Niobium and Zirconium Complexes Incorporating Asymmetrically Substituted <i>a</i><i>nsa</i> Ligands. X-ray Crystal Structures of [Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃R)]Nb(N<i>^t</i>Bu)Cl (R = Me, <i>ⁱ</i>Pr) and [Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃R)]ZrCl₂ (R = H, Me)

The asymmetric ansa-ligand precursors (C5Me4H)SiMe2(C5H4R) (R = H (1), Me (2), SiMe3 (3), iPr (4)) and their lithium derivatives [Me2Si(C5Me4)(C5H3R)]Li2 (R = H (5), Me (6), SiMe3 (7), iPr (8)) have been prepared. The ansa-niobocene imido complexes [Me2Si(η5-C5Me4)(η5-C5H3R)]Nb(NtBu)Cl (R = H (9), Me (10), SiMe3 (11), iPr (12)) were synthesized by the reaction of Nb(NtBu)Cl3(py)2 with [Me2Si(C5Me4)(C5H3R)]Li2 in THF. The ansa-zirconocene complexes [Me2Si(η5-C5Me4)(η5-C5H3R)]ZrCl2 (R = H (13), Me (14), SiMe3 (15), iPr (16)) were prepared by the reaction of ZrCl4 and the corresponding lithiated ansa-derivative. The molecular structures of 10, 12, 13, and 14 were determined by single-crystal X-ray diffraction studies. Preliminary results for the catalytic activity of 13−16 in the polymerization of ethylene are reported.