Synthesis and study of cyclic π-systems containing silicon ang germanium. The question of aromaticity in cyclopentadienyl analogues

Article abstract of DOI:10.1021/ja962103g

Synthetic routes to the metallole species C₄Me₄E(H)R (9, E = Si, R = Si(SiMe₃)₃; 10, E = Si, R = Mes (mesityl); 11, E = Ge, R = Si(SiMe₃)₃; 12, E = Ge, R = Mes), C₄R₄E(SiMe₃)₂ (13, E = Si, R = Me; 14, E = Ge, R = Me; 19, E = Si, R = Et; 20, E = Ge, R = Et), and C₄Me₄E(R)E(R)Me₄C₄(15, E = Si, R = SiMe₃; 16, E = Si, R = Me; 17, E = Ge, R = SiMe₃; 18, E= Ge, R = Me) are described. In the presence of 18-crown-6, dihalides 1 and 2 are reduced by potassium in tetrahydrofuran to give crystalline samples of the silole dianion [K(18-crown6)⁺]₂[C₄Me₄Si^2-] (21) and the germole dianion [K₄(18-crown-6)₃][C₄Me₄Ge]₂ (22). Compound 21 adopts an inverse sandwich geometry, while 22 is a dimer with a bridging [K(18-crown-6)K]²⁺ group and η⁵-binding modes for all of the potassium atoms. The metallole dianions in these structures appear to possess delocalized π-systems, as evidenced by nearly equivalent C-C bond lengths in the five-membered rings. Silolyl and germolyl anions have been obtained by various methods involving nucleophilic cleavage of bonds to germanium and silicon. Deprotonation of 11 and 12 in the presence of a crown ether gave the anions [K(18-crown-6)][C₄Me₄GeR] (23, R = Si(SiMe₃)₃; 24, R = Mes) and [Li(12-crown-4)₂] [C₄Me₄GeR] (25, R = Si(SiMe₃)₃; 26, R = Mes). NMR parameters for these species, and X-ray structures for 25 and 26, indicate that the anionic rings possess pyramidal germanium centers and bond localization in the diene portion of the ring. Spectroscopic and X-ray crystallographic data for [Na(15-crown-5)][C₄Me₄GeMe] (28), prepared by reductive cleavage of the Ge-Ge bond in 18, reveal a similar structure for the germolyl ring. The latter compound possesses a Na···Ge interaction in the solid state. Silolyl and germolyl anions M[C₄Me₄E(SiMe₃)] (30, E = Si, M = Li; 31, L = Si, M = K; 32, L = Si. M = Li( l2-crown-4)₂; 33, L = Si, M = K(18-crown-6); 34, E = Ge, M = K; 35, L = Ge, M = K(18-crown-6)) have been prepared by nucleophilic cleavage of the E-SiMe₃ bond in C₄Me₄E(SiMe₃)₂ with MCH₂Ph (M = Li, K). By similar methods, the monoanionic species [K(18-crown-6)] [C₄Me₄E(SiMe₃)C₄Me₄L] (36, E = Si; 37, E = Ge) were obtained. A crystal structure determination for 33 revealed a highly pyramidalized Si center (the angle between the C₄Si plane and the Si-Si bond is 99.6°) and pronounced double bond localization in the ring. Interaction between the [K(18-crown-6)]⁺ cation and the anion is rather weak, as indicated by the K···Si distance (3.604(2) A?) and the atomic position for K. By variable-temperature ¹H NMR spectroscopy, inversion barriers for the compounds [Li(12-crown-4)₂][C₄Et₄ESiMe₃] (38, E = Si; 40, E = Ge) and K[C₄Et₄ESiMe₃](39, E = Si; 41, E = Ge) were estimated. Barriers for the germolyl anions 40 and 41 (10.5(1) and 9.4(1) kcal mol^-1, respectively) are distinctly higher than those for the corresponding silolyl anions 38 and 39, as might be expected from periodic trends. The silolyl anions exhibited coalescence temperatures below the freezing point of tetrahydrofuran (165 K), but upper limits to the inversion barriers were estimated from spectra recorded at the lowest temperatures (≤ 8.4 kcal mol^-1 for 38 and < 8.4 kcal mol^-1 for 39). The measured inversion barriers for compounds 38-41 provide energy differences between the pyramidal anions and their corresponding planar (possibly aromatic) structures, and their low values may be attributed to stability imparted to the transition state by delocalization of π-electron density in the ring.

Full text of DOI:10.1021/ja962103g

J. Am. Chem. Soc. 1996, 118, 10457-10468
10457
Synthesis and Study of Cyclic π-Systems Containing Silicon
and Germanium. The Question of Aromaticity in
Cyclopentadienyl Analogues
William P. Freeman,† T. Don Tilley,*,† Louise M. Liable-Sands,‡ and
Arnold L. Rheingold*,‡
Contribution from the Department of Chemistry, UniVersity of California, Berkeley,
Berkeley, California 94720-1460, Department of Chemistry and Biochemistry,
UniVersity of Delaware, Newark, Delaware 19716-2522
ReceiVed June 21, 1996X
Abstract: Synthetic routes to the metallole species C4Me4E(H)R (9, E ) Si, R ) Si(SiMe3)3; 10, E ) Si, R ) Mes
(mesityl); 11, E ) Ge, R ) Si(SiMe3)3; 12, E ) Ge, R ) Mes), C4R4E(SiMe3)2 (13, E ) Si, R ) Me; 14, E ) Ge,
R ) Me; 19, E ) Si, R ) Et; 20, E ) Ge, R ) Et), and C4Me4E(R)E(R)Me4C4 (15, E ) Si, R ) SiMe3; 16, E )
Si, R ) Me; 17, E ) Ge, R ) SiMe3; 18, E ) Ge, R ) Me) are described. In the presence of 18-crown-6, dihalides
1 and 2 are reduced by potassium in tetrahydrofuran to give crystalline samples of the silole dianion [K(18-crown-
6)+]2[C4Me4Si2-] (21) and the germole dianion [K4(18-crown-6)3][C4Me4Ge]2 (22). Compound 21 adopts an inverse-
sandwich geometry, while 22 is a dimer with a bridging [K(18-crown-6)K]2+ group and η5-binding modes for all of
the potassium atoms. The metallole dianions in these structures appear to possess delocalized π-systems, as evidenced
by nearly equivalent C-C bond lengths in the five-membered rings. Silolyl and germolyl anions have been obtained
by various methods involving nucleophilic cleavage of bonds to germanium and silicon. Deprotonation of 11 and
12 in the presence of a crown ether gave the anions [K(18-crown-6)][C4Me4GeR] (23, R ) Si(SiMe3)3; 24, R )
Mes) and [Li(12-crown-4)2][C4Me4GeR] (25, R ) Si(SiMe3)3; 26, R ) Mes). NMR parameters for these species,
and X-ray structures for 25 and 26, indicate that the anionic rings possess pyramidal germanium centers and bond
localization in the diene portion of the ring. Spectroscopic and X-ray crystallographic data for [Na(15-crown-5)]-
[C4Me4GeMe] (28), prepared by reductive cleavage of the Ge-Ge bond in 18, reveal a similar structure for the
germolyl ring. The latter compound possesses a Na‚‚‚Ge interaction in the solid state. Silolyl and germolyl anions
M[C4Me4E(SiMe3)] (30, E ) Si, M ) Li; 31, E ) Si, M ) K; 32, E ) Si, M ) Li(12-crown-4)2; 33, E ) Si, M
) K(18-crown-6); 34, E ) Ge, M ) K; 35, E ) Ge, M ) K(18-crown-6)) have been prepared by nucleophilic
cleavage of the E-SiMe3 bond in C4Me4E(SiMe3)2 with MCH2Ph (M ) Li, K). By similar methods, the monoanionic
species [K(18-crown-6)][C4Me4E(SiMe3)C4Me4E] (36, E ) Si; 37, E ) Ge) were obtained. A crystal structure
determination for 33 revealed a highly pyramidalized Si center (the angle between the C4Si plane and the Si-Si
bond is 99.6°) and pronounced double bond localization in the ring. Interaction between the [K(18-crown-6)]+
cation and the anion is rather weak, as indicated by the K‚‚‚Si distance (3.604(2) Å) and the atomic position for K.
By variable-temperature 1H NMR spectroscopy, inversion barriers for the compounds [Li(12-crown-4)2][C4Et4ESiMe3]
(38, E ) Si; 40, E ) Ge) and K[C4Et4ESiMe3] (39, E ) Si; 41, E ) Ge) were estimated. Barriers for the germolyl
anions 40 and 41 (10.5(1) and 9.4(1) kcal mol-1, respectively) are distinctly higher than those for the corresponding
silolyl anions 38 and 39, as might be expected from periodic trends. The silolyl anions exhibited coalescence
temperatures below the freezing point of tetrahydrofuran (165 K), but upper limits to the inversion barriers were
estimated from spectra recorded at the lowest temperatures (e8.4 kcal mol-1 for 38 and <8.4 kcal mol-1 for 39).
The measured inversion barriers for compounds 38-41 provide energy differences between the pyramidal anions
and their corresponding planar (possibly aromatic) structures, and their low values may be attributed to stability
imparted to the transition state by delocalization of π-electron density in the ring.
There has been considerable recent interest in cyclic π-sys-
tems containing silicon and germanium.1 Much of this interest
concerns the possibility of aromatic character for such systems,
which may arise via delocalization of p-orbitals on silicon or
germanium with carbon-based π-systems. Theoretical calcula-
tions indicate that silabenzene is nearly as aromatic as benzene,2
whereas the silolyl (or silacyclopentadienyl) anion C4H4SiH-
is predicted to be only partially delocalized.3,4 Early calculations
indicated only a small degree of aromatic character for
C4H4SiH-,3 but more recent studies predict significant delo-
calization despite the presence of a pyramidal silicon atom in
the ground state.4 Hong and Boudjouk have provided NMR
parameters for the lithium and sodium salts of C4Ph4SitBu-
which suggest some delocalization of negative charge in the
ring.5 Germolyl anions may also be generated in solution,6 but
an analysis of NMR data for C4Me4GePh- suggested that the
negative charge was localized substantially on germanium.7 We
have reported the X-ray crystal structure for [Li(12-crown-4)2]-
[C4Me4GeSi(SiMe3)3], which possesses a highly pyramidal Ge
center and pronounced π-bond localization in the ring.8 How-
ever, it is interesting to compare the properties for (η5-C5-
Me5)Ru{η5-C4Me4ESi(SiMe3)3} (E ) Si9 and Ge10), which have
significantly delocalized electron density in the C4E rings, as
shown by NMR spectroscopy and X-ray crystallography. It
therefore seems that various substituent effects may greatly
† University of California, Berkeley.
‡ University of Delaware.
X Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0002-7863(96)02103-8 CCC: $12.00 © 1996 American Chemical Society

