(tBu2MeSi)2Sn=Sn(SiMetBu 2)2: A distannene with a > SN=SN < double bond that is stable both in the solid state and in solution

Article abstract of DOI:10.1021/ja063322x

(^tBu₂MeSi)₂Sn=Sn(SiMe^tBu ₂)₂ 1, prepared by the reaction of ^tBu ₂MeSiNa with SnCl₂-diox in THF and isolated as dark-green crystals, represents the first exa

Full text of DOI:10.1021/ja063322x

Published on Web 08/11/2006
(tBu2MeSi)2SndSn(SiMetBu2)2: A Distannene with a >SndSn<
Double Bond That Is Stable Both in the Solid State
and in Solution
Vladimir Ya. Lee,† Tomohide Fukawa,† Masaaki Nakamoto,† Akira Sekiguchi,*,†
Boris L. Tumanskii,‡ Miriam Karni,‡ and Yitzhak Apeloig‡
Contribution from Department of Chemistry,Graduate School of Pure and Applied Sciences,
UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, and Department of Chemistry,
Technion-Israel Institute of Technology, Haifa 32000, Israel
Received May 12, 2006; E-mail: sekiguch@staff.chem.tsukuba.ac.jp
Abstract: (tBu2MeSi)2SndSn(SiMetBu2)2 1, prepared by the reaction of tBu2MeSiNa with SnCl2-diox in THF
and isolated as dark-green crystals, represents the first example of acyclic distannene with a SndSn double
bond that is stable both in the crystalline form and in solution. This was proved by the crystal and NMR
spectral data of 1. Distannene 1 has these peculiar structural features: a shortest among all acyclic
distannenes SndSn double bond of 2.6683(10) Å, a nearly planar geometry around both Sn atoms, and
a highly twisted SndSn double bond. The reactions of 1 toward carbon tetrachloride and phenylacetylene
also correspond to the reactivity anticipated for the SndSn double bond. The one-electron reduction of 1
with potassium produced the distannene anion radical, the heavy analogue of alkene ion radicals, for which
the particular crystal structure and low-temperature EPR behavior are also discussed.
double bond in organic chemistry.3a The progressively increasing
difference between the size and energy of s- and p-orbitals
Introduction
The distannenes, that is, the compounds featuring SndSn
double bonds, have been known for nearly 30 years, since the
first stable alkene analogue of group 14 elements heavier than
carbon was reported by Lappert in 1976.1 This breakthrough
discovery of Dis2SndSnDis2 (Dis ) CH(SiMe3)2) has, in fact,
opened the door to a broad avenue of heavy alkene analogues
of group 14 elements, one of the most compelling topics of
modern organometallic chemistry. Since then the chemistry of
heavy alkene analogues, known also as dimetallaalkenes or
dimetallenes, has been greatly developed in all aspects: syn-
thetic, structural, reactivity. To date, many representatives of
both homonuclear (disilenes, digermenes, distannenes, di-
plumbenes) and heteronuclear (silenes, germenes, stannenes,
silagermenes, silastannenes, germastannenes) heavy alkene
analogues have been synthesized and, in a majority of cases,
structurally characterized.2 The striking progress in this field is
reflected in the number of recent reviews devoted to this topic.3
Very recently, unsaturated small (three- and four-membered)
cyclic compounds composed of heavier group 14 elements also
became synthetically accessible as a new class of cyclic heavy
alkene analogues.4
descending group 14 makes them more reluctant for sp2-type
hybridization in its classical sense, particularly for the heaviest
(2) Only the first stable representatives for each class of the heavy alkenes are
given below. Disilene: (a) West, R.; Fink. M. F.; Michl, J. Science 1981,
214, 1343. Digermene: (b) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J.;
Thorne, A. J. J. Chem. Soc., Chem. Commun. 1984, 480 and ref 1b.
Distannene: (c) see ref 1. Diplumbene: (d) Stu¨rmann, M.; Saak, W.;
Marsmann, H.; Weidenbruch, M. Angew. Chem., Int. Ed. 1999, 38, 187.
Silene: (e) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.;
Kallury, R. K. M. R. J. Chem. Soc., Chem. Commun. 1981, 191.
Germenes: (f) Couret, C.; Escudie´, J.; Satge, J.; Lazraq, M. J. Am. Chem.
Soc. 1987, 109, 4411 and (g) Meyer, H.; Baum, G.; Massa, W.; Berndt, A.
Angew. Chem., Int. Ed. Engl. 1987, 21, 221. Stannene: (h) Meyer, H.;
Baum, G.; Massa, W.; Berger, S.; Berndt, A. Angew. Chem., Int. Ed. Engl.
1987, 26, 546. Silagermene: (i) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A.;
Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 9034. Silastannene:
(j) Sekiguchi, A.; Izumi, R.; Lee, V. Ya.; Ichinohe, M. J. Am. Chem. Soc.
2002, 124, 14822. Germastannenes: (k) Schafer, A.; Saak, W.; Weiden-
bruch, M. Organometallics 2003, 22, 215. (l) Sekiguchi, A.; Izumi, R.;
Lee, V. Ya.; Ichinohe, M. Organometallics 2003, 22, 1483 and (m) Lee,
V. Ya.; Takanashi, K.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2003,
125, 6012.
(3) (a) Power, P. P. Chem. ReV. 1999, 99, 3463. (b) Weidenbruch, M. Eur. J.
Inorg. Chem. 1999, 373. (c) Escudie´, J.; Ranaivonjatovo, H. AdV.
Organomet. Chem. 1999, 44, 113. (d) Kira, M.; Iwamoto, T. J. Organomet.
Chem. 2000, 611, 236. (e) Weidenbruch, M. In The Chemistry of Organic
Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester,
U.K., 2001; Vol. 3, Chapter 5. (f) Klinkhammer, K. In The Chemistry of
Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.;
Wiley: Chichester, U.K., 2002; Vol. 2, Part 1, Chapter 4. (g) Tokitoh, N.;
Okazaki, R. In The Chemistry of Organic Germanium, Tin and Lead
Compounds; Rappoport, Z., Ed.; Wiley: Chichester, U.K., 2002; Vol. 2,
Part 1, Chapter 13. (h) Weidenbruch, M. Organometallics, 2003, 22, 4348.
(i) Lee V. Ya; Sekiguchi, A. Organometallics, 2004, 23, 2822.
It is now well-known and widely accepted that the nature of
the double bond between the heavier group 14 elements is
significantly different from that of the classical carbon-carbon
(4) Reviews on the three-membered ring unsaturated compounds of heavier
group 14 elements (heavy cyclopropenes): (a) Lee, V. Ya; Sekiguchi, A.
In The Chemistry of Organic Germanium, Tin and Lead Compounds;
Rappoport, Z., Ed.; Wiley: Chichester, U.K., 2002; Vol. 2, Part 1, Chapter
14. (b) Sekiguchi, A.; Lee V. Ya. Chem. ReV. 2003, 103, 1429. (c)
Sekiguchi, A.; Lee, V. Ya. In Organosilicon Chemistry V: From Molecules
to Materials; Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim, Germany,
2003; p 92.
