Proton electron nuclear double resonance spectra of oxovanadium(IV) complexes formed by salicylic and o-diphenolic ligands in aqueous solution

Abstract

The e.s.r. and 1H electron nuclear double resonance (ENDOR) spectra of oxovanadium(IV) complexes formed in aqueous solution by salicylic and 2,3-dihydroxybenzoic acids have been recorded and compared to those of catecholic ligands previously described. According to both spectroscopies, salicylic acid co-ordinates to the metal ion through the adjacent carboxyl and phenolate donors, while 2,3-dihydroxybenzoic acid behaves as either a catecholic or a salicylic ligand. The 1H ENDOR spectra allowed assignment of the resonances due to the ring protons which are adjacent to metal-bound carboxylate groups and those having contribution from more distant protons. Thus, while the e.s.r. spectra distinguish only the chromophores involved in metal co-ordination, the ENDOR features are distinctive of both the metal surrounding and the ligand structure. The presence of outer-sphere water molecules which, very likely, are hydrogen-bonded to the ligand has also been detected.