10458 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Freeman et al.
influence the degree of delocalization in silolyl and germolyl
rings.
over the years.19 For our investigations on stable η5-silolyl and
η5-germolyl transition metal complexes,9,10 we desired silole
and germole starting materials that did not contain alternative
binding sites for the metal (e.g., aryl groups). Therefore the
compounds C4Me4GeCl2 (1) and C4Me4SiBr2 (2), reported by
Fagan et al., seemed to offer ideal starting materials.20 The
synthesis of 1 was achieved by a modification of the Fagan
procedure, involving the synthesis of Cp2ZrC4Me4 and its
conversion to 1 in a single reaction flask. The analogous ethyl
derivative C4Et4GeCl2 (3) was obtained similarly as a colorless
oil in 70% yield, after distillation under vacuum. The reported
synthesis of 2 gives impure material in relatively low yield.
This method was modified by carrying out the reaction of Cp2-
ZrC4Me4 and neat SiBr4 at 135-140 °C over 12 h. In this way
compound 2, contaminated by 1-20% hexamethylbenzene, was
obtained in 30% yield as light pink crystals after purification
by short-path distillation, crystallization, and then sublimation.
Attempts to obtain C4Et4SiBr2 by the reaction of Cp2ZrC4Et4
with SiBr4 were unsuccessful. However, the analogous chloride
derivative C4Et4SiCl2 (4) was obtained by the sequence of
reactions shown in eq 1, using methods previously described
by Ashe21 and West.14
Related cyclic systems that have attracted more recent interest
are silole and germole dianions C4R4E2-. After the initial report
by Joo and co-workers11 on the generation of C4Ph4Si2-, a
number of investigations focused on the use of such species as
synthetic intermediates.12-16 Goldfuss and Schleyer published
a theoretical study on the silole dianion C4H4Si2- which
characterizes this cyclic system as highly aromatic,17 and this
view has been supported by recent crystallographic studies by
West15,18 and us.16 Given the level of interest in anions of the
types described above, and their potential to exhibit novel
electronic properties, we have sought to characterize such
systems in detail. Here we report a comprehensive study on
the synthesis, structure, and electronic properties for related C4-
(alkyl)4E-R- and C4(alkyl)4E2- (E ) Si, Ge) ring systems.
Results
Synthesis of Silole and Germole Precursors. A number
of synthetic routes to siloles and germoles have been reported
(1) (a) Colomer, E.; Corriu, R. J. P.; Lheureux, M. Chem. ReV. 1990,
90, 265-282. (b) Gru¨tzmacher, H. Angew. Chem., Int. Ed. Engl. 1995,
34, 295-298. (c) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov,
A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem.
Soc. 1994, 116, 2691-2692. (d) Heinemann, C.; Mu¨ller, T.; Apeloig, Y.;
Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023-2038. (e) Boehme, C.;
Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039-2046. (f) Blakeman, P.;
Gehrhus, B.; Green, J. C.; Heinicke, J.; Lappert, M. F.; Kindermann, M.;
Veszpre´mi, T. J. Chem. Soc., Dalton Trans. 1996, 1475-1480. (g)
Heinemann, C.; Herrmann, W. A.; Thiel, W. J. Organomet. Chem. 1994,
475, 73-84. (h) Barton, T. J.; Banasiak, D. J. Am. Chem. Soc. 1977, 99,
5199-5200. (i) Barton, T. J.; Burns, G. T. J. Am. Chem. Soc. 1978, 100,
5246. (j) Solouki, B.; Rosmus, P.; Bock, H.; Maier, G. Angew. Chem.,
Int. Ed. Engl. 1980, 19, 51-52. (k) Ma¨rkl, G.; Schlosser, W. Angew. Chem.,
Int. Ed. Engl. 1988, 27, 963-965. (l) Maier, G.; Mihm, G.; Reisenauer,
H. P. Angew. Chem., Int. Ed. Engl. 1980, 19, 52-53. (m) Nakadaira, Y.;
Sato, R.; Sakurai, H. Organometallics 1991, 10, 435-442. (n) Rich, J.
D.; West, R. J. Am. Chem. Soc. 1982, 104, 6884-6885. (o) Jutzi, P.; Meyer,
M.; Dias, H. V. R.; Power, P. P. J. Am. Chem. Soc. 1990, 112, 4841-
4846.
Desired precursors to silolyl and germolyl anions were the
monosubstituted derivatives C4Me4E(R)X and C4Me4E(R)H (E
) Si, Ge; R ) alkyl, aryl, or silyl). Compounds of these types
were obtained by the sequence of reactions shown in eq 2 (Mes
) mesityl). In general, this method is successful only with
sterically hindered R groups, as smaller nucleophiles lead to
mixtures of mono- and disubstituted products. Silyl bromide 6
was not isolated, but was generated in situ for the preparation
of 10.
(2) (a) Schlegel, H. B.; Coleman, B.; Jones, M. J. Am. Chem. Soc. 1978,
100, 6499-6501. (b) Blustin, P. H. J. Organomet. Chem. 1979, 166, 21-
24. (c) Baldrige, K. K.; Gordon, M. S. J. Am. Chem. Soc. 1988, 110, 4204-
4208.
(3) (a) Gordon, M. S.; Boudjouk, P.; Anwari, F. J. Am. Chem. Soc. 1983,
105, 4972-4976. (b) Damewood, J. R. J. Org. Chem. 1986, 51, 5028-
5029.
(4) Goldfuss, B.; Schleyer, P. von R. Organometallics 1995, 14, 1553-
1555.
(5) Hong, J.-H.; Boudjouk, P. J. Am. Chem. Soc. 1993, 115, 5883-5884.
(6) (a) Curtis, M. D. J. Am. Chem. Soc. 1967, 89, 4241-4242. (b) Curtis,
M. D. J. Am. Chem. Soc. 1969, 91, 6011-6018. (c) Jutzi, P.; Karl, A. J.
Organomet. Chem. 1981, 215, 19-25.
Routes to monoanions of interest were also based on the
starting compounds 13-18 shown in Scheme 1. These met-
allole rings are obtained by the in situ alkylation or silylation
of intermediate dianionic species obtained by the reduction of
1 and 2 with potassium metal. In a similar way, the silole C4-
Et4Si(SiMe3)2 (19) and the germole C4Et4Ge(SiMe3)2 (20) were
prepared. This versatile method was pioneered by Joo and co-
workers, who used it to obtain a number of tetraphenyl silole
derivatives.11
Isolation and Structural Characterization of Silole and
Germole Dianions. Given the strong interest in silole and
germole dianions as synthetic intermediates and as potentially
aromatic systems, we attempted to isolate and structurally
characterize examples of this class of compounds. Addition of
(7) Dufour, P.; Dubac, J.; Dartiguenave, M.; Dartiguenave, Y. Organo-
metallic 1990, 9, 3001-3003.
(8) Freeman, W. P.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L.;
Gantzel, P. K. Angew. Chem., Int. Ed. Engl. 1995, 34, 1887-1890.
(9) Freeman, W. P.; Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc.
1994, 116, 8428-8429.
(10) Freeman, W. P.; Tilley, T. D.; Rheingold, A. L.; Ostrander, R. L.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1744-1745.
(11) Joo, W.-C.; Hong, J.-H.; Choi, S.-B.; Son, H.-E.; Kim, C. H. J.
Organomet. Chem. 1990, 391, 27-36.
(12) Hong, J.-H.; Boudjouk, P.; Castellino, S. Organometallics 1994,
13, 3387-3389.
(13) Hong, J.-H.; Boudjouk, P. Bull. Soc. Chim. Fr. 1995, 132, 495-
498.
(14) Bankwitz, U.; Sohn, H.; Powell, D. R.; West, R. J. Organomet.
Chem. 1995, 449, C7-C9.
(15) West, R.; Sohn, H.; Powell, D. R.; Mu¨ller, T.; Apeloig, Y. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1002-1004.
(19) Dubac, J.; Laporterie, A.; Manuel, G. Chem. ReV. 1990, 90, 215-
263.
(16) Freeman, W. P.; Tilley, T. D.; Yap, G. A. P.; Rheingold, A. L.
Angew. Chem., Int. Ed. Engl. 1996, 35, 882-884.
(20) (a) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J. Am. Chem. Soc.
1994, 116, 1880-1889. (b) Fagan, P. J.; Nugent, W. A. J. Am. Chem.
Soc. 1988, 110, 2310-2312.
(21) Ashe, A. J.; Kampf, J. W.; Pilotek, S.; Rousseau, R. Organometallics
1994, 13, 4067-4071.
(17) Goldfuss, B.; Schleyer, P. von R.; Hampel, F. Organometallics 1996,
15, 1755-1757.
(18) West, R.; Sohn, H.; Bankwitz, U.; Calabrese, J.; Apeloig, Y.;
Mueller, T. J. Am. Chem. Soc. 1995, 117, 11608-11609.

Synthesis of Cyclic π-Systems Containing Si and Ge
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10459
18-crown-6 (2 equiv) to reaction mixtures obtained by adding
1 or 2 to potassium in THF resulted in an increased rate for
reduction to the dianions, as indicated by a more rapid
development of the red colors associated with these species.
Formation of the dianions from the dihalide starting materials
1
is accompanied by a dramatic downfield shift in the H NMR
resonances, which is consistent with considerable aromatic
1
character in the rings. For example, the H NMR shifts for 2
(δ 1.38, 1.70) are replaced by shifts for the dianion 21 at δ
2.81 and 3.27.
Small quantities of crystalline samples were obtained by the
slow (over 7-10 days), gas-phase diffusion of pentane into
THF/18-crown-6 solutions of the dianions. In this way, we have
isolated [K(18-crown-6)+]2[C4Me4Si2-] (21), which adopts an
“inverse sandwich” structure with both potassium atoms coor-
dinated by the silole dianion in an η5-fashion (Figure 1).16 Nearly
equivalent C-C distances in the five-membered ring suggest a
high degree of delocalization, as predicted by theory. Unfor-
tunately we were unable to obtain 13C and 29Si NMR data for
this silole dianion due to its low solubility in unreactive solvents
(e.g., benzene-d6 and THF-d8).
Figure 1. ORTEP diagram of the silole dianion [K(18-crown-6)+]2-
[C4Me4Si2-] (21).
Scheme 1
Crystals obtained by reduction of 1 contain the bis(germole
dianion) complex [K4(18-crown-6)3][C4Me4Ge]2 (22‚THF, Fig-
ure 2). Crystal and data collection parameters are given in Table
1, and selected bond distances and angles are listed in Table 2.
The two germole dianions in the unit cell are related by a
crystallographic point of inversion and are therefore identical.
The slight differences in the C-C bond lengths of the C4Ge
ring (1.45(1), 1.43(1), and 1.42(1) Å) point toward considerable
delocalization of π-electron density in the ring. The large
difference in Ge-C bond distances results from the rather high
Table 1. Crystallographic data for Compounds 22‚THF, 26, 28, and 33
compd
22‚THF
26
28
33
(a) Crystal Parameters
C33H55GeLiO8
659.3
formula
C56H104Ge2K4O19
1382.97
monoclinic
C2/c
20.681(3)
19.347(2)
20.887(2)
C19H35GeNaO5
439.05
monoclinic
P21/c
10.101(1)
14.270(1)
16.422(3)
C23H45KO6Si2
512.87
triclinic
P1h
9.999(2)
10.705(2)
14.208(3)
94.08(2)
92.79(2)
102.58(2)
1477.2(4)
2
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
16.037(6)
12.692(5)
17.893(6)
R (deg)
â (deg)
114.88(1)
103.25(3)
98.24(1)
γ (deg)
V (Å3)
7582(2)
4
3545(2)
4
2342.6(5)
4
Z
cryst color
D(calc), g cm-3
µ(Mo KR), cm-1
temp, K
red-orange
1.212
10.70
293
yellow
1.235
9.10
yellow
1.245
13.49
238
yellow
1.153
2.92
246
238
(b) Data Collection
diffractometer
monochromator
radiation
2θ range, deg
rflns collected
indpt rflns
Siemens P4
graphite
Mo KR (λ ) 0.71073 Å)
4-50
6870
6670
4-42
3913
3761
4-45
4024
3056
4-50
5923
4952
3 std/197 rflns
std rflns
3 std/197 rflns
3 std/197 rflns
3 std/197 rflns
(c) Refinementa
R(F), %
R(wF), %
∆/σ (max)
∆(F), e Å-3
No/NV
7.10b
16.23c
0.05
0.62
17.5
5.66
6.39
0.07
0.42
5.5
5.63b
14.22c
0.11
0.46
12.9
5.13b
12.54c
0.01
0.29
17.1
GOF
0.89
1.14
0.88
0.70
2
a Quantity minimized ) ∑w∆2; R ) ∑∆/∑(Fo); R(w) ) ∑∆w1/2/Σ(Fow1/2), ∆ ) |(Fo - Fc)|. b Quantity minimized ) R(wF2) ) ∑[w(Fo
-
2
Fc2)2]/∑[(wFo )2]1/2; R ) ∑∆/∑(Fo), ∆ ) |(Fo - Fc)|. c R(wF2), %.

