Reactivity of silyl-substituted Allyl compounds with group 4, 5, 9, and 10 metals: Routes to η3-allyls, alkylidenes, and sec-alkyl carbocations

Article abstract of DOI:10.1021/om0491692

Whereas the reaction of alkali-metal salts of silyl-allyls E ⁺[C₃H₃(SiMe₃)₂-1,3] ^- (E = Li, K) with group 4 and group 5 metal halides gave intractable reduction products, Co(acac)₃ and Ni(acac)₂ reacted with K[C₃H₃(SiMe₃)₂-1,3] to give Co{eta;³-C₃H₃(SiMe₃) ₂-1,3}₂ (1) and Ni{η³-C₃H ₃(SiMe₃)₂-1,3}₂ (2), respectively. The reaction of K[C₃H₃(SiMe₃)₂-1,3] with Me₃SnCl afforded Me₃SiCH=CHCH(SiMe ₃)(SnMe₃) (3), which reacted cleanly with TaCl₅ to give [η³C₃H₃(SiMe₃) ₂-1,3}TaCl₄ (4). Treatment of this complex with tetramethylethylenediamine led to HCl abstraction, and the allyl complex was transformed into the vinyl-alkylidene compound Me₃SiCH=CHC(SiMe ₃)=TaCl₃(TMEDA) (5). Whereas in the case of TaCl ₅ dehalostannylation was facile, the reaction of 3 with ZrCl ₄ and HfCl₄ took a different course, leading instead to the addition of Me₃Sn⁺ to 3 to give [HC{CH(SiMe ₃)(SnMe₃)}₂]⁺[M₂Cl ₉]^- (6, M = Zr; 7, M = Hf), the first examples of isolable sec-alkyl carbocations. These salts are surprisingly thermally stable and melt > 100°C; this stability is largely due to delocalization of the positive charge over the two tin atoms. The crystal structures of 1, 2, and 5-7 are reported.

Full text of DOI:10.1021/om0491692

1718
Organometallics 2005, 24, 1718-1724
Reactivity of Silyl-Substituted Allyl Compounds with
Group 4, 5, 9, and 10 Metals: Routes to η3-Allyls,
Alkylidenes, and sec-Alkyl Carbocations
Mark Schormann, Shaun Garratt, and Manfred Bochmann*
Wolfson Materials and Catalysis Centre, School of Chemical Sciences, University of East
Anglia, Norwich NR4 7TJ, U.K.
Received October 26, 2004
Whereas the reaction of alkali-metal salts of silyl-allyls E+[C3H3(SiMe3)2-1,3]- (E ) Li,
K) with group 4 and group 5 metal halides gave intractable reduction products, Co(acac)3
and Ni(acac)2 reacted with K[C3H3(SiMe3)2-1,3] to give Co{η3-C3H3(SiMe3)2-1,3}2 (1) and Ni-
{η3-C3H3(SiMe3)2-1,3}2 (2), respectively. The reaction of K[C3H3(SiMe3)2-1,3] with Me3SnCl
afforded Me3SiCHdCHCH(SiMe3)(SnMe3) (3), which reacted cleanly with TaCl5 to give {η3-
C3H3(SiMe3)2-1,3}TaCl4 (4). Treatment of this complex with tetramethylethylenediamine led
to HCl abstraction, and the allyl complex was transformed into the vinyl-alkylidene
compound Me3SiCHdCHC(SiMe3)dTaCl3(TMEDA) (5). Whereas in the case of TaCl5
dehalostannylation was facile, the reaction of 3 with ZrCl4 and HfCl4 took a different course,
leading instead to the addition of Me3Sn+ to 3 to give [HC{CH(SiMe3)(SnMe3)}2]+[M2Cl9]-
(6, M ) Zr; 7, M ) Hf), the first examples of isolable sec-alkyl carbocations. These salts are
surprisingly thermally stable and melt >100 °C; this stability is largely due to delocalization
of the positive charge over the two tin atoms. The crystal structures of 1, 2, and 5-7 are
reported.
Introduction
thanide metals of the type [LnI{C3H3(SiMe3)2}3]- 7 have
been reported, as well as tetraallyl actinide8 complexes.
The reaction of potassium silylallyl compounds with
group 4 metal halides may proceed with reduction of
the metal center; thus, Eisen reported the synthesis of
Zr(III) and Ti(III) 1,3-bis(tert-butyldimethylsilyl)allyl
complexes and their use as propene polymerization
catalysts.9 On the other hand, the reaction of group 4
halides with the ansa-bis(allyl) ligand [Me2Si-
(C3H3SiMe3)2]2- gave the Zr(IV) and Hf(IV) complexes
M{(η3-C3H3SiMe3)2SiMe2}2.10 We have recently de-
scribed the synthesis of a number of lanthanide com-
plexes containing the allyl ligands [1-C3H4SiMe3]-,
[C3H3(SiMe3)2-1,3]-, and [R2Si(C3H3SiMe3)2]2- (R ) Me,
Ph) and their application as catalysts for the polymer-
ization of butadiene,11 methyl methacrylate, and ꢀ-ca-
prolactone.12 Here we report the synthesis and struc-
tures of complexes of tantalum, cobalt, and nickel, as
well as the unexpected reaction of the bulky tin allyl
derivative Me3SiCHdCHCH(SiMe3)(SnMe3) with group
4 chlorides.
