Nonoxovanadium(IV) and Oxovanadium(V) Complexes with Mixed O, X, O-Donor Ligands (X = S, Se, P, or PO)

Ligating properties of four potentially tridentate bisphenol ligands containing [O, X, O] donor atoms (X = S 1, Se
2, P 3, or PO 4) toward the vanadium ions in +IV or +V oxidation states have been studied. Each ligand with
different heterodonor atoms yields as expected nonoxovanadium(IV) complexes, VIVL2, whose structures have been
determined by X-ray diffraction methods as having six-coordinate VIV, VO4X2, core. Compounds 1−4 have also
been studied with electrochemical methods, variable-temperature (2−295 K) magnetic susceptibility measurements,
X-band electron paramagnetic resonanace (EPR) (2−60 K) spectroscopy, and magnetic circular dichroism (MCD)
(5 K) measurements. Electrochemical results suggest metal-centered oxidations to VV (i.e., no formation of phenoxyl
radicals from the coordinated phenolates). A combination of density functional theory calculations and experimental
EPR investigations indicates a dramatic effect of the heteroatoms on the electronic structure of 1−4 with consequent
reordering of the energy levels; 1 and 3 possess a trigonal ground state (dz2)1, but 4 with the phosphoryl oxygen
as the heterodonor atom in contrast exhibits a tetragonal ground state, (dxy)1. On the basis of the intense electronic
transitions in absorption spectra, all electronic transitions observed for 4 have been assigned to ligand-to-metal
charge-transfer transitions, which have been confirmed by preliminary resonance Raman measurements and C/D
ratios obtained from low-temperature MCD spectroscopy. Moreover, diamagnetic complexes 5 and 6 containing
mononuclear and dinuclear oxovanadium(V) units have also been synthesized and structurally and spectroscopically
(51V NMR) characterized.