Synthesis of tBu3E+ nitrile complexes by oxidative cleavage of tBu3E-EtBu3 (E = Si, Ge, Sn)

Article abstract of DOI:10.1246/cl.2000.600

The reaction of hexa-tert-butyldimetallanes (^tBu₃E-E^tBu₃; E = Si, Ge, Sn) with trityl tetraarylborates in pivalonitrile or acetonitrile gave tri-terf-butylmetalyl cation nitrile complexes, [^tBu_3<

Full text of DOI:10.1246/cl.2000.600

600
Chemistry Letters 2000
Synthesis of tBu3E+ Nitrile Complexes by Oxidative Cleavage of tBu3E-EtBu3 (E = Si, Ge, Sn)
Masaaki Ichinohe, Hiroshi Fukui, and Akira Sekiguchi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571
(Received March 9, 2000; CL-000236)
The reaction of hexa-tert-butyldimetallanes (tBu3E-EtBu3;
E = Si, Ge, Sn) with trityl tetraarylborates in pivalonitrile or
acetonitrile gave tri-tert-butylmetalyl cation nitrile complexes,
[tBu3E+(NC-R)], by oxidative E-E bond cleavage. Their struc-
tures were determined by spectroscopic methods and X-ray
crystallography.
The chemistry of triorgano-substituted group 14 cations in
the condensed phase has developed in the last decade.1 For the
generation of triorgano-substituted heavier group 14 cations
(R3E+, E = Si, Ge, Sn), hydrogen abstraction of the correspon-
ding hydride with triphenylmethylium ion (trityl cation) has
usually been applied (the Corey method).2 However, the reac-
tion of hydrosilanes having bulky substituents, such as tBu3SiH,
Mes3SiH, with the trityl cation proceeds quite slowly or does
not occur, since the H atom is sterically protected from attack
by the trityl cation. Recently, Lambert et al. have reported the
synthesis of the trimesitylsilyl cation by using allyltrimesitylsi-
lane as a precursor, in which an electrophile such as Et3Si+(ben-
zene) or Et3SiCH2C+Ph2 is attached to the γ-carbon of the allyl
group to give a β-silyl carbocation, followed by elimination of
the trimesitylsilyl cation.3
solvent in vacuo, the residual orange solid was washed with
hexane to give a moisture- and air-sensitive colorless powder of
4+(NCtBu)·TFPB- (41 mg) in 71% yield (Scheme 1).8
Similarly, the reaction of 1 with Ph3C+·TFPB- in acetonitrile
produced 4+(NCMe)·TFPB−, but the product was not isolated in
a pure form due to the instability. The reaction of 1 in pivaloni-
trile or acetonitrile with other trityl salts, such as Ph3C+·TPFPB-
and Ph3C+·TSFPB− (TPFPB− = tetrakis(pentafluorophenyl)borate,
TSFPB- = tetrakis[4-(tert-butydimethylsilyl)-2,3,5,6-tetrafluo-
rophenyl]borate), also gave the nitrile complexes [tBu3Si+(NC-
R)] (R = tBu, Me) accompanying the corresponding borate as a
counter ion.8,9 Furthermore, we have carried out the reaction
with less hindered disilanes. The reaction of hexaisopropyldisi-
lane with Ph3C+·TFPB− in acetonitrile was quite slow, and
required over two weeks for completion. Hexaethyldisilane and
hexamethyldisilane did not react with Ph3C+·TFPB-. Hexa-tert-
butyldisilane (1) has the longest Si-Si bond (2.697 Å, normal
Si-Si bond: 2.34 Å), and therefore the HOMO (σ-orbital of the
Si-Si bond) of 1 is much higher than that of usual disilanes.7
Consequently, the oxidation reaction of 1 easily takes place.
However, the attempted generation of the tri-tert-butylsilyl
cation in aromatic hydrocarbon solvents did not succeed, since
tri-tert-butylsilyl fluoride was formed by fluorine abstraction
from TFPB- or TSFPB-.10 On the other hand, the reaction of 1
with Ph3C+·TPFPB- in toluene did not give tri-tert-butylsilyl
fluoride, but we could not confirm generation of the tri-tert-
butylsilyl cation.