Although metal allyl complexes have long been rec-
ognized as an important and versatile class of com-
pounds, many homoleptic compounds of the parent C3H5
ligand either are thermally very unstable, such as
(C3H5)nM (M ) Co, V, Ni), or are not known (M ) Fe,
Mn).1 In contrast, silyl-substituted allyl ligands provide
a striking stability increase. A variety of alkali-metal2
and alkaline-earth-metal3 silylallyl complexes have been
structurally characterized. In late-transition-metal chem-
istry various σ- and π-monosilylallyl complexes of Mn,
Fe, Co, Ni, Mo, Pd, and W were reported, and the
stabilizing effect of the silyl group was recognized.4 This
effect was clearly demonstrated by Hanusa’s isolation
of thermally stable M{η3-C3H3(SiMe3)2-1,3}2 complexes
(M ) Cr, Fe).5 More recently, related anionic tris(allyl)
complexes of manganese(II)6 and of a series of lan-
(1) Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.;
Kroner, M.; Oberkirch, W.; Tanaka, K.; Walter, D. Angew. Chem., Int.
Ed. Engl. 1966, 5, 151.
(2) (a) Fraenkel, G.; Chow, A.; Winchester, W. R. J. Am. Chem. Soc.
1990, 112, 1382. (b) Hitchcock, P. B.; Lappert, M. F.; Wang, Z.-X. Chem.
Commun. 1996, 1647. (c) Hitchcock, P. B.; Lappert, M. F.; Leung, W.-
P.; Liu, D.-S.; Mak, T. C. W.; Wang, Z.-X. J. Chem. Soc., Dalton Trans.
1999, 1257. (d) Hitchcock, P. B.; Lappert, M. F.; Leung, W. P.; Liu, D.
S.; Mak, T. C. W.; Wang, Z. X. J. Chem. Soc., Dalton Trans. 1999,
1263.
(7) Kuehl, C. J.; Simpson, C. K.; John, K. D.; Sattelberger, A. P.;
Carlson, C. N.; Hanusa, T. P. J. Organomet. Chem. 2003, 683, 149.
(8) Carlson, C. N.; Hanusa, T. P.; Brennessel, W. W. J. Am. Chem.
Soc. 2004, 126, 10550.
(9) Ray, B.; Neyroud, M. K.; Kapon, M.; Eichen, Y.; Eisen, M. S.
Organometallics 2000, 20, 3044.
(3) Harvey, M. J.; Hanusa, T. P.; Young, V. G., Jr. Angew. Chem.,
Int. Ed. 1999, 38, 217.
(10) Ferna´ndez-Gala´n, R.; Hitchcock, P. B.; Lappert, M. F.; Antin˜olo,
A.; Rodr´ıguez, A. M. Dalton 2000, 1743.
(4) (a) Pannell, K. H.; Lappert, M. F.; Stanley, K. J. Organomet.
Chem. 1976, 112, 37. (b) Ogoshi, S.; Ohe, K.; Chatani, N.; Kurosawa,
H.; Murai, S. Organometallics 1991, 10, 3813. (c) Ogoshi, S.; Yoshida,
W.; Ohe, K.; Murai, S. Organometallics 1993, 12, 578.
(5) Smith, J. D.; Hanusa, P. H.; Young, V. G., Jr. J. Am. Chem. Soc.
2001, 123, 6455.
(11) Woodman, T. J.; Schormann, M.; Bochmann, M. Isr. J. Chem.
2002, 42, 283.
(12) (a) Woodman, T. J.; Schormann, M.; Bochmann, M. Organo-
metallics 2003, 22, 2938. Woodman, T. J.; Schormann, M.; Hughes,
D. L.; Bochmann, M. Organometallics 2003, 22, 3028. (c) Woodman,
T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics
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(6) Layfield, R. A.; Humphrey, S. M. Angew. Chem., Int. Ed. 2004,
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10.1021/om0491692 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 03/02/2005

Reactivity of Silyl-Substituted Allyl Compounds
Organometallics, Vol. 24, No. 7, 2005 1719
Results and Discussion
The most straightforward method for introducing a
silylallyl ligand is the metathesis of metal halides with
the potassium and lithium compounds E[C3H3(SiMe3)2-
1,3] (E ) Li, K). However, the reaction of K[C3H3-
(SiMe3)2-1,3] with Ti, Zr, Hf, Nb, and Ta chlorides leads
to reduction. The products were collected as deeply
colored oils which gave broad NMR signals and resisted
all attempts at crystallization.