10460 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Freeman et al.
Scheme 2
Figure 2. ORTEP diagram for the bis(germole dianion) complex
[K4(18-crown-6)3][C4Me4Ge]2 (22).
have very similar structures in solution. For example, the 13C
NMR shifts for the ring carbons (CR and Câ) of compounds 23
and 25 are nearly equivalent (Table 3), and somewhat downfield
shifted relative to analogous resonances for the corresponding
germoles. Such shifts seem to be associated with bond
localization in the rings, and considerable localization of
negative charge onto the germanium atom.7,8 Note that the
analogous 13C shifts for 27 are similar (Table 3), indicating that
interaction of the lithium cation with the ring does not greatly
perturb its electronic structure.
X-ray quality crystals of 25 and 26 were obtained by slow
diffusion of pentane into diethyl ether solutions of the com-
pounds. The structure of 25 was communicated previously.8
Both structures consist of discrete cations and anions. An
ORTEP drawing of the anion in 26 is shown in Figure 3, and
important bond distances and angles are listed in Table 4. The
anion possesses a pyramidal germanium center, which results
in an angle between the C4Ge plane and the Ge-C(Mes) bond
of 113.4°. For comparison the analogous angle in 25 is
considerably smaller, at 100.1°. In addition the C4Ge ring
possesses pronounced bond localization, as indicated by the
variation in C-C bond lengths. Table 5 compares germole ring
C-C bond distances with related C-C distances in germolyl
anions. As can be seen from these data, the germolyl anions
25 and 26 exhibit similar differences (∆d) between the CR-Câ
and Câ-Câ bond distances, indicating that both compounds have
considerable diene character.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[K4(18-crown-6)3][C4Me4Ge]2‚THF (22‚THF)
(a) Bond Distances
Ge-C(1)
1.846(9)
1.959(8)
1.451(11)
1.431(10)
1.417(10)
3.302(2)
3.049(6)
2.885(7)
K(1)-C(3)
K(1)-C(4)
K(2)-Ge
K(2)-C(1)
K(2)-C(2)
K(2)-C(3)
K(2)-C(4)
2.881(7)
3.008(7)
3.348(2)
3.098(7)
3.007(7)
3.063(7)
3.169(7)
Ge-C(4)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
K(1)-Ge
K(1)-C(1)
K(1)-C(2)
(b) Bond Angles
C(1)-Ge-C(4)
Ge-C(1)-C(2)
Ge-C(4)-C(3)
C(1)-C(2)-C(3)
C(4)-C(3)-C(2)
Ge-C(1)-C(5)
Ge-C(4)-C(8)
86.4(4)
113.0(5)
112.2(5)
115.5(7)
112.9(7)
129.7(7)
124.4(7)
C(5)-C(1)-C(2)
C(8)-C(4)-C(3)
C(1)-C(2)-C(6)
C(4)-C(3)-C(7)
C(6)-C(2)-C(3)
C(7)-C(3)-C(2)
117.3(8)
123.2(8)
122.4(9)
122.8(8)
121.6(9)
124.1(8)
esd’s associated with this structure, and is therefore not
chemically meaningful.
Interestingly, silolyl and germolyl dianions were also pro-
duced (in only ca. 5% yield) when 3 equiv of potassium was
used in the reduction. From the reduction of C4Me4GeCl2 (1)
by 3 equiv of potassium, a mixture of poorly formed crystals
was obtained. This mixture reacts with Me3SiCl in benzene-d6
to give both 14 and 17 (3:1). Thus, it appears that the reactions
with 3 equiv of potassium give mixtures of the dianions C4-
Me4E(K)E(K)C4Me4 and C4Me4EK2. In preparative-scale reac-
tions, however, compounds 15-18 are readily separated from
the corresponding metalloles 13 and 14 by fractional crystal-
lization. Similar observations on silole dianion systems have
been described by Joo11 and Boudjouk.12
Reductive cleavage of the Ge-Ge bond in 18 with Na/15-
crown-5 in toluene produced 28, which is the germolyl anion
analog of pentamethylcyclopentadienide (C5Me5-) (eq 3). The
Synthesis and Isolation of Silolyl and Germolyl Anions.
We have employed three different methods for the synthesis of
these monoanionic species. The first method, pioneered by
Curtis in 1967,6 involves abstraction of a proton from germa-
nium. More recently, this method has been used to generate a
solution of [C4Me4GePh]-.7 The second route, introduced by
Boudjouk,5 is based on reductive cleavage of an E-E bond.
Finally, we have employed nucleophilic cleavage of E-E
bonds8,16 to produce both silolyl and germolyl anions.
The proton abstraction route was successfully employed only
for the hydrogermoles 11 and 12. Anions 23-26 form cleanly
in benzene-d6 solution by 1H NMR spectroscopy, and prepara-
tive scale reactions in toluene or diethyl ether allow isolation
of these salts as crystalline solids (Scheme 2). In addition the
base-free lithium germolyl Li[C4Me4GeSi(SiMe3)3] (27) was
isolated, as a yellow crystalline solid, from reaction of 11 with
nBuLi. The NMR parameters for 23-27 indicate that the anions
13C NMR data for 28 indicate that the anionic ring, like those
in 25 and 26, is bond-localized and nonaromatic (Table 3). This
species was crystallized by simply cooling the reaction solution
to -40 °C. The molecular structure is shown in Figure 4, and
important bond distances and angles are listed in Table 6. This
structure differs from those for 25 and 26, in that there is a
strong interaction between the cation and anion. This interaction
brings the sodium atom in contact with only the germanium
atom of the ring; there are no sodium-carbon bonding interac-
tions in the structure. Thus, the germanium atom is in a
distorted tetrahedral environment. Bond distances in the ring
reflect a nonaromatic diene structure, and are very similar to
the corresponding distances in 25 and 26, despite the presence
of the Na‚‚‚Ge bonding interaction. The angle between the Ge-
C(Me) bond and the C4Ge plane, 110.0°, is very close to the
corresponding value for the free anion 26 (113.4°).

Synthesis of Cyclic π-Systems Containing Si and Ge
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10461
Table 3. 13C and 29Si NMR Shifts for Ring Atoms of Selected Germoles, Siloles, Germolyl Anions, and Silolyl Anions
compd
δ(CR)
(a) Germoles
δ(Câ)
δ(ring Si)
C4Me4Ge(H)Si(SiMe3)3 (11)a
C4Me4Ge(H)Mes (12)b
132.89
144.69
134.86
135.83
133.79
144.34
146.45
146.65
145.67
145.47
145.22
151.18
C4Me4Ge(SiMe3)2 (14)b
C4Me4Ge(SiMe3)Ge(SiMe3)C4Me4 (17)b
C4Me4Ge(Me)Ge(Me)C4Me4 (18)b
C4Et4Ge(SiMe3)2 (20)b
(b) Siloles
C4Me4Si(H)Si(SiMe3)3 (9)
C4Me4Si(H)Mes (10)
129.53b
129.28b
130.99
131.83
131.25
139.78
150.94b
151.00b
149.81
150.11
149.11
155.56
-8.90a
-27.57a
-34.26
-34.71
-13.95
-37.50
C4Me4Si(SiMe3)2 (13)b
C4Me4Si(SiMe3)Si(SiMe3)C4Me4 (15)b
C4Me4Si(Me)Si(Me)C4Me4 (16)b
C4Et4Si(SiMe3)2 (19)b
(c) Germolyl Anions
137.13
[K(18-crown-6)][C4Me4GeSi(SiMe3)3] (23)b
[K(18-crown-6)][C4Me4GeMes] (24)b
[Li(12-crown-4)2][C4Me4GeSi(SiMe3)3] (25)c
[Li(12-crown-4)2][C4Me4GeMes] (26)c
Li[C4Me4GeSi(SiMe3)3] (27)d
157.06
156.40
157.03
155.21
156.90
151.5
156.89
159.96
157.40
158.56
156.43
166.84
168.62
145.94
136.95
145.49
141.29
138.7
136.10
145.83
141.10
136.72
140.63
142.45
141.05
Li[C4Me4GePh]7 d
[Na(15-crown-5)][C4Me4GeMe] (28)b
[K(18-crown-6)][C4Me4GeMe] (29)b
K[C4Me4GeSiMe3] (34)d
[K(18-crown-6)][C4Me4GeSiMe3] (35)b
[K(18-crown-6)][C4Me4Ge(SiMe3)C4Me4Ge] (37)b
[Li(12-crown-4)2][C4Et4GeSiMe3] (40)d
K[C4Et4GeSiMe3] (41)d
(d) Silolyl Anions
138.66
Li[C4Me4SiSiMe3] (30)d
146.38
148.97
148.62
149.60
149.08
158.27
158.42
-45.38
-42.70
-43.96
-41.52
-53.43
-53.12
-47.38
K[C4Me4SiSiMe3] (31)d
136.23
135.78
135.76
138.49
141.96
140.88
[Li(12-crown-4)2][C4Me4SiSiMe3] (32)b
[K(18-crown-6)][C4Me4SiSiMe3] (33)b
[K(18-crown-6)][C4Me4Si(SiMe3)C4Me4Si] (36)b
[Li(12-crown-4)2][C4Et4SiSiMe3] (38)d
K[C4Et4SiSiMe3] (39)d
a Dichloromethane-d2. b Benzene-d6. c Benzene-d6-THF-d8 (1:1). d THF-d8.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
[Li(12-crown-4)2][C4Me4GeMes] (26)
(a) Bond Distances
Ge-C(1)
2.007(9)
1.961(8)
1.340(15)
1.472(12)
1.346(14)
Ge-C(9)
2.039(10)
1.500(12)
1.497(15)
1.511(14)
1.521(11)
Ge-C(4)
C(1)-C(5)
C(2)-C(6)
C(3)-C(7)
C(4)-C(8)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
(b) Bond Angles
C(1)-Ge-C(9)
C(4)-Ge-C(9)
C(1)-Ge-C(4)
Ge-C(1)-C(5)
Ge-C(4)-C(3)
Ge-C(4)-C(8)
Ge-C(1)-C(2)
C(1)-C(2)-C(3)
98.5(4)
111.8(4)
84.8(4)
C(2)-C(3)-C(4)
C(3)-C(4)-C(8)
C(5)-C(1)-C(2)
C(4)-C(3)-C(7)
C(1)-C(2)-C(6)
C(2)-C(3)-C(7)
C(3)-C(2)-C(6)
116.9(9)
122.6(8)
125.7(8)
124.1(8)
124.7(8)
119.0(9)
119.1(9)
124.0(7)
110.6(6)
124.7(7)
110.2(6)
116.2(9)
Figure 3. ORTEP diagram for [Li(12-crown-4)2][C4Me4GeMes] (26).
in THF.23 We have extended this method to the synthesis of
tetraalkyl-substituted silolyl and germolyl anions. For example,
the Ge-Ge bond of C4Me4Ge(Me)Ge(Me)C4Me4 (18) is cleaved
by KCH2Ph/18-crown-6 to form [K(18-crown-6)][C4Me4GeMe]
(29).
Nucleophilic cleavage of the E-SiMe3 bonds in 13 and 14
with benzyl lithium or benzyl potassium proceeds cleanly in
toluene, benzene-d6, or tetrahydrofuran-d8 (eq 4). Complexing
agents for Li or K were readily incorporated by simply adding
them to the reaction mixture. Compounds 33 and 35 were
isolated in moderate yields by crystallization from toluene, as
orange-yellow and yellow crystals, respectively. The remaining
Attempted reduction of the silicon analog, C4Me4Si(Me)Si-
(Me)C4Me4 (16), with potassium/18-crown-6 in THF, gave
numerous products. However, reduction of 16 with sodium
dispersion/15-crown-5 in benzene-d6 produced a slightly impure
1
product which displayed H NMR shifts (at δ 2.30 and 2.65)
that were similar to those observed for 28.
The generation of silyl anion species via nucleophilic cleavage
of a Si-Si bond is a well-established synthetic method.22
Ishikawa et al. have previously described extension of this
method to generation of the 1-methyldibenzosilacyclopenta-
dieneide anion, by cleavage of a Si-Si bond with Ph2MeSiLi
(23) Ishikawa, M.; Tabohashi, T.; Okashi, H.; Kumada, M.; Iyoda, J.
Organometallics 1983, 2, 351-352.
(22) Tamao, K.; Kawachi, A. AdV. Organomet. Chem. 1995, 38, 1-58.

