The first metalladiene of group 14 elements with a silole-type structure with Si=Ge and C=C double bonds

Full text of DOI:10.1021/ja0030921

12604
J. Am. Chem. Soc. 2000, 122, 12604-12605
The First Metalladiene of Group 14 Elements with a
Silole-Type Structure with SidGe and CdC Double
Bonds
Vladimir Ya. Lee, Masaaki Ichinohe, and Akira Sekiguchi*
Department of Chemistry, UniVersity of Tsukuba
Tsukuba, Ibaraki 305-8571, Japan
ReceiVed August 21, 2000
1,3-Butadiene derivatives and their cyclic analogues constitute
one of the most fundamental classes of conjugated organic
compounds. After the discovery of the stable metallenes and
dimetallenes of Group 14 elements,1 experimental efforts have
been focused on the synthesis of the Group 14 elements containing
metalladienes. However, only two examples of the isolable
metalladienes of Group 14 elements have been reported to date:
hexakis(2,4,6-triisopropylphenyl)tetrasila-1,3-butadiene and hexakis-
(2,4,6-triisopropylphenyl)tetragerma-1,3-butadiene.2 Theoretical
calculations on the model 2,3-digerma-1,3-butadiene, H2CdGeH-
GeHdCH2, have predicted about half the degree of conjugation
compared with that of 1,3-butadiene.3 We report here the
synthesis, full characterization, X-ray structure, and reactivity of
the first cyclic metalladiene consisting of Group 14 elements of
the type -MdM′-CdC- (M ) Si, M′ ) Ge). This compound
represents also the previously unknown metallole with an un-
symmetrically composed skeleton, as well as the first example
of the structurally characterized SidGe double bond. The reaction
mechanism to form the metalladiene and the question of conjuga-
tion of the two double bonds will also be described.
Figure 1. ORTEP drawing of 4a. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å): Ge(1)-Si(2) 2.250(1), Si(1)-Si(2)
2.364(1), Si(1)-C(2) 1.888(3), C(1)-C(2) 1.343(5), Ge(1)-C(1)
1.972(3), Si(1)-Si(3) 2.431(1), Si(1)-Si(4) 2.432(1), Si(2)-Si(5)
2.380(1), Ge(1)-Si(6) 2.418(1). Selected bond angles (deg): C(1)-Ge-
(1)-Si(2) 101.8(1), Ge(1)-Si(2)-Si(1) 95.2(0), C(2)-Si(1)-Si(2)
98.1(1), C(1)-C(2)-Si(1) 126.9(2), C(2)-C(1)-Ge(1) 117.9(2),
Si(3)-Si(1)-Si(4) 124.7(0).
Scheme 1
The reaction of tetrakis[di-tert-butyl(methyl)silyl]-2-disilager-
mirene (1a)4 with excess phenylacetylene in deuteriobenzene at
room temperature in 5 h cleanly afforded an orange-red reaction
mixture, which exhibits a downfield signal in the 29Si NMR at
+75.8 ppm. Recrystallization from hexane gave air- and moisture-
sensitive bright orange crystals of 1,1,2,3-tetrakis[di-tert-butyl-
(methyl)silyl]-4-phenyl-1,2-disila-3-germacyclopenta-2,4-diene 4a
(Scheme 1).5 The 29Si NMR spectrum revealed five resonances,
of which three belong to the silyl substituents (19.4, 26.6, and
30.1 ppm); the downfield signal at +75.8 ppm is attributable to
the double bonded silicon atom, and the upfield signal at -45.6
ppm corresponds to the endocyclic sp3 Si atom.
The molecular structure of compound 4a was established by
X-ray crystallography (Figure 1).6 The five-membered ring is
almost planar, although the SidGe double bond has a twisted
(trans-bent) configuration with a torsion angle Si(6)-Ge(1)-
Si(2)-Si(5) of 38.6(1)°. The phenyl ring is almost perpendicular
to the plane of the silole ring, which is usual for phenyl-substituted
metalloles.7 The SidGe double bond length is 2.250(1) Å, which
is intermediate between the typical values for SidSi and
GedGe double bond lengths.8 This value is close to that of the
theoretically predicted 2.180 Å (MP2) for 1a.4 It is instructive to
estimate the degree of conjugation of the two double bonds in
the silole ring system. The length of the Ge(1)-C(1) bond is
1.972(3) Å, which is in the normal range for Ge-C bond lengths
of 1.95-2.00 Å.9 For the Si(2)dGe(1) double bond length, it was
impossible to compare with other examples, since no previous
(1) For the recent reviews on metallenes and dimetallenes of Group 14
elements, see: (a) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (b) Power,
P. P. Chem. ReV. 1999, 99, 3463. (c) Escudie´, J.; Ranaivonjatovo, H.; AdV.
Organomet. Chem. 1999, 44, 113.