Reduction is also involved in the reaction of Co(acac)3
(acac ) acetylacetonate) with K[C3H3(SiMe3)2-1,3], to
give the 15-electron Co(II) product Co{η3-C3H3(SiMe3)2-
1,3]}2 (1) (eq 1). The compound was obtained in moder-
Figure 1. Molecular structures of Co{η3-C3H3(SiMe3)2-
1,3}2 (1, left) and Ni{η3-C3H3(SiMe3)2-1,3}2 (2, right), show-
ing the atomic numbering schemes. Thermal ellipsoids are
shown at the 50% probability level. Methyl groups are
omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles
(deg) for M{η3-C3H3SiMe3)2-1,3}2
M ) Co (1)
M ) Ni (2)
M-C(1)
2.049(2)
2.004(2)
2.099(2)
1.417(3)
1.412(3)
2.387
2.057(3)
1.972(3)
2.042(3)
1.411(5)
1.427(4)
M-C(2)
M-C(3)
C(1)-C(2)
C(2)-C(3)
Co-H(3*)
Co-H(6*)
Ni-H(1*)
2.385
2.323
C(1)-C(2)-C(3)
H(1*)-C(1)-M
H(3*)-C(3)-M
H(6*)-C(6)-Co
123.8(2)
117.8(12)
94.2(11)
94.6(12)
122.7(3)
93(2)
118(2)
ate yield as orange crystals after recrystallization from
light petroleum and is thermally stable. In contrast, the
parent allyl compound Co(η3-C3H5)2 cannot be isolated
and disproportionates to give the Co(III) species Co(η3-
C3H5)3; even though the latter is an 18-electron complex,
it is thermally unstable and decomposes at >-50 °C.13
The analogous nickel complex Ni{η3-C3H3(SiMe3)2-
1,3}2 (2) was similarly prepared from K[C3H3(SiMe3)2-
1,3] and Ni(acac)2 in THF. A dark red solution was
initially obtained which gave a dark oil; this solidified
slowly on standing to give 2 as orange crystals. The
compound was very soluble in organic solvents, and
satisfactory elemental analyses could not be obtained.
The structures of 1 and 2 were confirmed by X-ray
diffraction (Figure 1). Selected bond lengths and angles
are collected in Table 1. Compound 1 crystallizes in the
monoclinic space group P21/c with one molecule per
asymmetric unit. All hydrogen atoms of the allyl cores
were located in the electron difference map and allowed
to refine freely. In the C2-symmetric molecule the allyl
ligands are η3 coordinated, with a variation in the Co-C
bond lengths from 2.00 to 2.10 Å. The planes through
the allylic C3 cores are oriented in an almost parallel
fashion, with a dihedral angle of 5.34(39)°. The metal
occupies the center of a plane defined by the four lateral
carbon atoms of the two allyl cores (C1-C3-C4-C6)
and deviates from the idealized position by only 0.098-
(10) Å. The silyl substituents are found in a cis,trans
arrangement in both ligands; this is in contrast with
the bis(silyl)allyl compounds of calcium and lanthanide
metals, where a trans,trans conformation is pre-
ferred.3,7,8,11,12 The structural features of 1 are in good
agreement with those of the Cr(II) complex Cr{C3H3-
(SiMe3)2-1,3}2 reported by Hanusa et al.5 The main
differences between the two structures are the R-CH
agostic interactions of the hydrogen atoms H3* and H6*
with the cobalt center, at a distance of 2.39 Å. These
two hydrogen atoms bend out of the allyl plane by 30°,
in contrast to the other four, which form an angle of
about 15° tipping in the opposite direction. The small
Co(1)-C(3)-H(3*) and Co(1)-C(6)-H(6*) angles of
94.2(11) and 94.6(12)°, respectively, are also a result of
these Co‚‚‚HC interactions.
Compound 2 is isostructural with Fe{η3-C3H3(SiMe3)2-
1,3}2.5 Here, too, the silyl substituents adopt a cis,trans
orientation. However, as in the cobalt analogue, we
observed an R-CH agostic interaction, with a Ni-H(1*)
bond length of 2.32 Å. H(1*) is bent out of the C(1)-
C(2)-C(3) plane by 27.6°; the Ni(1)-C(1)-H(1*) angle
is 93(2)°.
During the search for alternative synthetic routes to
allyl complexes, the dehalostannylation reactions of
transition-metal halides were investigated, using the
trisubstituted propene (Me3Si)CHdCHCH(SiMe3)(SnMe3)
(3). Compound 3 was prepared in a clean reaction from
K[C3H3(SiMe3)2-1,3] and Me3SnCl in high yield (eq 2).
(13) (a) Labinger, J. A. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
U.K., 1982; Vol. 3, pp 759-760. (b) Wigley, D. E.; Gray, S. D. In
Comprehensive Organometallic Chemistry II; Wilkinson, G., Stone, F.
G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 5, p 110.

1720 Organometallics, Vol. 24, No. 7, 2005
Schormann et al.
Scheme 1
Figure 2. Molecular structure of Me3SiCHdCHC(SiMe3)d
TaCl3(TMEDA) (5). Thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms are omitted for
clarity.
The tin allyl 3 is an air-stable colorless oil which
hydrolyzes slowly on exposure to moisture and can be
vacuum-distilled without decomposition. Compound 3
exists exclusively as the isomer with Sn in the 3-posi-
tion.
Table 2. Selected Bond Lengths (Å) and Angles
(deg) for Me3SiCHdCHC(SiMe3)dTaCl3((TMEDA)
(5)
The reaction of TaCl5 with 3 afforded the expected
dehalostannylation product {η3-C3H3(SiMe3)2-1,3}TaCl4
(4), which was isolated from light petroleum at -20 °C
as red microcrystals (Scheme 1). The reaction proceeded
cleanly, and the byproduct Me3SnCl could be easily
removed in vacuo and recycled after distillation. While
allyl niobium and tantalum entities carrying also other
π-ligands are well-known,13-15 monoallyl tantalum ha-
lides of the general formula ATaCl4 have, to our
knowledge, not been reported before. Compound 4 is
highly soluble in common organic solvents, is stable at
room temperature for days in the pure state, and melts
at 72-73 °C. The 1H NMR spectrum showed the
expected coupling pattern for the C3H3 allyl core, a
doublet at δ 5.30 and a triplet at δ 6.87, and the
elemental analysis confirmed the presence of one allyl
ligand per metal center.
