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 α-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 a β-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 β-abstraction product.