10462 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Freeman et al.
Table 5. Bond Length Data for Germoles and Germolyl and Silolyl Anions
compd
d[E-CR] (av) (Å)
d[CR-Câ] (av) (Å)
d[Câ-Câ] (Å)
∆da (Å)
C4Me4Ge(H)Si(SiMe3)3 (11)8
C4Ph4Ge(SiMe3)2
[Li(12-crown-4)2][C4Me4GeSi(SiMe3)3] (25)
[Li(12-crown-4)2][C4Me4GeMes] (26)
[Na(15-crown-5)][C4Me4GeMe] (28)
[K(18-crown-6)][C4Me4Si(SiMe3)] (33)
1.95
-
1.98
1.98
1.96
1.88
1.34
1.361(4)
1.36
1.34
1.33
1.504(4)
1.494(4)
1.46(6)
1.47(1)
1.48(1)
1.450(5)
0.16
0.13
0.10
0.13
0.15
0.09
b
1.36
a ∆d is the difference between the Câ-Câ and CR-Câ bond lengths. b West, R., personal communication.
Figure 4. ORTEP diagram for [Na(15-crown-5)][C4Me4GeMe] (28).
Figure 5. ORTEP diagram for [K(18-crown-6)][C4Me4SiSiMe3] (33).
Table 6. Selected Bond Distances (Å) and Angles (deg) for
[Na(15-crown-5)][C4Me4GeMe] (28)
shifts that are very similar to those observed for the related
germanium-centered anions described above. Generally, forma-
tion of the silolyl anions results in a downfield shift of the ring
carbons with respect to the parent silole compounds, especially
for the CR carbon atoms. Note also that the silolyl anions
possess 29Si NMR shifts for the ring silicon atoms that are quite
upfield, and shifted considerably upfield with respect to
analogous neutral silole compounds. These data are entirely
consistent with silyl anion character for these species,24 and
significant charge localization on silicon. Not that the NMR
shift parameters for 30 and 31 (in tetrahydrofuran-d8), which
may have significant metal-ring interactions in solution, are
very similar to the shifts observed for the anion in 32, which
should have no coordinated metal atom.
Given the complete lack of structural data for silolyl anions,
and reported suggestions that they might be delocalized,3-5 we
determined the structure of compound 33. A view of the
structure is given in Figure 5, and important metrical data are
listed in Table 7. The [C4Me4SiSiMe3]- anion in this structure
is clearly nonaromatic, as indicated by the pronounced bond
localization which is reflected in a difference between the CR-
Câ and Câ-Câ distances of almost 0.1 Å. In particular, the
sharp angle of 99.6° between the C4Si plane and the Si-Si bond
indicates a high degree of pyramidalization at silicon. The
[K(18-crown-6)]+ cation interacts with the anion, but only via
contact with the ring silicon atom, at a long distance of 3.604-
(2) Å. For comparison, the K-Si bond distances in 21 are
3.387(3) and 3.364(3) Å.16 Further evidence for a relatively
weak K‚‚‚Si interaction is seen in the atomic position for K,
which is far removed from the remaining tetrahedral position
about Si(1). This can be seen for example in the inequivalent
K(1)-Si(1)-C(1) and K(1)-Si(1)-C(4) bond angles of 146.8-
(1) and 95.9(1)°, respectively. The Si-CR distances (1.890(4)
(a) Bond Distances
Ge-C(1)
Ge-C(4)
Ge-Na
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
1.957(7)
1.967(7)
2.975(3)
1.335(11)
1.484(11)
1.331(9)
Ge-C(9)
2.006(6)
1.525(10)
1.522(10)
1.536(10)
1.493(9)
C(1)-C(5)
C(2)-C(6)
C(3)-C(7)
C(4)-C(8)
(b) Bond Angles
C(1)-Ge-C(9)
C(4)-Ge-C(9)
C(1)-Ge-C(4)
Ge-C(1)-C(5)
Ge-C(4)-C(3)
Ge-C(4)-C(8)
Ge-C(1)-C(2)
C(1)-C(2)-C(3)
103.0(3)
102.0(3)
85.7(3)
C(2)-C(3)-C(4)
C(3)-C(4)-C(8)
C(5)-C(1)-C(2)
C(4)-C(3)-C(7)
C(1)-C(2)-C(6)
C(2)-C(3)-C(7)
C(3)-C(2)-C(6)
Na-Ge-C(9)
116.9(7)
126.4(7)
124.8(7)
123.8(9)
123.8(9)
119.3(8)
120.4(9)
142.4(2)
124.0(6)
109.9(5)
123.5(6)
110.5(6)
115.9(6)
silolyl and germolyl anions were generated in solution and
characterized by NMR spectroscopy. Similarly, 36 and 37 were
formed by cleavage of an E-SiMe3 bond in 15 and 17 (eq 5).
(24) (a) Scholl, R. L.; Maciel, G. E.; Musker, W. K. J. Am. Chem. Soc.
1972, 94, 6376-6385. (b) Stanislawski, D. A.; West, R. J. Organomet.
Chem. 1981, 204, 295-305. (c) Sharp, K. G.; Sutor, P. A.; Williams, E.
A.; Cargioli, J. D.; Farrar, T. C.; Ishibitsu, K. J. Am. Chem. Soc. 1976, 98,
1977-1979. (d) Sekiguchi, A.; Nanjo, M.; Kabuto, C.; Sakurai, H.
Organometallics 1995, 14, 2630-2632.
The 13C NMR shift data for the ring carbons of these anions
are listed in Table 3. The germolyl anions 34, 35, and 37 exhibit

Synthesis of Cyclic π-Systems Containing Si and Ge
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10463
Table 7. Selected Bond Distances (Å) and Angles (deg) for
[K(18-crown-6)][C4Me4Si(SiMe3)] (33)
(a) Bond Distances
Si(1)-C(1)
Si(1)-C(4)
Si(1)-K(1)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
1.880(3)
1.890(4)
3.604(2)
1.360(5)
1.450(5)
1.354(5)
Si(1)-Si(2)
C(1)-C(5)
C(2)-C(6)
C(3)-C(7)
C(4)-C(8)
2.334(2)
1.515(5)
1.525(5)
1.527(5)
1.507(5)
(b) Bond Angles
C(1)-Si(1)-Si(2)
C(4)-Si(1)-Si(2)
C(1)-Si(1)-C(4)
Si(1)-C(1)-C(5)
Si(1)-C(4)-C(3)
Si(1)-C(4)-C(8)
Si(1)-C(1)-C(2)
C(1)-C(2)-C(3)
K(1)-Si(1)-C(1)
95.2(1)
96.2(1)
C(2)-C(3)-C(4)
C(3)-C(4)-C(8)
C(5)-C(1)-C(2)
C(4)-C(3)-C(7)
C(1)-C(2)-C(6)
C(2)-C(3)-C(7)
C(3)-C(2)-C(6)
K(1)-Si(1)-Si(2)
K(1)-Si(1)-C(4)
115.6(3)
123.9(3)
122.4(3)
123.8(4)
124.4(3)
120.6(3)
120.2(3)
117.01(5)
95.9(1)
87.9(2)
127.0(3)
110.3(3)
125.7(3)
110.5(2)
115.3(3)
146.8(1)
Figure 6. Coalescence behavior for the diastereotopic R-methylene
1
hydrogens in K[C4Et4GeSiMe3] (41) in H NMR spectra.
Table 8. Inversion Barriers for 38-41
compd
Tc, Ka barrier, kcal mol-1
be useful in the synthesis of a number of cyclic main group
compounds,20,21,26 and in this work we show that it provides a
convenient synthetic pathway to silolyl and germolyl anions,
and silole and germole dianions.
[Li(12-crown-4)2][C4Et4SiSiMe3] (38)b e170
e8.4
<8.4
10.5(1)
9.4(1)
K[C4Et4SiSiMe3] (39)c
<170
210
[Li(12-crown-4)2][C4Et4GeSiMe3] (40)
K[C4Et4GeSiMe3] (41)
190
Via alkali metal reduction of dihalide starting materials 1-4,
solutions of the dianions [R4C4E]2- may be conveniently
generated in solution. This synthetic method, pioneered by Joo
and co-workers,11 has proven to be a valuable synthetic method
for the synthesis of various silole and germole derivatives. The
long reaction times associated with dianion formation may be
reduced with the use of ultrasound,13 or by addition of a crown
ether. The dianions are formed cleanly, as demonstrated by
derivatization reactions carried out in situ, such as those
represented in Scheme 1. In the presence of 18-crown-6,
crystalline samples of the dianion complexes 21 and 22 may
be obtained in low yield. The structural analyses of these
compounds reveal the presence of delocalized π-systems, as
indicated by roughly equivalent C-C distances in the ring. This
was also observed for the dianion rings in [Li(THF)2][Li(THF)3]-
[η5,η1-C4Ph4Si],18 [Li(dioxane)2]2[η5-C4Ph4Ge], and [Li(diox-
ane)2]2[η5,η1-C4Ph4Ge].15 In addition, Hong and Boudjouk have
recently reported NMR data for the dianions [Ph4C4Ge]2- and
[Ph4C4Si]2- which is consistent with delocalized π-electron
density in the rings.13 Aromaticity in silole dianions has recently
been investigated theoretically by Goldfuss et al. via ab initio
calculations.17 These calculations indicate that C4H4Si2- would
a Coalescence temperature. Data were obtained at 300 MHz in THF-
d8. b The chemical shift difference between the coalescing peaks was
estimated to be the same as for the germanium analog 40 (∆νc ) 26.2
Hz). c ∆νc estimated to be the same as for the germanium analog 41
(24.8 Hz).
and 1.880(3) Å) and the Si-Si distance (2.334(2) Å) represent
typical single bond lengths.
Inversion Barriers in Silolyl and Germolyl Anions. The
structural and spectroscopic data described above clearly
characterize germolyl and silolyl anions as metallole-like
species, with minimal delocalization of π-electron density in
the five-membered rings. To probe the stability of these
pyramidal structures, we sought to measure their inversion
barriers. For this purpose we synthesized ion pairs 38-41
(Table 8), using the methods described above. These species
possess diastereotopic methylene hydrogens which are inter-
converted by inversion at silicon or germanium. Thus, the
measured inversion barriers provide energy differences between
the ground state pyramidal and planar (possibly delocalized)
structures.
Inversion barriers were determined by variable-temperature
1H NMR spectroscopy, which allowed analysis of the coales-
cence behavior for the diastereotopic hydrogens of the methylene
groups bonded to the CR carbons (Figure 6). Barriers for the
germolyl anions 40 and 41 are distinctly higher than those for
the corresponding silolyl anions 38 and 39, as might be expected
from periodic trends.25 The silolyl anions exhibited coalescence
temperatures below the freezing point of tetrahydrofuran (165
K), but upper limits to the inversion barriers were estimated
from spectra recorded at the lowest temperatures. Spectra
recorded at 170 K indicated that compound 38 was very close
to coalescence, and closer than 39. Note that the inversion
barriers are slightly higher for the “free anion” species 38 and
40.
be even more aromatic than isoelectronic phosphole and
-
thiophene systems, and about as aromatic as C5H5
. This
aromaticity apparently reflects the absence of a pyramidal silicon
atom in the dianion. Consistent with aromatic character in
C4H4Si2-, the calculations indicate that the dilithium, disodium,
and dipotassium salts would adopt “inverse-sandwich” structures
with simultaneous η5-binding of the silole dianion to both metal
atoms. This bonding mode is in fact observed for three of the
five structurally characterized silole and germole dianions.
Various methods were employed for the synthesis of silolyl
and germolyl anions. Of particular note is the utility of benzyl
potassium as a nucleophilic reagent which generally gives short
reaction times and clean conversions. The most general routes
(26) (a) Fagan, P. J.; Burns, E. G.; Calabrese, J. C. J. Am. Chem. Soc.
1988, 110, 2979-2981. (b) Buchwald, S. L.; Fisher, R. A.; Foxman, B.
A. Angew. Chem., Int. Ed. Engl. 1990, 29, 771-772. (c) Spence, R. E. v.
H.; Hsu, D. P.; Davis, W. M.; Buchwald, S. L.; Richardson, J. F.
Organometallics 1992, 11, 3492-3493. (d) Hsu, D. P.; Warner, B. P.;
Fisher, R. A.; Davis, W. M.; Buchwald, S. L. Organometallics 1994, 13,
5160-5162. (e) Kanji, A.; Meunier, P.; Gautheron, B.; Dubac, J.; Daran,
J.-C. J. Organomet. Chem. 1993, 454, 51-58. (f) Cowley, A. H.; Gabba¨ı,
F. P.; Decken, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 1370-1372.
Discussion
We describe here a general approach to the synthesis of silole
and germole derivatives, based on Fagan’s zirconocene-mediated
route to the dihalides 1 and 2.20 This approach has proven to
(25) (a) Pitzer, K. S. Acc. Chem. Res. 1979, 12, 271-276. (b) Pyykko¨,
P.; Desclaux, J.-P. Acc. Chem. Res. 1979, 12, 276-281.

