Allyldimethylsilylcyclopentadienyl hafnium complexes: Formation and trapping of hafnocenium cations

Article abstract of DOI:10.1021/om050437l

The mixed silyl-stannyl-cyclopentadiene C₅H ₄[SiMe₂(CH₂CH=CH₂)][Sn(nBu) ₃] was synthesized by reaction of the lithium salt Li[C ₅H₄{SiMe₂(CH₂CH=C

Full text of DOI:10.1021/om050437l

4782
Organometallics 2005, 24, 4782-4787
Allyldimethylsilylcyclopentadienyl Hafnium Complexes:
Formation and Trapping of Hafnocenium Cations
Gema Mart´ınez and Pascual Royo*
Departamento de Quı´mica Inorga´nica, Universidad de Alcala´, Campus Universitario,
E-28871-Alcala´ de Henares (Madrid), Spain
Received June 1, 2005
The mixed silyl-stannyl-cyclopentadiene C5H4[SiMe2(CH2CHdCH2)][Sn(nBu)3] was syn-
thesized by reaction of the lithium salt Li[C5H4{SiMe2(CH2CHdCH2)}] with the corresponding
chlorotrialkyltin and then reacted with HfCl4 to prepare the monocyclopentadienyl complex
[Hf{η5-C5H4SiMe2(CH2CHdCH2)}Cl3‚DME] (DME ) dimethoxyethane). Reaction of this
compound with Na(C5H5) or Li(C5H5) afforded low yields of the dichloro hafnocene [Hf(η5-
C5H5){η5-C5H4SiMe2(CH2CHdCH2)}Cl2], which was also isolated more efficiently by trans-
metalation of the lithium salt Li[C5H4{SiMe2(CH2CHdCH2)}] with [Hf(η5-C5H5)Cl3‚DME].
Alkylation of the mono- and dicyclopentadienyl complexes with MgCl(CH2Ph) or LiMe gave
the tribenzyl monocyclopentadienyl [Hf{η5-C5H4SiMe2(CH2CHdCH2)}(CH2Ph)3] or the dialkyl
hafnocene derivatives [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}R2] (R ) Me, CH2Ph). The
reaction of the dibenzyl hafnocene with the Lewis acid B(C6F5)3 was monitored by NMR
spectroscopy at variable temperature. Formation of the hafnocenium cationic species [Hf-
(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CHdCH2)}(CH2Ph)]+ was initially observed, which then
slowly transformed into the metallacyclic alkyl [Hf(η5-C5H5){η5-C5H4SiMe2CH2CH(CH2Ph)-
η1-CH2}]+ cation, resulting from the benzyl migratory insertion of the allylic alkene moiety.
Reaction of these intermediate cationic species with KOtBu gave the neutral dicyclopenta-
dienyl hafnocene compounds [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}(CH2Ph)(OtBu)] and
[Hf(η5-C5H5){η5-C5H4SiMe2[CH2CH(CH2Ph)-η1-CH2]}(OtBu)], which were characterized by
elemental analyses and NMR spectroscopy.
Introduction
cation followed by alkyl migration to the coordinated
alkene is the generally accepted mechanism of propaga-
tion. Studies of the group 4 metallocene alkyl-alkene
cations [MCp2R(alkene)]+ have generated a great deal
of interest because the cations are key intermediates
in metallocene-catalyzed alkene polymerization. Many
efforts have been made to evaluate the kinetic barriers
for alkene complexation, inversion at the metal, and
migratory alkene insertion into the metal-alkyl bond.
Models required for these studies have been based on
d0 neutral yttrium22-24 and cationic zirconium metal-
locenes with chelating alkoxo-alkene25-27 and alkyl-
Group 4 metallocenes are well-known precursors for
single-site alkene polymerization catalysts.1-9 The ac-
tive metal alkyl cation is formed by reaction of these
precursors with different alkylating agents and strong
Lewis acid activators10-21 (MAO, B(C6F5)3, [CPh3]-
[B(C6F5)4]). Alkene coordination to the metal alkyl
* To whom correspondence should be addressed. Tel: 34 91 8854765.
Fax: 34 91 8854683. E-mail: pascual.royo@uah.es
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10.1021/om050437l CCC: $30.25 © 2005 American Chemical Society
Publication on Web 08/25/2005

Allyldimethylsilylcyclopentadienyl Hf Complexes
Organometallics, Vol. 24, No. 20, 2005 4783
(C5H5),40 B(C6F5)3,41 and [Hf(η5-C5H5)Cl3‚DME]42 were isolated
by reported methods. 1H and 13C NMR spectra were recorded
on a Varian Unity VXR-300 or Varian Unity 500 Plus instru-
ment. Chemical shifts, (δ) in ppm, are measured relative to
residual 1H and 13C resonances for C6H6-d6, CHCl3-d1, and CH2-
Cl2-d2 used as solvents and coupling constants are in Hz. C,
H analyses were carried out with a Perkin-Elmer 240-C
analyzer.
alkene28-34 ligands, which provide additional stability
to the weak metal-alkene interaction, which lacks d-π*
back-donation. These species show a highly polarized
and unsymmetrically coordinated alkene with a partial
positive charge on the internal olefinic carbon.
