synthesis of the first stable neutral
stannaaromatic compound, 2-stannanaphthalene 1a, which is kineti-
cally stabilized by the combination of a Tbt group on the tin atom
and a t-Bu group on the adjacent carbon atom (Chart 1).
Sn1-C2 2.081(6), Sn1-C14 2.132(5), C2-C3 1.372(9), C2-C10 1.522-
(9), C3-C9 1.443(9), C4-C9 1.417(9), C4-C5 1.356(9), C5-C6 1.415-
(10), C6-C7 1.361(9), C7-C8 1.419(9), C8-C9 1.436(9), C1-C8
1.394(8), C1-Sn1-C2 100.0(3), C1-Sn1-C14 123.0(2), C2-Sn1-C14
137.0(2), Sn1-C2-C3 114.3(5), C2-C3-C9 132.7(6), C3-C9-C8 126.7-
(6), C1-C8-C9 124.6(6), Sn1-C1-C8 121.4(5), C5-C4-C9 123.5(7).
C4-C5-C6 119.0(6), C5-C6-C7 119.5(7), C6-C7-C8 123.0(6), C7-
C8-C9 117.5(6), C4-C9-C8 117.4(6).
Chart 1
2-stannanaphthalene unit and the completely trigonal planar ge-
ometry around the tin atom. In addition, the benzene ring of the
Tbt group was found to be almost perpendicular to the 2-stanna-
naphthalene plane; hence, it is considered that there is very little
conjugative interaction between the two aromatic units. The lengths
of the two endocyclic Sn-C bonds [2.029(6) and 2.081(6) Å] are
shorter than those of typical single bonds (av 2.14 Å).7 In particular,
the former Sn-C bond length is close to the Sn-C double bond
length of the previously reported stannene, Tbt(Mes)SndCR2
[CR2 ) fluorenylidene; 2.016(5) Å],8 which is stabilized by the
conjugation of the Sn-C double bond with the fluorenylidene
moiety. The C-C bond lengths of the 2-stannanaphthalene ring of
1a [1.356(9)-1.443(9) Å] are also roughly intermediate between
those of C-C double and single bonds. These results suggest that
the π-electrons are delocalized in the 2-stannanaphthalene skeleton
of 1a. Furthermore, theoretical calculations for the model molecules
1b-d supported the experimental results, indicating the effective
1,2-Dihydro-2-stannanaphthalene 4 was prepared by taking
advantage of (E)-o-(2′-lithiovinyl)benzyllithium 3, which can be
readily generated from isotellurochromene 2 according to the
method reported by Sashida.6 The following bromination of 4 with
NBS afforded bromostannane 5, a suitable precursor of 2-stanna-
naphthalene 1a (Scheme 1). 2-Stannanaphthalene 1a was synthe-
sized as pale-yellow crystals by the dehydrobromination of
bromostannane 5 with LDA in hexane at -40 °C (Scheme 1).
2-Stannanaphthalene 1a is thermally stable under an inert atmo-
sphere either in the solid state (decomposed at 144 °C) or in solution
(C6D6, at 80 °C for 1 h in a sealed tube).
The molecular structure of 1a was determined by X-ray crystal-
lographic analysis (Figure 1), which revealed the planarity of the
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J. AM. CHEM. SOC. 2006, 128, 1050-1051
10.1021/ja057531d CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have succeeded in the synthesis and character-
ization of 2-stannanaphthalene 1a, the first example of a stable,
neutral aromatic compound containing a tin atom. The results of
spectroscopic and crystallographic structural analyses and theoretical
calculations strongly suggest that 1a has a delocalized 10-π-electron
ring system as does naphthalene. These findings are of great
importance for the chemistry of aromatic compounds containing
heavier group 14 elements.
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research (Nos. 12CE2005, 14078213, 14204064,
16750033, 17GS0207, and 17•50542), and the 21st Century COE
Program on Kyoto University Alliance for Chemistry from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. We are grateful to Prof. Yukio Furukawa, Waseda Univer-
sity, for the measurement of FT-Raman spectra. Y. M. thanks
Research Fellowships of the Japan Society for the Promotion of
Science for Young Scientists.