10464 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Freeman et al.
involve nucleophilic cleavage of E-E bonds. This method was
pioneered by Hong and Boudjouk, who generated THF solutions
of M[C4Ph4SitBu]- (M ) Li, Na) by reductive cleavage of the
Si-Si bond in C4Ph4(tBu)SiSi(tBu)C4Ph4.5 In general, we have
introduced crown ethers as metal-complexing agents that (1)
accelerate formation of the anions by solubilizing and/or
activating the nucleophilic reagent, (2) render the final products
more crystalline and therefore more amenable to structural
analysis, and (3) serve to weaken or cancel metal‚‚‚anion
interactions, so that the inherent electronic properties of the
anions may be probed directly.
By NMR spectroscopy, it seems that the nature of the
countercation has little effect on the electronic structure of the
silolyl and germolyl anions (e.g., compare data for compounds
30-33 in Table 3). The 13C chemical shifts for the ring carbons
in these anions are very similar to the corresponding shifts for
related siloles and germoles (Table 3). The 29Si NMR shifts
for the silolyl anions reported here are displaced upfield with
respect to values for the corresponding neutral starting materials,
and appear in the region expected for classical silyl anions of
the type RR′R′′Si-.24 Thus, the NMR data are consistent with
significant localization of charge on the heavy group 14 element
(Si or Ge) and a nonaromatic, bond-localized structure. Similar
results for related anions have been reported for Li[C4Me4GePh]7
and 1-lithio-1-methyl-1-silafluorenide.27 This picture is sup-
ported by X-ray crystal structures that have been determined
for three germolyl anions (25, 26, and 28) and the silolyl anion
33, which reveal highly pyramidalized Ge and Si centers and
π-bond localization that rivals that found in isolated dienes.
Boudjouk and co-workers have recently determined the structure
of a novel trisgermole dianion, [Li(THF)(tmed)][C4Et4Ge-
(GeEt4C4)2Li], which also displays pyramidal Ge centers and
bond localization.28
Theoretical studies have also concluded that silolyl and
germolyl anions should be pyramidal and nonaromatic.3,4,8 We
have reported RHF-level calculations on isolated [C4H4ESiH3]-
ions, which predict a high degree of localization in the π-systems
for E ) Si, Ge, and Sn, and give good agreement with the bond
parameters for 25. This is consistent with a high degree of
p-character in the bonding orbitals for E, as might be expected.25
Earlier theoretical studies have described the silolyl anion
[C4H4SiH]- as being pyramidal at silicon, but delocalized in
the ring to some degree.3 Most recently, Goldfuss and Schleyer
have come to the interesting conclusion that although [C4H4SiH]-
would be pyramidal, it should possess roughly half of the
aromaticity associated with C5H5-. The planar [C4H4SiH]- ion
was calculated to be destabilized relative to the ground state
by only 3.8 kcal mol-1, which corresponds to the inversion
barrier for the silolyl anion. These calculations also indicate
that Li+ would coordinate to [C4H4SiH]- in an η5 fashion, to
give a structure that would be significantly more aromatic than
the free ion.4
Figure 7. Reaction coordinate diagram for inversion in the complex
[Li(12-crown-4)2][C4Et4GeSiMe3] (40).
state properties of silolyl and germolyl anions can be greatly
influenced by the nature of substituents on the five-membered
ring.
It is interesting to compare the results obtained for the free
anions with those observed for the metal complexes (η5-C5-
Me5)Ru{η5-C4Me4ESi(SiMe3)3} (E ) Si9 and Ge10). For the
germolyl complex, the C4Ge ring appears to be highly delo-
calized, as indicated by the summation of bond angles at Ge
(358.1°) and the equivalent C-C distances of 1.42 Å. In
addition, the germolyl ring carbons in this molecule have similar
chemical shifts, at δ ) 80.23 and 87.32. Also, NMR parameters
for the silolyl complex indicate significant delocalization in the
C4Si ring. Therefore coordination of a transition metal fragment
induces considerable π-delocalization and apparent aromatic
character for silolyl and germolyl rings.
The measured inversion barriers for compounds 38-41
provide energy differences between the pyramidal anions and
their corresponding planar (possibly aromatic) structures (Figure
7). For the germolyl anions investigated, this barrier is
approximately 10 kcal mol-1, and the corresponding silolyl
anions have inversion barriers that we estimate to be on the
order of 7-8 kcal mol-1
. Thus, the observed barriers are
significantly higher than the value of 4 kcal mol-1 predicted
for [C4H4SiH]-.4 However, they are much lower than the
-
estimated inversion barrier for SiH3 (26 kcal mol-1),29 and
Lambert and Urdaneta-Perez have concluded that inversion
barriers for LiSiR3 and LiGeR3 anions are greater than 24 kcal
mol-1 30
Therefore the low barriers observed for 38-41 may
.
be attributed to stability imparted to the transition state by
delocalization of π-electron density in the ring. It should be
noted, however, that the -SiMe3 substituents in these anions
may also play a role in lowering the barrier to inversion.31 This
possibility will be addressed in future experiments intended to
broaden the database for such inversion barriers.
The difference in barriers between analogous silolyl and
germolyl anions apparently reflects a slightly greater degree of
delocalization in the silicon compounds. The barriers also seem
to be influenced slightly by the nature of the countercation.
Although the structures of the K[C4Et4ESiMe3] species in
tetrahydrofuran solution are unknown, it seems possible that
these derivatives possess some kind of potassium-anion
interaction, as was observed in 28 and 33. It is therefore
tempting to speculate that a weak interaction of this kind may
stabilize a delocalized transition state, as predicted by Goldfuss
In light of the above results, it is somewhat surprising that
the silolyl anions M[C4Ph4SitBu] (M ) Li, Na)5 exhibit NMR
parameters that are consistent with significant delocalization in
the ring. Perhaps most significantly, the downfield 29Si NMR
shifts suggest delocalization of the negative charge throughout
the ring. These results therefore stand in sharp contrast to what
we observe for the silolyl anions prepared in our laboratory,
which exhibit upfield shifts for the silicon atoms. The only
silolyl anion to be structurally characterized, compound 33,
possesses a strongly pyramidalized silicon center. An important
question that remains, therefore, is whether or not the ground
(29) Eades, R. A.; Dixon, D. A. J. Chem. Phys. 1980, 72, 3309-3313.
(30) Lambert, J. B.; Urdaneta-Pe´rez, M. J. Am. Chem. Soc. 1978, 100,
157-162.
(31) (a) Apeloig, Y.; Godleski, S. A.; Heacock, D. J.; McKelvey, J. M.
Tetrahedron Lett. 1981, 22, 3297-3300. (b) Godleski, S. A.; Heacock, D.
J.; McKelvey, J. M. Tetrahedron Lett. 1981, 23, 4453-4456.
(27) Hong, J.-H.; Boudjouk, P.; Stoenescu, I. Organometallics 1996, 15,
2179-2181.
(28) Hong, J. H.; Pan, Y. L.; Boudjouk, P. Angew. Chem., Int. Ed. Engl.
1996, 35, 186-188.

Synthesis of Cyclic π-Systems Containing Si and Ge
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10465
and Schleyer.4 Clearly there is much more to be learned about
the electronic structures of this type of anion, and how their
electronic properties may be manipulated via changes in ring
substituents and the nature of the cation. Future investigations
will address these issues.
δ 14.52, 16.15, 21.54, 29.50 (s, CH2CH3), 109.84 (s, C5H5), 134.00,
191.02 (s, C4Et4Si).
C4Et4I2. A modification of the literature procedure for C4Me4I2 was
followed,21 using crystalline Cp2ZrC4Et4. The product was purified
by column chromatography on silica gel (200-400 mesh) with
petroleum ether. The product was isolated as a slightly impure, light-
sensitive, colorless oil in 70% yield. 1H NMR (400 Mz, benzene-d6):
δ 1.00, 1.06 (t, 6 H, CH2CH3), 2.01, 2.19, 2.37, 2.46 (m, 2 H, CH2-
CH3). 13C{1H} NMR (100 MHz, benzene-d6): δ 13.49, 13.99, 25.70,
34.81 (s, CH2CH3), 108.99, 149.39 (s, C4Et4Si).
Experimental Section
All manipulations were conducted under an atmosphere of nitrogen
or argon using standard Schlenk techniques or a Vacuum Atmospheres
glovebox. Dry, deoxygenated solvents were employed for all manipu-
lations (except chromatography). Olefin impurities were removed from
pentanes by treatment with concentrated H2SO4, 0.5 N KMnO4 in 3 M
H2SO4, saturated NaHCO3, and then MgSO4. Removal of residual
thiophenes from benzene and toluene was accomplished by washing
each with concentrated H2SO4, saturated NaHCO3, and then MgSO4.
Solvents were distilled from sodium benzophenone ketyl. The NMR
solvents benzene-d6 and toluene-d8 were purified by vacuum distillation
from Na/K alloy. Dichloromethane-d2 was stirred over CaH2 for 2
days, transferred by vacuum distillation onto P2O5, stirred for 2 h, and
then vacuum-distilled for purification. The compounds LiMes,32
(THF)3LiSi(SiMe3)3,33 KCH2Ph,34 and (Et2O)LiCH2Ph34 were synthe-
sized by literature methods. The reagents n-butyllithium (1.6 M in
hexanes) and Cp2ZrCl2 were used as received. GeCl4, SiBr4, ClSiMe3,
and MeI were distilled before use, and LiAlH4 was purified by
extraction into diethyl ether, followed by removal of the ether by
vacuum transfer. 12-Crown-4 and 15-crown-5 were distilled from P2O5,
and 18-crown-6 was sublimed twice. Elemental analyses were
performed by Desert Analytics or the College of Chemistry Microana-
lytical Laboratory at UC Berkeley. NMR spectra were recorded on
GE QE-300 or Bruker AMX-300 instruments at 300 MHz (1H), 75.5
MHz (13C), 59.6 MHz (29Si), or on a Bruker AMX-400 instrument at
400 MHz (1H) and 100 MHz (13C). Infrared spectra were recorded
with a Perkin-Elmer 1330 spectrometer.
C4Et4SiCl2 (4). This procedure is a modification of the one used
by West et al. to prepare C4Me4SiCl2.14 To a cold (-80 °C) diethyl
ether (100 mL) solution of C4Et4I2 (7.41 g, 14.4 mmol) was added a
hexane solution of n-butyllithium (1.6 M, 18.9 mL, 30.3 mmol) which
had been diluted with 100 mL of diethyl ether. The latter reagent was
added slowly over 1 h. The reaction mixture was allowed to warm to
room temperature (over 1 h), and was then stirred for an additional 20
min. After the reaction mixture was cooled to -110 °C with a liquid
nitrogen/pentanes slurry, SiCl4 (16.5 mL, 144 mmol) was added. The
resulting mixture was stirred for 15 min before the cold bath was
removed, and stirring was continued for 12 h. The volatiles were
removed by vacuum transfer and then the product was extracted with
pentane (4 × 50 mL). The pentane was evaporated from the combined
extracts to give the product as a yellow oil. Distillation (45 °C; 0.001
Torr) gave a 75% yield of somewhat impure material. Anal. Calcd
for C12H20Cl2Si: C, 54.73; H, 7.67. Found: C, 51.08; H, 7.69. 1H
NMR (400 MHz, benzene-d6): δ 0.75, 1.18 (t, 6 H, CH2CH3), 1.97,
2.21 (q, 4 H, CH2CH3). 13C{1H} NMR (100 MHz, benzene-d6): δ
14.06, 14.64, 20.90, 20.92, (s, CH2CH3), 131.15, 155,76 (s, C4Et4Si).
C4Me4Si(Br)Si(SiMe3) (5). A solution of (THF)3LiSi(SiMe3)3 (6.57
g, 15.5 mmol) in 50 mL of toluene was added to 2 (4.17 g, 15.5 mmol)
in cold (0 °C) toluene (50 mL). The cold bath was removed after 5
min and the reaction solution was stirred for 90 min at room
temperature. The volatiles were then removed by vacuum transfer,
and the residue was extracted with pentane (3 × 40 mL). The combined
pentane extracts were reduced to 60 mL and cooled (-80 °C) to give
light pink crystals in 50% yield. Anal. Calcd for C17H39BrSi5: C,
44.01; H, 8.49. Found: C, 42.20; H, 8.68. 1H NMR (300 MHz,
benzene-d6): δ 0.32 (s, 27 H, SiMe3), 1.61, 1.98 (s, 6 H, C4Me4Si).
13C{1H} NMR (100 MHz, benzene-d6): δ 2.75 (s, SiMe3), 14.04, 14.35
(s, C4Me4Si), 130.57, 149.61 (s, C4Me4Si). 29Si{1H} NMR (59.6 MHz,
benzene-d6): δ -130.82 (s, Si(SiMe3)3), -8.72 (s, SiMe3), 8.21 (s, C4-
Me4Si).
C4Me4GeCl2 (1). A modification of the literature procedure20 was
followed. Cp2ZrCl2 (30.0 g, 102 mmol) in 250 mL of THF was cooled
to -80 °C, and then 2-butyne (16.1 mL, 205 mmol) was added. A
hexane solution of n-butyllithium (1.6 M, 128 mL, 205 mmol) was
then added dropwise to the reaction flask over 90 min, and the solution
was stirred for 15 min at -80 °C. The cold bath was removed, and
stirring of the solution was continued for 3 h. The reaction mixture
was then cooled to 0 °C, and GeCl4 (19.3 mL, 195 mmol) was added
over 2 min by syringe. After the reaction mixture was stirred for 12
h at room temperature, the volatiles were removed by vacuum transfer,
and the resulting residue was extracted with pentane (4 × 50 mL).
The combined pentane extracts were concentrated to 100 mL and cooled
C4Me4Ge(Cl)Si(SiMe3)3 (7). The method for 5 was followed, using
1 (0.998 g, 3.97 mmol) and (THF)3LiSi(SiMe3)3 (1.68 g, 3.97 mmol),
to give colorless crystalline product in 70% yield. Anal. Calcd for
C17H39ClGeSi4: C, 44.01; H, 8.49. Found: C, 44.26; H, 8.66. 1H
NMR (300 MHz, benzene-d6): δ 0.32 (s, 27 H, SiMe3), 1.59, 2.07 (s,
6 H, C4Me4Ge). 13C{H} NMR (75.5 MHz, benzene-d6): δ 2.42 (s,
SiMe3), 14.53, 15.47 (s, C4Me4Ge), 133.63, 146.02 (s, C4Me4Ge). Mp
159-161 °C.
1
to -80 °C to yield the oily crystalline product which was pure by H
NMR spectroscopy. Sublimation gave colorless, non-oily crystals of
the product in 60% yield. 1H NMR (300 MHz, benzene-d6): δ 1.31,
1.70 (s, 6 H, C4Me4Ge). 13C{1H} NMR (75.5 MHz, benzene-d6): δ
12.49, 13.69 (s, C4Me4Ge), 124.58, 145.81 (s, C4Me4Ge).
C4Me4Ge(Cl)Mes (8). A pressure bottle containing 1 (6.01 g, 23.88
mmol), LiMes (3.01 g, 23.88 mmol), and toluene (75 mL) was heated
at 70 °C for 6 h, and then the volatile materials were removed by
vacuum transfer. The white residue was extracted with pentane (4 ×
40 mL), and the combined extracts were concentrated and cooled (-80
°C) to give the product as a colorless powder in 56% yield. Anal.
Calcd for C17H23ClGe: C, 60.87; H, 6.92. Found: C, 56.15; H, 6.44.
1H NMR (300 MHz, benzene-d6): δ 1.57, 1.96 (s, 6 H, C4Me4Ge),
2.05 (s, 3 H, p-Me), 2.53 (s, 6 H, o-Me), 6.69 (s, 2 H, m-H). 13C{H}
NMR (100 MHz, benzene-d6): δ 14.24, 14.80 (s, C4Me4Ge), 24.21 (s,
o-Me), 20.94 (s, p-Me), 129.42, 130.61, 130.85, 140.04 (s, Ph), 143.77,
146.33 (s, C4Me4Ge).
C4Me4Si(H)Si(SiMe3)3 (9). A solution of 5 (4.58 g, 9.87 mmol) in
50 mL of THF was added to LiAlH4 (0.370 g, 9.87 mmol) in 50 mL
of cold (0 °C) THF. After stirring at 0 °C for 10 min, the cold bath
was removed and the reaction solution was then stirred for an additional
90 min before the volatiles were removed by vacuum transfer. The
product was extracted with pentane (4 × 40 mL) and the combined
extracts were concentrated and cooled (-80 °C) to give the product as
a colorless powder in 85% yield. Anal. Calcd for C17H40Si5: C, 53.03;
H, 10.49. Found: C, 52.87; H, 10.65. 1H NMR (300 MHz; benzene-
d6): δ 0.30 (s, 27 H, SiMe3), 1.75, 2.04 (s, 6 H, C4Me4Si), 4.79 (s, 1
C4Me4SiBr2 (2). The literature procedure was followed.20 The
product was distilled (40-70 °C; 0.001 Torr), dissolved in 3 times its
volume of pentane, and crystallized at -40 °C. The product was then
sublimed (35 °C; 0.001 Torr) and isolated as light pink transparent
crystals that contain 1-20% hexamethylbenzene. The yield was
typically ca. 30%. 1H NMR (400 MHz, benzene-d6): δ 1.38, 1.70 (s,
6 H, C4Me4Si).
C4Et4GeCl2 (3). The method described for 1 was followed. 1H
NMR (400 MHz, benzene-d6): δ 0.70, 1.18 (t, 6 H, CH2CH3), 1.92,
2.22 (q, 4 H, CH2CH3). 13C{1H} NMR (100 MHz, benzene-d6): δ
14.11, 14.76, 20.97, 21.97 (s, CH2CH3), 132.61, 149.81 (s, C4Et4Ge).
Cp2ZrC4Et4. This compound was obtained in 70% yield by the
method reported by Fagan for Cp2ZrC4Me4.20 Anal. Calcd for C22H30-
Zr: C, 68.50; H, 7.86. Found: C, 68.25; H, 7.80. 1H NMR (400 MHz,
benzene-d6): δ 0.88, 1.00 (t, 6 H, CH2CH3), 2.21, 2.32 (q, 4 H, CH2-
CH3), 5.92 (s, 10 H, C5H5). 13C{1H} NMR (100 MHz, benzene-d6):
(32) Fink, M. J.; Michalczyk, M. J.; Haller, K. J.; West, R.; Michl, J.
Organometallics 1984, 3, 793-800.
(33) Gutekunst, G.; Brook, A. G. J. Organomet. Chem. 1982, 225, 1-3.
(34) Schlosser, M.; Hartmann, J. Angew. Chem., Int. Ed. Engl. 1973,
12, 508-509.

