Synthesis, characterization, and polymerization activity of the scandium half-sandwich complex [Sc(η5-C5Me 4{SiMe2(C6F5)})(CH 2SiMe3)2(THF)]

Article abstract of DOI:10.1002/zaac.200600144

The scandium half-sandwich (bis)alkyl complex [Sc(η⁵-C ₅Me₄{SiMe₂(C₆F₅)}) (CH₂SiMe₃)₂(THF)] has been prepared by σ-bond metathesis between [Sc(CH₂SiMe₃) ₃(THF)₂] and C₅Me₄H{SiMe ₂(C₆F₅)}. Structural characterization by single crystal X-ray diffraction revealed the typical piano-stool geometry for this type of complex. The addition of [Ph₃C][B(C₆F ₅)] resulted in the formation of a half-sandwich alkyl cation which is an efficient catalyst for the syndiospecific polymerization of styrene in the presence of aluminium alkyls.

Full text of DOI:10.1002/zaac.200600144

DOI: 10.1002/zaac.200600144
Synthesis, Characterization, and Polymerization Activity of the Scandium
Half-Sandwich Complex [Sc(η5-C5Me4{SiMe2(C6F5)})(CH2SiMe3)2(THF)]1)
Julia Hitzbleck and Jun Okuda*
Aachen, Institut für Anorganische Chemie, RWTH Aachen
Received May 5th, 2006.
Dedicated to Professor Glen B. Deacon on the Occasion of his 70th Birthday
Abstract. The scandium half-sandwich (bis)alkyl complex [Sc(η5-
C5Me4{SiMe2(C6F5)})(CH2SiMe3)2(THF)] has been prepared
by σ-bond metathesis between [Sc(CH2SiMe3)3(THF)2] and
C5Me4H{SiMe2(C6F5)}. Structural characterization by single cry-
stal X-ray diffraction revealed the typical piano-stool geometry for
this type of complex. The addition of [Ph3C][B(C6F5)4] resulted in
the formation of a half-sandwich alkyl cation which is an efficient
catalyst for the syndiospecific polymerization of styrene in the pre-
sence of aluminium alkyls.
Keywords: Rare-earth metals; Half-sandwich compounds; Homoge-
neous catalysis
The structural investigation of rare-earth metal half-sandwich
(bis)alkyl complexes has received attention in the last few years
[1Ϫ3], due to their isostructural relationship with the cationic
titanium(III) complex [CpЈTiMe]ϩ, which has been proposed as the
active species in syndiospecific styrene polymerization by the com-
mercially used titanium catalyst systems CpЈTiCl3/MAO (CpЈ ϭ
C5H5 or C5Me5, MAO ϭ methylalumoxane) [4]. The latter catalysts
only show poor activity for the copolymerization of styrene with
ethylene [5], resulting from the different oxidation states of the
active species [6]. An improved system [Sc(η5-C5Me4SiMe3)(CH2Si-
Me3)2(THF)]/[Ph3C][B(C6F5)4] was recently reported showing high
activity and stereoselectivity in the homo-/copolymerization of
styrene with ethylene [7].
vestigation by extending the series of structurally characterized
scandium, yttrium and lutetium compounds as precatalysts of type
[(η5-C5Me4{SiMe2R})Ln(CH2SiMe3)2(THF)] (Ln ϭ Sc, Y, Lu)
with different dimethylsilyl-substituents on the cyclopentadienyl
ring including R ϭ Me, Ph, 2-pyridyl, C6F5, 2-furyl, 2-(furyl-5-Me)
[11]. These compounds show reasonably good stability as crystal-
line solids as well as in solution. Catalyst activation with
[Ph3C][B(C6F5)4] in toluene results in highly sensitive, charge sepa-
rated species [7]. We report on the synthesis and structural charac-
terization of such a scandium half-sandwich (bis)alkyl complex and
its activity in syndiospecific styrene polymerization.
