Doublet Ground State in a Vanadium(II) Complex: Redox and Coordinative Noninnocence of Tripodal Ligand Architecture.

We report on the geometric and electronic structures of a series of V 2+/3+ tren-bridged iminopyridine complexes [tren = tris(2-aminoethyl)amine] that enable the observation of an unexpected doublet ground state for a nominally 3d 3 species. Tren undergoes condensation reactions with picolinaldehyde or methyl-6-formylnictonate to form the respective tripodal ligand sets of (py) 3 tren and (5-CO 2 Mepy) 3 tren. The (py) 3 tren ligand is coordinated to V 2+ and V 3+ metal centers to form complex salts [1-H] (OTf) 2 and [1-H] (OTf) 3 , respectively (OTf - = CF 3 SO 3 - ). For [1-H] 2+ , strong metal-ligand π-covalency with respect to the V 2+ (3d 3 ) and iminopyridine ligands weakens its interelectronic repulsion. For [1-H] 3+ , the bridgehead nitrogen of the tren scaffold forms a seventh coordinate covalent bond with a V 3+ (3d 2 ) metal center. The coordination of (5-CO 2 Mepy) 3 tren to a V 2+ metal center results in the redox noninnocent and heptacoordinate compound [1-CO 2 Me] (OTf) 2 with a doublet ( S = 1 / 2 ) ground state that we support with magnetic susceptibility and spectroscopy measurements. The complexes are uniformly characterized experimentally with single-crystal X-ray diffraction, electronic absorbance, and electrochemistry, and electronic structures are corroborated by computational techniques. We present a new computational procedure that we term the spin-optimized approximate pair (SOAP) method that enables the visualization and quantification of electron-electron interactions.