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 π-donor interactions from ancillary ligands giving C_s symmetry, whereas three strong π-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 π-bonding, whereas in cases where tantalum−phenolate π-bonding is overridden by stronger Ta−N π-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 π-donor to exert dominant control over the electronic and geometric properties of the complex.
Additional InformationCopyright © 2009 American Chemical Society. Received November 25, 2008. Publication Date (Web): May 7, 2009. We thank Dr. Nilay Hazari (Caltech) for assistance with the DFT calculations. DFT calculations were carried out using the Molecular Graphics and Computation Facility, College of Chemistry, University of California, Berkeley, California, with equipment support from NSF grant CHE-0233882. This work has been supported by USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-85ER13431). The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF CRIF:MU award to the California Institute of Technology, CHE-0639094. Supporting Information: Tables of bond lengths, angles, and anisotropic displacement parameters for the presented solid-state structures. Structural drawing of 5, which due to its twinned nature cannot be refined anisotropically. X-ray crystallographic data files (CIF) of compounds 1−7. This material is available free of charge via the Internet at http://pubs.acs.org.
Supplemental Material - Tonksic802234x_si_001.pdf
Supplemental Material - Tonksic802234x_si_002.cif