ansa-metallocene derivatives : XVII. Racemic and meso diastereomers of group IV metallocene derivatives with symmetrically substituted, dimethylsilanediyl-bridged ligand frameworks. Crystal structure of R,S-Me₂Si(3-t-Bu-5-MeC₅H₂)₂ZrCl₂

The formation of racemic and meso diastereomers of the disubstituted ansa-zirconocenes (CH3)2Si(1-C5H3-3-R)2ZrCl2 (R = t-butyl, trimethylsilyl, α,α-dimethylbenzyl and 1-phenylcyclohexyl) from dimethylsilanediyl-bridged, symmetrically substituted bis-cyclopentadienyl dianion derivatives and ZrCl4·2THF has been studied under a variety of conditions. Rac/meso ratios close to 1 were obtained in all cases. Similar product ratios were observed for analogous titanium and hafnium complexes with R = trimethylsilyl. A significant increase in the rac/meso product ratio was observed when an additional substituent was present next to the dimethylsilyl bridge as in (CH3)2Si(1-C5H2-3-R-5-CH3)2ZrCl2 (R = CH(CH3)2, C(CH3)3). It is suggested that intermediates with one η1-bound ring ligand control the stereochemical course of the ansa-metallocene formation. The crystal structure of R,S-(CH3)2Si(1-C5H2-3-C(CH3)3-5-CH3)2ZrCl2 is reported