Synthesis and properties of a stable 6-stannapentafulvene

Article abstract of DOI:10.1039/b512097g

The first donor-free 6-stannapentafulvene 1a stable at ambient temperature was synthesized and isolated, exhibiting the shortest tin-carbon bond length among those previously reported. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b512097g

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Synthesis and properties of a stable 6-stannapentafulvene{
Yoshiyuki Mizuhata, Nobuhiro Takeda, Takahiro Sasamori and Norihiro Tokitoh*
Received (in Cambridge, UK) 25th August 2005, Accepted 26th September 2005
First published as an Advance Article on the web 20th October 2005
DOI: 10.1039/b512097g
n- or tert-butyllithium as a base resulted in some side reactions,
The first donor-free 6-stannapentafulvene 1a stable at ambient
temperature was synthesized and isolated, exhibiting the
shortest tin–carbon bond length among those previously
reported.
such as direct substitution on the tin atom or reduction of the
Sn–Cl bond.
6-Stannapentafulvene 1a was successfully synthesized by the
dehydrofluorination of 7 using tert-butyllithium as a base in Et2O
at 240 uC (Scheme 2).{ Initially the reaction mixture became
yellow solution indicating the formation of the lithiated compound
8, and then it turned deep violet at room temperature. Removal of
In the past few decades, much attention has been focused on the
chemistry of heavier congeners of alkenes, i.e., ‘‘metallenes and
dimetallenes’’ (.MLC, and .MLM,: M 5 Si, Ge, Sn, Pb),1
and a number of reports have appeared on their syntheses,
structures, and properties. In the field of metallenes, the chemistry
of stannenes (.SnLC,) has been undeveloped due to their high
reactivity toward self-oligomerization in contrast to the relatively
well-established chemistry of silenes (.SiLC,) and germenes
(.GeLC,). In view of the small overlap between the p orbitals of
tin and carbon atoms, one can make a natural question whether a
concrete SnLC double bond can exist as a stable stannene in spite
of the weak p-bond. Although several reports on the theoretical
calculations of a parent stannene, H2SnLCH2, predicted its double
bond character,2 no clear conclusion can be drawn from only
theoretical studies and the experimental verification has long been
sought for an SnLC double bond. The structural analyses of the
SnLC double-bond compounds ever reported are limited to only
two types, both of which are thermodynamically stabilized by
heteroatoms such as B and/or N. While Berndt et al. reported a
LiF and the solvents afforded violet crystals of 1a, the 1H and 13
C
NMR spectra of which showed no coordinated Et2O. Compound
1a was found to be thermally stable under an inert atmosphere in
solution (benzene-d6, at 80 uC in a sealed tube) and highly
moisture- and air-sensitive. Indeed, 1a undergoes a ready addition
reaction with water across its SnLC moiety to give the
corresponding hydroxystannane 9 (76%, Scheme 2).
The molecular geometry of 1a was determined by X-ray
crystallographic analysis (Fig. 2).§ The structural parameters for 1a
are summarized in Table 1 together with the calculated values for a
series of diboryl-substituted stannenes 2 having an extremely short
3
˚
˚
˚
SnLC bond [2a (2.025 A), 2c (2.036 A) and 2d (2.032 A)], the
formal SnLC compounds such as 3,4 4,5 and 5,6 the Sn–C bond
˚
lengths of which (3: 2.314 A, 4a: 2.290 A, 4b: 2.379 A, 5: 2.303 A)
˚
˚
˚
are markedly longer than the typical Sn–C single bond lengths (av.
7
2.14 A), have also been synthesized and characterized (Fig. 1).
˚
Although 6-stannapentafulvene 1b bearing only carbon substi-
tuents on the SnLC moiety was synthesized by Escudie´ et al.,8 1b
was not structurally characterized due to its ready dimerization at
room temperature. In order to study the chemical behavior of tin–
carbon double bond in detail, we examined the synthesis of
stannene 1a kinetically stabilized by the combination of an
extremely bulky, effective steric protection group, 2,4,6-tris[bis-
(trimethylsilyl)methyl]phenyl (denoted as Tbt), and Mes group.
Here, we report the synthesis and structure of the donor-free
6-stannapentafulvene 1a, the first tetraarylstannene stable at
ambient temperature.
Fig. 1
Fluorenyl(fluoro)stannane 7, a suitable precursor of 1a, was
prepared according to Scheme 1. Although chlorostannane 6
was also considered to be a good precursor, the reactions of 6 with
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto,
611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp;
Fax: (+81) 774-38-3209; Tel: (+81) 774-38-3200
{ Electronic supplementary information (ESI) available: Experimental
procedures and physical properties for new compounds and the optimized
structure of 1c. See DOI: 10.1039/b512097g
Scheme 1 Synthesis of the precursor for 1a.
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were assigned to the skeletal vibrations on the 6-stannapentafulvene
plane based on the theoretical calculations. The experimentally
observed spectrum showed close resemblance with the simulated
spectrum for 1c as shown in Fig. 3, indicating that the calculated
structure for 1c reproduces the characters of 1a adequately.
