Formation of new non-oxido vanadium(IV) species in aqueous solution and in the solid state by tridentate (O, N, O) ligands and rationalization of their EPR behavior

The systems formed by the (VO2+)-O-IV ion with tridentate ligands provided with the (O, N-imine, O) donor set were described. The ligands studied were 2,2'-dihydroxyazobenzene (Hdhab), alpha-(2-hydroxy-5-methylphenylirnino)-o-cresol (Hhmpic), calmagite (H(2)calm), anthracene chrome red A (H(3)anth), calcon (H(2)calc), and calconcarboxylic acid (H(3)calc(C)). They can bind vanadium with the two deprotonated phenol groups and the imine nitrogen to give (5,6)-membered chelate rings. The systems were studied with EPR, UV-vis and IR spectroscopy, pH-potentiometry, and DFT methods. The ligands form unusual non-oxido V-IV compounds both in aqueous solution and in the solid state. [V(anthH(-1))(2)](4-) and [V(calmH(-1))(2)](2-) (formed in water at the physiological pH) and [V(dhabH(-1))(2)] and [V(hmpicH(-1))(2)] (formed in the solid state in MeOH) are hexa-coordinated with geometry intermediate between the octahedron and the trigonal prism and an unsymmetric facial arrangement of the two ligand molecules. DFT calculations were used to predict the structure and V-51 hyperfine coupling tensor A of the non-oxido species. The EPR behavior of 13 non-oxido V-IV species was put into relationship with the relevant geometrical parameters and was rationalized in terms of the spin density on the d(xy) orbital. Depending on the geometric isomer formed (meridional or facial), d(z)(2) mixes with the d(xy) and this effect causes the lowering of the highest V-51 A value.