First successful reaction of a silyl anion with hafnium tetrachloride

Article abstract of DOI:10.1039/b201508k

Hafnium tetrachloride reacts with the tris(trimethylsilyl)silyl potassium tmen adduct (1) to form a [tris(trimethylsilyl)silyl]trichlorohafnium tmen complex (2); reaction of 2 with 2,6-dimethylphenylisonitrile leads to insertion into the silicon hafnium bond (4).

Full text of DOI:10.1039/b201508k

First successful reaction of a silyl anion with hafnium tetrachloride
Dieter Frank, Judith Baumgartner and Christoph Marschner*
Institut für Anorganische Chemie, Technische Universität Graz, Stremayrgasse 16, A-8010 Graz, Austria.
E-mail: marschner@anorg.tu-graz.ac.at
Received (in Cambridge, UK) 11th February 2002, Accepted 15th April 2002
First published as an Advance Article on the web 30th April 2002
Hafnium tetrachloride reacts with the tris(trimethylsilyl)si-
lyl potassium tmen adduct (1) to form a [tris(trimethylsi-
lyl)silyl]trichlorohafnium tmen complex (2); reaction of 2
with 2,6-dimethylphenylisonitrile leads to insertion into the
silicon hafnium bond (4).
angle of the chloro ligands in trans-position to the nitrogen
atoms [102.98(18)°] are the only cis-dihedral angles differing
significantly from the ideal 90°. All other angles range between
86.8(3) and 91.9(3)°. The slightly elongated bond length
between hafnium and Cl(2) [2.382(3) Å] and Cl(4) [2.385(4) Å]
compared to those between hafnium and Cl(1) [2.362(3) Å] and
Cl(3) [2.368(4) Å] suggest that the chloro ligand exhibits a
stronger trans effect than tmen. This is consistent to what has
been found for a related zirconium compound.8 The crystal
structure of 2‡ shows again distorted octahedral geometry with
the silyl ligand in the trans-position to a nitrogen atom (Fig. 2).
The silyl hafnium bond length [2.802(6) Å] lies within the range
of the values found so far.6a,9 The bond distance between the
nitrogen in trans-position to the silyl ligand and hafnium
[2.473(14) Å] is substantially elongated compared to the one
with the one trans to the chloro ligand [2.378(14) Å].
The chemistry of group 4 silyl compounds is a rapidly
developing area,1 which contributes important aspects to the
research on catalysis2,3 and new materials.4 However, most of
this chemistry studied so far involves compounds with cyclo-
pentadienyl (Cp) or related ligands.1 Although there are a few
examples of Cp-free silyl Zr5,6 and Hf6 complexes, the
seemingly most straightforward way to access this class of
compounds, namely the reaction of a silyl anion with a group 4
tetrahalide has not been reported so far. A likely reason for this
is that this reaction under the conditions usually employed
suffers from severe side reactions. The strongly Lewis acidic
character of the metal halides activates ethereal solvents present
from the generation of the silyl reagents and makes them
susceptible for nucleophilic attack by the silyl anion.
In the course of our studies of oligosilyl anions7 we have
investigated alternative donor molecules for oligosilyl po-
tassium compounds. For example we have prepared the
tris(trimethylsilyl)silyl potassium tmen adduct (1) which is
obtained in the reaction of tetrakis(trimethylsilyl)silane and
potassium tert-butoxide in toluene in the presence of three
equivalents of tetramethylethylenediamine (tmen).†
The reaction of two equivalents of 1 with hafnium tetrachlo-
ride in toluene gave, in a clean reaction, a hafnium trichloride
tris(trimethylsilyl)silyl tmen complex (2) besides one equiva-
lent of tris(trimethylsilyl)silane. Reaction of only one equiva-
lent of 1 leads to incomplete formation of 2 accompanied by the
hafnium tetrachloride tmen complex (3). This seems to indicate
the formation of the latter compound as the first step in the
course of reaction. Interestingly, no crystal structure analysis of
a hafnium tmen complex has been reported so far, so that the
compounds in this report constitute the first examples of this
class of compounds.
