Structure, Bonding, and Reactivity of Ti and Zr Amidate Complexes: DFT and X-Ray Crystallographic Studies

Easily prepared and highly modular organic amide proligands have been used to synthesize a series of new
bis(amidate)-bis(amido) Ti and Zr complexes via protonolysis. These complexes have been structurally characterized
by NMR spectroscopy and X-ray crystallography. The solid-state molecular structures of these complexes indicate
that the amidate ligands bind to the metal centers in an exclusively bidentate fashion, resulting in discrete monomeric
species. Geometric isomerism in these species is highly dependent upon the steric characteristics of the proligands
utilized in the synthesis. In solution, these complexes are observed to isomerize on the NMR time scale, with one
isomer being predominant. Bonding in the bis(amidate)-bis(amido) complexes was investigated by DFT calculations.
The geometric isomers predicted by theory matched the experimentally observed results, within experimental error.
The orbitals associated with amidate−metal bonding are energetically well below the frontier orbitals. The HOMO
in these complexes is a π orbital associated with amido ligand-to-metal bonding character, while the LUMO in all
cases is a vacant d orbital on the metal center.