Synthesis, characterization and catalytic behaviour of ansa-zirconocene complexes containing tetraphenylcyclopentadienyl rings: X-ray crystal structures of [Zr{Me2Si(η5-C5Ph4)(η5-C5H3R)}Cl2] (R = H, But)

The ansa-bis(cyclopentadiene) compounds, Me2Si(C5HPh4)(C5H4R) (R = H (2); But (3)), have been prepared by the reaction of C5HPh4(SiMe2Cl) (1) with Na(C5H5) or Li(C5H4But), respectively, and transformed to the di-lithium derivatives, Li2{Me2Si(C5Ph4)(C5H3R)} (R = H (4); But (5)), by the action of n-butyllithium. The ansa-zirconocene complexes, [Zr{Me2Si(η5-C5Ph4)(η5-C5H3R)}Cl2] (R = H (6); But (7)), were synthesized from the reaction of ZrCl4 with 4 or 5, respectively. Compounds 6 and 7 have been tested in the polymerization of ethylene and compared with their methyl-substituted analogues, [Zr{Me2Si(η5-C5Me4)(η5-C5H3R)}Cl2] (R = H (8); But (9)). Whilst 8 and 9 are catalytically active, the tetraphenyl-substituted complexes 6 and 7 proved to be inactive in the polymerization of ethylene. This phenomenon has been explained by DFT calculations based on the reaction intermediates in the polymerization processes involving 6 and 7, which showed that the extraction of a methyl group from the zirconocene complex to form the cationic active specie is endothermic and therefore unfavourable.