We reported35 that formation of the cationic species [Zr-
(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CHdCH2)}(CH2Ph)]+ was
stabilized by the â-Si substituent of the allylsilyl alkene
tethered to one of the cyclopentadienyl ligands, which
favors a partial positive charge on the internal olefinic
carbon36 of the d0 metal-alkene complex. This effect was
verified more recently37,38 by comparing the coordination
of allyltrimethylsilane and 4,4-dimethyl-1-pentene to
[Zr(η5-C5H4Me)2(OtBu)(ClCD2Cl)]+, which shows ad-
ditional stabilization against insertion due to the pres-
ence of the alkoxo ligand.
Synthesis of [C5H4{SiMe2(CH2CHdCH2)}{Sn(nBu)3}]
(1). Sn(nBu)3Cl (5.63 mL, 15.00 mmol) was slowly added at 0
°C to a suspension of Li[C5H4{SiMe2(CH2CHdCH2)}] (3.00 g,
17.60 mmol) in hexane (150 mL). The mixture was stirred
overnight. After filtration to remove LiCl, the solvent was
removed under vacuum to give 1 as a yellow viscous liquid
1
(6.10 g, 13.46 mmol, 89%). H NMR (300 MHz, C6D6, 25 °C):
δ 0.11 (s, 6H, Si(CH3)2), 0.88 (m, 9H, -CH3(nBu)), 1.29 (m,
6H, -CH2-(nBu)), 1.42 (m, 12H, -CH2-(nBu)), 1.50 (d, 2H, J
) 8.2, SiCH2), 4.82 (2m, 2H, dCH2), 5.70 (m, 1H, CHd), 6.14
(m, 2H, C5H4), 6.59 (m, 2H, C5H4).
Similar studies were also reported39 for related neu-
tral titanium and zirconium dialkyl compounds and
their cationic derivatives [M(η5-C5H5){η5-C5H4SiMe2-
(CH2-η2-CHdCH2)}R]+ (M ) Ti, R ) Me, CH2Ph; M )
Zr, R ) Me), although kinetic parameters could not be
evaluated.
In this paper we describe the synthesis and charac-
terization of new monocyclopentadienyl hafnium com-
plexes [Hf{η5-C5H4SiMe2(CH2CHdCH2)}X3](X)Cl, CH2-
Ph) and neutral hafnocenes [Hf(η5-C5H5){η5-C5H4-
SiMe2(CH2CHdCH2)}X2] (X ) Cl, Me, CH2Ph), studies
related to the formation of the cationic hafnocenium spe-
cies [Hf(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CHdCH2)}(CH2-
Ph)]+[(CH2Ph)B(C6F5)3]- and [Hf(η5-C5H5){η5-C5H4Si-
Me2CH2CH(CH2Ph)-η1-CH2}]+[(CH2Ph)B(C6F5)3]- be-
tween -80 and -20 °C, and their trapping and charac-
terization as the neutral tert-butyl alkoxides [Hf(η5-
C5H5){η5-C5H4SiMe2(CH2CHdCH2)}(CH2Ph)(OtBu)]-
and [Hf(η5-C5H5){η5-C5H4SiMe2CH2CH(CH2Ph)-η1-CH2}-
(OtBu)].
Synthesisof[Hf{η5-C5H4SiMe2(CH2CHdCH2)}Cl3‚DME]-
(2). Dimethyl sulfide (1.64 mL, 22.33 mmol) was added at room
temperature to a stirred suspension of HfCl4 (3.57 g, 11.16
mmol) in 150 mL of dichloromethane. The resulting clear
solution was then treated dropwise with 1 (4.98 g, 11.00 mmol).
The reaction mixture was stirred for 18 h, and then DME (2.00
mL, 19.10 mmol) was added and stirred for an additional 1 h.
The suspension was filtered, and the solvent was removed
under reduced pressure. The residue was washed with hexane
(4 × 20 mL) and dried under vacuum to yield 2 as a grayish
white solid (3.51 g, 6.52 mmol, 59%). 1H NMR (300 MHz, C6D6,
25 °C): δ 0.58 (s, 6H, Si(CH3)2), 1.88 (d, 2H, J ) 8.4, SiCH2),
3.00-3.40 (m, 10H, DME), 4.80 (m, 2H, dCH2), 5.82 (m, 1H,
CHd), 6.34 (m, 2H, C5H4), 6.60 (m, 2H, C5H4). 13C NMR (75
MHz, C6D6, 25 °C): δ -2.1 (Si(CH3)2), 25.3 (SiCH2), 60.0-70.0
(DME), 113.4 (dCH2), 120.0 (C5H4), 124.1 (C5H4), 126.4 (Cipso
-
Si C5H4), 135.3 (CHd). Anal. Calcd for C14H25SiO2Cl3Hf: C,
31.24; H, 4.68. Found: C, 31.22; H, 4.67.