† University of Tsukuba.
‡ Technion-Israel Institute of Technology.
(1) (a) Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas, K. M. J. Chem.
Soc., Chem. Commun. 1976, 261. (b) Goldberg, D. E.; Hitchcock, P. B.;
Lappert, M. F.; Thomas, K. M.; Thorne, A. J.; Fjeldberg, T.; Haaland, A.;
Schilling, B. E. R. J. Chem. Soc., Dalton Trans. 1986, 2387.
9
10.1021/ja063322x CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 11643-11651
11643

A R T I C L E S
Scheme 1
Lee et al.
should be ascribed to the different coordinating abilities of THF
vs diethyl ether.
The crystal (X-ray) and spectral (119Sn NMR, UV) charac-
teristics of distannene 1 are particularly important for the
discussion of its structure both in the solid state and in solution.
Crystal Structure of 1. The crystal structure of 1 was
determined by X-ray crystallographic analysis (Figure 1, Table
1), which revealed several rather unusual structural features.
First: the SndSn double bond is very short, 2.6683(10) Å,
which should be recognized as the shortest among all structurally
characterized stable distannenes reported to date3a,f,g,8 (Table
2) (the calculated value of the SndSn double bond in the model
(Me3Si)2SndSn(SiMe3)2 distannene is 2.6805 Å).9 This value
is nearly 5% shorter than the average value of the SnsSn single
bond (2.81 Å).10 In light of the above-mentioned inherent
weakness and stretching of SndSn double bonds (see Introduc-
tion), the rather short and strong SndSn double bond in
distannene 1 is particularly amazing. Indeed, such a phenomenon
should originate from the electronic influence of the four
σ-donating silyl substituents, because it is well-known that the
electropositive groups effectively strengthen the π-bonds be-
tween the heavier group 14 elements, whereas electronegative
groups have the opposite effect.3a
The second structural characteristic worthy of mention is the
nonpyramidal geometry around the SndSn double bond: trans-
bend angle ) 1.22(5)°, the sum of the bond angles around the
tricoordinate tin atoms ) 359.98°. This is the first example of
a distannene with a nonpyramidal SndSn skeleton (Table 2).3f
Such an unusual geometry for the SndSn double bond (together
with its shortening) closely resembles the structural properties
of alkenes (typically, planar and short CdC double bond), in
marked contrast to all other structurally characterized distan-
nenes having a highly pronounced degree of pyramidalization
at the SndSn double bond (trans-bend angles range from 21.4
to 64.4°).3a,f,g,8 Such planarization of the double bond in 1 should
also be ascribed to the electropositive influence of the silyl
substituents.
group 14 elements (Sn, Pb). In other words, on going down
group 14 from C to Pb, there is an apparent and strong tendency
for a steady increase in the lone-pair character versus double-
bond character of the elements forming the double bond.3a This
finally results in the highly pronounced pyramidalization of the
doubly bonded elements, stretching of the double bond, and,
consequently, decrease in its strength.
It is, therefore, not surprising that typical disilenes and
digermenes maintain their relatively short and strong SidSi and
GedGe double bonds both in the crystalline form and in
solution.3a,f,g In marked contrast, all distannenes and diplumbenes
known to date possess rather long SndSn and PbdPb double
bonds, which are very close to (or even longer than) the Sns
Sn and PbsPb single bonds.3a,f,g Accordingly, the heaviest
alkenes of the type >EdE< (E ) Sn, Pb) dissociate in solution
into a pair of corresponding heavy carbene analogues, >E:. In
fact, all distannenes known to date, even those structurally
characterized in the crystalline form as doubly bonded species,
undergo unavoidable double bond breaking in solution to
produce the corresponding stannylenes.3f,g This was clearly
demonstrated by their spectral (NMR, UV) and chemical
properties in solution corresponding to the behavior of stan-
nylenes rather than real distannenes. As theoretical support,
recent ELF and AIM calculations attributed the weakness of
the SndSn double bond to the lower attraction and decreased
localization of the valence shell electrons into the bonding region
because of the larger size and lower electronegativity of the tin
atoms.5 In the latest review dealing with distannenes, it was
pointed out: “A distannene which does not dissociate into
stannylenes in solution is still unknown.”3g Now this situation
is remedied because of our synthesis of tetrakis(di-tert-butyl-
methylsilyl)distannene, the first compound featuring a SndSn
double bond that is stable both in the solid state and in solution.6
In this paper we wish to report a full account of the synthesis,
spectral and structural elucidation, and specific reactivity of this
distannene, as well as the structure of its reduction product, the
distannene anion radical.
However, such a great shortening of the SndSn double bond
is associated with a significant repulsion between the four highly
sterically demanding tBu2MeSi substituents, which in turn results
in the significant twisting of the SndSn double bond (twisting
angle of 44.62(7)°). In effect, distannene 1 has a rather unusual
combination of the structural characteristics of the SndSn
bonds: it is short, nonpyramidal, and highly twisted (Table 2).
It should be mentioned that the combination of such features is
rather unusual in the structural chemistry of distannenes. Two
bonding models for the double bonds between group 14
elements are now commonly accepted (Scheme 2).1-4 The first
Results and Discussion
Synthesis of Tetrakis(di-tert-butylmethylsilyl)distannene
1. Tetrakis(di-tert-butyl-methylsilyl)distannene 1 was synthe-
sized by the straightforward reaction of SnCl2-diox with 2.5
t
equiv of Bu2MeSiNa in THF at -78 °C and was isolated by
recrystallization from hexane as dark-green crystals in 43% yield
(Scheme 1).
(8) Structurally characterized acyclic distannenes: (a) see ref 1. (b) Klinkham-
mer, K. W.; Niemeyer, M.; Klett, J. Chem.sEur. J. 1999, 5, 2531. (c)
Sturmann, M.; Saak, W.; Klinkhammer, K. W.; Weidenbruch, M. Z. Anorg.
Allg. Chem. 1999, 625, 1955. (d) Klinkhammer, K. W.; Schwarz, W. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1334. (e) Klinkhammer, K. W.; Fassler, T.
F.; Grutzmacher, H. Angew. Chem., Int. Ed. 1998, 37, 124.
It is interesting that the reaction of the same compounds
previously reported by us carried out in diethyl ether instead of
THF resulted in the formation of the tris(di-tert-butylmethyl-
silyl)stannyl radical (tBu2MeSi)3Sn‚27 instead of distannene 1.
Definitely, such a dramatic difference in the reaction course
(9) The theoretical calculations were performed with the GAUSSIAN program
package at the B3LYP/6-31G(d) level for H, C, Si and LANL2DZd level
for Sn atoms. Frisch, M. J. et al. Gaussian 98 (revision A.11) and Gaussian
03 (revision B.05); Gaussian, Inc.: Pittsburgh, PA, 2001. For the B3LYP
method: (a) Stephens, P. J.; Devlin, F.; Chabalowski, C. F.; Frisch, M. J.