Fig. 2 Molecular structure of 2 with thermal ellipsoids at the 30 %
probability level. Selected bond distances (Å) and angles (°): Hf–Si(1)
2.802(6), Hf–N(1) 2.473(14), Hf–N(2) 2.378(14), Hf–Cl(1) 2.367(5), Hf–
Cl(2) 2.361(5), Hf–Cl(3) 2.392(5); N(1)–Hf–N(2) 74.8(6), Cl(1)–Hf–Cl(3)
177.52(19), Si(1)–Hf–N(1) 167.3(4), Si(4)–Si(1)–Si(3) 104.2(3), Si(4)–
Si(1)–Si(2) 101.8(3), Si(3)–Si(1)–Si(2) 105.3(3).
Crystal structure analysis of 3‡ reveals a distorted octahedron
(Fig. 1). The bite angle of the tmen ligand [76.9(4)°] and the
Fig. 3 Molecular structure of 4 with thermal ellipsoids at the 30%
probability level. Selected bond distances (Å) and angles (°): Hf–C(7)
2.212(6), Hf–N(1) 2.523(5), Hf–N(2) 2.469(6), Hf–N(3) 2.180(5), Hf–Cl(1)
2.4338(19), Hf–Cl(2) 2.4253(18), Hf–Cl(3) 2.4483(18), C(7)–N(3)
1.273(7); N(1)–Hf–N(2) 74.0(2), Cl(1)–Hf–Cl(2) 169.25(7), Si(4)–Si(1)–
Si(3) 107.83(10), Si(4)–Si(1)–Si(2) 108.48(10), Si(3)–Si(1)–Si(2)
109.12(10).
Fig. 1 Molecular structure of 3 with thermal ellipsoids at the 30%
probability level. Selected bond distances (Å) and angles (°): Hf–N(1)
2.372(11), Hf–N(2) 2.369(9), Hf–Cl(1) 2.362(3), Hf–Cl(2) 2.382(3), Hf–
Cl(3) 2.368(4), Hf–Cl(4) 2.385(4); N(1)–Hf–N(2) 76.9(4), Cl(1)–Hf–Cl(3)
102.98(18), Cl(2)–Hf–Cl(4) 175.33(14).
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= 2.162 g cm23, m = 8.54 mm21 (Mo-Ka, l = 0.71073 Å), T = 273 K;
the structure was solved by direct methods and refined by full matrix least
squares procedures: R1 = 0.0329 and 0.0450 (wR2 = 0.0574 and 0.0598)
for 1907 unique measured reflections. CCDC reference number 177992.
For HfC30H58N3Cl3Si4 4: orthorhombic, space group P212121 (no. 19), a
= 11.625(2), b = 12.164(2), c = 29.975(6) Å, V = 4238.8(15) Å3, Z = 4,
Dc = 1.344 g cm23, m = 2.79 mm21 (Mo-Ka, l = 0.71073 Å), T = 223
K; the structure was solved by direct methods and refined by full matrix
least squares procedures: R1 = 0.0305 and 0.0331, (wR2 = 0.0786 and
0.0797) for 6073 unique measured reflections. CCDC reference number
177993.
The Si–Hf bond in 2 can be cleaved by reaction with water or
hydrogen to form tris(trimethylsilyl)silane. Insertion into the
silyl–metal bond can be accomplished with 2,6-dimethyl-
phenylisonitrile5b,10 to give compound 4.† Structure analysis‡
of this compound exhibits a distorted octahedron with the tmen
ligand almost coplanar with the metallazirine ring (Fig. 3). Both
bonds of chloro [2.4253(18), 2.4338(19), 2.4443(18) Å] and
nitrogen [2.469(6), 2.523(5) Å] atoms to hafnium are sig-
nificantly elongated compared to the structures of 2 and 3. Due
to the more electron donating properties of the imine ligand the
hafnium nitrogen bond length increases from 2.369(9) Å in 3 to
2.523(5) Å.
See http://www.rsc.org/suppdata/cc/b2/b201508k/ for crystallographic
data in CIF or other electronic format.
Experiments to test for the catalytic activity of 2 with respect
to dehydropolymerisation of phenylsilane2 and 1,2-dimethyldi-
silane11 proved to be not successful.
