Temperature and solvent structure dependence of VO2+ complexes of pyridine-N-oxide derivatives and their interaction with human serum transferrin

The behaviour of the systems formed by VO2+, 2-hydroxypyridine-N-oxide (Hhpo) and 2-mercaptopyridine-N-oxide (Hmpo) was studied both in solution and in the solid state through the combined application of spectroscopic (EPR and UV-Vis spectroscopy) and DFT methods. The geometry of solid bis-chelated complexes [VOL2], with L = hpo and mpo, is square pyramidal, but it can change to cis-[VOL2S], where S is a solvent molecule, when these are dissolved in a coordinating solvent. The equilibrium between the square pyramidal and cis-octahedral forms is strongly affected by solvent and temperature. At room temperature, the predominant species is [VOL2], which gives a pink colour to the solutions; at lower temperatures, the equilibrium is shifted—partially or completely—toward the formation of cis-[VOL2S], which is green. In an acidic environment and in the presence of an excess of ligand, [VOL2] can transform into the tris-chelated complex [VL3]+, in which vanadium loses the oxido ligand and adopts a hexa-coordinated geometry intermediate between octahedral and trigonal prismatic. 1-Methylimidazole (1-MeIm), which represents a model for His–N coordination, forms mixed complexes with stoichiometry cis-[VOL2(1-MeIm)], occupying an equatorial position. In the ternary systems VO2+–Hhpo–hTf and VO2+–Hmpo–hTf at room temperature and pH 7.4, besides (VO)hTf and (VO)2hTf, the mixed species cis-VO(hpo)2(hTf) and VO(mpo)(hTf) are observed, with the equatorial binding of an accessible histidine residue. Finally, the contribution of the N-oxide group to 51VAz and Aiso hyperfine coupling constants, which can be important in the characterisation of similar species, is discussed.