Synthesisof[Hf{η5-C5H4SiMe2(CH2CHdCH2)}(CH2Ph)3]-
(3). A 2 M THF solution of MgCl(CH2Ph) (1.40 mL, 2.90 mmol)
was added at -78 °C to a solution of 2 (0.50 g, 0.98 mmol) in
Et2O (20 mL), and the suspension was stirred for 18 h while
warming slowly to room temperature. After filtration the
solvent was removed under reduced pressure, and the residue
was extracted into hexane (40 mL) to separate the white
insoluble solid. The solvent was removed under reduced
pressure and the residue dried under vacuum to give 3 as a
yellow oily solid (0.19 g, 0.32 mmol, 33%). 1H NMR (300 MHz,
C6D6, 25 °C): δ 0.16 (s, 6H, Si(CH3)2), 1.58 (d, 2H, J ) 8.3,
SiCH2), 1.64 (s(a), 6H, CH2Ph), 4.90 (m, 2H, dCH2), 5.68 (m,
2H, C5H4), 5.69 (m, 1H, CHd), 5.96 (m, 2H, C5H4), 6.70, 6.86-
7.26 (3m, 15H, C6H5). 13C NMR (75 MHz, C6D6, 25 °C): δ -2.3
(Si(CH3)2), 25.1 (SiCH2), 69.7 (CH2Ph), 114.1 (dCH2), 117.5
(C5H4), 118.6 (C5H4), 125.6 (Cipso-Si C5H4), 126.1 (Cpara C6H5),
128.7 (Cmeta C6H5), 129.2(Cortho C6H5), 134.2 (CHd), 144.2 (Cipso
C6H5). Anal. Calcd for C31H36SiHf: C, 60.52; H, 5.90. Found:
C, 59.88; H, 5.09.
Experimental Section
General Procedures. All manipulations were performed
under an inert atmosphere of argon using standard Schlenk
techniques or an M. Braun drybox. Solvents used were
previously dried and freshly distilled under argon. Deuterated
solvents from Scharlau were degassed, dried, and stored over
molecular sieves. SiMe2(CH2CHdCH2)Cl, Sn(nBu)3Cl, Mg(CH2-
Ph)Cl, nBuLi, MeLi, HfCl4, KOtBu, and SMe2 were obtained
from commercial sources and used as received with the
exception of Sn(nBu)3Cl, which was previously distilled. Na-
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1999, 121, 9483-9484.
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3970-3977.
Synthesis of [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}-
Cl2] (4). A THF (20 mL) solution of Li[C5H4{SiMe2(CH2CHd
CH2)}] (3.35 g, 19.7 mmol) was added dropwise to a suspension
of [Hf(η5-C5H5)Cl3‚DME] (8.70 g, 19.7 mmol) in THF (40 mL)
at -78 °C. The cooling bath was removed, and the reaction
mixture was stirred for 18 h at room temperature. After
solvent removal under vacuum the residue was extracted into
toluene (150 mL) and the solution was concentrated to 10 mL
and treated with hexane (80 mL) to give 4 as a colorless
crystalline solid, which was separated by filtration and dried
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J. Am. Chem. Soc. 2001, 123, 1483-1489.
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2001, 20, 4253-4261.
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2001, 20, 4262-4265.
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21, 473-483.
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Chem. Soc. 2001, 123, 10762-10763.
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11170-11171.
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2427.

4784 Organometallics, Vol. 24, No. 20, 2005
Mart´ınez and Royo
under vacuum (5.70 g, 12.90 mmol, 65%). 1H NMR (300 MHz,
C6D6, 25 °C): δ 0.34 (s, 6H, Si(CH3)2), 1.69 (d, 2H, JH-H ) 8.1,
SiCH2), 4.90 (2m, 2H, dCH2), 5.73 (m, 1H, CHd), 5.80 (m, 2H,
19F NMR (CD2Cl2, -80 °C): δ -169.3 (m-F), -164.9 (p-F),
-120.9 (o-F).
This reaction mixture was heated to -40 °C, kept for 1 h at
this temperature, and then heated to -20 °C to give complex
8 as a mixture of two diastereomers, 8a and 8b. 1H NMR (500
MHz, CD2Cl2, -20 °C): δ 0.08×bb/0.07b (s, 3H, Si(CH3)2),
0.24×bb,b (m, 1H, HfCH2), 0.27×bb/0.26b (s, 3H, Si(CH3)2),
0.99×bb,b (m, 2H, SiCH2), 1.44×bb,b (m, 1H, HfCH2), 1.93×bb,b
(m, 1H, CH), 2.24×bb,b (m, 1H, CH2Ph), 2.78×bb,b (s.a, 2H
CH2B), 2.84×bb,b (m, 1H, CH2Ph), 5.92×bb/5.96b (s, 5H, C5H5),
6.18×bb/6.02b (m, 1Ha, 2Hb, C5H4), 6.22×bb/6.46b (m, 1Ha, 2Hb,
C5H4), 6.57×bb,b (m, 1H, C5H4), 6.63×bb,b (m, 1H, C5H4),
6.80×bb/7.40b (m, 15H, C6H5). 13C NMR (75 MHz, CD2Cl2, -20
°C): δ -4.8a/-4.9b (Si(CH3)2), 0.3a,b (Si(CH3)2), 24.8×bb,b (SiCH2),
30.2a,b (CH2B), 42.3×bb/42.6b (CH), 52.5×bb/52.6b (CH2Ph),
78.7a/78.9b (CH2Hf), 110.2a/110.3b (C5H4), 114.0a/114.1b (C5H4),
114.2×bb,b (C5H4), 114.3×bb,b (C5H4), 114.5, 122.2, 122.5, 122.9,
1
C5H4), 5.86 (s, 5H, C5H5), 6.25 (m, 2H, C5H4). H NMR (300
MHz, CDCl3, 25 °C): δ 0.31 (s, 6H, Si(CH3)2), 1.69 (d, 2H, JH-H
) 8.1, SiCH2), 4.82 (2m, 2H, dCH2), 5.71 (m, 1H, CHd), 6.34
(s, 5H, C5H5), 6.43(m, 2H, C5H4), 6.59 (m, 2H, C5H4). 13C NMR
(75 MHz, C6D6, 25 °C): δ -2.4 (Si(CH3)2), 24.9 (SiCH2), 113.9
(dCH2), 114.4 (C5H5), 114.7 (C5H4), 114.9 (Cipso-Si C5H4), 124.3
(C5H4), 134.6 (CHd). Anal. Calcd for C15H20SiCl2Hf: C, 37.73;
H, 4.23. Found: C, 37.17; H, 4.10.