In 1997, we successfully synthesized and characterized a
"free" germyl cation, [(tBu3E)3Ge3]+ (E = Si, Ge), by reaction
t
of Bu3E-substituted cyclotrigermenes with trityl tetraarylbo-
rates.4 In this reaction, the trityl cation acts as a one-electron
oxidizing reagent, and the resulting radical cation of cyclotri-
germene decomposes to the cyclotrigermenium ion by cleavage
of the exocyclic Ge-Si or Ge-Ge bond on the saturated ring Ge
atom. Furthermore, we have also reported the reaction of
tetrakis(tri-tert-butylsilyl)tetrasilatetrahedrane with the trityl
cation to give the bis-silyl-substituted oxonium ion,
[(tBu3Si)4Si4OH]+, which is formed by oxidative Si-Si bond
cleavage of the Si4 skeleton, followed by reaction with residual
water.5 Although both compounds mentioned above are very
crowded systems, the bond between heavier group 14 elements
is easily cleaved by chemical oxidation to form cationic
species. Of course, the generation of silyl and germyl cations
by electrochemical oxidation of disilane and digermane, respec-
tively, has been suggested by cyclic voltammetry, but has not
been well characterized.6
The germanium and tin analogues (2, E = Ge; 3, E = Sn)
also react with Ph3C+·TFPB- in nitrile solvents to give the corre-
sponding cation-nitrile complexes, 5+(NCtBu)·TFPB- (80%),
5+(NCMe)·TFPB- (78%), and 6+(NCtBu)·TFPB- (62%) (Scheme
1).9
The structures of 4+(NCtBu)·TFPB- and 5+(NCtBu)·TFPB-
were established by X-ray crystallography of a single crystal
obtained by recrystallization from dichloromethane/toluene
mixed solvent.11,12 The crystal structure of 4+(NCtBu)·TFPB- is
shown in Figure 1 and reveals four-coordinate silicon, which is
greatly distorted from tetrahedral angles (109.5°), the average C-
Si-C bond angle being 115.9(3)°. This angle is very similar to
that in the water complex of the tri-tert-butylsilyl cation,
[tBu3Si+(OH2)] (116.0°).13 By comparison with typical Si-N
Here we report that the reaction of hexa-tert-butyldimetal-
lanes [tBu3E-EtBu3; 1 (E = Si), 2 (E = Ge), 3 (E = Sn)] with
trityl tetraarylborates in pivalonitrile or acetonitrile leads to
oxidative E-E bond cleavage and formation of tBu3E+(NCR) [4+
(E = Si), 5+ (E = Ge), and 6+ (E = Sn)].
The mixture of hexa-tert-butyldisilane7 (1, 10 mg, 25
µmol) and two equimolar amounts of Ph3C+·TFPB- (TFPB- =
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) (54 mg, 50
µmol) was stirred in dry oxygen-free pivalonitrile (1 mL) at 60
°C for two days to give an orange solution. After removal of
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
601
bond lengths (1.70-1.76 Å),14 the Si-N distance of 1.822(5) Å is
very long, and almost equal to that in [iPr3Si+(NCCH3)]
(1.82(2) Å).15
The degree of distortion around the Ge atom of
5+(NCtBu)·TFPB- is quite similar to the silicon analog,
4+(NCtBu)·TFPB-, and the average C-Ge-C bond angle is
116.9(3)°. Of course, the Ge-N distance of 1.975(7) Å is longer
Tse, and B. Kuhimann, J. Am. Chem. Soc., 121, 5001
(1999).
4
a) A. Sekiguchi, M. Tsukamoto, and M. Ichinohe, Science,
275, 60 (1997). b) A. Sekiguchi, M. Tsukamoto, M.
Ichinohe, and N. Fukaya, Phosphorus, Sulfur, and Silicon
and the Related Elements, 124 & 125, 323 (1997). c) M.
Ichinohe, N. Fukaya, and A. Sekiguchi, Chem. Lett., 1998,
1045. d) A. Sekiguchi, N. Fukaya, M. Ichinohe, and Y.
Ishida, Eur. J. Inorg. Chem., in press.
5
6
M. Ichinohe, N. Takahashi, and A. Sekiguchi, Chem. Lett.,
1999, 553.
a) W. G. Boberski and A. L. Allred, J. Organomet. Chem.,
88, 65 (1975). b) M. Okano and K. Mochida, Chem. Lett.,
1991, 819.
7
8
N. Wiberg, K. Amelunxen, H. W. Lerner, H. Schuster, H.
Nöth, I. Krossing, M. S. Amelunxen, and T. Seifert, J.
Organomet. Chem., 542, C1 (1997).
1
tBu3Si+(NCCD3)·TFPB-: H NMR (CD3CN, 298 K, δ) 1.31
(s, 27 H), 7.73 (s, 4 H), 7.80 (s, 8 H); 13C NMR (CD3CN,
298 K, δ) 23.4 (SiCMe3), 29.5 (SiCMe3), 118.3 (s, para),
125.9 (q, 1J13C-19F = 271 Hz, CF3), 129.9–131.4 (m, meta),
1
135.8 (s, ortho), 162.7 (q, J13C-11B = 50 Hz, ipso); 29Si
NMR (CD3CN, 298 K, δ) 29.9.