J. Phys. Chem. 1994, 98, 11623. (b) Hertwig, R. H.; Koch, W. Chem. Phys.
Lett. 1997, 268, 345. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (d)
Becke, A. D. Phys. ReV. A 1988, 38, 3098. (e) Lee, C.; Yang, W.; Parr, R.
G. Phys. ReV. B 1988, 37, 785.
(5) Malcolm, N. O. J.; Gillespie, R. J.; Popelier, P. L. A. J. Chem. Soc., Dalton
Trans. 2002, 3333.
(6) Fukawa, T.; Lee, V. Ya.; Nakamoto, M.; Sekiguchi, A. J. Am. Chem. Soc.
2004, 126, 11758.
(7) Sekiguchi, A.; Fukawa, T.; Lee, V. Ya.; Nakamoto, M. J. Am. Chem. Soc.
2003, 125, 9250.
(10) Mackay, K. M. In The Chemistry of Organic Germanium, Tin, and Lead
Compounds; Patai, S., Ed.; Wiley: Chichester, U.K., 1995; Chapter 2.
9
11644 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006

A Distannene with a Stable >SndSn< Double Bond
A R T I C L E S
Figure 1. Crystal structure of 1 (ORTEP plot; thermal ellipsoids are shown at the 30% probability; hydrogen atoms are omitted): (a) side view; (b) view
along the Sn-Sn bond.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of 1a
1
1
stannylenes (tBu2MeSi)2Sn: (σ1-σ1 and pπ -pπ ) resulting in
the formation of a nonpyramidal but a highly twisted SndSn
double bond (Scheme 2, mode C). Indeed, the triplet state of
(tBu2MeSi)2Sn: might be easily accessible due to its relatively
small singlet-triplet energy gap, ∆ES-T ) 8.5 kcal/mol, in favor
of the singlet state calculated at the B3LYP/SDD14 level of
theory (however, this value may be overestimated by ca. 2-4
kcal/mol).15 The significant decrease in the ∆ES-T value for
(tBu2MeSi)2Sn: relative to that of Me2Sn: (29.5 kcal/mol) can
be definitely attributed to the electronic effect of the electro-
positive silyl substituents and to their large steric bulk, which
widens the Si-Sn-Si bond angle from 96.7° in Me2Sn: to
109.6° in (tBu2MeSi)2Sn:. This trend is quite similar to the
influence of substituents on the ∆ES-T value of silylenes R2Si:,
in which the electropositive and voluminous silyl groups R also
widen the R-Si-R bond angle, thus decreasing ∆ES-T values.
Such significantly smaller ∆ES-T values for bulky silyl-
substituted silylenes make the singlet-triplet promotion acces-
sible,16 and the multiplicity order may even be reversed, as was
found experimentally for (tBu3Si)2Si:, which has a triplet ground
state.12 As one can expect, when the size of the substituents R
in stannylenes R2Sn: is successively decreased and the elec-
tronegativity of the substituents is increased, the ∆ES-T value
progressively increases; namely, for R ) tBu2MeSi, Me3Si, H3Si,
H, and Me, the calculated ∆ES-T values are 8.5, 10.3, 18.0,
26.5, and 29.2 kcal/mol, respectively, at the B3LYP/SDD level
(earlier ab initio calculations at MCSCF and CCSD(T) levels
predicted ∆ES-T values of 22.7 and 27.9 kcal/mol for H2Sn:17
and Me2Sn:18 stannylenes, respectively).
Sn(1)-Sn(1#)
Sn(1)-Si(1)
Sn(1)-Si(2)
Si(1)-C(1)
Si(1)-C(2)
Si(1)-C(6)
Si(2)-C(10)
Si(2)-C(11)
Si(2)-C(15)
2.6683(10)
2.631(2)
2.630(2)
1.954(10)
1.904(9)
1.872(10)
1.915(10)
1.897(10)
1.898(9)
Sn(1#)-Sn(1)-Si(1)
Sn(1#)-Sn(1)-Si(2)
Si(1)-Sn(1)-Si(2)
Sn(1)-Si(1)-C(1)
Sn(1)-Si(1)-C(2)
Sn(1)-Si(1)-C(6)
Sn(1)-Si(2)-C(10)
Sn(1)-Si(2)-C(11)
Sn(1)-Si(2)-C(15)
124.21(7)
126.50(5)
109.27(8)
105.6(3)
115.8(3)
109.2(2)
104.3(3)
108.6(4)
116.8(3)
a Atomic numbers are given in Figure 1. Standard deviations are in
parentheses.
one is applicable to alkenes as formal carbene dimers. Because
typical carbenes have either a triplet ground state, 3B1, or a small
singlet-triplet energy separation,11 the formation of the CdC
double bond is considered to be a result of interaction between
the two triplet carbenes: σ1-σ1 (σ-bond) and pπ -pπ1 (π-bond)
1
to form a classical strong covalent double bond with a planar
geometry around it (Scheme 2, mode A). Another bonding
model is employed for the description of the double bond
between the heavier group 14 elements. Because all experi-
mentally known heavy carbene analogues (except for
(tBu3Si)2Si:12) possess a singlet ground state, 1A1, with largely
different (by both size and energy) s- and p-orbitals,13 the
interaction between them to form a double bond is not possible
in the classical mode, but rather by the double donor-acceptor
interaction between the doubly occupied σ-orbitals and empty
pπ-orbitals (σ2-pπ) of the two singlet carbene analogues to form
a weak dative EdE bond with the characteristic trans-bending
of substituents (Scheme 2, mode B).
It is apparent that in the particular case of distannene 1 (short,
nonpyramidal, and twisted SndSn double bond) one cannot
adequately describe the bonding situation using either of the
two models discussed above (Scheme 2, modes A and B). Such
an unusual bonding motif in 1 can alternatively be explained
as a result of out-of-plane (because of the high steric hindrances
due to the bulky silyl substituents) interaction of the two triplet
119Sn NMR Spectrum of 1. In solution, distannene 1
expectedly exhibited very simple NMR spectra in accord with
its symmetrical composition. Most essential for discussion of
the structure of 1 is its 119Sn NMR spectrum measured in
benzene-d6, which revealed a single resonance at +630.7 ppm.
Such a chemical shift lies in the region expected for the sp2-Sn
atoms in distannene, we can compare it with the only two
(14) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993,
80, 1431.
(15) Based on a comparison between the ∆ES-T values of H2Sn: and Me2Sn:
calculated by us at the B3LYP/SDD level of theory and the corresponding
values calculated previously17,18 at the higher MCSCF and CCSD(T) levels
of theory.
(16) Holthausen, M. C.; Koch, W.; Apeloig, Y. J. Am. Chem. Soc. 1999, 121,
2623.
