Amine, Amido, and Imido Complexes of Tantalum Supported by a Pyridine-Linked Bis(phenolate) Pincer Ligand: Ta-N π-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,T-donor interactions from ancillary ligands giving C, 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.