10466 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Freeman et al.
H, SiH). 13C{1H} NMR (75.5 MHz; benzene-d6): δ 2.86 (s, SiMe3),
14.77, 15.69 (s, C4Me4Si), 129.53, 150.94 (s, C4Me4Si). 29Si{1H} NMR
(59.6 MHz; dichloromethane-d2): δ -138.64 (s, Si(SiMe3)3), -32.68
(s, C4Me4Si), -8.90 (s, SiMe3).
was extracted with pentane (3 × 40 mL) and the combined pentane
extracts were concentrated and cooled to -40 °C. The product was
isolated as colorless crystals in 60% yield. Anal. Calcd for C22H42-
Si4: C, 63.06; H, 10.12. Found: C, 62.89; H, 10.24. 1H NMR (400
MHz, benzene-d6): δ 0.13 (s, 18 H, SiMe3), 1.87, 2.12 (s, 12 H, C4Me4-
Si). 13C{1H} NMR (100 MHz, benzene-d6): δ -0.61 (s, SiMe3), 14.81,
15.74 (s, C4Me4Si), 131.83, 150.11 (s, C4Me4Si). 29Si{1H} NMR (59.6
MHz, benzene-d6): δ -34.71 (s, C4Me4Si), -13.24 (s, SiMe3).
C4Me4Si(Me)Si(Me)C4Me4 (16). The procedure for 15 was fol-
lowed, using 2 (1.28 g, 4.76 mmol), potassium (0.558 g, 14.3 mmol),
and MeI (1.50 mL, 24.0 mmol), to give the product as colorless crystals
in 15% yield. Anal. Calcd for C18H30Si2: C, 71.43; H, 10.01.
Found: C, 69.78; H, 10.10. 1H NMR (400 MHz, benzene-d6): δ 0.28
(s, 6 H, SiMe), 1.78, 1.92 (s, 12 H, C4Me4Si). 13C{1H} NMR (100
MHz, benzene-d6): δ -8.06 (s, SiMe), 14.14, 14.24 (s, C4Me4Si),
131.25, 149.11 (s, C4Me4Si). 29Si{1H} NMR (59.6 MHz, benzene-
d6): δ -13.95 (s, C4Me4Si).
C4Me4Ge(SiMe3)Ge(SiMe3)C4Me4 (17). The procedure for 15 was
followed, with 1 (1.21 g, 4.83 mmol), potassium (0.585 g, 14.9 mmol),
and ClSiMe3 (8.20 mL, 64.5 mmol), to give the product as colorless
crystals in 33% yield. Anal. Calcd for C22H38Ge2Si2: C, 52.02; H,
8.35. Found: C, 51.69; H, 8.47. 1H NMR (300 MHz, benzene-d6):
δ 0.19 (s, 9 H, SiMe3), 1.87, 2.20 (s, 6 H, C4Me4Ge). 13C{H} NMR
(100 MHz, benzene-d6): δ 0.12 (s, SiMe3), 14.80, 17.58 (s, C4Me4-
Ge), 135.83, 145.47 (s, C4Me4Ge). 29Si{1H} NMR (59.6 MHz,
benzene-d6): δ -4.93 (s, SiMe3).
C4Me4Si(H)Mes (10). A solution of 6 was generated from the
reaction of 2 (1.60 g, 5.93 mmol) and MesLi (0.735 g, 5.93 mmol) in
toluene (40 mL). After reaction at 65 °C for 14 h, the volatile materials
were removed, and the resulting residue was dissolved in 50 mL of
THF. This solution was then added to a cold (0 °C) solution of LiAlH4
(0.225 g, 5.93 mmol) in 30 mL of THF. The reaction mixture was
stirred at room temperature for 60 min, the volatiles were removed by
vacuum transfer, and then the residue was extracted with pentane (3 ×
40 mL). Evaporation of the pentane gave an oil which was purified
by column chromatography on silica gel (200-400 mesh) with hexanes.
The hexanes were removed by vacuum transfer and the product was
dried under vacuum for 12 h. The silane was then crystallized from
pentane at -80 °C as colorless crystals in 45% yield. Anal. Calcd
for C17H24Si: C, 79.60; H, 9.45. Found: C, 79.71; H, 9.55. 1H NMR
(300 MHz, benzene-d6): δ 1.75, 1.88 (s, 6 H, C4Me4Si), 2.10 (s, 3 H,
p-Me), 2.44 (s, 6 H, o-Me), 5.29 (s, 1 H, SiH), 6.76 (s, 2 H, m-H).
13C{1H} NMR (75.5 MHz, benzene-d6): δ 14.03, 14.44 (s, C4Me4Si),
21.16, 23.16 (s, o- and p-Me), 128.59, 128.92, 139.67, 146.07 (s, Ph),
129.28, 151.00 (s, C4Me4Si). 29Si{1H} NMR (59.6 MHz, dichloro-
methane-d2): δ -27.57 (s, C4Me4Si).
C4Me4Ge(H)Si(SiMe3)3 (11). The method for 9 was followed, using
7 (3.68 g, 7.73 mmol) and LiAlH4 (0.293 g, 7.73 mmol), to give the
product as colorless crystals in 80% yield. Anal. Calcd for C17H40-
GeSi4: C, 47.55; H, 9.41. Found: C, 46.95; H, 9.70. 1H NMR (300
MHz, benzene-d6): δ 0.29 (s, 27 H, SiMe3), 1.77, 2.13 (s, 6 H, C4-
Me4Ge), 4.96 (s, 1 H, GeH). 13C{H} NMR (75.5 MHz, dichloro-
methane-d2): δ 2.46 (s, SiMe3), 14.80, 17.36 (s, C4Me4Ge), 132.89,
146.45 (s, C4Me4Ge). 29Si{H} NMR (59.6 MHz, dichloromethane-
d2): δ -127.62 (s, Si(SiMe3)3), -8.83 (s, SiMe3).
C4Me4Ge(Me)Ge(Me)C4Me4 (18). The procedure for 15 was
followed, with 1 (1.59 g, 6.32 mmol), potassium (0.741 g, 18.9 mmol),
and MeI (1.50 mL, 24.0 mmol), to give the product as colorless crystals
in 35% yield. Anal. Calcd for C18H30Ge2: C, 55.20; H, 7.74.
Found: C, 55.07; H, 7.80. 1H NMR (400 MHz, benzene-d6): δ 0.50
(s, 6 H, GeMe), 1.78, 2.03 (s, 12 H, C4Me4Ge). 13C{1H} NMR (100
MHz, benzene-d6): δ -6.27 (s, GeMe), 14.37, 15.89 (s, C4Me4Ge),
133.79, 145.22 (s, C4Me4Ge).
C4Me4Ge(H)Mes (12). The method for 9 was followed, with 8 (3.98
g, 11.9 mmol) and LiAlH4 (0.450 g, 11.9 mmol), to give the product
as a colorless powder in 73% yield. Anal. Calcd for C17H24Ge: C,
67.83; H, 8.05. Found: C, 68.04; H, 8.15. 1H NMR (300 MHz,
benzene-d6): δ 1.76, 1.95 (s, 6 H, C4Me4Ge), 2.11 (s, 3 H, p-Me),
2.39 (s, 6 H, o-Me), 5.62 (s, 1 H, GeH), 6.76 (s, 2 H, m-Me). 13C{H}
NMR (75.5 MHz, benzene-d6): δ 14.51, 15.90 (s, C4Me4Ge), 21.10
(s, p-Me), 23.41 (s, o-Me), 128.81, 129.92, 130.95, 138.76 (s, Ph),
144.69, 146.65 (s, C4Me4Ge).
C4Et4Si(SiMe3)2 (19). The procedure for 13 was followed, using 4
(2.87 g, 10.9 mmol), potassium (1.73 g, 44.3 mmol), and ClSiMe3 (6.93
mL, 54.7 mmol), to give the product as a yellow oil in 45% yield.
Anal. Calcd for C18H38Si3: C, 63.80; H, 11.33. Found: C, 64.04; H,
11.38. 1H NMR (400 MHz, benzene-d6): δ 0.22 (s, 18 H, SiMe3),
1.00, 1.12 (t, 6 H, CH2CH3), 2.33, 2.42 (q, 4 H, CH2CH3). 13C{1H}
NMR (100 MHz, benzene-d6): δ 0.10 (s, SiMe3), 15.50, 17.21, 21.79,
23.29 (s, CH2CH3), 139.78, 155.56 (s, C4Et4Si). 29Si{1H} NMR (59.6
MHz, benzene-d6): δ -37.50 (s, C4Et4Si), -14.78 (s, SiMe3).
C4Et4Ge(SiMe3)2 (20). The procedure for 13 was followed, using
3 (1.52 g, 4.98 mmol), potassium (0.792 g, 20.2 mmol), and ClSiMe3
(6.00 mL, 47.2 mmol), to give the product as a slightly yellow oil in
85% yield. Anal. Calcd for C18H38GeSi2: C, 56.39; H, 10.01.
Found: C, 56.38; H, 10.11. 1H NMR (400 MHz, benzene-d6): δ 0.26
(s, 18 H, SiMe3), 1.03, 1.14 (t, 6 H, CH2CH3), 2.35, 2.49 (q, 4 H,
CH2CH3). 13C{1H} NMR (100 MHz, benzene-d6): δ 0.77 (s, SiMe3),
15.66, 17.59, 21.87, 25.06 (s, CH2CH3), 144.34, 151.18 (s, C4Et4Ge).
29Si{1H} NMR (59.6 MHz, benzene-d6): δ -7.74 (s, SiMe3).
[K(18-crown-6)]2[C4Me4Si] (21). 2 (0.404 g, 1.50 mmol), 18-
crown-6 (0.793 g, 3.00 mmol), small potassium chunks (0.240 g, 6.15
mmol), and 70 mL of THF were combined, and the reaction solution
was stirred for 3 days. The product was crystallized from the red
solution by pentane diffusion as red crystals in 5% yield. Anal. Calcd
C4Me4Si(SiMe3)2 (13). Compound 2 (0.533 g, 1.98 mmol) in 100
mL of THF was added to chunks of potassium (0.310 g, 7.92 mmol),
and the resulting reaction solution was stirred for 10 days at room
temperature. The initially clear, colorless solution slowly turned white
and then ruby red as the potassium was consumed. The reaction
solution was cooled to -80 °C and ClSiMe3 (1.50 mL, 11.8 mmol)
was added. The volatiles were removed and the product was extracted
with pentane (4 × 30 mL). The pentane was removed from the
combined extracts and the product was isolated as a slightly yellow
oil in 75% yield. 1H NMR (400 MHz, benzene-d6): δ 0.19 (s, 18 H,
SiMe3), 1.84, 2.01 (s, 6 H, C4Me4Si). 13C{1H} NMR (100 MHz,
benzene-d6): δ -0.34 (s, SiMe3), 14.65, 15.38 (s, C4Me4Si), 130.99,
149.81 (s, C4Me4Si). 29Si{1H} NMR (59.6 MHz, benzene-d6):
-34.26 (s, C4Me4Si), -14.83 (s, SiMe3).
δ
C4Me4Ge(SiMe3)2 (14). The method for 13 was followed, using 1
(1.27 g, 5.04 mmol), potassium (0.808 g, 20.7 mmol) and ClSiMe3
(9.00 mL, 70.8 mmol), to give the product as a slightly yellow oil in
75% yield. Anal. Calcd for C14H30GeSi2: C, 51.39; H, 9.26. Found:
C, 50.40; H, 8.98. 1H NMR (400 MHz, benzene-d6): δ 0.23 (s, 18 H,
SiMe3), 1.86, 2.11 (s, 6 H, C4Me4Ge). 13C{1H} NMR (100 MHz,
benzene-d6): δ 0.51 (s, SiMe3), 14.71, 17.53 (s, C4Me4Ge), 134.86,
145.67 (s, C4Me4Ge). 29Si{1H} NMR (59.6 MHz, benzene-d6): δ
-7.55 (s, SiMe3).
C4Me4Si(SiMe3)Si(SiMe3)C4Me4 (15). 2 (0.620 g, 2.30 mmol) in
50 mL of THF was added to chunks of potassium (0.270 g, 6.90 mmol),
and the reaction solution was stirred for 10 days at room temperature.
The initially clear colorless solution slowly turned white and then bright
red as the potassium was consumed. Then the reaction solution was
cooled to -80 °C and ClSiMe3 (1.50 mL, 11.8 mmol) was added, and
after 5 min the cold bath was removed and the reaction solution was
stirred for 60 min longer. After removal of the volatiles, the residue
1
for C32H60K2O12Si: C, 51.71; H, 8.15. Found: C, 49.12; H, 7.94. H
NMR (400 MHz, benzene-d6): δ 2.81, 3.27 (s, 6 H, C4Me4Si), 3.47
(br s, 24 H, 18-crown-6). Attempts to obtain acceptable elemental
analysis data for this compound failed, perhaps due to its extreme air-
sensitivity.
[K4(18-crown-6)3][C4Me4Ge]2‚THF (22‚THF). 1 (0.748 g, 2.97
mmol), 18-crown-6 (1.57 g, 5.94 mmol), small potassium chunks (0.476
g, 12.2 mmol), and 80 mL of THF were stirred together for 3 days.
The product was crystallized from the red solution by pentane diffusion
as orange crystals in 5% yield. Anal. Calcd for C56H104Ge2K4O19: C,
48.62; H, 7.59. Found: C, 45.20; H, 7.08. 1H NMR (400 MHz,
benzene-d6): δ 2.80, 3.56 (s, 6 H, C4Me4Ge), 3.44 (br s, 24 H, 18-
crown-6). Attempts to obtain acceptable elemental analysis data for
this compound failed, perhaps due to its extreme air-sensitivity.
[K(18-crown-6)][C4Me4GeSi(SiMe3)3] (23). To 11 (0.202 g, 0.470
mmol), 18-crown-6 (0.124 g, 0.470 mmol), and KN(SiMe3)2 (0.094 g,

