Group 4 ketimide complexes: Synthesis, reactivity and catalytic applications

Abstract

The synthesis and reactivity of Group 4 ketimide complexes, comprising bent-metallocene, half-sandwich and cyclopentadienyl-free compounds is described. Neutral compounds of general formulas C p 2 ′ M(N CR 1R 2)X and C p 2 ′ M(N CR 1R 2) 2 have been prepared by transmetallation reactions of metal halide derivatives with appropriate lithium, tin or silicon ketimide derivatives and by acid–base reactions between ketimines and anionic ligands attached to the metals. Ti(III) and cationic M(IV) bent-metallocene complexes have been mainly prepared by nitrile insertion into metal–carbon or metal–hydride bonds of bis-Cp systems. Metallacyclic ketimide complexes are also described. These were obtained as a result of oxidative addition reactions, nitrile insertions and nitrile additions to unsaturated C C bonds in bent-metallocene systems. Half-sandwich and cyclopentadienyl-free complexes were prepared aiming at the synthesis of olefin polymerization catalyst precursors through transmetallation reactions. Titanium systems containing one Cp ring and one ketimide ligand in particular have proven to be highly active ethylene, propylene and 1-hexene homopolymerization catalysts when activated by MAO. The activities reported in styrene homopolymerization are slightly lower although Cp *Ti(N C tBu 2)Cl 2 catalyses living styrene/ethylene copolymerization. This emerging role of ketimide ligands as ancillary ligands in the reactivity of early transition metals is quite recent and comes from their inertness towards insertion of unsaturated substrates in M N bonds and electrophile attack.