Amine, amido, and imido complexes of tantalum supported by a pyridine-linked bis(phenolate) pincer ligand: Ta-N pi-bonding influences pincer ligand geometry.

A series of tantalum imido and amido complexes supported by a pyridine-linked bis(phenolate) ligand has been synthesized. Characterization of these complexes via X-ray crystallography reveals both C(s) and C(2) binding modes of the bis(phenolate)pyridine ligand, with complexes containing two or fewer strong pi-donor interactions from ancillary ligands giving C(s) symmetry, whereas three strong pi-donor interactions (e.g., three amido ligands or one amido ligand and one imido ligand) give C(2)-symmetric binding of the bis(phenolate)pyridine ligand. DFT calculations and molecular orbital analyses of the complexes have revealed that the preference for C(s)-symmetric ligand binding is a result of tantalum-phenolate pi-bonding, whereas in cases where tantalum-phenolate pi-bonding is overridden by stronger Ta-N pi-bonding, C(2)-symmetric ligand binding is preferred, likely because conformationally this is the lowest-energy arrangement. This electronically driven change in geometry indicates that, unlike analogous metallocene systems, the bis(phenolate)pyridine pincer ligand is not a strong enough pi-donor to exert dominant control over the electronic and geometric properties of the complex.