Synthesis of Cyclic π-Systems Containing Si and Ge
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10467
0.47 mmol) were added 50 mL of toluene, and the resulting reaction
solution was stirred for 30 min at room temperature. The product was
crystallized at -40 °C as yellow crystals in 45% yield. Anal. Calcd
for C29H63GeKO6Si4: C, 47.59; H, 8.68. Found: C, 47.44; H, 8.57.
1H NMR (300 MHz, benzene-d6): δ 0.59 (s, 27 H, SiMe3), 2.22, 2.62
(s, 6 H, C4Me4Ge), 3.09 (s, 24 H, 18-crown-6). 13C{1H} NMR (75.5
MHz, benzene-d6): δ 3.98 (s, SiMe3), 15.96, 20.46 (s, C4Me4Ge), 69.91
(s, 18-crown-6), 137.13, 157.06 (s, C4Me4Ge). 29Si{1H} NMR (59.6
MHz, benzene-d6): δ -125.03 (s, Si(SiMe3)3), -8.22 (s, SiMe3).
[K(18-crown-6)][C4Me4GeMes] (24). To an NMR tube containing
12 (0.018 g, 0.061 mmol), 18-crown-6 (0.016 g, 0.061 mmol), and
0.35 mL of benzene-d6 was added KCH2Ph (0.0079 g, 0.061 mmol),
then the NMR tube was shaken for 20 min. 1H NMR (400 MHz,
benzene-d6): δ 2.27, 2.61 (s, 6 H, C4Me4Ge), 2.33 (s, 3 H, p-Me),
2.96 (s, 6 H, o-Me), 3.15 (s, 24 H, 12-crown-4), 7.03 (s, 2 H, m-H).
13C{1H} NMR (100 MHz, benzene-d6): δ 15.56, 19.23 (s, C4Me4Ge),
21.39, 26.04 (s, o- and p-Me), 70.00 (s, 18-crown-6), 127.15, 133.00,
134.09, 152.05 (s, Mes), 145.94, 156.40 (s, C4Me4Ge).
Li[C4Me4SiSiMe3] (30). To an NMR tube containing 13 (0.0408
g, 0.144 mmol) in 0.35 mL of THF-d8 was added (Et2O)LiCH2Ph
(0.0260 g, 0.151 mmol), then the tube was shaken for 2 min. 1H NMR
(300 MHz, THF-d8): δ -0.29 (s, 9 H, SiMe3), 1.83, 2.02 (s, 6 H,
C4Me4Si). 13C{1H} NMR (100 MHz, THF-d8): δ 1.55 (s, SiMe3),
14.78, 16.91 (s, C4Me4Si), 138.66, 146.38 (s, C4Me4Si). 29Si{1H} NMR
(59.6 MHz, THF-d8): δ -45.38 (C4Me4Si), -12.47 (SiMe3).
K[C4Me4SiSiMe3] (31). To an NMR tube containing 13 (0.026 g,
0.094 mmol) in 0.35 mL of THF-d8 was added KCH2Ph (0.013 g, 0.098
mmol), then the tube was shaken for 2 min. 1H NMR (400 MHz, THF-
d8): δ -0.03 (s, 9 H, SiMe3), 1.81, 2.09 (s, 6 H, C4Me4Si). 13C{1H}
NMR (100 MHz, THF-d8): δ 1.90 (s, SiMe3), 14.84, 17.72 (s, C4Me4-
Si), 136.23, 148.97 (s, C4Me4Si). 29Si{1H} NMR (59.6 MHz, THF-
d8): δ -42.70 (C4Me4Si), -12.44 (SiMe3).
[Li(12-crown-4)2][C4Me4SiSiMe3] (32). To an NMR tube contain-
ing 13 (0.0291 g, 0.103 mmol) and 12-crown-4 (33.3 µL, 0.206 mmol)
in 0.35 mL of benzene-d6 was added (Et2O)LiCH2Ph (0.0177 g, 0.103
mmol), then the tube was shaken for 2 min. 1H NMR (400 MHz,
benzene-d6): δ -0.49 (s, 9 H, SiMe3), 2.30, 2.63 (s, 6 H, C4Me4Si),
3.37 (s, 32 H, 12-crown-4). 13C{1H} NMR (100 MHz, benzene-d6):
δ 2.74 (s, SiMe3), 14.42, 18.35 (s, C4Me4Si), 68.82 (s, 12-crown-4),
135.78, 148.62 (s, C4Me4Si). 29Si{1H} NMR (59.6 MHz, benzene-
d6): δ -43.96 (C4Me4Si), -11.68 (SiMe3).
[K(18-crown-6)][C4Me4SiSiMe3] (33). A toluene (10 mL) solution
of 13 (0.465 g, 1.64 mmol) and 18-crown-6 (0.435 g, 1.64 mmol) was
added to KCH2Ph (0.224 g, 1.72 mmol) in 5 mL of toluene, and the
resulting solution was stirred for 30 min at room temperature. Cooling
to -40 °C produced orange-yellow crystals of the product in 20% yield.
Anal. Calcd for C23H45KO6Si2: C, 53.85; H, 8.86. Found: C, 54.13;
H, 8.94. 1H NMR (400 MHz, benzene-d6): δ 0.57 (s, 9 H, SiMe3),
2.34, 2.67 (s, 6 H, C4Me4Si), 3.20 (s, 24 H, 18-crown-6). 13C{1H}
NMR (100 MHz, benzene-d6): δ 2.50 (s, SiMe3), 15.55, 18.34 (s,
C4Me4Si), 70.05 (s, 18-crown-6), 135.76, 149.60 (s, C4Me4Si). 29Si-
{1H} NMR (59.6 MHz, benzene-d6): δ -41.52 (s, C4Me4Si), -11.00
(s, SiMe3).
K[C4Me4GeSiMe3] (34). To an NMR tube containing 14 (0.029 g,
0.090 mmol) in 0.35 mL of THF-d8 was added KCH2Ph (0.012 g, 0.095
mmol), then the tube was shaken for 1 min. 1H NMR (400 MHz, THF-
d8): δ -0.02 (s, 9 H, SiMe3), 1.87, 2.06 (s, 6 H, C4Me4Ge). 13C{1H}
NMR (100 MHz, THF-d8): δ 2.03 (s, SiMe3), 15.16, 19.42 (s, C4Me4-
Ge), 141.10, 157.40 (s, C4Me4Ge).
[K(18-crown-6)][C4Me4GeSiMe3] (35). A mixture of compound
14 (0.203 g, 0.620 mmol) and 18-crown-6 (0.164 g, 0.620 mmol) in
10 mL of toluene was added to KCH2Ph (0.085 g, 0.65 mmol) in 5
mL of toluene, and the reaction solution was stirred for 30 min at room
temperature. The reaction solution was cooled to -40 °C to obtain
the product as yellow crystals in 30% yield. Anal. Calcd for
C23H45GeKO6Si: C, 49.55; H, 8.15. Found: C, 49.55; H, 8.01. 1H
NMR (400 MHz, benzene-d6): δ 0.54 (s, 9 H, SiMe3), 2.26, 2.67 (s,
6 H, C4Me4Ge), 3.19 (s, 24 H, 18-crown-6). 13C{1H} NMR (100 MHz,
benzene-d6): δ 2.49 (s, SiMe3), 15.86, 20.06 (s, C4Me4Ge), 70.07 (s,
18-crown-6), 136.72, 158.56 (s, C4Me4Ge). 29Si{1H} NMR (59.6 MHz,
benzene-d6): δ -3.86 (s, SiMe3).
[K(18-crown-6)][C4Me4Si(SiMe3)C4MeSi] (36). To an NMR tube
containing 15 (0.0202 g, 0.048 mmol), 18-crown-6 (0.0127 g, 0.048
mmol), and 0.35 mL of benzene-d6 was added KCH2Ph (0.0063 g, 0.048
mmol), then the tube was shaken for 20 min. 1H NMR (400 MHz,
benzene-d6): δ 0.44 (s, 9H, SiMe3), 2.08, 2.39 (s, 6 H, C4Me4SiSiMe3),
2.33, 2.65 (s, 6 H, C4Me4Si-K+), 3.24 (s, 24 H, 18-crown-6). 13C-
{1H} NMR (100 MHz, benzene-d6): δ 0.60 (s, SiMe3), 14.60, 16.45
(s, C4Me4SiSiMe3), 15.71, 18.43 (s, C4Me4Si-K+), 69.95 (s, 18-crown-
6), 137.23, 143.80 (s, C4Me4SiSiMe3), 138.49, 149.08 (s, C4Me4Si-K+).
29Si{1H} NMR (59.6 MHz, benzene-d6): δ -53.43 (s, C4Me4Si-K+),
-25.47 (s, C4Me4SiSiMe3), -13.49 (s, SiMe3).
[Li(12-crown-4)2][C4Me4GeSi(SiMe3)3] (25). To 11 (0.258 g, 0.602
mmol) and 12-crown-4 (0.194 mL, 1.20 mmol) in 25 mL of Et2O was
added a hexane solution of n-butyllithium (1.6 M, 0.396 mL, 0.632
mmol). Diffusion of pentane into the reaction solution yielded the
product as yellow crystals in 42% yield. Anal. Calcd for C33H71-
GeLiO8Si4: C, 50.30; H, 9.10. Found: C, 50.22; H, 9.10. 1H NMR
(300 MHz, benzene-d6): δ 0.62 (s, 27 H, SiMe3), 2.26, 2.69 (s, 6 H,
C4Me4Ge), 3.18 (s, 32 H, 12-crown-4). 13C{1H} NMR (100 MHz,
benzene-d6; THF-d0): δ 3.77 (s, SiMe3), 15.59, 20.30 (s, C4Me4Ge),
69.19 (s, 12-crown-4), 136.95, 157.03 (s, C4Me4Ge). 29Si{1H} NMR
(59.6 MHz, benzene-d6): δ -125.57 (s, Si(SiMe3)3), -8.43 (s, SiMe3).
[Li(12-crown-4)2][C4Me4GeMes] (26). The procedure for 25 was
followed, with 12 (0.204 g, 0.677 mmol), 12-crown-4 (0.219 mL, 1.35
mmol), and n-butyllithium (1.6 M, 0.440 mL, 0.712 mmol), to yield
the product as yellow crystals in 35% yield. Anal. Calcd for C33H55-
GeLiO8: C, 60.10; H, 8.42. Found: C, 60.01; H, 8.47. 1H NMR (300
MHz, benzene-d6): δ 2.29, 2.60 (s, 6 H, C4Me4Ge), 2.31 (s, 3 H, p-Me),
2.92 (s, 6 H, o-Me), 3.20 (s, 32 H, 12-crown-4), 7.04 (s, 2 H, m-H).
13C{1H} NMR (100 MHz, benzene-d6, THF-d0): δ 15.12, 18.83 (s,
C4Me4Ge), 21.14, 25.67 (s, o- and p-Me), 69.44 (s, 12-crown-4), 126.71,
132.60, 133.84, 152.13 (s, Mes), 145.49, 155.21 (s, C4Me4Ge).
Li[C4Me4GeSi(SiMe3)3] (27). The procedure for 25 was followed,
with 11 (0.35 g, 0.81 mmol) and n-butyllithium (1.6 M in hexanes,
0.53 mL, 0.85 mmol), to yield the product as yellow crystals in 20%
yield. Anal. Calcd for C17H39GeLiSi4: C, 46.89; H, 9.04. Found:
C, 47.22; H, 9.51. 1H NMR (400 MHz, THF-d8): δ 0.09 (s, 27 H,
SiMe3), 1.77, 2.16 (s, 6 H, C4Me4Ge). 13C{1H} NMR (100 MHz, THF-
d8): δ 6.50 (s, SiMe3), 18.11, 22.78 (s, C4Me4Ge), 141.29, 156.90 (s,
C4Me4Ge).
[Na(15-crown-5)][C4Me4GeMe] (28). A solution of 18 (0.212 g,
0.540 mmol) and 15-crown-5 (0.238 g, 1.08 mmol) in 10 mL of toluene
was added to a sodium dispersion (0.038 g, 1.62 mmol) in 5 mL of
toluene. The reaction solution was stirred for 4 days at room
temperature, and then the product was extracted with toluene (5 × 15
mL) and the extracts were combined, concentrated, and cooled to -40
°C. The product formed as yellow crystals in 40% yield. Anal. Calcd
for C19H35GeNaO5: C, 51.97; H, 8.05. Found: C, 51.62; H, 7.82. 1H
NMR (400 MHz, benzene-d6): δ 0.95 (s, 3 H, SiMe), 2.27, 2.67 (s, 6
H, C4Me4Si), 3.17 (s, 20 H, 15-crown-5). 13C{1H} NMR (100 MHz,
benzene-d6): δ 0.36 (s, GeMe), 15.30, 18.64 (s, C4Me4Ge), 69.82 (s,
15-crown-5), 136.10, 156.89 (s, C4Me4Ge).
[K(18-crown-6)][C4Me4GeMe] (29). To an NMR tube containing
18 (0.021 g, 0.053 mmol), 18-crown-6 (0.014 g, 0.058 mmol), and
0.35 mL of benzene-d6 was added KCH2Ph (0.069 g, 0.058 mmol),
then the NMR tube was shaken for 20 min. For 29: 1H NMR (400
MHz, benzene-d6): δ 1.01 (s, 3 H, GeMe), 2.25, 2.69 (s, 6 H, C4Me4-
Ge), 3.16 (s, 24 H, 18-crown-6). 13C{1H} NMR (100 MHz, benzene-
d6): δ 0.96 (s, GeMe), 15.36, 18.92 (s, C4Me4Ge), 69.99 (s, 18-crown-
6), 145.83, 159.96 (s, C4Me4Ge). For C4Me4Ge(Me)CH2Ph: 1H NMR
(400 MHz, benzene-d6): δ 0.25 (s, 3 H, GeMe), 1.71, 1.82 (s, 6 H,
C4Me4Ge), 2.32 (s, 2 H, CH2Ph), 6.98-7.11 (m, 5 H, Ph). 13C{1H}
NMR (100 MHz, benzene-d6): δ -7.21 (s, GeMe), 14.07, 15.57 (s,
C4Me4Ge), 23.35 (s, CH2Ph), 124.44, 128.06, 128.46, 140.87 (s, Ph),
131.31, 135.26 (s, C4Me4Ge).
[K(18-crown-6)][C4Me4Ge(SiMe3)C4Me4Ge] (37). A mixture of
17 (0.206 g, 0.405 mmol) and 18-crown-6 (0.107 g, 0.405 mmol) in
10 mL of toluene was added to KCH2Ph (0.555 g, 0.426 mmol) in 5
mL of toluene, and the reaction solution was stirred at room temperature
for 30 min. The solution was then cooled to -40 °C to obtain the
product as yellow crystals in 20% yield. Anal. Calcd for C31H57Ge2-
KO6Si: C, 50.43; H, 7.80. Found: C, 50.59; H, 7.49. 1H NMR (400
MHz, benzene-d6): δ 0.38 (s, 9 H, SiMe3), 2.04, 2.45 (s, 6 H, C4Me4-
GeSiMe3), 2.25, 2.69 (s, 6 H, C4Me4Ge-K+), 3.16 (s, 24 H, 18-crown-