The 119Sn NMR spectrum of 1a in C6D6 showed a signal at
270 ppm, which is characteristic of doubly bonded tin derivatives.
This value is slightly upfield-shifted in comparison to that of 1b
(288 ppm). The assignments of the 1H, 13C and 119Sn NMR signals
are listed in Table 2 along with the calculated values for 1c. The
observed values are in good agreement with the calculated ones
except for the case of 119Sn NMR at B3LYP level (central row). At
MPW1PW91 level (right row), which was evidenced to be effective
for the GIAO calculations for Tip3Sn+,11 the calculated values for
1c are consisted with the observed ones for 1a. The 13C NMR
signal of the SnLC (144.9 ppm) reasonably appeared in the sp2
region. It is noteworthy that all the signals of the fluorenylidene
unit were observed nonequivalent probably due to the restricted
rotation of the Sn–C bond, indicating that doubly bonded
structure 1a rather than 1a9 (Scheme 3) might be dominant in
solution. In addition, 1a reacts with 2,3-dimethyl-1,3-butadiene at
Scheme 2
Fig. 2 ORTEP drawing of 6-stannapentafulvene 1a (50% probability).
Hydrogen atoms are omitted for clarity.
˚
Table 1 Observed and calculated bond lengths (A) and angles (u) for
6-stannapentafulvenes
1a: obs.
2.016(5)
1.452(6)/1.456(6)
1.410(6)/1.425(6)
1.446(6)
1c: calc.a
Fig. 3 Raman spectra of 6-stannapentafulvenes. Solid line: FT-Raman
spectrum of 1a measured with the excitation by He–Ne laser (833 nm).
Dashed line: Spectrum of 1c simulated by the theoretical calculation at the
B3LYP/6-31G(d) [TZ(2d) on Sn] level.
Sn–C1
1.998
1.465
1.426
1.457
C1–C2/C1–C13
C2–C7/C8–C13
C7–C8
Table 2 Observed and calculated 1H, 13C and 119Sn NMR chemical
shifts (ppm) for 6-stannapentafulvenes
C14–Sn1–C41
C1–Sn1–C14/C1–Sn1–C41
C2–C1–C13
C2–C1–Sn1/C13–C1–Sn1
a
Calculated at the B3LYP/6-31G(d) (LANL2DZ on Sn) level.
118.27(17)
110.15
124.92
106.19
126.93
127.92(19)/113.67(19)
106.3(4)
128.3(4)/125.3(3)
model compound 1c. The bond length of the Sn1–C1 bond
[2.016(5) s] is the shortest among those of tin–carbon bonds ever
reported. In addition, it is ca. 6% shorter than the typical Sn–C
1a: obs.a
1c: calc.b
1c: calc.c
7
˚
single bond length (ca. 2.14 A) and is close to the observed values
˚
for 2 and that calculated for 1c (1.998 A). The structural analysis
Sn
270
185
259
revealed the completely trigonal planar geometry around the Sn1
(359.9u) and C1 (359.9u) atoms. The large twisted angle between
the C14–Sn1–C41 plane and the fluorenylidene moiety (28.5u) is
probably due to the steric reason, since such twisted structures
have already been observed in bifluorenylidene9 (43u) and
Mes2GeLC(fluorenylidene)10 (5.9u). Although the calculated para-
meters for 1c are almost similar to those observed for 1a, the
twisted angle around the SnLC unit of 1c (13.1u) is smaller than
that of 1a probably due to the lower bulkiness of the substituents
on the tin atom.
H3 (H12)
H4 (H11)
H5 (H10)
H6 (H9)
C1
C2 (C13)
C3 (C12)
C4 (C11)
C5 (C10)
C6 (C9)
C7 (C8)
a
7.61 (7.78)
8.22
7.11
7.18
7.94
141.16
147.11
120.18
120.49
125.44
125.13
136.87
7.97
7.02
7.07
7.70
137.23
138.76
115.27
120.06
119.06
114.90
128.88
7.29 (7.06)
7.25 (7.15)
7.91 (7.89)
144.91
145.04 (146.39)
119.78 (120.18)
124.13 (120.59)
125.23 (126.64)
122.84 (122.99)
134.24 (134.40)
b
Measured in benzene-d6. Calculated at the GIAO-B3LYP/6-
311+G(2d,p) (TZV on Sn)//B3LYP/6-31G(d) (LANL2DZ on Sn)
level. Calculated at the GIAO-MPW1PW91/6-31G(d) (TZV on
In-plane vibration modes for the 6-stannapentafulvene skeleton
of 1a were observed by the Raman spectra. A part of the Raman
spectrum of 1a is shown in Fig. 3. The signals at 287 and 674 cm21
c
Sn)//B3LYP/6-31G(d) (LANL2DZ on Sn) level.
