Angewandte
Chemie
Germyl Cation
germanylium tetrakis(pentafluorophenyl)borate [(tBu2-
MeSi)3Ge]+ [TPFPB]ꢀ (2), was isolated as extremely mois-
[(tBu2MeSi)3Ge]+: An Isolable Free Germyl
Cation LackingConjugation to p Bonds**
ture-sensitive dark red crystals in 75% yield (Scheme 1).
Akira Sekiguchi,* Tomohide Fukawa, Vladimir Ya. Lee,
Masaaki Nakamoto, and Masaaki Ichinohe
Carbocations, together with carbanions, free radicals, and
carbenes, are the most fundamental species in organic
chemistry.[1] In contrast to the well-established chemistry of
carbocations, we still do not know much about the chemistry
of their heavier Group 14element congeners. The crystal-
structure data of the two isolable silyl “cation-like” species,
Scheme 1. Synthesis of the germyl cation 2.
The NMR spectra of 2 are very simple, in accordance with
its highly symmetrical structure. It is interesting that the Me
groups on the Si substituents are greatly deshielded (d =
1.17 ppm), appearing in the region of tBu groups (d =
1.19 ppm). The 29Si NMR resonance signal of the silyl
substituents is also greatly shifted downfield (d = +
49.9 ppm) compared with the related compounds (tBu2Me-
Si)3GeX (d = 25.6 ppm for X = H, d = 29.9 ppm for X = Cl).[10]
This effect can be explained by the positive charge not being
localized on the central germanium atom, but being trans-
ferred to the electropositive silicon atoms, similar to the case
of the cyclotrigermenylium ion [{(tBu3Si)3Ge}3]+.[7,11] Indeed,
DFT calculations at the B3LYP/6-31G(d) level on the model
compound [(H3Si)3Ge]+ have demonstrated the distribution
of the positive charge over the three Si atoms: Ge ꢀ0.12, Si +
0.37.
[Et3Si(toluene)]+[B(C6F5)4]ꢀ[2] and [iPr3Si]d+ [CB11H6Br6]dꢀ [3]
,
were reported by groups of Lambert and Reed, respectively.
Such silyl cations were not “free”, they were coordinated by
either solvent molecules or counteranions. Later,
[Mes3Si]+ [B(C6F5)4]ꢀ[4] was reported by Lambert and co-
workers as a free silyl cation in solution, and just recently
crystallographic evidence for the free [Mes3Si]+ ion was given
by Reed and Lambert and co-workers.[5] Examples of Ge-
centered three-coordinate cations are still very rare, and
include the germanium poly(pyrazolyl)borate complex
[HB(3,5-Me2pz)3Ge]+Iꢀ, which synthesis and crystal structure
were reported in 1996 by Reger and Coan.[6] In 1997 we
synthesized
an
isolable
free
germyl
cation,
[(tBu3E)3Ge3]+ [B(C6H5)4]ꢀ (E = Si, Ge) as a 2p-electron
aromatic system[7] and then prepared the stable free silyl
cation, [(tBu2MeSiSi)3SitBu2]+ [TPFPB]ꢀ (TPFPBꢀ = tetra-
kis(pentafluorophenyl)borate) which has a homoaromatic
character.[8,9] However, in both cases these free germyl and
The crystal structure of 2 was determined by X-ray
analysis to have a completely planar geometry around the Ge
center (Figure 1).[12] Similar to the precursor Ge radical 1,[10]
the methyl substituents on the Si atoms all lie in the Ge(1)-
Si(1)-Si(2)-Si(3) plane, apparently to minimize steric hin-
¼
silyl cations were stabilized by conjugation with Ge Ge or
¼
ꢀ
Si Si double bonds. Recently, we reported the synthesis of the
drance. The Si Ge bond lengths in 2 are very long, ranging
isolable Ge-centered radical (tBu2MeSi)3GeC (1), which lacks
stabilization by p-conjugation effects.[10] This compound
seemed to be an ideal starting point for a one-electron
oxidation reaction to form the corresponding germyl cation
species. Herein we report our successful oxidation of Ge
radical 1 with [Ph3C]+[TPFPB]ꢀ to produce the correspond-
ing free germyl cation that can be isolated as a stable
compound, the crystal structure and reactivity of which are
also given.
from 2.5036(10) to 2.5379(10) (av. 2.5195(10) ), which
ꢀ
greatly exceeds the normal values of Si Ge bond lengths
(2.384–2.462 ),[13] and the corresponding Si Ge bond
ꢀ
lengths
in
radical
1
(2.4514(4)–2.4553(4)
(av.
ꢀ
2.4535(4) ).[10] We can explain the elongation of the Si Ge
bonds in 2 by the different extent of hyperconjugation
between the cationic Ge center (4pz) and s* orbitals of the
ꢀ
Si C(tBu) bonds. The degree of such interaction depends on
the 4pz orbital occupancy: 0 for the cation 2 (2.5195(10) ), 1
Reaction of the free radical (tBu2MeSi)3GeC (1) with the
stoichiometric amount of [Ph3C]+ [TPFPB]ꢀ was complete in
five minutes, which results in the formation of a dark red
reaction mixture, from which tris[di-tert-butyl(methyl)silyl]-
for the radical
1
(2.4535(4) ),
2
for the anion
(tBu2MeSi)3Geꢀ (2.4332(10) ).
The closest distance between the Ge center of the cation
and the F atoms of the aromatic rings of the anion is greater
than 5.4, which shows the absence of any interaction
between the cation and anion. Thus, 2 represents a free Ge
cation in the solid state, it lacks any interaction with either
solvent molecules or counteranion. This independence of 2 is
retained in solution. The 29Si NMR resonance signal of 2 is
nearly the same in different solvents, that is, at 49.9 ppm in
[D2]dichoromethane, 49.9 ppm in CDCl3, and 50.3 ppm in
[D6]benzene, which indicates that 2 is indeed a free cation in
solution. Compound 2 possesses a much higher electro-
philicity towards nucleophilic solvents than the [(tBu3-
Si)3Ge3]+ ion. For example, decomposition of 2 in [D8]THF
[*] Prof. Dr. A. Sekiguchi, Dipl.-Chem. T. Fukawa, Dr. V. Ya. Lee,
Dr. M. Nakamoto, Dr. M. Ichinohe
Department of Chemistry
University of Tsukuba
Tsukuba, Ibaraki 305-8571 (Japan)
Fax: (+81)298-53-4314
E-mail: sekiguch@staff.chem.tsukuba.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research
(Nos. 13440185, 14044015, 14078204) from the Ministry of Edu-
cation, Science and Culture of Japan, TARA (Tsukuba Advanced
Research Alliance), and the COE (Center of Excellence) program.
Angew. Chem. Int. Ed. 2003, 42, No. 10
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1433-7851/03/4210-1143 $ 20.00+.50/0
1143
Sekiguchi, Akira