As we were not able to obtain crystals suitable for
X-ray diffraction, we reacted 4 with neutral electron
donor molecules. Thus, the addition of tetramethyleth-
ylenediamine (TMEDA) afforded the tantalum alky-
lidene complex Me3SiCHdCHC(SiMe3)dTaCl3(TMEDA)
(5), evidently the product of dehydrochlorination by the
base, accompanied by the formation of TMEDA‚HCl.
Tantalum alkylidene complexes are of course a well-
known class of compounds; however, an allyl-to-alky-
lidene transformation has to our knowledge not been
observed before.16,17 In vinyl alkylidene complexes
Schrock et al. observed an equilibrium with the corre-
sponding metallacyclobutene with loss of a neutral
donor ligand in solution.17e However, the 13C NMR
Ta(1)-C(1)
Ta(1)-N(2)
Ta(1)-Cl(2)
C(1)-C(2)
1.953(7)
2.507(6)
2.382(2)
1.488(9)
Ta(1)-N(1)
Ta(1)-Cl(1)
Ta(1)-Cl(3)
C(2)-C(3)
2.374(5)
2.355(2)
2.374(2)
1.320(10)
C(1)-Ta(1)-N(2)
C(1)-Ta(1)-Cl(2)
N(1)-Ta(1)-Cl(1) 160.42(13) Cl(2)-Ta(1)-Cl(3) 167.73(6)
164.1(2)
89.3(2)
C(1)-Ta(1)-Cl(1)
C(1)-Ta(1)-Cl(3)
105.9(2)
100.6(2)
spectra of 5 were almost unchanged over a temperature
range of from 25 to -80 °C; here only the noncyclic form
is present. The 13C NMR signal for the R-C atom at δ
265.8 (-80 °C) or δ 268.2 (25 °C) is in good agreement
with those for other tantalum alkylidene complexes.
Dark cubic crystals of 5 were obtained from petroleum
ether at -20 °C. The structure is shown in Figure 2;
selected bond lengths and angles are collected in Table
2. The tantalum center possesses a distorted-octahedral
environment with an trans alkylidene-nitrogen ar-
rangement: C(1)-Ta(1)-N(2) ) 164.1(2)°. The Ta-C(1)
bond length of 1.953(7) Å is in good agreement with
literature values.16,17 The TMEDA ligand is unsym-
metrically bonded, with the Ta(1)-N(2) bond trans to
the alkylidene ligand being significantly longer (2.507-
(6) Å) than the Ta(1)-N(1) bond which is trans to Cl
(2.375(5) Å). The vinylalkylidene-metallacyclobutene
equilibrium referred to above requires the creation of a
vacant cis position; here this is inhibited because it
would involve the breaking of the stronger tantalum-
(16) (a) Labinger, J. A. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
U.K., 1982; Vol. 3, pp 719-730. (b) Wigley, D. E.; Gray, S. D. In
Comprehensive Organometallic Chemistry II, Wilkinson, G., Stone, F.
G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 5, pp 83-
89. (c) Schrock, R. R. ACS Symp. Ser. 1983, No. 211, 369. Schrock, R.
R. In Reactions of Coordinated Ligands; Braterman, P. S., Ed.;
Plenum: New York, 1986.
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Reactivity of Silyl-Substituted Allyl Compounds
Organometallics, Vol. 24, No. 7, 2005 1721
Scheme 2
nitrogen bond. The C(2)-C(3) bond length of 1.320(10)
Å indicates the presence of a nondelocalized double bond
and confirms the formation of a vinyl alkylidene struc-
ture.
ate Me3SnCl, which in turn reacted with further MCl4
accompanied by chloride abstraction. While it did not
prove possible to identify any organometallic byproducts
arising from that reaction, the participation of Me3SnCl
in the reaction sequence was supported by the observed
increase in yield once 1 equiv Me3SnCl had been added
to the 3/MCl4 mixture.
The reaction of 3 with group 4 metal chlorides takes
a rather different course. Treatment of 3 with ZrCl4 or
HfCl4 in dichloromethane at room temperature over
4-16 h gave a straw-colored solution. When the solution
was cooled, pale yellow cubes separated in moderate
yield, which were soluble in dichloromethane but in-
soluble in hydrocarbons. These products were identified
crystallographically as the salts of carbocations, [HC{CH-
(SiMe3)(SnMe3)}2]+M2Cl9- (M ) Zr (6), Hf (7)). Evidently
these compounds are formed by attack of Me3Sn+ on 3
to give a sterically highly hindered, tin-substituted
carbocation, in preference to the expected dehalostan-
nylation and the formation of zirconium and hafnium
allyl species (Scheme 2). To our knowledge these are
the first examples of isolable and thermally stable sec-
alkyl carbocations. A preliminary account of this reac-
tion has appeared.18
The choice of the correct metal halide is obviously
important and is possibly dictated by the stability of the
resulting halometalate anion. For example, the at-
tempted reaction of 3 with either TiCl4 or AlCl3 took a
different course and led to intractable materials.