Synthesis of [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}-
(CH2Ph)2] (5). A 2 M THF solution of MgCl(CH2Ph) (2.30 mL,
4.60 mmol) was added at -78 °C to a solution of 4 (1.04 g,
2.10 mmol) in Et2O (50 mL), and the suspension was stirred
for 18 h while warming slowly to room temperature. After
filtration the solvent was removed under reduced pressure and
the residue extracted with hexane to separate the white
insoluble solid. The solution was concentrated to 20 mL and
cooled to -35 °C. Complex 5 was collected as a yellow solid
123.0, 126.9, 128.3, 128.6, 129.4, a,b(C5H6), 141.5×bb/141.6 (Cipso
b
C5H6). 19F NMR (CD2Cl2, -20 °C): δ -167.3 (m-F), -164.3 (p-
F), -120.9 (o-F).
Synthesis of [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}-
(CH2Ph)(OtBu)] (9). Toluene (20 mL) was added at -78 °C
to a mixture of 5 (0.24 g, 0.40 mmol) and B(C6F5)3 (0.21 g, 0.40
mmol), and the reaction mixture was stirred for an additional
1 h at this temperature. The resulting clear yellow solution
was then treated with a THF solution of KOtBu (0.40 mL, 0.40
mmol) at -78 °C. The reaction mixture was allowed to warm
to room temperature and stirred for 18 h. The solvent was
removed at reduced pressure, and the residue was extracted
into hexane (20 mL). After filtration, the solvent was removed
at reduced pressure and the residue was dried under vacuum
to give a yellow oily solid characterized as 9 (0.09 g, 0.15 mmol,
1
and dried under vacuum (0.42 g, 0.70 mmol, 45%). H NMR
(300 MHz, C6D6, 25 °C): δ 0.05 (s, 6H, Si(CH3)2), 1.69 (m, 4H,
HfCH2), 1.74 (d, 2H, JH-H ) 7.9, SiCH2), 4.88 (2m, 2H, dCH2),
5.58 (m, 1H, CHd), 5.61 (s, 5H, C5H5), 5.79 (m, 2H, C5H4), 5.82
(m, 2H, C5H4), 6.93 (m, 6H, Hmeta, Hpara C6H5), 7.26 (m, 4H,
Hortho C6H5). 1H NMR (500 MHz, CD2Cl2, 25 °C): δ 0.27 (s,
6H, Si(CH3)2), 1.54 (d, 2H, JH-H ) 10.7, HfCH2), 1.61 (d, 2H,
JH-H ) 10.7, HfCH2), 1.67 (d, 2H, JH-H ) 7.9, SiCH2), 4.89
(m, 2H, dCH2), 5.75 (m, 1H, CHd), 5.89 (s, 5H, C5H5), 6.04
(m, 2H, C5H4), 6.20 (m, 2H, C5H4), 6.83 (m, 2H, Hpara C6H5),
6.84 (m, 4H, Hmeta C6H5), 7.21 (m, 4H, Hortho C6H5). 13C NMR
(75 MHz, CD2Cl2, 25 °C): δ -2.5 (Si(CH3)2), 25.1 (SiCH2), 65.3
(CH2Ph), 111.8 (C5H5), 113.1 (dCH2), 114.1 (Cipso-Si C5H4),
116.0 (C5H4), 119.9 (C5H4), 121.7, 126.7, 128.5 (C6H5), 134.3
(CHd), 152.5 (Cipso C6H5). Anal. Calcd for C29H34SiHf: C, 59.02;
H, 5.97. Found: C, 59.23; H, 6.00.
Synthesis of [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}-
Me2] (6). A 1.6 M Et2O solution of methyllithium (2.60 mL,
4.20 mmol) was added at -78 °C to a solution of 4 (1.04 g,
2.00 mmol) in Et2O (20 mL).The reaction mixture was stirred
for 18 h while it was warmed slowly to room temperature. The
solvent was removed under vacuum, and the residue was
extracted into hexane (30 mL). After solvent removal under
reduced pressure the residue was washed with cold hexane
(-78 °C) and complex 6 (0.43 g, 0.98 mmol, 45%) was obtained
as a colorless oily solid, which could not be crystallized. 1H
NMR (300 MHz, C6D6, 25 °C): δ -0.29 (s, 6H, HfMe2), 0.14 (s,
6H, Si(CH3)2), 1.58 (d, 2H, JH-H ) 7.9, SiCH2), 4.92 (2m, 2H,
dCH2), 5.38 (m, 2H, C5H4), 5.63 (m, 1H, CHd), 5.71 (s, 5H,
C5H5), 5.90 (m, 2H, C5H4). 13C NMR (75 MHz, C6D6, 25 °C): δ
-2.5 (Si(CH3)2), 25.3 (SiCH2), 36.9 (HfMe2), 110.1 (C5H5), 110.3
(C5H4), 113.6 (dCH2), 113.7 (Cipso-Si C5H4), 129.3 (C5H4), 134.8
(CHd). Anal. Calcd for C17H26SiHf: C, 46.35; H, 6.05. Found:
C, 46.72; H, 5.99.