(17) (a) Balasubramanian, K. J. Chem. Phys. 1988, 89, 5731. (b) Matsunaga,
N.; Koseki, S.; Gordon, M. S. J. Chem. Phys. 1996, 104, 7988.
(18) Su, M.-D. Chem.sEur. J. 2004, 10, 6073.
(11) (a) Kirmse, W. Carbene Chemistry, 2nd ed.; Academic Press: New York,
USA, 1971. (b) Moss, R. A.; Jones, M., Jr. Carbenes; Wiley: Chichester,
U.K., 1975; Vol. 2. (c) Bourissou, D.; Guerret, O.; Gabbai, F.; Bertrand,
G. Chem. ReV. 2000, 100, 39.
(12) Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Tero-Kubota, S. J.
Am. Chem. Soc. 2003, 125, 4962.
(13) (a) Neumann, W. P. Chem. ReV. 1991, 91, 311. (b) Tokitoh, N.; Okazaki,
R. Coord. Chem. ReV. 2000, 210, 251. (c) Gaspar, P. P.; West, R. In The
Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.;
Wiley: Chichester, U.K., 1998; Vol. 2, Part 3, Chapter 43.
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11645

A R T I C L E S
Lee et al.
Table 2. Comparison of RR′SndSnRR′ Distannene Structures
R
)
R
′ )
R
)
R
(Me3Si)3Si
′ ) Arb
R
)
R
′ )
R
)
R
(Me3Si)3Si
′ ) Ard
R
)
R
′ )
R
)
R
(Me3Si)3Si
R, R
′
tBu2MeSia
(Me3Si)2CHc
(Me3Si)3Sie
′ ) Arf
SndSn (Å)
2.6683(10)
359.98
2.702
not available
2.768(1)
343.5
2.7914(4)
337.4
2.8247(6)
351.96
2.833(1)
337.9
sum of bond angles
around Sn atoms (deg)
trans-bent angle θ (deg)
SndSn geometry
twisting angle τ (deg)
color
1.22(5)
39.4
trans-bent
0
41.0
trans-bent
0
44.9
trans-bent
0
28.6
trans-bent
63.2
41.5
trans-bent
0
nonbent
44.62(7)
dark green
not available
brick red
brown-violet
black
violet-red
a This work. b Ar ) 2,4,6-Me3-C6H2, ref 8b. c Reference 8a. d Ar ) 2-tBu-4,5,6-Me3-C6H, ref 8c. e Reference 8d. f Ar ) 2,4,6-(CF3)3-C6H2, ref 8e.
Scheme 2
UV Spectrum of 1. It is interesting that distannene 1 has a
rather unusual dark-green color. Accordingly, the longest
wavelength absorption maximum in the UV spectrum of 1 was
observed at 670 nm, whose position did not change in the
temperature interval from 273 to 203 K. This absorption is
greatly red-shifted compared with that of Tip2SndSnTip2 (Tip
) 2,4,6-triisopropylphenyl), the only distannene relatively stable
in solution at low temperature, which has a red color and
absorption maximum λmax ) 494 nm (-70 °C).19 There are
several reasons which may contribute to such an experimental
observation of the bathochromic shift of 1, which all originate
from the high twisting of the SndSn double bond substituted
by the electropositive σ-donating silyl groups. The first one is
the possibility of the 5pπ(Sn)-σ(Sn-Si) orbitals mixing, raising
the energy of the HOMO level and, consequently, decreasing
the HOMO-LUMO energy gap of 1. Such an 5pπ(Sn)-σ(Sn-
Si) interaction of orbitals might be possible upon the experi-
mentally observed twisting of the SndSn double bond in 1.
Another important reason of the bathochromic shift is the
reduced overlap of the two 5pπ(Sn) orbitals upon the twisting
of the SndSn double bond, which should also lead to the
destabilization of the HOMO. To gain further insight into the
electronic structure of distannene 1 we have performed theoreti-
cal calculations on the model compound (Me3Si)2SndSn-
(SiMe3)2 (Figure 2).9 Geometry optimization provided us a
trans-bent (bend angle 32.5°) very slightly twisted (twist angle
3.7°) conformation A as a minimum structure (Figure 2, A).
The discrepancy between the experimental (nonpyramidal,
highly twisted) and calculated (trans-bent, very slightly twisted)
geometries of distannene could be attributed to the different
substitution patterns at the Si atoms (Me3Si in calculated model
available examples of 119Sn NMR spectra of distannenes
measured at room temperature in solution: +427.3 ppm (for
tetrakis(2,4,6-triisopropylphenyl)distannene)19 and +412 ppm
(for cyclotristannene).20 As was discussed above, distannenes
R2SndSnR2 typically tend to dissociate in solution into a pair
of the corresponding stannylenes R2Sn:, which was clearly
manifested by their diagnostic downfield shifted resonances of
the 119Sn NMR chemical shifts.3f,g The only distannene that
preserves its SndSn double bond undissociated in solution at
room temperature was cyclotristannene, reported recently by
Wiberg.20 However, this compound has a specific structure
whose properties are greatly affected by its unique cyclic
skeleton. Thus, our distannene 1 represents the first example
of a SndSn double bond in the isolable acyclic distannenes
that is stable in solution at room temperature and even upon
heating.21
t
vs much more bulky Bu2MeSi in the real compound 1).
Inspection of the MOs revealed that in the slightly twisted
structure A the HOMO (-4.54 eV) and LUMO (-2.07 eV)
energy gap is rather small (∆E ) 2.47 eV), thus making
HOMO-LUMO promotion easily accessible (Figure 2, A).
Then we performed single-point energy calculations based on
(21) The only examples of heavy alkene analogues containing sp2-Sn atoms,
which are stable both in the solid state and in solution, were recently
synthesized by us: silastannene >SidSn< (see ref 2j) and germastannenes
>GedSn< (see ref 2l,m). The example of the transient distannene, Me2Snd
SnMe2, which does not dissociate in solution, was also recently reported:
Becerra, R.; Gaspar, P. P.; Harrington, C. R.; Leigh, W. J.; Vargas-Baca,
I.; Walsh, R.; Zhou, D. J. Am. Chem. Soc. 2005, 127, 17469.
(19) Masamune, S.; Sita, L. R. J. Am. Chem. Soc. 1985, 107, 6390.
(20) Wiberg, N.; Lerner, H.-W.; Vasisht, S.-K.; Wagner, S.; Karaghiosoff, K.;
No¨th, H.; Ponikwar, W. Eur. J. Inorg. Chem. 1999, 1211.
9
11646 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006

A Distannene with a Stable >SndSn< Double Bond
A R T I C L E S
Figure 2. Molecular orbitals of the model (Me3Si)2SndSn(SiMe3)2 distannene: (A) trans-bent, nontwisted conformation; (B) nonbent, highly twisted
conformation.