1
tBu3Si+(NCtBu)·TFPB-: H NMR (CD2Cl2, 298 K, δ) 1.26
(s, 27 H), 1.70 (s, 9 H), 7.56 (s, 4 H), 7.72 (s, 8 H); 13C
NMR (CD2Cl2, 298 K, δ) 23.5 (SiCMe3), 27.7 (CMe3), 29.5
1 13
(SiCMe3), 30.3 (CMe3), 118.3 (s, para), 125.4 (q, J C-19F
= 271 Hz, CF3), 129.1–131.0 (m, meta), 135.6 (s, ortho),
163.3 (q, 1J13C-11B = 50 Hz, ipso); 29Si NMR (CD2Cl2, 298
K, δ) 33.6.
All the new products obtained here showed the satisfactory
spectral data.
9
10 Fluorine abstraction by silylcation is precedent, see: M. Kira,
T. Hino, and H. Sakurai, J. Am. Chem. Soc., 114, 6697
(1992).
than that of normal Ge-N bond lengths (1.85 Å).16
11 Crystal data of 1a+(NCtBu)·TFPB-: C49H48BF24NSi, FW =
1145.80, orthorhombic, space group P212121, a = 13.4550(7)
Å, b = 18.8530(8) Å, c = 21.0850(5) Å, V = 5348.6(4) Å3, Z
= 4, dcalc = 1.423 g·cm-3, temperature 120 K. Full matrix
least-squares refinement yielded the final R value of 0.0815
for 5759 independent reflections [θ < 27.92°, I > 2.00σ(I)].
12 Crystal data of 2a+(NCtBu)·TFPB-: C49H48BF24GeN, FW =
1190.31, orthorhombic, space group P212121, a =
13.5140(5), b = 18.9460(6) Å, c = 21.2390(4) Å, V =
5438.0(3) Å3, Z = 4, dcalc = 1.395 g·cm-3, temperature 150 K.
Full matrix least-squares refinement yielded the final R
value of 0.0717 for 6282 independent reflections [θ <
27.92°, I > 2.00σ(I)].
The 29Si NMR resonance of 4+(NCMe)·TPFPB- in CD2Cl2
appears at 35.9 ppm, in the typical region for solvated tri-
alkylsilyl cations (e.g., 37.2 ppm for iPr3Si+(NCCH3)·[Br5-
CB9H5]- in CD2Cl2).15 Thus far, 29Si chemical shifts of solvated
silyl cation have been discussed, but coordinating solvents,
such as nitrile, have not been noted. In the 13C NMR spectrum,
the sp hybridized carbon of coordinated acetonitrile in
4+(N13CMe)·TPFPB-, prepared by using 13C labeled acetoni-
trile, is observed at 126.8 ppm, which is shifted downfield by
approximately 10 ppm compared with that of free acetonitrile.
References and Notes
1
For reviews, see: a) R. J. P. Corriu and M. Henner, J.
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VCH, Weinheim (1996), Vol. II. d) P. v. R. Schleyer,
Science, 275, 39 (1997). e) J. Belzner, Angew. Chem., Int.
Ed. Engl., 36, 1277 (1997). f) P. D. Lickiss, “The Chemistry
of Organic Silicon Compounds,” ed. by S. Patai and Z.
Rappoport, Wiley, New York (1998), Vol. II.
13 Z. Xie, R. Bau, and C. A. Reed, J. Chem. Soc., Chem.
Commun., 1994, 2519.
14 W. S. Sheldrick, “The Chemistry of Organic Silicon
Compounds,” ed. by S. Patai and Z. Rappoport, Wiley, New
York (1989) p. 227.
15 a) Z. Xie, D. J. Liston, T. Jelínek, V. Mitro, R. Bau, and C.
A. Reed, J. Chem. Soc., Chem. Commun., 1993, 384. b) S.
Bahr and P. Boudjouk, J. Am. Chem. Soc., 115, 4514 (1993).
16 K. M. Mackay, “The Chemistry of Organic Germanium, Tin
and Lead Compounds,” ed. by S. Patai, Wiley, New York
(1995) p. 101.
2
3
J. Y. Corey, J. Am. Chem. Soc., 97, 3237 (1975).
a) J. B. Lambert and Y. Zhao, Angew. Chem., Int. Ed. Engl.,
36, 400 (1997). b) J. B. Lambert, Y. Zhao, H. Wu, W. C.