1
42%). H NMR (300 MHz, CD2Cl2, 25 °C): δ 0.23 (s, 3H, Si-
(CH3)2), 0.25 (s, 3H, Si(CH3)2), 1.07 (s, 9H, tBuO), 1.61 (d, 2H,
J ) 7.9, SiCH2), 2.13 (d, 1H, J ) 11.4, CH2Ph), 2.32 (d, 1H, J
) 11.4, CH2Ph), 4.93 (m, 2H, dCH2), 5.70 (m, 1H, CHd), 5.74
(s, 5H, C5H5), 5.81 (m, 1H, C5H4), 5.92 (m, 2H, C5H4), 5.95 (m,
1H, C5H4), 6.99 (m, 1H, Hpara C6H5), 7.16 (m, 2H, Hmeta C6H5),
7.30 (m, 2H, Hortho C6H5). 13C NMR (75 MHz, CD2Cl2, 25 °C):
δ -1.9 (Si(CH3)2), -1.8 (Si(CH3)2), 25.6 (SiCH2), 32.3 (CMe3-
tBu), 50.3 (CMe3-tBu), 70.8 (CH2Ph), 110.4 (C5H5), 112.3 (d
CH2), 114.5 (Cipso-SiC5H4), 119.9 (C5H4), 121.2 (C5H4), 121.5
(C5H4), 125.6 (C5H4), 126.1 (Cpara C6H5), 128.5 (Cortho C6H5),
128.7 (Cmeta C6H5), 134.7 (dCH), 154.2 (Cipso C6H5). Anal. Calcd
for C26H36SiOHf: C, 54.70; H, 6.30. Found: C, 53.83; H, 6.24.
Synthesis of [Hf(η5-C5H5){η5-C5H4SiMe2[CH2CH(CH2Ph)-
η1-CH2]}(OtBu)] (10). The same procedure described to
prepare 9 was followed using 5 (0.24 g, 0.40 mmol) and
B(C6F5)3 (0.21 g, 0.40 mmol). The reaction mixture was stirred
for an additional 2 h at -20 °C, and then a THF solution of
KOtBu (0.40 mL, 0.40 mmol) was added to give 10 as a yellow-
orange oil (0.06 g, 0.12 mmol, 30%). 1H NMR (300 MHz, CD2-
Cl2, 25 °C): δ 0.06 (s, 3H, Si(CH3)2), 0.21 (s, 3H, Si(CH3)2),
0.40 (m, 1H, CH2Hf), 0.75 (m, 1H, SiCH2), 0.90 (m, 1H, SiCH2),
1.02 (m, 9H, tBuO), 1.25 (m, 1H, CH2Hf), 2.05 (m, 1H, CH),
2.65 (q, 1H, J ) 13.1, 11.8, CH2Ph), 2.80 (q, 1H, J ) 13.1, 11.8,
CH2Ph), 5.28 (m, 1H, C5H4), 5.68 (m, 5H, C5H5), 5.90 (m, 1H,
C5H4), 5.93 (m, 1H, C5H4), 6.20 (m, 1H, C5H4), 6.99 (m, 2H,
Hmeta C6H5), 7.05 (m, 1H, Hpara C6H5), 7.25 (m, 2H, Hortho C6H5).
13C NMR (75 MHz, CD2Cl2, 25 °C): δ -3.6 (SiCH3), 0.6 (SiCH3),
24.8 (SiCH2), 31.7 (CMe3 (tBu)), 38.1 (CH), 50.1 (CMe3 (tBu)),
50.7 (HfCH2), 54.3 (CH2Ph), 105.4 (C5H4), 109.8 (C5H5), 115.1
(C5H4), 115.8 (C5H4), 119.6 (C5H4), 126.1 (Cmeta C6H5), 129.2
(Cpara C6H5), 129.9 (Cortho C6H5), 140.2 (Cipso-Si C5H4), 140.2
(Cipso C6H5). Anal. Calcd for C26H36SiOHf: C, 54.70; H, 6.30.
Found: C, 53.21; H, 5.90.
NMR Characterization of [Hf(η5-C5H5){η5-C5H4SiMe2-
(CH2-η2-CHdCH2)}(CH2Ph)] [(CH2Ph)B(C6F5)3] (7) and
[Hf(η5-C5H5){η5-C5H4SiMe2[CH2CH(CH2Ph)-η1-CH2]}] [(CH2-
Ph)B(C6F5)3] (8). An equimolar mixture of 5 (20.00 mg, 0.037
mmol) and B(C6F5)3 (19.00 mg, 0.037 mmol) was charged into
a Teflon-valved NMR tube, and after being cooled to -78 °C
1
CD2Cl2 was transferred under vacuum to give complex 7. H
NMR (500 MHz, CD2Cl2, -80 °C): δ 0.27 (s, 3H, Si(CH3)2),
0.52 (s, 3H, Si(CH3)2), 2.19 (m, 2H, SiCH2), 2.38 (m, 1H, d
CHHtrans), 2.59 (d, 1H, JH-H ) 10.7, HfCH2), 2.69 (s. b., CH2B),
2.97 (d, 1H, JH-H ) 10.7, HfCH2), 3.39 (m, 1H, dCHHcis), 5.36
(m, 1H, C5H4), 6.07 (s, 5H, C5H5), 6.29 (m, 1H, C5H4), 6.63 (m,
1H, C5H4), 6.65 (m, 1H, C5H4), 7.35 (m, 1H, CHid), 6.61, 6.80-
6.90, 7.09-7.15, 7.20, 7.50 (C6H5). 13C NMR (75 MHz, CD2-
Cl2, -80 °C): δ 1.3 (Si(CH3)2), 2.1 (Si(CH3)2), 28.1 (SiCH2), 45.7
(HfCH2), 92.3 (dCH2), 109.2 (C5H5), 108.2, 108.5, 110.0, 112.5,
114.6, 118.5, 118.9, 119.7, 149.1 (C5H4+C6H5), 171.1 (CHd).
