New ansa-niobocene complexes. X-ray crystal structure of [Me2Si(η5-C5H4) 2]Nb(=NtBu)Cl. Stereoselective synthesis of meso-[Me2Si(η5-C5H3SiMe 3)2]Nb(=NtBu)Cl

Article abstract of DOI:10.1021/om980561n

The ansa-niobocene complex [Me₂Si(η⁵-C₅-H₄) ₂]Nb(=N^tBu)Cl (1) has been prepared and its structure determined by single-crystal X-ray diffraction. The alkyl complexes [Me₂Si(η⁵-C₅-H₄) ₂]Nb(=N^tBu)(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(η⁵-C₅-H₄) ₂]Nb(=N^tBu)Cl (6) was prepared and isolated uniquely as the meso-isomer. Nevertheless the synthesis of Me₂Si(η⁵-C₅H₃SiMe ₃)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(η⁵-C₅H₃SiMe ₃)₂]NbCl(RC=CR) (R = Me (8), Ph (9)). Both complexes were initially isolated as mixtures of meso- and racisomers and in the case of 9 could be separated due to their differing solubilities in hexane.