The 1H and 13C{1H} NMR spectra of 6 and 7 in CD2-
Cl2 indicate complex equilibria in solution, and even
measuring solution NMR spectra at low temperature
allowed no conclusive identification. However, the solid-
state magic-angle spinning 13C NMR spectrum sup-
ported the formation of the proposed species and showed
a peak at δ 217 for the cationic R2CH+ carbon (Figure
3). This chemical shift indicates significant stabilization
of the cationic carbon, very similar to that found in
+
The addition of Me3Sn+ implies that some 3 must
have undergone dehalostannylation with MCl4 to gener-
CPh3 (δ 210.4).24 By comparison, the corresponding
resonance of the isopropyl cation Me2CH+ in SO2ClF/
SbF5 at low temperature was observed at δ 317.25 The
charge delocalization to the tin substituents is apparent
in the solid-state magic-angle spinning 119Sn NMR
chemical shift of δ 173.9, significantly deshielded com-
(18) Schormann, M.; Garratt, S.; Hughes, D. L.; Green, J. C.;
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Angew. Chem., Int. Ed. 2004, 43, 2908.
(23) For a structurally characterized vinyl cation see: Mu¨ller, T.;
Juhasz, M.; Reed, C. A. Angew. Chem., Int. Ed. 2004, 43, 1543.
(24) Olah, G. A.; White, V. J. Am. Chem. Soc. 1969, 91, 5801.
(25) Olah, G. A.; Westerman, P. W.; Nishimura, J. J. Am. Chem.
Soc. 1974, 96, 3548.
Figure 3. Solid-state 13C MAS NMR spectrum of 6 (75.4
MHz). Spinning sidebands are indicated by an asterisk.

1722 Organometallics, Vol. 24, No. 7, 2005
Schormann et al.
C-C single bond, 1.422(8) Å. The C-Si bonds are
normal. However, the C(4)-Sn bonds of 2.213(2) Å
are long compared to the average Sn-CH3 distance
of 2.04 Å. These bond length variations are accom-
panied by characteristic changes in bond angles. There
are significant differences between Si and Sn. Whereas
the C-C-Si angle of 116.3(4)° is unremarkable, the
C-C-Sn angle is much smaller, 105.6(2)°. It is evi-
dent, therefore, that the elongation of the C(4)-Sn
bond is accompanied by an inclination of the SnMe3
substituents toward the positively charged carbon C(8).
Although this inclination is not sufficient to bring C(8)
and Sn into a bonding contact (Sn‚‚‚C(8) ) 2.935 Å),
the structure shows an almost linear Sn‚‚‚C+‚‚‚Sn
arrangement (Scheme 3). This allows the empty p
orbital on C(8) and the two C-Sn bonds to be aligned
parallel to one another. The structural features in
these carbocations are a nice illustration of the effect
of charge delocalization by hyperconjugation (Scheme
3), in agreement with the observed 13C and 119Sn
chemical shifts.
Compounds 6 and 7 are surprisingly thermally stable
and melt at 109 and 120 °C, respectively, without
decomposition. For comparison, carbocations can also
be stabilized by delocalization over aromatic rings,19 by
a heteroatom carrying lone electron pairs (Me2C+-E,
where E ) OR, NR2, halide)20 or in superacidic SbF5-
based media by C+‚‚‚F interactions.21 While the delo-
calization over an aromatic ring or by a heteroatom
leads to thermally stable products, there are only a
handful of examples of structurally characterized ali-
phatic carbocations; all are tertiary and are thermally
unstable.22,23
Figure 4. Structure of the ion pair [HC{CH(SiMe3)-
-
(SnMe3)}2]+Hf2Cl9 (7), showing the atomic numbering
scheme. Ellipsoids are drawn at the 50% probability level.
Table 3. Selected Bond Lengths (Å) and Angles
-
(deg) for [HC{CH(SiMe3)(SnMe3)}2]+M2Cl9 (M ) Zr
(6), Hf (7))
6
7
Sn-C(1)
Sn-C(2)
Sn-C(3)
Sn-C(4)
C(4)-Si
C(4)-C(8)
M-Cl(1)
M-Cl(2)
M-Cl(3)
M-Cl(4)
1.96(3)
2.07(4)
2.04(3)
2.14(3)
2.130(9)
2.213(2)
1.963(6)
1.422(8)
2.5985(14)
2.632(2)
2.3545(14)
2.350(2)
2.128(14)
2.216(3)
1.982(9)
1.419(12)
2.576(2)
2.611(2)
2.341(2)
2.335(3)
C(1)-Sn-C(2)
C(1)-Sn-C(3)
C(1)-Sn-C(4)
C(8)-C(4)-Sn
C(8)-C(4)-Si
Si-C(4)-Sn
C(4A)-C(8)-C(4)
Cl(1A)-M-Cl(1)
Cl(1)-M-Cl(2)
Cl(3)-M-Cl(1)
Cl(3A)-M-Cl(1)
Cl(4)-M-Cl(1)
M(A)-Cl(1)-M
M(A)-Cl(2)-M
114.5(9)
110.9(6)
105.3(6)
105.6(2)
116.3(4)
112.3(3)
128.8(9)
77.23(7)
77.12(5)
90.24(5)
164.27(5)
90.11(6)
88.35(6)
86.94(7)
110.3(13)
110.8(9)
107.2(8)
106.0(3)
116.1(6)
111.4(5)
126.2(13)
77.19(11)
77.05(6)
90.47(8)
164.34(7)
90.48(8)
88.45(8)
86.95(10)
pared to SnMe4 (δ 0.00) and even low field from Bun -
Conclusion
3
SnCl (δ 144.0). In contrast, the silyl substituents are
not involved in this stabilizing interaction, as indicated
by the 29Si chemical shift of δ 1.0.