Scheme 3
the X-ray structure of 1 to verify the properties of the highly
twisted geometry of the SndSn double bond in conformation
B (Figure 2, B). It was found that upon twisting the SndSn
double bond on going from structure A to structure B the energy
of the HOMO level increased to -4.22 eV, whereas the energy
of the LUMO level remained almost unchanged (-2.04 eV).
In the twisted structure B the HOMO is the 5pπ-5pπ (SndSn)
orbital with a contribution of σ(Sn-Si) orbitals (Figure 2, B).
rostannane R2SnCl2 as the final compound, which can be
considered as the chlorinated product of stannylenes R2Sn:
In effect, the HOMO-LUMO gap decreased upon twisting the
SndSn double bond to 2.18 eV (in B) vs 2.47 eV (in A). In
total, these calculation results provide some theoretical explana-
tion to the experimentally observed bathochromic shift in the
UV spectrum of distannene 1.
Finally, the manifestation of the double bond between the
Sn atoms was also clearly demonstrated by the characteristic
reactivity of distannene 1, which will be discussed in the
following section.
Reactivity of 1. Because all previously known distannenes
dissociate in solution into the corresponding stannylenes, the
reactivity of distannenes described thus far was, in fact, the
reactivity of >Sn: species rather than the SndSn double bond.3f,g
Below, we will present the first convincing examples of the
reactivity of the SndSn double bond of distannene 1.
Reaction of 1 with Carbon Tetrachloride. It is now well
established that the halogenation of the disilenes R2SidSiR2
and digermenes R2GedGeR2 with simple alkyl halides such as
CCl4 results in the formation of the stable 1,2-dichlorodisilanes
R2(Cl)SisSi(Cl)R222a and -digermanes R2(Cl)GesGe(Cl)R2.22b
However, the reaction of distannene Dis2SndSnDis2 (Dis )
CH(SiMe3)2) with either CCl4 or Cl2 gave only 1,1-dichlo-
formed upon the dissociation of the R2SndSnR2 double bond.23
Our distannene 1 reacts with carbon tetrachloride with an
immediate color change from dark green to pale yellow. The
product, 1,2-dichloro-1,1,2,2-tetrakis(di-tert-butylmethylsilyl)-
distannane 3, was isolated by recrystallization from hexane as
1
pale-yellow crystals in 75% yield (Scheme 3). The H NMR
spectrum of 3 exhibited the two distinct resonances of diaste-
t
reotopic Bu groups, which is expected for 1,2-dichloride
(tBu2MeSi)2(Cl)Sn-Sn(Cl)(SiMetBu2)2 but not for 1,1-dichloride
(tBu2MeSi)2SnCl2. The structure of 3 was undoubtedly estab-
lished by the X-ray crystallographic analysis. However, we do
not discuss the structural details because of the unsatisfactory
refinement.
Reaction of 1 with Phenylacetylene. The [2 + 2] cycload-
dition of the simple acetylenes across the >EdE< bonds of
heavy alkenes is a powerful tool in elucidating the nature of
the double bond. In the case of disilenes and digermenes, the
1,2-disila- and 1,2-digermacyclobut-3-enes are the anticipated
and virtual products of these reactions.22c,d In the case of
distannenes, formation of the four-membered [2 + 2] cycload-
duct competes with formation of the three-membered [1 + 2]
cycloadduct due to the intrinsic distannene-stannylene equi-
librium in solution (Scheme 4).
(22) (a) Kira, M.; Iwamoto, T. J. Organomet. Chem. 2000, 611, 236. (b)
According to our experimental data, tetrasilyl-substituted digermenes
smoothly react with CCl4 to form the corresponding 1,2-dichlorodigermanes
without breaking the central Ge-Ge bond. These data will be published in
forthcoming papers. (c) Okazaki, R.; West. R. AdV. Organomet. Chem.
1996, 39, 231. (d) Escudie´, J.; Ranaivonjatovo, H. AdV. Organomet. Chem.
1999, 44, 113.
Distannene 1 smoothly reacted with an excess of phenyl-
acetylene to produce immediately the [2 + 2] cycloaddition
(23) Cotton, J. D.; Davidson, P. J.; Lappert, M. F. J. Chem. Soc., Dalton Trans.
1976, 2275.
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11647

A R T I C L E S
Scheme 4
Lee et al.
Table 3. Selected Bond Lengths (Å) and Angles (deg) of 4a
Bond Length
Sn(1)-Sn(2)
Sn(1)-C(2)
Sn(1)-Si(1)
Sn(1)-Si(2)
Sn(2)-C(1)
Sn(2)-Si(3)
Sn(2)-Si(4)
C(1)-C(2)
Si(1)-C(3)
Si(1)-C(4)
2.9004(3)
Si(1)-C(8)
Si(2)-C(12)
Si(2)-C(13)
Si(2)-C(17)
Si(3)-C(21)
Si(3)-C(22)
Si(3)-C(26)
Si(4)-C(30)
Si(4)-C(31)
Si(4)-C(35)
C(1)-C(39)
1.932(2)
1.891(3)
1.936(3)
1.930(3)
1.884(3)
1.939(3)
1.935(3)
1.886(3)
1.934(3)
1.924(3)
1.498(4)
2.188(3)
2.6496(7)
2.6400(7)
2.228(3)
2.6581(7)
2.6709(7)
1.327(4)
1.884(3)
1.925(3)
Bond Angle
C(2)-Sn(1)-Sn(2)
Sn(1)-Sn(2)-C(1)
Sn(2)-C(1)-C(2)
C(1)-C(2)-Sn(1)
68.95(8)
68.88(8)
108.9(2)
111.9(2)
C(2)-Sn(1)-Si(2)
99.81(7)
Sn(1)-Sn(2)-Si(3) 111.377(16)
Sn(1)-Sn(2)-Si(4) 129.427(18)
Scheme 5
C(1)-Sn(2)-Si(3)
101.17(7)
125.19(8)
Sn(2)-Sn(1)-Si(1) 133.324(17) C(1)-Sn(2)-Si(4)
Sn(2)-Sn(1)-Si(2) 110.420(18) Si(3)-Sn(2)-Si(4) 111.85(2)
Si(1)-Sn(1)-Si(2) 113.47(2) Sn(2)-C(1)-C(39) 134.9(2)
C(2)-Sn(1)-Si(1) 116.98(8) C(2)-C(1)-C(39) 115.2(3)
a Atomic numbers are given in Figure 3. Standard deviations are in
parentheses.
The X-ray analysis of 4 clearly confirms its 1,2-distannacy-
clobut-3-ene composition. The four-membered ring of 4 is nearly
planar with the sum of the internal bond angles being 358.63°,
similar to other 1,2-distannacyclobut-3-enes.24 The Sn-Sn bond
in 4 of 2.9004(3) Å is significantly longer than the average Sn-
Sn single bond of ca. 2.81 Å10 and Sn-Sn distances in other
known 1,2-distannacyclobut-3-enes (2.817(1)24a and 2.840(1)
Å),24b which can be ascribed to the high steric demand of the
t
bulky Bu2MeSi groups on the Sn atoms.