Results and Discussion
Interest in co- and terpolymers of styrene with a variety of α-olefins
stems from their improved mechanical properties during processing
compared to the brittleness of syndiotactic polystyrene (sPS) [8].
Nonetheless it is a promising polymer material due to its high melt-
ing temperature and crystallinity as well as exceptional heat and
chemical resistance [9].
The scandium half-sandwich (bis)alkyl complex [Sc(η5-
C5Me4{SiMe2(C6F5)})(CH2SiMe3)2(THF)]
1 was obtained by
σ-bond metathesis in pentane between the scandium tris(trimethyl-
silylmethyl) complex [Sc(CH2SiMe3)3(THF)2] and the substituted
cyclopentadiene C5Me4H{SiMe2(C6F5)} (Scheme 1) [2, 3]. Recrys-
tallization from pentane at Ϫ40 °C afforded the complex as a color-
less crystalline solid in good yield.
Analogous rare-earth metal half-sandwich complexes have been
prepared previously by our group [3, 10], thus prompting their rein-
* Prof. Dr. Jun Okuda
Institut für Anorganische Chemie
RWTH Aachen
Landoltweg 1
D-52074 Aachen
Scheme 1
Fax 0241 80 92644
E-mail jun.okuda@ac.rwth-aachen.de
Complex 1 has been fully characterized by analytical and spectro-
scopic methods. Determination of the solid-state structure by X-ray
crystallography revealed a trigonal pyramidal structure as shown in
Figure 1.
1) Paper presented at the XVIIIth Tage der Seltenen Erden (Terrae
Rarae 2005) at Bonn-Röttgen/Germany, November 30th Ϫ Decem-
ber 2nd, 2005 (www.Terrae-rarae.de).
Z. Anorg. Allg. Chem. 2006, 632, 1947Ϫ1949
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1947

J. Hitzbleck, J. Okuda
Experimental Section
All operations were carried out under argon using standard
Schlenk-line and glovebox techniques. Pentane was distilled from
sodium/triglyme benzophenone ketyl under argon. NMR spectra
were recorded on Bruker Avance II (1H, 400.1 MHz; 13C,
100.6 MHz) and Varian Mercury (19F, 188.1 MHz) spectrometers
in [D6]benzene at 25 °C; the chemical shifts were referenced to the
residual solvent resonances. Metal analysis was performed by com-
plexometric titration [12].
[Sc(η5-C5Me4{SiMe2(C6F5)})(CH2SiMe3)2(THF)] (1).
A solution of C5Me4H{SiMe2(C6F5)} (0.70 g, 2.0 mmol) in pentane
(10 mL) was added to [Sc(CH2SiMe3)3(THF)2] [13] (0.90 g,
2.0 mmol) and stirred at room temperature for 3 h. The solution
was filtered and the solvent volume reduced to ϳ 4 mL. Cooling
to -40 °C gave colorless crystals of 1 (1.03 g, 1.6 mmol, 81 %).
C29H48F5OScSi3 (636.90) Sc 7.06 (calc. 6.98) %.
1H NMR δ ϭ Ϫ0.27 (d, 4H, CH2SiMe3, 14.7 Hz), 0.27 (s, 18H, CH2SiMe3),
0.84 (t, 6H, SiMe2, 1.6 Hz), 1.18 (bs, 4H, β-THF), 1.78, 2.20 (s, 2 x 6H,
C5Me4), 3.59 (t, 4H, α-THF, 6.4 Hz). 13C{1H} NMR δ ϭ 2.6 (SiMe2), 4.3
(CH2SiMe3), 11.7, 14.8 (C5Me4), 25.1 (β-THF), 41.2 (b, Sc-CH2), 71.2 (α-
THF), 111.8 (ipso-C5Me4), 112.2 (ipso-C6F5), 124.7, 128.5 (C5Me4), 136.2,
148.8, 140.6, 143.1, 147.9, 150.3 (m, C6F5); 19F NMR (C6D6) δ ϭ Ϫ161.2
(m, 2F, o-C6F5), Ϫ152.6 (t, 1F, p-C6F5, 20.8 Hz), Ϫ126.8 (dd, 2F, m-C6F5,
9.8 Hz).
Fig.
