Hypersilyl-substituted complexes of group 1 and 2 metals: Syntheses, structures and use in styrene polymerisation

Alkali and alkaline-earth metal complexes with sterically demanding hypersilyl-substituted cyclopentadienyl or fluorenyl ligands [hSi-Cp or hSi-Flu; hSi = (Me3Si)(3)Si] were prepared. The crystal structures of [Li(hSi-Flu)(THF)(2)], [K(hSi-Flu)(THF)], [Ca(hSi-Flu)(2)(THF)(2)], [Sr(hSi-Flu)(2)(THF)(2)] and [Ca(hSi-Cp)(2)(THF)] were determined. All structures show the enormous steric bulk of the hypersilyl substituent, which either results in a reduction in the number of solvent molecules in the coordination sphere of the metal or in a distortion of the ligand-metal interaction. Heteroleptic calcium and strontium complexes with a benzyl ligand (2-Me2N-alpha-Me3Si-benzyl) and a sterically demanding hSi-Flu ligand were prepared and used as initiators in the polymerization of styrene. The polymers produced with the heteroleptic benzylcalcium initiator are enriched in syndiotactic sequences, although they show the same tacticities as polymers obtained with benzylcalcium initiators containing the less bulky Me3Si-Flu ligand. Introduction of the bulky hypersilyl group in heteroleptic benzylstrontium initiators resulted in a slight increase of the syndiotacticity of the polymers. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).