Results and Discussion
Mixed metallocenes containing two different cyclo-
pentadienyl rings may be prepared by any of the two

Allyldimethylsilylcyclopentadienyl Hf Complexes
Organometallics, Vol. 24, No. 20, 2005 4785
Scheme 1
alternative methods based on reactions of monocyclo-
pentadienylmetal halides with a cyclopentadienyl-trans-
fer reagent. Reactions of either [M{η5-C5H4SiMe2(CH2-
CHdCH2)}Cl3] with Li(C5R5) and Na(C5R5) or [M(η5-
C5R5)Cl3] with Li[C5H4SiMe2(CH2CHdCH2)] may there-
fore be appropriate methods to isolate the group 4
metallocenes [M(η5-C5R5){η5-C5H4SiMe2(CH2CHdCH2)}-
Cl2]. The monocyclopentadienyl titanium complex was
isolated39 by dechlorosilylation of the symmetric disi-
lylcyclopentadiene C5H4[SiMe2(CH2CHdCH2)]2 with
TiCl4, whereas a similar reaction of the asymmetric
C5H4(SiMe3)[SiMe2(CH2CHdCH2)] was not a convenient
method, as it afforded a mixture of the two possible
desilylation products, [Ti{η5-C5H4SiMe2(CH2CHdCH2)}-
Cl3] and [Ti(η5-C5H4SiMe3)Cl3]. However no reaction
was observed when the same procedure was carried out
using the less reactive zirconium and hafnium halides
(MCl4). Transmetalation of these compounds with the
corresponding lithium cyclopentadienide Li[η5-C5H4-
SiMe2(CH2CHdCH2)] was also unsuccessful, as it pro-
vided only the dicyclopentadienyl derivative.
The related tin compounds are usually better cyclo-
pentadienyl-transfer reagents and have been exten-
sively used43-45 to prepare many mono- and dicyclopen-
tadienyl transition metal complexes. The mixed silyl-
stannyl-cyclopentadiene C5H4[SiMe2(CH2CHdCH2)][Sn-
(nBu)3] (1) was prepared by reaction of the lithium salt
Li[C5H4{SiMe2(CH2CHdCH2)}] with 1 equiv of the
chlorotrialkyltin as shown in Scheme 1 and isolated as
a yellow liquid, which was characterized by NMR
spectroscopy as a single 1-silyl-1-stannylcyclopentadi-
enyl isomer.
using the zirconium halide ZrCl4, making this method
unsuitable to prepare the related monocyclopentadienyl
zirconium complex. As shown in Scheme 1, benzylation
of 2 with 3 equiv of MgCl(CH2Ph) afforded the tribenzyl
compound [Hf{η5-C5H4SiMe2(CH2CHdCH2)}(CH2Ph)3]
(3), which was isolated as a highly air sensitive yellow
oily solid, soluble in all of the usual organic solvents. It
was also a thermally unstable compound, which could
not be stored for long periods at room temperature
under an inert atmosphere. Compound 3 was character-
ized by elemental analysis and NMR spectroscopy.
Using the monocyclopentadienyl hafnium compounds
[Hf{η5-C5H4SiMe2(CH2CHdCH2)}Cl3‚DME] (2) and [Hf-
(η5-C5H5)Cl3‚DME],42 alternative methods may be fol-
lowed to isolate the mixed hafnocene complex by reac-
tion with the Li(C5H5) or Na(C5H5) and Li[C5H4SiMe2-
(CH2CHdCH2)] salts, respectively. However the silyl-
ated compound 2 resulted in a less advantageous and
experimentally more demanding method, which always
gave lower yields of the required metallocene. The most
efficient method used the lithium salt of the allylsilyl-
cyclopentadiene, which when reacted with the adduct
[Hf(η5-C5H5)Cl3‚DME], gave the hafnocene compound
[Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}Cl2] (4), as
shown in Scheme 1. Complex 4 was isolated as a white
solid and was characterized by elemental analysis and
NMR spectroscopy.
Alkylation of the dichlorometallocene 4 with 2 equiv
of MgCl(CH2Ph) afforded the dibenzyl hafnium deriva-
tive [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}(CH2-
Ph)2] (5), and similar methylation with LiMe afforded
the dimethyl derivative [Hf(η5-C5H5){η5-C5H4SiMe2(CH2-
CHdCH2)}Me2] (6). Formation of a monomethyl com-
plex similar to the titanium derivative reported previ-
ously39 was not observed when 1 equiv of the alkylating
agent was used.
