Tribenzyl tantalum(V) complexes of amine bis(phenolate) ligands:: Investigation of α-abstraction vs ligand backbone β-abstraction paths

The synthesis, structure, and reactivity of tribenzyl Ta(V) complexes of amine bis(phenolate) ligands were investigated as a function of the ligands' structure: i.e., the steric bulk of the phenolate substituents and the presence of a "side arm" donor. The tribenzyl complexes were prepared by toluene-elimination reactions of Ta(CH2Ph)(5) with the ligand precursors. They were found to be of octahedral mer geometry, with the side-arm donor (if present) unbound. These complexes underwent two well-defined toluene elimination reactions determined by the structural features of the ligands. An a-abstraction reaction, leading to alkyl-alkylidene complexes, occurred only if the phenolate ortho substituents were small (H, Cl) and a side-arm donor was present. Bulkier phenolate substituents (Me) or the lack of a side-arm donor led to beta-abstraction process from the ligand backbone, forming a dibenzyl complex with a metallaazacyclopropane ring. The side-arm donor, if present, was found to bind to the metal in both paths. A bulkier ligand precursor having t-Bu groups on the two phenolate rings did not react with Ta(CH2Ph)(5) at room temperature, whereas an asymmetric "half-bulky" ligand precursor carrying a Me group and a t-Bu group on the two rings led directly to the beta-abstraction product.