Trimethylsilyl substituents on allyl ligands signifi-
cantly increase the thermal stability of the resulting
complexes. The successful introduction of this ligand is,
however, strongly dependent on the synthetic route,
with alkali-metal allyls being prone to reduction. Al-
though the dehalostannylation method was successful
for the synthesis of the first example of a tantalum
monoallyl complex of the type LTaCl4 from Me3SiCHd
CHCH(SiMe3)(SnMe3) and TaCl5, the analogous reac-
tion with zirconium and hafnium tetrachloride unex-
pectedly gave M2Cl9- salts of tin-substituted carbocations,
[HCR2]+ (R ) CH(SiMe3)(SnMe3)). These salts are
surprisingly thermally stable due to charge delocaliza-
tion to Sn; unlike previous examples of structurally
characterized alkyl carbocations, there are no close
contacts with the counteranion. The effect of stannyl
substitution as the stabilizing principle in carbocation
chemistry is currently under investigation.
The crystal structure of 7 is shown in Figure 4.
Selected bond distances and angles for 6 and 7 are
collected in Table 3. Since the CHR1R2 moieties in the
cation [HC{CHR1R2}2]+ are chiral, the ion could in
principle exist as two diastereomers with a rac (R,R/
S,S) or meso (R,S) configuration. In the solid state only
the rac isomer is found, and models show that the meso
isomer would have several sterically unfavorable close
contacts between the two trimethylstannyl substituents.
Unlike previous crystal structures of aliphatic carboca-
tions, 6 and 7 contain no close contacts between cations
and anions. There are several C-H‚‚‚Cl contacts at
normal van der Waals distances, while the minimum
C‚‚‚Cl distance is 3.60 Å.
As might be expected in a carbocation, the C-C bonds
of the C3 core are significantly shorter than a normal
Scheme 3

Reactivity of Silyl-Substituted Allyl Compounds
Organometallics, Vol. 24, No. 7, 2005 1723
Me3SnCl, were removed in vacuo over 4 h. The residue
consisted of a deep red oil. Addition of light petroleum (5 mL)
and cooling to -30 °C afforded 4 as an analytically pure
microcrystalline red solid: yield 0.71 g (49%), mp 72-73 °C.
Another slightly impure batch was collected as a sticky solid
after reducing the amount of solvent to 2-3 mL (0.36 g, total
yield 74%) and subsequent cooling. Anal. Calcd for C9H21Cl4-
Experimental Section
All manipulations were performed under nitrogen, using
standard Schlenk techniques. Solvents were predried over
sodium wire (toluene, light petroleum, THF, diethyl ether) or
calcium hydride (dichloromethane) and distilled under nitro-
gen from sodium (toluene), sodium-potassium alloy (light
petroleum, bp 40-60 °C), sodium-benzophenone (THF, diethyl
ether), or calcium hydride (dichloromethane). Deuterated
solvents were stored over activated 4 Å molecular sieves and
degassed by several freeze-thaw cycles. NMR spectra were
recorded using a Bruker Avance DPX-300 spectrometer. 1H
NMR spectra (300.1 MHz) were referenced to the residual
solvent proton of the deuterated solvent used. 13C NMR spectra
(75.5 MHz) were referenced internally to the D-coupled 13C
resonances of the NMR solvent. Co(acac)3, Ni(acac)2, Me3SnCl,
and TaCl5 were used as supplied; ZrCl4 and HfCl4 were
sublimed prior to use. Tetramethylethylenediamine (TMEDA)
was dried over KOH. K[C3H3(SiMe3)2-1,3] was prepared by
following a literature procedure.11
Preparation of Co{η3-C3H3(SiMe3)2-1,3]}2 (1). A solution
of K[C3H3(SiMe3)2-1,3] (7.02 g, 31.3 mmol) in diethyl ether (50
mL) was added dropwise to suspension of Co(acac)3 (3.36 g,
9.43 mmol) also in diethyl ether (100 mL) at -78 °C. The
reaction mixture was warmed slowly to room temperature,
during which time the color changed from green to orange,
until all the Co(acac)3 had dissolved. The volatiles were
removed in vacuo, and the resulting orange powder was
extracted with light petroleum (100 mL). The extract was
concentrated to ca. 5 mL and stored at -30 °C to give 1 as
orange crystals: yield 1.33 g (32.8%). A satisfactory elemental
analysis could not be obtained. The identity of the product was
confirmed by single-crystal X-ray diffraction.
1
Si2Ta: C, 21.27; H, 4.16. Found: C, 20.78; H, 4.01. H NMR
3
(CDCl3, 300 MHz): δ 0.34 (s, 18 H, SiMe3), 5.30 (d, JH-H
)
3
15.0 Hz, 2 H, CHCHCH), 6.89 (t, JH-H ) 15.0 Hz, H,
CHCHCH). 13C NMR (CDCl3, 75.5 MHz): δ 0.1 (SiMe3), 134.6
(CHCHCH), 137.2 (CHCHCH). IR (Nujol, KBr plates, cm-1):
1462 (s), 1250 (s), 935 (s), 840 (vs), 786 (s), 765 (s), 748 (s),
732 (s), 654 (s).
Synthesis of Me3SiCHdCH(SiMe3)CdTaCl3‚TMEDA (5).