One-Electron Reduction of 1. The heavy analogues of
alkenes of the type >EdE< (E ) Si, Ge, Sn, and Pb) are
expected to be reduced more smoothly than the corresponding
alkenes >CdC< because of the low-lying and easily accessible
LUMOs of the former compared with those of the latter.
However, to date the generation of only disilene anion radicals
has been reported in the scientific literature. Thus, Weidenbruch
reported in 1985 the generation of the first representatives of
disilene anion radicals [R2SidSiR2]•- (R ) Pr, Bu) by the
reduction of 1,2-dihalodisilanes R2(X)SisSi(X)R2 with alkali
metals in THF.25 Such anion radical species with a lifetime of
several minutes at room temperature exhibited an EPR spectrum
with hyperfine coupling constant (hfcc) a(29Si) ) 3.36 mT.
Following this original report, Kira commented on the generation
of disilene anion radicals by the direct reduction of stable
tetrasilyldisilenes with potassium in DME.22a And finally, the
first virtually stable disilene anion radical was synthesized and
structurally characterized by the reduction of the disilene (tBu2-
i
t
Figure 3. Crystal structure of 4 (ORTEP plot; thermal ellipsoids are shown
at the 30% probability; hydrogen atoms are omitted). The position of the
Ph group is disordered between the C(1) and C(2) atoms, and the major
contribution (78% probability) is shown.
product, 1,1,2,2-tetrakis(di-tert-butylmethylsilyl)-3-phenyl-1,2-
distannacyclobut-3-ene 4, isolated by recrystallization from
hexane as yellow plates in 64% yield (Scheme 5). The structure
of 4 was elucidated on the basis of NMR spectral data and X-ray
crystallography (Figure 3, Table 3).
In the 119Sn NMR spectrum of 4 the two signals of the skeletal
Sn atoms were observed at -329.6 and -175.7 ppm (1J(119Sn-
119,117Sn) ) 1354 Hz). Such a coupling constant is smaller than
those of the similar [2 + 2] cycloadducts of phenylacetylene
with other distannenes (1J(119Sn-119,117Sn) ranging from 2436
to 4462 Hz).24 This gives evidence for the longer Sn-Sn bond
separation in 4 compared with other similar [2 + 2] cycload-
ducts.
t
MeSi)2SidSi(SiMetBu2)2 with BuLi in THF.26
The reduction of distannene 1 was achieved by its treatment
with an equivalent amount of potassium mirror in THF in the
presence of [2.2.2]cryptand to produce the first and still only
example of a distannene anion radical (Scheme 6). The tetrakis-
(di-tert-butylmethylsilyl)distannene anion radical 5 was isolated
as a dark-red solid in 82% yield. The crystal structure of 5 is
represented by the highly twisted (twist angle 74°) Sn-Sn
(25) Weidenbruch, M.; Kramer, K.; Scha¨fer, A.; Blum, J. K. Chem. Ber. 1985,
(24) (a) Sita, L. R.; Kinoshita, I.; Lee, S. P. Organometallics 1990, 9, 1644. (b)
Weidenbruch, M.; Scha¨fer, A.; Kilian, H.; Pohl, S.; Saak, W.; Marsmann,
H. Chem. Ber. 1992, 125, 563.
118, 107.
(26) Sekiguchi, A.; Inoue, I.; Ichinohe, M.; Arai, Y. J. Am. Chem. Soc. 2004,
126, 9626.
9
11648 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006

A Distannene with a Stable >SndSn< Double Bond
A R T I C L E S
Scheme 6
triplet biradicals leads to the fine structure of the EPR spectrum,
and three pairs of sidelines are expected to appear (∆Ms ) 1
transition).29 However, in the EPR spectrum of 5 the species
with spin S ) 1/2 dominate, which does not allow the observation
of sidelines and determination of the zero-field splitting
parameters |D′| and |E′| for interaction between the two electrons
in paramagnetic species having spin S ) 1. We can estimate
|D′| from the fact that the total width of the EPR spectrum
signals of 5 occupies a range of ∼200 G; that is, the 2|D′| value
cannot exceed 200 G, or otherwise sidelines could be observed.
Assuming the spin-spin dipolar interactions, |D′| is ap-
3
proximately given by the equation |D′| ) /2g2b2r-3, where g
skeleton with a Sn-Sn bond length of 2.8978(3) Å, which is
0.2295 Å longer than that of the starting distannene 1 due to
the decrease in bond order upon one-electron reduction of the
double bond (Figure 4, Table 4). The geometry around the two
Sn atoms is different: one of them is essentially planar (sum
of the bond angles 355.39°, trans-bend angle 19.50(4)°) as a
radical center, whereas the other one is profoundly pyramidal
(sum of the bond angles 323.16°, trans-bend angle 60.05(4)°)
being an anionic center. The characteristic planarity of the silyl-
substituted stannyl radicals (for example, (tBu2MeSi)3Sn•)7 and
pyramidalization of the silyl-substituted stannyl anions (for
example, (tBu2MeSi)3Sn-)27 was established in our previous
studies. This implies that the negative charge and the unpaired
electron are effectively separated between the two Sn atoms in
distannene anion radical 5. It is interesting that such charge-
electron separation is also maintained in the solution of the
distannene anion radical. This was manifested in the EPR
spectrum of 5 exhibiting a single resonance (g ) 2.0517) with
two distinct pairs of satellite signals with hfcc values of 34.0
mT (R-119,117Sn) and 18.7 mT (â-119,117Sn), respectively, imply-
ing localization of the unpaired electron on one of the two
Sn atoms. In marked contrast, in disilene anion radical
[(tBu2MeSi)2Si-Si(SiMetBu2)2]•- the unpaired electron is quickly
exchanged between both skeletal silicon atoms on the EPR time
scale, which, however, can be suppressed at low temperature
(120 K).26
is the g-factor value, r is the distance between the two interacting
electrons, and b is the Bohr magneton.29 If |D′| ) 100 G, the
minimum distance between the two electrons in the triplet
species can be estimated as 5.3 Å. Therefore, in our case there
is evidence that the distance between the two interacting
electrons in the triplet species is greater than 5.3 Å, but on the
other hand this distance should be shorter than 12 Å (|D′| < 8
G), the maximum distance at which the signal in the half-field
region is detectable by the X-band EPR spectrometer.
One can suggest that the molecules of distannene anion radical
5 can partially aggregate at low temperature to form paramag-
netic triplet dimers 6 (Scheme 7) through the coordination of
the anionic Sn center of one molecule of 5 to the potassium
countercation of another molecule of 5.
