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 V2+/3+ tren-bridged iminopyridine complexes [tren =
tris­(2-aminoethyl)­amine] that enable the observation of an unexpected
doublet ground state for a nominally 3d3 species. Tren
undergoes condensation reactions with picolinaldehyde or methyl-6-formylnictonate
to form the respective tripodal ligand sets of (py)3tren
and (5-CO2Mepy)3tren. The (py)3tren
ligand is coordinated to V2+ and V3+ metal centers
to form complex salts [1-H](OTf)2 and [1-H](OTf)3, respectively (OTf– = CF3SO3–). For [1-H]2+, strong metal–ligand π-covalency
with respect to the V2+ (3d3) 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 V3+ (3d2) metal center. The coordination of (5-CO2Mepy)3tren to a V2+ metal center results in the redox
noninnocent and heptacoordinate compound [1-CO2Me](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.