The procedure above for the synthesis of compound 4 was
followed, and the reaction product was used without workup.
The red oil was dissolved in light petroleum (30 mL), and
tetramethylethylenediamine (TMEDA, 0.8 g, 6.9 mmol) was
added at 0 °C. The solution turned black. Crystallization at
-30 °C afforded 5 as dark block-shaped crystals, yield 0.48 g
(28%), mp 89 °C. Anal. Calcd for C15H36Cl3N2Si2Ta: C, 30.64;
H, 6.17; Cl, 18.09. Found: C, 30.47; H, 6.32; Cl, 17.14. 1H NMR
(CDCl3, 300 MHz): δ 0.37, 0.79 (s, 9 H, SiMe3), 1.81-1.83,
2.05-2.07 (m, 2 H, CH2 (TMEDA)), 2.38, 2.74 (s, 3 H, CH3
(TMEDA)), 4.47 (d, 3JH-H ) 19.0 Hz, H, CHdCH(SiMe3)), 10.95
3
(d, JH-H ) 19.0 Hz, H, CHdCH(SiMe3)). 13C NMR (CD2Cl2,
75.5 MHz, -80 °C): δ 0.4, 4.3 (SiMe3), 45.0, 50.9, 57.0, 61.3
(TMEDA), 121.0 (Cipso), 153.7 (Câ), 265.8 (CR).
-
Synthesis of [CH{CH(SiMe3)(SnMe3)}2]+Zr2Cl9 (6). A
mixture of Me3SnCl (0.6 g, 3.0 mmol) and 3 (1.0 g, 2.9 mmol)
in CH2Cl2 (30 mL) was added to solid ZrCl4 (1.4 g, 6.0 mmol).
When the mixture was stirred for 4 h at room temperature,
the solid dissolved and the solution turned yellow. After
filtration the amount of solvent was reduced to 10 mL.
Crystallization at -30 °C afforded the title compound as
colorless crystals in 50% yield (1.5 g, 1.5 mmol), mp 109 °C.
Solid-state magic angle spinning NMR: 13C, δ -1.0, 3 (SiMe3,
SnMe3), 70 (CH), 217 (CH+); 29Si, δ 1. 119Sn NMR (standard
SnMe4): δ 173.9. Anal. Calcd for C15H39Cl9Si2Sn2Zr2: C, 17.76;
H, 3.87; Cl, 31.45. Found: C, 17.82; H, 3.74; Cl, 32.15.
Synthesis of [CH{CH(SiMe3)(SnMe3)}2]+Hf2Cl9- (7). By
the procedure for 6, compound 7 was prepared from Me3SnCl
(0.6 g, 3.0 mmol), 3 (1.0 g, 2.9 mmol), and HfCl4 (1.9 g, 6.0
mmol) as colorless crystals in 50% yield (1.7 g, 1.5 mmol), mp
120 °C. The 13C, 29Si, and 119Sn solid-state NMR spectroscopic
data were essentially identical with those for 6. Anal. Calcd
for C15H39Cl9Hf2Si2Sn2: C, 15.15; H, 3.31; Cl, 26.83. Found:
C, 14.87; H, 3.19; Cl, 27.67.
X-ray Crystallography. Crystals coated in dried perfluo-
ropolyether oil were mounted on glass fibers and fixed in a
cold nitrogen stream. Diffraction intensities were measured
on a Rigaku R-Axis IIc image-plate diffractometer equipped
with a rotating-anode X-ray source, Mo KR radiation, and
graphite monochromators. Crystal and refinement data are
collected in Table 4. Data were processed using the DENZO/
SCALEPACK programs.26 The structure was determined by
the direct-methods routines in the XS27 and SHELXS28 pro-
grams and refined by full-matrix least-squares methods, on
F2’s, in XL or SHELXL. Non-hydrogen atoms were refined with
anisotropic displacement parameters. Scattering factors for
neutral atoms were taken from the literature.29 Computer
programs were run on a Silicon Graphics Indy computer at
Preparation of Ni{η3-C3H3(SiMe3)2-1,3]}2 (2). A solution
of K[C3H3(SiMe3)2-1,3] (5.00 g, 22.3 mmol) in THF (40 mL) was
added dropwise to a solution of Ni(acac)2 (2.60 g, 10.1 mmol)
in THF (60 mL) at -78 °C. The mixture was warmed to room
temperature, at which time the solution had become dark red.
After a further 16 h, the solvent was evaporated, the residue
extracted with light petroleum (100 mL), and the extract
filtered. Concentrating the filtrate left an oil which slowly
crystallized on standing. The product was obtained as orange
crystals, yield 1.78 g (41.2%). Satisfactory analysis could not
be obtained. The identity of the product was confirmed by
single-crystal X-ray diffraction.
Preparation of (Me3Sn)(Me3Si)CHCHdCHSiMe3 (3). A
suspension of K[C3H3(SiMe3)2-1,3] (5.0 g, 22 mmol) in diethyl
ether (100 mL) at 0 °C was treated with a solution of Me3-
SnCl (4.4 g, 22 mmol) in diethyl ether (30 mL). There was
immediate formation of a voluminous KCl precipitate. The
reaction mixture was warmed to room temperature and
filtered, and the residue was extracted with ether (30 mL).