We would like to thank Dr Guido Kickelbick (Technische
Universität Wien) for helpful discussions. Financial support of
this work was provided by the START program ‘Chiral
Polysilanes’ (Y-120) of the Austrian Federal Ministry for
Education, Science and Culture.
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Notes and references
†
Preparation of 1: all experimental manipulations were performed
under rigorously anaerobic conditions using standard Schlenk techniques or
a nitrogen filled glovebox. Tetrakis(trimethylsilyl)silane (1.50 g, 4.68
mmol) and potassium tert-butanolate (0.55 g, 4.90 mmol) were suspended
in 20 mL of toluene. Tetramethylethylenediamine (tmen) (1.63 g, 14.0
mmol) was added at 0 °C and then the solution was stirred at r.t. for 2 h. The
resultant green solution was concentrated, redissolved in pentane, filtered
and cooled to 236 °C to obtain colorless crystals of 1 which were isolated
by filtration (yield: 1.95 g, 80%). 1H NMR (500 MHz, C6D6): d 1.94 (s, 8H),
1.89 (s, 24H), 0.64 (s, 27H). 13C NMR (125.7 MHz, C6D6): d 57.3, 45.6,
7.52. 29Si NMR (59.3 MHz, C6D6): d 25.1, 2189.7. Anal. Calc. for
C21H59KN4Si4: C, 48.58; H, 11.45. Found: C, 48.09; H: 11.33%.
Preparation of 2: HfCl4 (0.50 g, 1.56 mmol) and 1 (1.62 g, 3.12 mmol)
were dissolved in 6 mL cold toluene (-36°C). The orange solution was
allowed to warm slowly to r.t., then treated with 3 mL pentane and the
obtained precipitate was removed by filtration. The solution was reduced in
volume and thin plate orange crystals could be isolated by filtration (yield:
0.63 g, 62%). 1H NMR (300 MHz, C6D6): d 2.23 (s, 12H), 1.61 (s, 4H), 0.65
(s, 27H). 13C NMR (75.4 MHz, C6D6): d 57.9, 51.6, 5.4. 29Si NMR (59.3
MHz, C6D6): d 22.6, 256.3. Anal. Calc. for C15H43Cl3HfN2Si4: C, 27.77;
H, 6.68. Found: C, 28.27; H: 6.68%.
Preparation of 4: in an NMR-tube, 2 (50 mg, 0.077 mmol) was dissolved
in C6D6 and treated with 2,6-dimethylphenylisocyanide (11 mg, 0.085
mmol) leading to an immediate change from bright orange to pale yellow,
and after filtration crystalline 4 was obtained. 1H NMR (300 MHz, C6D6):
d 6.97 (s, 1H), 6.90 (s, 2H), 2.67 (s, 3H), 2.48 (s, 2H), 2.36 (s, 3H), 2.22 (s,
3H), 2.10 (s, 3H), 2.05 (s, 2H), 0.43 (s, 6H), 0.29 (s, 27 H). 13C NMR (75.4
MHz, C6D6): d 246.9, 153.2, 128.2, 128.0, 127.8, 127.7, 127.6, 59.0, 57.3,
51.3, 19.9, 18.5, 2.8, 2.7, 2.6. 29Si NMR (59.3 MHz, C6D6): d 211.2,
280.8. Anal. Calc. for C24H52Cl3HfN3Si4: C, 36.96.77; H, 6.72. Found: C,
36.67; H: 6.88%.
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‡
Crystal data for HfC15H43N2Cl3Si4 2: orthorhombic, space group Pbca
(no. 61), a = 14.212(3), b = 13.760(3), c = 30.415(6) Å, V = 5948(2) Å3,
Z = 8, Dc = 1.449 g cm23, m = 3.94 mm21 (Mo-Ka, l = 0.71073 Å), T
= 223 K; the structure was solved by direct methods and refined by full
matrix least squares procedures: R1 = 0.0814 and 0.1409 (wR2 = 0.1693
and 0.1721) for 4345 unique measured reflections. CCDC reference number
177994.
For HfC6H16N2Cl4 3: orthorhombic, space group Pna21 (no. 33), a =
14.797(5), b = 7.552(3), c = 12.000(4) Å, V = 1341.0(8) Å3, Z = 4, Dc
CHEM. COMMUN., 2002, 1190–1191
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