Cationic Group 4 Complexes (M = Ti, Zr, Hf): Modifications and Limitations in the Design of Tridentate Cp,O,P-Ligand Frameworks Built Directly in the Coordination Sphere of the Metal

The reactions of monopentafulvene complexes Ti1, Zr1, and Hf1 with bidentate O,P-ligand precursors L1–L3 to form the corresponding cationic complexes employing an established three-step synthetic protocol [insertion, methylation, activation with B(C6F5)3] are investigated. Ligands L1–L3 are designed to have different sized spacers between the carbonyl and diphenylphosphine functional groups. The attempts to react Ti1, Zr1, and Hf1 with acetyldiphenylphosphine (L1) proved to yield undesired products at various steps in the synthetic sequence. When Ti1 is used, Ti2 is formed and diphenylphosphine is released at the same time. Compound Ti2, with the exocyclic double bond, is the formal product of insertion of the smallest ketene (H2C=C=O) into the Ti–Cexo bond. Starting with Zr1 results in isolation of the insertion product Zr2 without loss of diphenylphosphine, but a byproduct is formed during the reaction with L1. Subsequent methylation with methyllithium yields a complex reaction mixture. Hf1 reacts cleanly with L1 to the insertion product Hf2. Also, the methylation reaction selectively yields Hf3 as the result of chloride/methyl exchange, but final activation with B(C6F5)3 causes decomposition and release of diphenylphosphine. The use of the ligand precursors L2 and L3 with two methylene groups or an aryl group as linkers between the functional groups selectively provides the desired cationic complexes Ti6, Zr6, Hf6, and Ti9 in good to excellent overall yields.