The low-temperature aggregation of anion radicals of similar
type, involving participation of countercations and solvent
molecules, was previously observed for paramagnetic ketyl
dimers, though it was found to be a minor process in strongly
solvating solvents such as DMF.28a To compare the
low-temperature EPR data of 5, we have measured the EPR
spectrum of the previously synthesized neutral stannyl radical
(tBu2MeSi)3Sn•7 2 at the same conditions (2-Me-THF, 100 K).
The EPR spectrum of 2 is typical for a system with axial
symmetry, the g-factor being characterized by the two compo-
nents g ) 2.074 and gII ) 1.993 (Figure 5b). In this case the
signal in the half-field region corresponding to a forbidden ∆Ms
) 2 transition was not observed, in contrast to the above case
of distannene anion radical 5, because such a type of paramag-
netic aggregation of the neutral radicals 2, lacking the anionic
center, is unlikely.
The EPR spectra of the distannene anion radical were also
measured in 2-methyl-THF at glass matrix conditions (100 K).
Expectedly, the low symmetry of the distannene anion radical
caused the three-axis anisotropy of the g-factor of 5, in which
the contributors were determined as gxx ) 2.109, gyy ) 2.057,
and gzz ) 1.988 (Figure 5a).
Experimental Section
Most importantly, at high concentration and high gain, a very
characteristic single resonance (1631 G) corresponding to a
forbidden ∆Ms ) 2 electronic transition was observed at a field
strength approximately half of the center of the allowed ∆Ms
) 1 electronic transition (the so-called half-field region).28 Such
∆Ms ) 2 transitions are intrinsically less anisotropic than the
∆Ms ) 1 transitions, and therefore only a single broad signal is
typically observed. It is known that such ∆Ms ) 2 forbidden
transitions are characteristic for randomly oriented triplet
systems being the result of the magnetic interaction between
the two parallel spins.28 The spin-spin dipolar interaction in
General Procedures. All manipulations with the air-sensitive
compounds were carried out using an MBRAUN MB 150B-G glovebox,
a high-vacuum line and Schlenk techniques, and dry, oxygen-free
solvents. NMR spectra were recorded on Bruker AC-300FT (1H NMR
at 300.13 MHz; 13C NMR at 75.47 MHz; 29Si NMR at 59.63 MHz;
119Sn NMR at 111.92 MHz) and ARX-400FT NMR spectrometers (1H
NMR at 400.23 MHz; 13C NMR at 100.65 MHz; 29Si NMR at 79.51
MHz; 119Sn NMR at 149.32 MHz). The EPR spectra were measured
on a Bruker EMX-T EPR spectrometer, UV-vis and mass spectra were
recorded on SHIMADZU UV-3150 and JEOL JMS-SX102A spec-
trometers, respectively. Starting materials SnCl2-diox30 and tBu2-
MeSiNa31 were synthesized according to the previously reported
experimental procedures.
(27) Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Sekiguchi, A. Organometallics
2004, 23, 2376.
(28) For the discussion concerning the nature of half-field EPR signals,
corresponding to a forbidden ∆Ms ) 2 electronic transition, see: (a) Hirota,
N.; Weissman, S. I. J. Am. Chem. Soc. 1964, 86, 2538. (b) Hirota, N. J.
Am. Chem. Soc. 1967, 89, 32. (c) Takeshita, T.; Hirota, N. J. Am. Chem.
Soc. 1971, 93, 6421. (d) Jain, R.; Sponsler, M. B.; Coms, F. D.; Dougherty,
D. A. J. Am. Chem. Soc. 1988, 110, 1356.
(29) Wertz, J.; Bolton, J. Electron Spin Resonance; McGraw-Hill: New York,
USA, 1972; Chapter 10.
(30) Hough, E.; Nicholson, D. G. J. Chem. Soc., Dalton Trans. 1976, 1782.
(31) Sekiguchi, A.; Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Ichinohe, M. J.
Am. Chem. Soc. 2002, 124, 9865.
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11649

A R T I C L E S
Lee et al.
Figure 4. Crystal structure of 5 (ORTEP plot; thermal ellipsoids are shown at the 30% probability; hydrogen atoms are omitted).
Table 4. Selected Bond Lengths (Å) and Angles (deg) of 5a
Bond Length
Sn(1)-Sn(2)
Sn(1)-Si(1)
Sn(1)-Si(2)
Si(1)-C(1)
Si(1)-C(2)
Si(1)-C(6)
Si(2)-C(10)
Si(2)-C(11)
Si(2)-C(15)
2.8978(3)
2.6679(8)
2.6781(8)
1.901(3)
1.954(3)
1.949(3)
1.900(3)
1.946(3)
1.937(3)
Sn(2)-Si(3)
Sn(2)-Si(4)
Si(3)-C(19)
Si(3)-C(20)
Si(3)-C(24)
Si(4)-C(28)
Si(4)-C(29)
Si(4)-C(33)
2.6432(9)
2.6507(9)
1.890(3)
1.943(3)
1.937(3)
1.891(4)
1.937(4)
1.924(3)
Bond Angle
Sn(2)-Sn(1)-Si(1) 107.51(2) Sn(1)-Sn(2)-Si(3) 107.34(2)
Sn(2)-Sn(1)-Si(2) 105.836(19) Sn(1)-Sn(2)-Si(4) 134.39(2)
Si(1)-Sn(1)-Si(2)
Sn(1)-Si(1)-C(1)
Sn(1)-Si(1)-C(2)
Sn(1)-Si(1)-C(6)
Sn(1)-Si(2)-C(10) 113.34(10)
Sn(1)-Si(2)-C(11) 104.61(10)
Sn(1)-Si(2)-C(15) 118.14(11)
109.81(3)
106.99(11)
102.63(10)
124.52(10)
Si(3)-Sn(2)-Si(4)
113.66(3)
Sn(2)-Si(3)-C(19) 114.48(11)
Sn(2)-Si(3)-C(20) 110.72(10)
Sn(2)-Si(3)-C(24) 109.69(11)
Sn(2)-Si(4)-C(28) 109.65(12)
Sn(2)-Si(4)-C(29) 114.15(11)
Sn(2)-Si(4)-C(33) 110.15(11)
a Atomic numbers are given in Figure 4. Standard deviations are in
parentheses.
Synthesis of Tetrakis(di-tert-butylmethylsilyl)distannene 1. Dry
THF (30 mL) was vacuum transferred to a mixture of SnCl2-diox (924
t
mg, 3.33 mmol) and Bu2MeSiNa (1.5 g, 8.32 mmol). The reaction
mixture was initially stirred at -78 °C and then overnight with gradual
warming to room temperature to produce a dark-green solution. After
removal of inorganic salt and polymeric materials, the solvent was
evaporated in a vacuum. The dark-green residue was recrystallized from
hexane at -30 °C to give distannene as dark-green crystals (620 mg,
43%): mp 172-174 °C (dec); 1H NMR (C6D6, δ) 0.83 (s, 12 H, CH3),
1.34 (s, 72 H, (CH3)3C); 13C NMR (C6D6, δ) 4.6 (CH3), 32.3 ((CH3)3C),
36.4 ((CH3)3C); 29Si NMR (C6D6, δ) 46.9; 119Sn NMR (C6D6, δ) 630.7;
UV-vis (hexane) λmax/nm (ꢀ): 309 (8300), 395 (4200), 670 (3100).