10468 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Freeman et al.
6). 13C{1H} NMR (100 MHz, benzene-d6): δ 1.08 (s, SiMe3), 14.82,
18.41 (s, C4Me4GeSiMe3), 15.96, 20.01 (s, C4Me4Ge-K+), 70.16 (s,
18-crown-6), 138.46, 143.92 (s, C4Me4GeSiMe3), 140.63, 156.43 (s,
C4Me4Ge-K+). 29Si{1H} NMR (59.6 MHz, benzene-d6): δ -5.99 (s,
SiMe3).
[Li(12-crown-4)2][C4Et4SiSiMe3] (38). To an NMR tube containing
19 (0.0407 g, 0.120 mmol) and 12-crown-4 (38.8 µL, 0.240 mmol) in
0.35 mL of THF-d8 was added (Et2O)LiCH2Ph (0.0217 g, 0.126 mmol),
then the tube was shaken for 2 min. 1H NMR (400 MHz, THF-d8): δ
-0.02 (s, 9 H, SiMe3), 0.99, 1.12 (t, 6 H, CH2CH3), 2.36, 2.52 (q, 4 H,
CH2CH3), 3.63 (s, 32 H, 12-crown-4). 13C{1H} NMR (100 MHz, THF-
d8): δ 2.73 (s, SiMe3), 17.20, 20.03, 22.47, 26.20 (s, C4Et4Si), 70.70
(s, 12-crown-4), 141.96, 158.27 (s, C4Et4Si). 29Si{1H} NMR (59.6
MHz, THF-d8): δ -53.12 (C4Me4Si), -14.27 (SiMe3).
K[C4Et4SiSiMe3] (39). To an NMR tube containing 19 (0.026 g,
0.076 mmol) in 0.35 mL of THF-d8 was added KCH2Ph (0.010 g, 0.079
mmol), then the tube was shaken for 2 min. 1H NMR (400 MHz, THF-
d8): δ -0.02 (s, 9 H, SiMe3), 0.98, 1.14 (t, 6 H, CH2CH3), 2.40, 2.55
(q, 4 H, CH2CH3). 13C{1H} NMR (100 MHz, THF-d8): δ 2.56 (s,
SiMe3), 16.99, 20.56, 22.26, 25.90 (s, C4Et4Si), 140.88, 158.42 (s, C4-
Et4Si). 29Si{1H} NMR (59.6 MHz, THF-d8): δ -47.38 (C4Me4Si),
-14.22 (SiMe3).
THF-d8): δ -0.03 (s, 9 H, SiMe3), 0.97, 1.14 (t, 6 H, CH2CH3), 2.34,
2.55 (q, 4 H, CH2CH3). 13C{1H} NMR (100 MHz, THF-d8): δ 2.52
(s, SiMe3), 16.79, 20.06, 22.42, 27.54 (s, CH2CH3), 141.05, 168.62 (s,
C4Et4Ge).
Crystallographic Structure Determinations. Crystallographic data
are collected in Table 1. All data crystals were sealed in Lindemann
capillary tubes under nitrogen. The Laue symmetry of each was
photographically determined and the space groups assigned unambigu-
ously for 26 and 28 from systematic absences. For 22‚THF and 33
the centrosymmetric alternatives were chosen initially based on the E
statistics; the results of subsequent refinement supported these choses.
ψ-scan data indicated that no corrections for absorption were required.
All structures were solved by direct methods, refined with anisotropic
thermal parameters (except as noted), and include idealized hydrogen-
atom contributions. In 22‚THF, a partially occupied, multiply posi-
tioned molecule of disordered THF was found to accompany the
asymmetric unit. The disorder was modeled as two concentric
orientations in different planes with one atom in common with a
combined occupancy of 0.5. The atoms of the solvent molecule were
isotropically refined and hydrogen atoms were ignored. All computa-
tions used SHELXTL software (version 4.2 for 26, and version 5.1 for
the others; G. Sheldrick, Siemens XRD, Madison, WI.).
[Li(12-crown-4)2][C4Et4GeSiMe3] (40). To an NMR tube contain-
ing 20 (0.0517 g, 0.135 mmol) and 12-crown-4 (43.7 µL, 0.270 mmol)
in 0.35 mL of THF-d8 was added (Et2O)LiCH2Ph (0.0244 g, 0.142
mmol), then the tube was shaken for 2 min. 1H NMR (400 MHz, THF-
d8): δ -0.02 (s, 9 H, SiMe3), 0.97, 1.11 (t, 6 H, CH2CH3), 2.31, 2.54
(q, 4 H, CH2CH3), 3.68 (s, 32 H, 12-crown-4). 13C{1H} NMR (100
MHz, THF-d8): δ 2.97 (s, SiMe3), 16.96, 19.84, 22.59, 27.74 (s, CH2-
CH3), 69.64 (s, 12-crown-4), 142.45, 166.84 (C4Et4Ge).
Acknowledgment is made to the National Science Founda-
tion for their generous support of this work.
Supporting Information Available: Tables of crystal, data
collection, and refinement parameters, bond distances and
angles, and anisotropic displacement parameters (29 pages). See
any current masthead page for ordering and Internet access
instructions.
K[C4Et4GeSiMe3] (41). To an NMR tube containing 20 (0.0543
g, 0.142 mmol) in 0.35 mL of THF-d8 was added KCH2Ph (0.0194 g,
0.149 mmol), then the tube was shaken for 2 min. 1H NMR (400 MHz,
JA962103G