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 [0, X, 0] 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, (VL2)-L-IV, whose structures have been determined by X-ray diffraction methods as having six-coordinate V-IV, 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 V-V (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 (d(Z)(2))(1), but 4 with the phosphoryl oxygen as the heterodonor atom in contrast exhibits a tetragonal ground state, (d(xy))(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 (V-51 NMR) characterized.