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Notes and references
{ 1a: Purple crystalline solid; mp 167–171 uC (decomp.); 1H NMR
(300 MHz, C6D6, 25 uC): d 20.02 (s, 9H), 0.02 (s, 9H), 0.12 (s, 18H), 0.18
(s, 9H), 0.26 (s, 9H), 1.56 (s, 1H), 1.91 (s, 1H), 2.06 (s, 1H), 2.10 (s, 3H),
2.59 (br s, 3H), 2.66 (s, 3H), 6.73 (br s, 1H), 6.84 (br s, 1H + 2H), 7.06 (ddd,
3J 5 8 Hz, 3J 5 8 Hz, 4J 5 2 Hz, 1H), 7.15 (dd, 3J 5 8 Hz, 3J 5 8 Hz, 1H),
7.25 (ddd, 3J 5 7 Hz, J 5 6 Hz, 4J 5 2 Hz, 1H), 7.29 (ddd, J 5 7 Hz,
3J 5 7 Hz, 4J 5 2 Hz, 1H), 7.61 (dd, 3J 5 7 Hz, 4J 5 2 Hz, 1H), 7.78 (d,
3J 5 8 Hz, 1H), 7.89 (dd, 3J 5 8 Hz, 4J 5 2 Hz, 1H), 7.91 (dd, 3J 5 6 Hz,
4J 5 2 Hz, 1H); 13C NMR (75 MHz, C6D6, 25 uC): d 1.07 (q), 1.20 (q), 1.45
(q), 1.65 (q), 21.13 (q), 27.62 (q), 28.63 (q), 31.25 (d), 35.96 (d), 37.59 (d),
119.78 (d), 120.18 (d), 120.59 (d), 122.84 (d), 122.99 (d), 123.20 (d), 124.13
(d), 125.23 (d), 125.64 (d), 128.29 (d), 129.12 (d), 129.40 (d), 134.24 (s),
134.40 (s), 140.61 (s), 142.75 (s), 143.35 (s), 143.62 (s), 144.91 (s), 145.04 (s),
145.87 (s), 146.39 (s), 150.73 (s), 151.33 (s); 119Sn NMR (111 MHz, C6D6,
25 uC): d 270; UV/vis (hexane, rt) 552 nm (e 1 6 104); High resolution
FAB-MS m/z: calc. for C49H79Si6120Sn: 955.3827, found: 955.3851.
§ Crystal data for 1a: C49H78Si6Sn, M 5 953.34; monoclinic, space group
3
3
Scheme 3
Scheme 4
˚
C2/c (no. 15); a 5 21.0447(6), b 5 13.5070(5), c 5 38.1717(13) A,
3
b 5 96.1949(12)u, V 5 10787.0(6) A ; Z 5 8; Dc 5 1.175 g cm23; m 5
˚
0.636 mm21; 2hmax 5 50u; T 5 103 K; R1(I . 2s(I)) 5 0.0488; wR2 (all
data) 5 0.1065; GOF 5 1.001 for 32670 reflections and 526 parameters.
Crystal data for 10: C55H88Si6Sn?0.5C6H6 M 5 1075.54; triclinic, space
¯
˚
group P1 (no. 2); a 5 12.347(2), b 5 13.0052(18), c 5 20.418(4) A;
3
˚
a 5 71.537(7), b 5 81.570(7), c 5 74.856(7)u, V 5 2994.7(8) A ; Z 5 2;
Dc 5 1.193 g cm23; m 5 0.581 mm21; 2hmax 5 50u; T 5 103 K; R1(I .
2s(I)) 5 0.0350; wR2 (all data) 5 0.0750; GOF 5 1.068 for 19733
reflections and 667 parameters.
CCDC 282727 and 282728. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b512097g
1 For reviews, see: (a) K. M. Baines and W. G. Stibbs, Adv. Organomet.
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Fig. 4 ORTEP drawing of 10 (50% probability). Hydrogen atoms and
benzene molecule are omitted for clarity.
room temperature to afford the [2 + 4] cycloadduct 10 in 51% yield
(Scheme 4, Fig. 4§), suggesting that 1a has an SnLC double-bond
character rather than an ionic character (1a9) from the viewpoints
of the chemical reactivity.
In summary, we succeeded in the synthesis and X-ray crystal-
lographic analysis of 6-stannapentafulvene 1a stable at ambient
temperature for the first time. Judging from the NMR spectra,
molecular structure, and reactivities, 1a has a sufficient SnLC
double-bond character. These results demonstrate a possibility of
forming a p-bond between tin and carbon atoms, when it is
kinetically well stabilized.
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This work was supported by Grants-in-Aid for Scientific
Research (Nos 12CE2005, 14078213, 14204064, 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 University, for the
measurement of FT-Raman spectra. This manuscript was written
at TU Braunschweig during the tenure of a von Humboldt Senior
Research Award of one of the authors (N. Tokitoh), who is
grateful to Alexander von Humboldt Stiftung for their generosity
and to Professors Reinhard Schmutzler, Wolf-Walther du Mont,
and Matthias Tamm for their warm hospitality at TU
Braunschweig. Y. M. thanks Research Fellowships of the Japan
Society for the Promotion of Science for Young Scientists.
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5878 | Chem. Commun., 2005, 5876–5878
This journal is ß The Royal Society of Chemistry 2005