Anal. Calcd for C36H84Si4Sn2: C, 49.88; H, 9.77. Found: C, 49.59; H,
9.65.
Synthesis of 1,2-Dichloro-1,1,2,2-tetrakis(di-tert-butylmethylsilyl)-
distannane 3. Dry CCl4 (1 mL) was vacuum transferred to distannene
1 (63 mg, 0.073 mmol), and then the reaction mixture was stirred for
5 min at room temperature. Excess CCl4 was evaporated in a vacuum
to give 3 as pale-yellow crystals (51 mg, 75%): mp 147-148 °C (dec);
1H NMR (C6D6, δ) 0.74 (s, 12 H, CH3), 1.24 (s, 36 H, (CH3)3C), 1.26
(s, 36 H, (CH3)3C); 13C NMR (C6D6, δ) -2.2 (CH3), 23.0 ((CH3)3C),
24.3 ((CH3)3C), 29.5 ((CH3)3C), 29.9 ((CH3)3C); 29Si NMR (C6D6, δ)
44.8; 119Sn NMR (C6D6, δ) 59.1; UV-vis (hexane) λ max/nm (ꢀ): 271
(54 600), 318 (46 200); Anal. Calcd for C36H84Cl2Si4Sn2: C, 46.11; H,
9.03. Found: C, 46.10; H, 8.92.
Figure 5. (a) The EPR spectrum of anion radical 5 in 2-methyl-THF at
100 K. The insert shows the resonance corresponding to a forbidden ∆Ms
) 2 transition. (b) The EPR spectrum of neutral stannyl radical (tBu2-
MeSi)3Sn• in 2-methyl-THF at 100 K.
Synthesis of 1,2-Tetrakis(di-tert-butylmethylsilyl)-3-phenyl-1,2-
distannacyclobut-3-ene 4. Phenylacetylene (0.5 mL, 4.55 mmol) was
vacuum transferred to a benzene solution (1 mL) of distannene 1 (90
mg, 0.104 mmol). The solution color gradually changed from dark green
9
11650 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006

A Distannene with a Stable >SndSn< Double Bond
A R T I C L E S
Scheme 7
X-ray Crystal Structure Analyses. Single crystals suitable for an
X-ray diffraction study were grown from the following solvents: 1
and 5 from toluene; 4 from hexane. Diffraction data were collected on
a Mac Science DIP2030K image plate diffractometer employing
graphite-monochromatized Mo KR radiation (λ ) 0.710 70 Å). The
structures were solved by direct method and refined by the full-matrix
least-squares method using an SHELXL-97 program. Crystal data for
4 at 120 K: MF ) C44H90Si4Sn2, MW ) 968.90, triclinic, P1h, a )
11.7250(3) Å, b ) 13.0250(6) Å, c ) 18.7090(8) Å, R ) 75.877(2)°,
â ) 87.714(3)°, γ ) 68.845(4) °, V ) 2580.44(18) Å3, Z ) 2, Dcalcd
) 1.247 g‚cm-3. The final R factor was 0.0335 for 9339 reflections
with I > 2σ(I) (Rw ) 0.0914 for all data, 25 788 reflections), GOF )
1.034. For the detailed information of the X-ray structure determination
for 1 and 5, see the Supporting Information in ref 7.
to reddish purple. After 3 h of stirring, solvent and excess phenylacety-
lene were evaporated in a vacuum. The residue was recrystallized from
hexane to give 4 as yellow crystals (68 mg, 64%): mp 184-187 °C;
1H NMR (C6D6, δ) 0.70 (s, 6 H, CH3), 0.74 (s, 6 H, CH3), 1.10 (s, 18
H, (CH3)3C), 1.16 (s, 18 H, (CH3)3C), 1.26 (s, 18 H, (CH3)3C), 1.27 (s,
18 H, (CH3)3C), 7.00 (t, J ) 7.3 Hz, 1 H, Hpara), 7.15 (t, J ) 7.3 Hz,
2 H, Hmeta), 7.43 (d, J ) 7.3 Hz, 2 H, Hortho), 8.43 (s, 1 H, CPhdCH);
13C NMR (C6D6, δ) 0.6 (CH3), 0.7 (CH3), 23.0 ((CH3)3C), 23.3-
((CH3)3C), 23.9 (2 C, (CH3)3C)), 30.8 ((CH3)3C), 31.0 ((CH3)3C), 31.3
((CH3)3C), 31.6 ((CH3)3C), 126.2 (Carom), 127.4 (Carom), 128.1 (Carom),
149.4 (Cipso), 154.0 (PhCdCH), 169.3 (PhCdCH); 29Si NMR (C6D6,
δ) 30.2, 31.3; 119Sn NMR (C6D6, δ) -330.0, -176.1; UV-vis (hexane)
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research (Research Nos. 16205008, 17550029,
17750031, 17655014, 18037008, 18039004) from the Ministry
of Education, Science and Culture of Japan, JSPS Research
Fellowship for Young Scientists (T.F.), COE (Center of Excel-
lence) program, Israel Science Foundation administrated by
Israel Academy of Sciences and Humanities, Fund for the
Promotion of Research at the Technion, and Minerva Foundation
in Munich. B.T. is grateful for support to the Center for
Absorption in Science, Israel Ministry of Immigrant Absorption,
State of Israel.
λmax/nm (ꢀ): 220 (51 500), 286 (15 600), 316 (15 200). Anal. Calcd
for C44H92Si4Sn2: C, 54.43; H, 9.55. Found: C, 54.37; H, 9.35.
Synthesis of Tetrakis(di-tert-butylmethylsilyl)distannene Anion-
Radical 5. Dry THF (2 mL) was vacuum transferred to a mixture of
distannene 1 (300 mg, 0.346 mmol) and [2.2.2]cryptand (140 mg, 0.345
mmol). Then the resulting solution was shortly contacted to potassium
mirror (14 mg, 0.358 mmol), and the color of solution immediately
changed from dark green to dark red. Solvent was evaporated in a
vacuum, and the residue was washed with dry hexane in a glovebox to
give distannene anion radical 5 as a dark-red powder (375 mg, 82%):
EPR (2-Me-THF, 298 K) g ) 2.0517, hfccs a(R-119,117Sn) ) 34.0 mT,
a(â-119,117Sn) ) 18.7 mT.
Supporting Information Available: Tables giving the details
of the X-ray structure determination, thermal ellipsoid plots,
fractional atomic coordinates, anisotropic thermal parameters,
bond lengths, and bond angles for 4 (PDF and CIF), complete
ref 9. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA063322X
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J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11651