1
Molecular structure of [Sc(η5-C5Me4{SiMe2(C6F5)})-
(CH2SiMe3)2(THF)] (1).
Crystal structure determination of 1
˚
Selected bond distances /A and angles /°: Sc(1)-C(24, 29) 2.201(2), 2.263(2);
Sc(1)-O(34) 2.164(2); Sc(1)-C(1-5) 2.463(2)-2.542(2); C(24)-Sc(1)-C(29)
106.36(7); O(34)-Sc(1)-C(24) 100.96(7); O(34)-Sc(1)-C(29) 97.10(6).
A
colorless fragment (0.48
ϫ
0.47
ϫ
0.43 mm) of
1
C29H48F5OScSi3 (636.90 g molϪ1) was selected for data collection
˚
˚
at Ϫ153 °C. Monoclinic, P21/c, a ϭ 11.378(3) A, b ϭ 12.267(3) A,
3
˚
˚
c ϭ 25.898(6) A, βϭ 106.83(1)°, V ϭ 3459.9(15) A , Z ϭ 4, ρcalcd ϭ
1.223 g cmϪ3, µ ϭ 0.363 mmϪ1. X-ray diffraction data was col-
lected on a Bruker CCD area-detector diffractometer with Mo-KͰ
˚
radiation (graphite monochromator, λ ϭ 0.71073 A) using ϕ and
The scandium half-sandwich complex 1 displays the commonly ob-
served piano-stool geometry in the solid-state, with an η5-coordi-
nated cyclopentadienyl ring in the apical position of a trigonal
pyramid and the remaining two alkyl ligands and the THF donor
in the basal plane (Figure 1). Noteworthy is the preferential coordi-
nation of the THF donor trans to the silyl-substitutent of the cyclo-
pentadienide ring. This arrangement has been observed before in
the related complex [Sc(η5-C5Me4{SiMe3})·(CH2SiMe3)2(THF)],
ω scans, θmax ϭ 27.54°. Indices: Ϫ14<ϭh<ϭ14, Ϫ15<ϭk<ϭ15,
Ϫ33<ϭl<ϭ33; F(000) 1352; 50070 reflections collected, 7960
unique [R(int) ϭ 0.0434], 7105 [I > 2σ(I)]. The SMART program
package was used for the data collection and unit cell determi-
nation; processing of the raw frame data was performed using
SAINT; absorption corrections were applied with SADABS [14].
Structure solution and refinement against F02 using direct methods
with SHELXL-97 [15] led to R1 ϭ 0.0469 [I > 2σ(I)] and wR2 ϭ
0.1137 for all 7960 independent reflections.
˚
which shows nearly identical structural features (Sc-O 2.158(2) A,
˚
˚
˚
Sc-C 2.206(2), 2.240(2) A; Sc-Cpcent 2.189 A, c.f. 2.196 A 1) [7].
Crystallographic data for the structure has been deposited with
the Cambridge Crystallographic Data Centre, CCDC 603507.
Copies of the data can be obtained free of charge on application
to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (Fax: int.code ϩ(1223)336-033; e-mail for inquiry:
fileserv@ccdc.cam.ac.uk).
