New ansa-niobocene complexes.: X-ray crystal structure of [Me2Si(η5-C5H4)2]Nb (=NtBu)Cl.: Stereoselective synthesis of meso-[Me2Si(η5-C5H3SiMe3)2]Nb (=NtBu)Cl

The ansa-niobocene complex [Me2Si(eta(5)-C5H4)(2)]Nb(=(NBu)-Bu-t)Cl (1) has been prepared and its structure determined by single-crystal X-ray diffraction. The alkyl complexes [Me2Si(eta(5)-C5H4)(2)]Nb(=(NBu)-Bu-t)(R) (R = Me (2), Et (3), CH2Ph (4), CH2CH=CH2 (5)) were prepared by the reaction of 1 and the corresponding Grignard reagent. Using the trimethylsilyl-substituted ansa ligand Me2Si(C5H3SiMe3)(2), [Me2Si(eta(5)-C5H3SiMe3)(2)]Nb(=(NBu)-Bu-t)Cl (6) was prepared and isolated uniquely as the meso-isomer. Nevertheless the synthesis of [Me2Si(eta(5)-C5H3SiMe3)(2)]NbCl2 (7) yielded a mixture of meso- and rac-isomers. Reduction of 7 in the presence of ligand (RC=CR) (R = Me, Ph) gave [Me2Si(eta(5)-C5H3SiMe3)(2)]NbCl(RC=CR) (R = Me (8), Ph (9)). Both complexes were initially isolated as mixtures of meso- and rac-isomers and in the case of 9 could be separated due to their differing solubilities in hexane.