Exploring routes to tantalum(V) alkylidene complexes supported by amine tris(phenolate) ligands

Two basic organometallic species, tris(neopentyl)mono(neopenylidene) tantalum and pentabenzyltantalum, were evaluated as starting materials on the route to alkyl and alkylidene complexes with amine tris(phenolate) ligands. The reaction of the former with the ligand precursor proceeded via the addition of the O-H functionality to the Ta=C double bond at the first step, forming a sterically congested tetraneopentyl complex, that was characterized by X-ray crystallography. In contrast, the reaction of the less bulky pentabenzyltantalum with a variety of the ligand precursors led directly and in a high yield to the dibenzyl Ta(V) complexes. These species were surprisingly stable toward thermolysis, decomposing only at 120-130°C after prolonged periods of time. The unexpected product of this decomposition was found to be a dinuclear μ-benzylidene Ta(V) complex.