The title compound was obtained as a pale yellow oil; yield
7.5 g (98%). The compound was used without further purifica-
tion; however, an analytically pure sample was obtained after
condensation to a cold trap. Anal. Calcd for C12H30Si2Sn: C,
41.27; H, 8.66. Found: C, 41.67; H, 8.61. 1H NMR (CDCl3, 300
MHz): δ -0.01 (s, 9 H, SnMe3), 0.00 (s, 9 H, SiMe3), 0.07 (s, 9
3
H, SiMe3), 1.52 (d, JH-H ) 11.5 Hz, H, CHdCHCH), 5.23 (d,
3JH-H ) 18.0 Hz, H, CHdCHCH), 5.97 (dd, H, CHdCHCH).
13C NMR (CDCl3, 75.5 MHz): δ -8.5 (satellites, SnMe3), -0.2
(SiMe3), 28.1 (CHdCHCH), 125.5 (CHdCHCH), 147.3 (CHd
CHCH). MS (EI): m/z 350 (M+), 335 (M+ - Me), 165 (SnMe3+).
IR (neat, KBr plates, cm-1): 2955 (vs), 1583 (s), 1259 (s), 1248
(vs), 1051 (s), 1011 (s), 875 (vs), 837 (vs), 766 (s), 724 (s), 689
(s), 526 (s).
(26) Otwinowski, Z.; Minor, W. Methods in Enzymology, Macromo-
lecular Crystallography, Part A; Carter, C. W., Sweet, R. M., Eds.;
Academic Press: New York, 1997; Vol. 276, pp 307-326.
(27) Sheldrick, G. M. SHELXTL package, including XS for structure
determination, XL for refinement, and XP for molecular graphics;
Siemens Analytical Inc., 1995.
(28) Sheldrick, G. M. SHELX-97-Programs for crystal structure
determination (SHELXS) and refinement (SHELXL); University of
Go¨ttingen, Go¨ttingen, Germany, 1997.
Synthesis of [CH(CHSiMe3)2]TaCl4 (4). Neat 3 (1.0 g, 2.9
mmol) was added dropwise via syringe to a suspension of TaCl5
(1.3 g, 3.6 mmol) in CH2Cl2 (30 mL) at 0 °C. The reaction
mixture turned red. After the mixture was warmed to room
temperature, all volatile components, including the byproduct

1724 Organometallics, Vol. 24, No. 7, 2005
Table 4. Crystal Data and Refinement Details
Schormann et al.
1
2
5
6
7
formula
C
18H42CoSi4
C
18H42NiSi4
C
15H36Cl3Si4Ta
C15H39Si2Sn2‚Zr2Cl9
1014.5
C
15H39Si2Sn2‚Hf2Cl9
fw
429.81
140(1)
429.59
140(1)
587.94
140(1)
1189.1
140(1)
temp, K
140(1)
cryst syst
monoclinic
P21/c
monoclinic
C2/c
monoclinic
P21/c
orthorhombic
Cmcm
14.899(1)
12.310(1)
19.894(1)
90
orthorhombic
Cmcm
14.860(1)
12.280(1)
19.901(1)
90
space group
a, Å
12.323(3)
10.322(3)
21.059(4)
99.59(3)
2641.2(9)
1.081
16.578(3)
12.430(3)
12.698(3)
90.84(3)
2616.3(9)
1.091
18.928(4)
9.059(2)
14.410(3)
100.39(3)
2430.4(8)
1.607
b, Å
c, Å
â, deg
V, Å3
3648.7(4)
1.847
3631.6(5)
2.175
Dcalcd, g cm-3
Z
4
4
4
4
4
µ, mm-1
0.830
0.924
4.952
2.640
7.792
no. of indep/obsd (I > 2σI) rflns
no. of data/restraints/params
F(000)
4832/4071
4832/0/232
932
0.0399
0.0332
0.0961
1.059
2365/2125
2365/0/117
936
0.0499
0.0463
0.1306
1.156
4397/4301
4397/0/235
1168
0.0353
0.0346
0.1015
1.255
1595/1548
1594/0/113
1960
1760/1667
1757/0/113
2216
a
Rint
0.043
0.065
R1 (I > 2σ(I))b
wR2 (all data)c
GOF on F2
0.045
0.058
0.110
0.145
1.257
1.218
2
2
2
a ∑|Fo2 - Fo2(mean)|/∑Fo
.
b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑w(Fo2 - Fc )2/∑w(Fo2)2]1/2; w ) [σ2(Fo2) + (aP)2 + bP]-1, where P ) [2Fc
+ max(Fo2, 0)]/3.
the University of East Anglia or on a DEC-Alpha Station 200
4/100 computer in the Biological Chemistry Department, John
Innes Centre. Crystal data are deposited with the Cambridge
Crystallographic Data Centre, under the following CCDC
numbers: 253620 (compound 1), 253621 (2), 253623 (5),
177524 (6) and 177525 (7). These can be obtained without
charge from www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; deposit@ccdc.cam.ac.uk).
Acknowledgment. This work was supported by the
European Commission (Contract No. HPRN-CT2000-
00004) and Bayer Inc., Sarnia, Canada. We thank
Dr. N. J. Clayden for solid-state MAS NMR measure-
ments.
Supporting Information Available: Crystallographic
data for compounds 1, 2, and 5-7. This material is available
free of charge via the Internet at http://pubs.acs.org.
(29) International Tables for X-ray Crystallography; Kluwer Aca-
demic: Dordrecht, The Netherlands, 1992; Vol. C, pp 193, 219, and
500.
OM0491692