(2) (a) Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2503. (b) Scha¨fer, H.; Saak, W.; Weidenbruch, M.
Angew. Chem., Int. Ed. 2000, 39, 3703.
(3) Jouany, C.; Mathieu, S.; Chaubon-Deredempt, M. A.; Trinquier, G. J.
Am. Chem. Soc. 1994, 116, 3973.
(4) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Nagase, S. J.
Am. Chem. Soc. 2000, 122, 9034.
(5) Spectral data for 4a: bright orange crystals; mp 161-163 °C; 1H NMR
(C6D6, δ) 0.37 (s, 6 H), 0.40 (s, 3 H), 0.59 (s, 3 H), 0.99 (s, 18 H), 1.15 (s,
18 H), 1.19 (s, 18 H), 1.31 (s, 18 H), 7.06-7.09 (m, 1 H, ArH), 7.19-7.24
(m, 2 H, ArH), 7.34 (s, 1 H, CdCH), 7.42 (dd, J1 ) 8.24 Hz, J2 ) 8.15 Hz,
2 H, ArH); 13C NMR (C6D6, δ) -2.9, -2.3 (3C), 22.1, 22.5, 23.2, 23.7, 29.9,
30.96, 31.12, 31.4, 126.3, 126.8, 128.6, 149.8 (CdCH), 151.7 (ipso C), 173.3
(CdCH); 29Si NMR (C6D6, δ) -45.6, 19.4, 26.6, 30.1, 75.8; MS (EI, 70 eV)
856-866 (M+ cluster, 31), 803 (M+ - tBu, 2), 703 (M+ - SiMetBu2, 4), 73
(100); UV/vis (hexane) λmax/nm (ꢀ) 243 (37330), 307 (6570), 472 (5540).
(6) Crystal data for 4a at 120 K: MF ) C44H90GeSi6, MW ) 860.29,
triclinic, P1h, a ) 12.2070(8) Å, b ) 13.214(1) Å, c ) 18.815(1) Å, R )
74.551(4)°, â ) 73.514(5)°, γ ) 64.841(5)°, V ) 2596.7(4) Å3, Z ) 2, Dcalcd
) 1.100 g‚cm-3. The final R factor was 0.0562 for 7813 reflections with Io >
2σ(Io) (Rw ) 0.1277 for all data, 10815 reflections), GOF ) 1.015.
(7) For the reviews on metalloles chemistry, see: (a) Dubac, J.; Laporterie,
A.; Manuel, G. Chem. ReV. 1990, 90, 215. (b) Colomer, E.; Corriu, R. J. P.;
Lheureux, M. Chem. ReV. 1990, 90, 265. (c) Dubac, J.; Gue´rin, C.; Meunier,
P. The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig,
Y., Eds.; John Wiley & Sons Ltd.: New York, 1998; Vol. 2, Part 3, Chapter
34.
(8) Typical values for a SidSi double bond length range from 2.138 to
2.289 Å, whereas GedGe double bond lengths lie in the region of 2.213-
2.460 Å: see, for example, ref 1b and the following: Schmedake, T. A.;
Haaf, M.; Apeloig, Y.; Mu¨ller, T.; Bukalov, S.; West, R. J. Am. Chem. Soc.
1999, 121, 9479.
(9) Baines, K. M.; Stibbs, W. Coord. Chem. ReV. 1995, 145, 157.
10.1021/ja0030921 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/01/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 50, 2000 12605
Scheme 2
structural data were available. The C(1)dC(2) double bond length
is 1.343(5) Å, which is similar to that of 2,4-cyclopentadiene
derivatives and 1-sila-2,4-cyclopentadienes (1.34-1.35 Å).7a None
of these data give any evidence for noticeable conjugation between
the SidGe and CdC double bonds in the cyclopentadiene unit
of 4a, despite the planarity of the five-membered ring. The same
conclusion has been drawn from the UV/vis spectrum of 4a, which
showed no significant bathochromic shift in comparison with an
isolated SidGe double bond in compound 1a (472 vs 467 nm,
respectively).
