Proton Electron Nuclear Double Resonance Spectra of Oxovanadium(IV) Complexes Formed by Salicylic and o-Diphenolic Ligands in Aqueous Solution

The e.s.r. and 'H 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 'H 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.