Reaction of 1 with 1 equiv of the hafnium adduct
HfCl4(SMe2)2 in dichloromethane and further addition
of DME afforded the DME adduct of the hafnium
monocyclopentadienyl complex [Hf{η5-C5H4SiMe2(CH2-
CHdCH2)}Cl3‚DME] (2), isolated as a grayish white
solid and characterized by elemental analysis and NMR
spectroscopy (see Experimental Section). No reaction
was observed when a similar procedure was followed
Formation and Dynamic Behavior of Cationic
Species. Reaction of the dibenzyl hafnocene complex 5
with B(C6F5)3 was carried out in CD2Cl2 using a Teflon-
1
valved NMR tube and monitored by H, 13C, and 19F
NMR spectroscopy at variable temperature.
Addition of 1 equiv of B(C6F5)3 at -78 °C gave the
alkene-coordinated species [Hf(η5-C5H5){η5-C5H4SiMe2-
(CH2-η2-CHdCH2)}(CH2Ph)]+[(CH2Ph)B(C6F5)3]- (7) (see
Scheme 2). The 1H NMR spectrum of 7 at -80 °C shows
the expected two SiMe singlets (δ 0.27, 0.52) and four
(43) Chirik, P. J.; Zubris, D. L.; Ackerman, L. J.; Henling, L. M.;
Day, M. W.; Bercaw, J. E. Organometallics 2003, 22, 172-187.
(44) Huttenhofer, M.; Weeber, A.; Brintzinger, H.-H. J. Organomet.
Chem. 2002, 663, 58-62.
(45) Larkin, S. A.; Golden, J. T.; Shapiro, P. J.; Yap, G. P. A.; Foo,
D. M. J.; Rheingold, A. L. Organometallics 1996, 15, 2393-2398.

4786 Organometallics, Vol. 24, No. 20, 2005
Mart´ınez and Royo
Scheme 2
resonances (δ 5.36, 6.29, 6.63, 6.65) for the silyl-
substituted cyclopentadienyl ring protons corresponding
to an asymmetric cation and a broad methylene reso-
nance (δ 2.69) characteristic of the benzylborate [(CH2-
Ph)B(C6F5)3]- anion. The 19F NMR spectrum shows
signals for the meta (δ -169.3), para (δ -164.9), and
ortho (δ -120.9) fluorine atoms with a difference ∆δ-
(meta- - para-F) ) 4.4, consistent46 with the presence
of the noncoordinated borate anion. The methylenic
protons of the metal-bonded benzyl ligand are observed
as an AB spin system shifted downfield (δ 2.59, 2.97)
with respect to the value observed for the precursor
neutral compound 5, whereas the terminal vinylic Ht
and Hc signals of the allyl moiety are shifted high-field
(δ 2.38 and 3.39) and the internal Hi signal is shifted
downfield (δ 7.35), demonstrating the high degree of
polarization of the coordinated alkene known25,35 for this
type of compound. Similar behavior was observed in the
13C NMR spectrum, which shows the resonances for the
external (dCH2) and internal (-CHd) carbon atoms
shifted high-field [δ 92.3, ∆δ ) δ(7) - δ(5) ) -20.8] and
downfield [δ 171.1, ∆δ ) 36.8], respectively, with respect
to those found for compound 5.
The NMR behavior observed at variable temperature
between -80 and -45 °C for the hafnocenium cation 7
was similar to that found35 for the related zirconocenium
derivative, with both showing the broadening of signals
expected for rapid interconversion between the endo-
exo diastereomers, formed by dissociation of the alkene
followed by recomplexation to the opposite alkene enan-
tioface, on the NMR time scale. Variable-temperature
1H NMR spectra between -80 and -10 °C for the
zirconocenium cation were also consistent with revers-
ible interconversion between the two enantiomers formed
by inversion at zirconium, with a combined barrier for
alkene dissociation and zirconium inversion estimated35
at 11.7 kcal mol-1. However, as expected from the higher
energetic barriers evaluated by DFT calculations,47 the
same dynamic process was much slower for the hafno-
cenium cation 7, for which a different irreversible
transformation before coalescence was observed at -40
°C.
When a solution of 7 was warmed to -40 °C, a slow
and irreversible transformation into a new species, [Hf-
(η5-C5H5){η5-C5H4SiMe2[CH2CH(CH2Ph)-η1-CH2]}]+[(CH2-
Ph)B(C6F5)3]- (8), was observed (Scheme 2). This trans-
formation was almost quantitative after 30 min, or in
shorter periods at temperatures higher than -40 °C,
and 8 was the unique species present in solution at -20
°C. The complex could not be isolated because decom-
position to unidentified products took place rapidly at
temperatures higher than -10 °C.
Compound 8 exhibited two stereogenic centers at
hafnium and at the benzyl-bound tertiary carbon atom
1
and was characterized by H, 13C, 19F, DEPT, HOMO-
DEC, TOCSY-1D, and HMQC-2D NMR techniques (see
Supporting Information) as a mixture of two diastere-
omers 8a and 8b in a 1:1 molar ratio of the metallacyclic
cation, resulting from the migratory insertion of the
alkene into the hafnium-benzyl bond in both enantio-
mers of the endo-diastereoisomer, giving two series of
close and identical signals in the 1H and 13C NMR
spectra. A slight stabilization by a weak interaction of
this coordinatively unsaturated cation with the ben-
zylborate anion is consistent with the 19F NMR spec-
trum, which shows resonances for meta- (δ -167.3),
para- (δ -164.3), and ortho- (δ -120.9) fluorine atoms
with a value of ∆δ(meta- - para-F) ) 3.0. Both diaster-
eomers are easily distinguished by the singlets observed
for the cyclopentadienyl ring (C5H5) and the silicon-
methyl (Si-CH3) protons (see Experimental Section).
