Oxovanadium(IV) complexes with pyrazinecarboxylic acids: The coordinating properties of ligands with the (N-aromatic, COO-) donor set

Complex formation between the (VO)-O-IV ion and four pyrazine derivatives, 2-pyrazinecarboxylic acid (pzc), 5-methyl-2-pyrazinecarboxylic acid (5-Mepzc), 2,3-pyrazinedicarboxylic acid (3-COOHpzc) and 5-hydroxy-2-pyrazinecarboxylic acid (5-OHpzc), was studied in aqueous solution and in the solid state through the combined application of potentiometric and spectroscopic (EPR and FT-IR) techniques. The results indicate that in acidic and neutral aqueous solution all the ligands form mono(chelated), bis(chelated) and dinuclear species of composition VOL, VOL2 and (VO)(2)L2H-2. Hexacoordinated VOL2 complexes are characterised by a cis/trans isomerism, where cis and trans are the species with a water molecule bound in the cis or trans position with respect to the V=O group. The trans arrangement is favoured over the cis arrangement. Three solid derivatives, [VO(5-Mepzc)(2)] (1), cis-[VO(pzc)(H2O)] (2) and cis-[VO(3-COOHpzc(H2O)] (3), were isolated and characterised. Based on the experimental results and on the data in the literature, the stability of cis and trans isomers in aqueous solution and in the solid state has been discussed, showing that with ligands of comparable basicity and size of the chelate ring the hydrophilicity favours the cis species and determines the relative amount of the two isomers. The analysis of the magnetic properties of the hydroxo-bridged (VO)-O-IV dimers suggests that for the (VO)(2)L2H-2 species the anti-coplanar arrangement is realised.