Figure 2. ORTEP drawing (30% probability) of 6a with nonhydrogen
atoms. Selective bond lengths (Å) and angles (deg): Sn1-C1 2.035(5),
Sn1-C2 2.093(4), Sn1-C17 2.128(4), C2-C3 1.379(6), C2-C10 1.533-
(6), C3-C9 1.443(6), C4-C9 1.439(6), C4-C5 1.353(6), C5-C6 1.408-
(7), C6-C7 1.363(6), C7-C8 1.448(6), C8-C9 1.427(6), C1-C8 1.425(6),
Sn1-Cr1 2.7537(8), C1-Sn1-C2 94.36(18), C1-Sn1-C17 124.63(18),
C2-Sn1-C17 140.05(17), Sn1-C2-C3 118.3(3), C2-C3-C9 132.5(4),
C3-C9-C8 124.5(4), C1-C8-C9 123.1(4), Sn1-C1-C8 125.9(4), C5-
C4-C9 122.1(4). C4-C5-C6 119.8(4), C5-C6-C7 120.8(5), C6-C7-
C8 121.5(5), C7-C8-C9 117.2(4), C4-C9-C8 118.5(4).
Supporting Information Available: Experimental procedures and
spectral data for new compounds, crystallographic data for 1a and 6a,
Tables S1 and S2, Figure S1, UV/vis spectrum of 1a (Figure S2), and
the optimized structures of 1b-d (Tables S3-S5). This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 2. Synthesis of 6a
References
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The 119Sn NMR spectrum of 1a in C6D6 showed a signal at 264
ppm, which is characteristic of the low-coordinated tin atom.9 All
the 1H NMR signals of the 2-stannanaphthalene ring protons of 1a
(7.05-9.28 ppm) were observed in the aromatic region, and the
13C NMR signals of the stannanaphthalene ring carbons (120.0-
174.0 ppm) were located in the sp2 region. Thus, these results clearly
indicate the delocalized π-electronic system of 1a even in solution.
The Raman spectrum of 1a (Figure S1, Supporting Information)
showed planar skeletal vibration as the most intense Raman signal
at 1331 cm-1, which corresponds to those of 1382, 1368, and 1360
cm-1 for naphthalene, 2-silanaphthalene,2a and 2-germanaphthalene,2g
respectively. The calculated vibration modes of 1c resemble
considerably those of naphthalene, suggesting the structural similar-
ity between 2-stannanaphthalene 1a and naphthalene.10
(3) (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,
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Tbt-substituted sila- and germabenzene are known to behave as
well as the conventional arene systems in the complexation with
transition metal carbonyl complexes.11 Similarly, the ligand ex-
change reaction of 1a with [Cr(CH3CN)3(CO)3] at room temperature
in THF resulted in the regioselective formation of the first η6-2-
stannanaphthalene chromium complex 6a as brown crystals in 89%
yield (Scheme 2). The X-ray crystallographic analysis of 6a (Figure
2) revealed that 6a still keeps the planarity for the 2-stannanaph-
thalene moiety [sum of the interior bond angles in the ring A (Chart
1): 718.7°] and the trigonal planar geometry around the tin atom
(sum of the bond angles: 359.0°). The lengths of the two endocyclic
Sn-C bonds of 6a [2.035(5) and 2.093(4) Å] are slightly longer
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(9) The assignments of the 1H, 13C, and 119Sn NMR signals are listed in Table
S2 (Supporting Information) along with the calculated values for 1b-d.
The observed values are in good agreement with the calculated ones.
1
than those of 1a [2.029(6) and 2.081(6) Å]. In H, 13C, and 119Sn
NMR spectra of 6a, the signals corresponds to the atoms in the
ring A (δSn: 106, δH1: 5.07, δH2: 6.44, δC: 88.4-131.3) were
shifted upfield relative to those for the free 1a (δSn: 264, δH1: 9.28,
(10) This strongest vibrational frequency observed for 1a is in good agreement
with the theoretically calculated one [1378 cm-1 for 1b, computed at the
B3LYP/6-31G(d) (LANL2DZ on tin atom) level].
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δH2: 8.75, δC: 120.0-174.0). The IR spectrum (KBr) of 6a showed
the presence of three intense ν(CO) bands at 1941, 1862, and 1851
cm-1, which were observed in the region similar to those of [η6-
(naphthalene)Cr(CO)3] [1941 and 1864 cm-1 (KBr)12]. The result
suggests that 1a has coordination ability as an arene ligand as well
as that of naphthalene.
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