The most relevant feature of these spectra is the absence
of the low-field signal for the internal olefinic -CHd
group, whereas signals [δ 1.93 (1H) and 42.3-42.6 (13C)]
are observed for a tertiary CH group.
The structural assignments discussed above were
confirmed when these intermediate cationic species 7
and 8 were trapped as the stable neutral complexes by
addition of the strong nucleophile KOtBu.37 Addition of
1 equiv of KOtBu to a toluene solution containing an
equimolar mixture of complex 7 and B(C6F5)3 at -80
(46) Horton, A. D.; deWith, J. Chem. Commun. 1996, 1375-1376.
(47) Nicolas, P.; Royo, P.; Galakhov, M. V.; Blacque, O.; Jacobsen,
H.; Berke, H. Dalton Trans. 2004, 2943-2951.

Allyldimethylsilylcyclopentadienyl Hf Complexes
Organometallics, Vol. 24, No. 20, 2005 4787
Figure 1. TOCSY-1D NMR spectrum for the major 10 (a) and minor 10 (b) isomers.
°C gave the tert-butoxo complex [Hf(η5-C5H5){η5-C5H4-
SiMe2(CH2CHdCH2)}(CH2Ph)(OtBu)] 9 (Scheme 2) as
a yellow oily solid after purification. Similar addition
of KOtBu, after maintaining the solution at -20 °C for
1 h, gave the tert-butoxo compound [Hf(η5-C5H5){η5-
C5H4SiMe2[CH2CH(CH2Ph)-η1-CH2]}(OtBu)], 10 (see
Scheme 2), which was isolated as a yellow-orange oil.
In both reactions the nucleophile is bound to the metal,
easily displacing the very weakly coordinated allylic
alkene moiety of 7 and the borate anion of 8. Complexes
9 and 10 could not be crystallized and were isolated as
oily products, for which unsatisfactory analytical data
were found for samples characterized as spectroscopi-
cally pure compounds by NMR spectroscopy.
Conclusions
New chloro and alkyl mono-[Hf{η5-C5H4SiMe2(CH2-
CHdCH2)}X3] (X ) Cl, CH2Ph) and dicyclopentadienyl
[Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHdCH2)}X2] (X ) Cl,
Me, CH2Ph) hafnium complexes containing a silyl-
substituted cyclopentadienyl ligand with a pendant allyl
moiety have been isolated and characterized by elemen-
tal analyses and NMR spectroscopy.
The dibenzyl hafnium complex reacts with the Lewis
acid B(C6F5)3 at -80 °C to generate the cationic spe-
cies [Hf(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CHdCH2)}(CH2-
Ph)]+, which exhibits dynamic behavior similar to that
known for the zirconium derivative between -80 and
-45 °C, as monitored by 1H and 13C NMR spectroscopy.
However at temperatures higher than -45 °C this
cationic species is irreversibly transformed into the
metallacyclic alkyl cation [Hf(η5-C5H5){η5-C5H4SiMe2-
CH2CH(CH2Ph)-η1-CH2}]+ by migratory insertion of the
alkene into the metal-benzyl bond.
Both cationic species were characterized by NMR
techniques, and their structures were confirmed after
being trapped by reactions with KOtBu. In both cases
the nucleophile displaces the coordinated olefin to form
[Hf(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CHdCH2)}(OtBu)-
(CH2Ph)] or the borate anion to yield [Hf(η5-C5H5){η5-
C5H4SiMe2CH2CH(CH2Ph)-η1-CH2}(OtBu)].
Complex 9 shows the typical vinylic signals of the free
allyl moiety (δ 4.93 and 5.70) with the signals for the
diastereotopic methylenic protons of the benzyl ligand
shifted downfield (δ 2.13, 2.32) with respect to those
observed for 7.
Compound 10 shows the two stereogenic centers
observed for its precursor complex 8 and was character-
ized by 1H, 13C, DEPT, TOCSY-1D, and HMQC-2D
NMR techniques as a mixture of two diastereomers. The
1H NMR spectrum of the major component 10(a) shows
two SiCH3 (δ 0.06, δ 0.21) and four silylcyclopentadienyl
ring proton resonances (δ 5.28, 5.90, 5.93, 6.20) along
with signals due to the aliphatic chain protons (see
Experimental Section), whereas almost undetectable
amounts of a second diastereomer 10(b) were present.
As shown in Figure 1, the TOCSY-1D NMR spectrum
of 10(a) shows the expected seven signals of the cyclic
alkyl system, whereas only six were observed for the
minor component 10(b), as the signals due to the H4
and H5 protons overlapped, probably due to the aniso-
tropic effect of the phenyl ring.
Acknowledgment. We gratefully acknowledge Min-
isterio de Educacio´n y Ciencia (Project MAT2004-02614)
for financial support of our work. G.M. acknowledges
MEC for a Fellowship.
1
Supporting Information Available: Figures giving H
NMR spectra for 7, 8, and 10 and TOCSY-1D and HMQC-2D
for 8 and 10 are available free of charge via the Internet at
http://pubs.acs.org.
OM050437L