Polymerization experiments of styrene, ethylene and α-olefins with
the catalyst system [Ln(η5-C5Me4{SiMe2R})(CH2SiMe3)2(THF)]/
[Ph3C][B(C6F5)4]/AlRЈ3 reveal that the polymerization activity
strongly depends on metal size (Ln ϭ Sc > Y ϳ Lu), but less on
the nature of the substituent R (TMS ϳ C6F5 > Ph > 2-py >
2-furyl). Moreover, they clearly demonstrate the crucial role of the
aluminum alkyl cocatalysts AlRЈ3 [11]. Small alkyl or hydride
groups, as in AlMe3, MAO, or AliBu2H, can potentially block
the active site of the catalyst whereas AlEt3, AliBu3, and
Al(CH2SiMe3)3 showed highly active catalyst systems. Whether this
effect is attributable to varying scavenger qualities of the different
aluminium alkyls or to their individual capability of stabilizing the
active species still remains to be elucidated. Studies towards the
structural elucidation of the corresponding cationic complexes are
in progress.
Acknowledgements. This work has been supported by the EC in the
FP6 Project: NMP3-CT-2005-516972 and the Fonds der Chem-
ischen Industrie.
[1] C. J. Schaverien, Organometallics 1992, 11, 3476; S. Arndt, J.
Okuda, Chem. Rev. 2002, 102, 1953; L. D. Henderson, G. D.
MacInnis, W. E. Piers, M. Parvez, Can. J. Chem. 2004, 82, 162.
1948
www.zaac.wiley-vch.de
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 1947Ϫ1949

Half-Sandwich Complex [Sc(η5-C5Me4{SiMe2(C6F5)})(CH2SiMe3)2(THF)]
[2] K. C. Hultzsch, T. P. Spaniol, J. Okuda, Angew. Chem. 1999,
111, 163; Angew. Chem., Int. Ed. 1999, 38, 227.
[3] S. Arndt, T. P. Spaniol, J. Okuda, Organometallics 2003, 22,
775.
[8] J. F. Craymer, R. M. Kasi, E. B. Coughlin, Polyhedron 2005,
24, 1347; P. S. Chum, W. J. Kruper, M. J. Guest, Adv. Mater.
2000, 12, 1759.
[9] N. Tomotsu, M. Malanga, J. Schellenberg, Modern Styrenic
Polymers: Polystyrenes and Styrenic Copolymers 2003, 365.
[10] K. C. Hultzsch, P. Voth, K. Beckerle, T. P. Spaniol, J. Okuda,
Organometallics 2000, 19, 228.
[11] J. Hitzbleck, K. Beckerle, J. Okuda, T. Halbach, R. Mülhaupt,
Macromol. Symp. 2006, 236, 23.
[12] B. Das, S. C. Shome, Anal. Chim. Acta 1971, 56, 483.
[13] M. F. Lappert, R. Pearce, J. Chem. Soc., Chem. Commun.
1973, 126.
[14] Siemens, ASTRO, SAINT and SADABS. Data Collection and
Processing Software for the SMART System, 1996, Madison,
WI.
[15] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, 1997, Göttingen.
[4] N. Ishihara, T. Seimiya, M. Kuramoto, M. Uoi, Macro-
molecules 1986, 19, 2464; A. Grassi, A. Zambelli, F. Laschi,
Organometallics 1996, 15, 480; A. Grassi, S. Saccheo, A. Zam-
belli, F. Laschi, Macromolecules 1998, 31, 5588.
[5] P. Longo, A. Grassi, L. Oliva, Makromol. Chem. 1990, 191,
2387; A. L. McKnight, R. M. Waymouth, Chem. Rev. 1998,
98, 2587; M. Malanga, Adv. Mat. 2000, 12, 1869; Z. Hou, Y.
Zhang, H. Tezuka, P. Xie, O. Tardif, T.-a. Koizumi, H.
Yamazaki, Y. Wakatsuki, J. Am. Chem. Soc. 2000, 122,
10533.
[6] G. Xu, D. Cheng, Macromolecules 2000, 33, 2825.
[7] Y. Luo, J. Baldamus, Z. Hou, J. Am. Chem. Soc. 2004, 126,
13910.
Z. Anorg. Allg. Chem. 2006, 1947Ϫ1949
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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