Controlled Benzylation of α-Diimine Ligands Bound to Zirconium and Hafnium: An Alternative Method for Preparing Mono- and Bis(amido)M(CH2Ph)n (n=2, 3) Complexes as Catalyst Precursors for Isospecific Polymerization of α-Olefins

Reactions of M(CH2Ph)(4)(M = Zr, Hf) with various alpha-diimine ligands afforded amido-imino or diamido complexes through intramolecular benzylation of the C=N bonds of the ligands. Selective benzylation of alpha-diimine ligands, i.e., single- and double-benzylation, was accomplished by varying the substituent on the nitrogen atom of the imine moiety or the ligand backbone. Kinetic analysis of the second benzylation step indicated that the benzyl group migrated from the metal center to the C=N moiety via an ordered four-center transition state (Delta S-double dagger = -3(4) eu for 3a; -5(9) eu for 5b). Upon activation with B(C6F5)(3) or [Ph3C] [B(C6F5)(4)], amido-imino (3, 6) and diamido (2, 4) complexes became active catalysts for 1-hexene polymerization, and the resulting poly(1-hexene)s had moderate isotacticity ([mmmm], up to 90%). Polymerization of vinylcyclohexane was also catalyzed with moderate activity to give highly isotactic poly(vinylcyclohexane) ([mmmm] > 95%) via a chain-end control mechanism.