VIVO Versus VIV Complex Formation by Tridentate (O, Narom, O) Ligands: Prediction of Geometry, EPR 51V Hyperfine Coupling Constants, and UV-Vis Spectra

Systems formed using the (VO2+)-O-IV ion with tridentate ligands containing a (O, N-arom, O) donor set were described. Examined ligands were 3,5-bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole (H(2)hyph(Ph)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H(3)hyph(C)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzenesulfonic acid (H(3)hyph(S)), and 2,6-bis(2-hydroxyphenyl)pyridine (H(2)bhpp), with H(3)hyph(C) being an orally active iron chelator that is commercially available under the name Exjade (Novartis) for treatment of chronic iron overload arising from blood transfusions. The systems were studied using EPR. UV-Vis, and IR spectroscopies, pH potentiometry, and DFT methods. The ligands bind vanadium with the two terminal deprotonated phenol groups and the central aromatic nitrogen to give six-membered chelate rings. In aqueous solution the main species were the mono- and bis-chelated (VO)-O-IV complexes, whereas in the solid state neutral non-oxido V-IV compounds were formed. [V(hyph(Ph))(2)] and [V(bhpp)(2)] are hexacoordinated, with a geometry close to the octahedral and a meridional arrangement of the ligands. DFT calculations allow distinguishing (VO)-O-IV and V-IV species and predicting their structure the V-51 hyperfine coupling constant tensor A, and the electronic absorption spectra. Finally, EPR spectra of several non-oxido V-IV species were compared using relevant geometrical parameters to demonstrate that in the case of tridentate ligands the V-51 hyperfine coupling constant is related to the geometric isomerism (meridional or facial) rather than the twist angle Phi, which measures the distortion of the hexacoordinated structure toward a trigonal prism.