For the mechanism of the formation of 4a, the following
reaction sequences can be considered (Scheme 2). In the first step,
[2+2] cycloaddition of one molecule of phenylacetylene occurs
across the SidGe double bond to form the bicyclic compound
2a.10 This compound then quickly undergoes valence isomeriza-
tion to form the silole type structure 3a with one SidC and one
GedC double bond, which in turn rapidly isomerizes to give the
thermodynamically more stable silole 4a with one SidGe and
one CdC double bond.11,12 Cycloaddition of a second molecule
of phenylacetylene to the SidGe double bond of silole 4a can be
achieved only at elevated temperature. Thus, heating at 80 °C
for 6 h caused the complete disappearance of 4a and formation
of the new bicyclic compound 1,2,2,5-tetrakis[di-tert-butyl-
(methyl)silyl]-4,7-diphenyl-1,2-disila-5-germabicyclo[3.2.0]hepta-
3,6-diene (5a) in 55% yield.13 It is interesting that the regio-
chemistry of the PhCCH cycloaddition to the SidGe double bond
of 4a is different from the expected one; according to the polarity
of the SidGe double bond one can expect the opposite regiose-
lectivity.10 Apparently, in this case the reaction is governed by a
steric effect, rather than an electronic one. Since there is no
conjugation in a silole ring system, it is not surprising that 4a
and a second molecule of phenylacetylene react by the [2+2]
rather than the [4+2] cycloaddition mechanism. That is, silole
4a represents an unusual system with two formally conjugated,
but actually isolated double bonds, in which one (SidGe)
possesses a much higher reactivity than the other (CdC).
The reaction of 1-disilagermirene (1b) with phenylacetylene
proceeds in a similar way at room temperature finally to give the
bicyclic compound, 1,2,2,5-tetrakis[di-tert-butyl(methyl)silyl]-4,7-
diphenyl-2,5-disila-1-germabicyclo[3.2.0]hepta-3,6-diene 5b, in
63% yield (Scheme 2).13 Even at room temperature, 4b was not
observed.
Acknowledgment. This work was supported by a Grant-in-Aid for
Scientific Research (Nos. 10304051, 12020209, and 12042213) from the
Ministry of Education, Science and Culture of Japan, and the TARA
(Tsukuba Advanced Research Alliance) fund.
(10) The cycloaddition of phenylacetylene to the transient germasilenes
was found to be regioselective to form only one isomer, in which the phenyl
group-substituted carbon atom is attached to a germanium atom, which is in
accordance with the polarity of the SidGe double bond, see: Baines, K. M.;
Dixon, C. E.; Langridge, J. M.; Liu, H. W.; Zhang, F. Organometallics 1999,
18, 2206.
(11) Such a type of isomerization was found for the first time for 1-methyl-
1-trimethylsilyl-2,5-diphenyl-1-silacyclopenta-2,4-diene, which undergoes
isomerization at 150 °C through a 1,5-silyl shift to form 1-methyl-2,5-diphenyl-
5-trimethylsilyl-1-silacyclopenta-1,3-diene. The last compound is unstable and
was evidenced only by trapping reactions, see: Barton, T. J.; Wullf, W. D.;
Arnold, E. V.; Clardy, J. J. Am. Chem. Soc. 1979, 101, 2733.
Supporting Information Available: Tables of crystallographic data
including atomic positional and thermal parameters for 4a, 5a, and 5b,
as well as the experimental procedure and spectral data of 4a, 5a, and
5b (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) The stability of the isomeric compounds 2a, 3a, and 4a was evaluated
by ab initio calculations of the parent compounds (hydrogen atoms instead of
all substituents) at the B3LYP/6-31G* level. This theory predicted the most
stable compound to be 4a. Compound 2a is less stable by 13.4 kcal/mol and
compound 3a is the most unfavorable isomer by 14.3 kcal/mol.
JA0030921
(13) For the spectral data and X-ray structures of 5a and 5b, see Supporting
Information.