Vanadium(IV and V) complexes of Schiff bases and reduced Schiff bases derived from the reaction of aromatic o-hydroxyaldehydes and diamines:: Synthesis, characterisation and solution studies

Several sal(2)en-type reduced Schiff bases have been prepared from the reaction of 2 equiv. of salicylaldehyde (or derivatives) with ethylenediamine followed by reduction with NaBH4 and subsequently characterised. The water-soluble ligands (SO3-sal)(2)en and R(SO3-sal)(2)en (SO3-sal = salicylaldehyde-5-sulphonate) have been studied by pH potentiometry and H-1 NMR spectroscopy and the protonation constants of R(SO3-sal)(2)en have been determined. The Schiff bases are more susceptible to hydrolysis than the corresponding reduced Schiff bases. The crystal structures of H-2(o-van)(2)en and KH(o-van)(2)en (o-van = o-vanillin) have been determined by X-ray diffraction. The vanadium(IV) complexes have been prepared and characterised by magnetic measurements and IR, UV/Vis and EPR spectroscopy. The colours, magnetic susceptibilities and IR spectra of the new vanadium(IV) complexes of the reduced Schiff bases now obtained suggest oligomeric/polymeric structures for the compounds while the corresponding (VO)-O-IV-Schiff-base complexes are monomeric. The complexation of [(VO)-O-IV](2+) and [(VO)-O-IV](2+) with R(SO3-sal)(2)-en in aqueous solution has been studied by pH potentiometry, UV/Vis as well as by EPR spectroscopy for the (VO)-O-IV system and H-1 and V-51 NMR spectroscopy for the (VO2)-O-V system. Complex formation constants have been determined and binding modes have been proposed. N,N'-ethylenebis(pyridoxylaminato) (Rpyr(2)en) has also been obtained and the structure of Na[(VO2)-O-V(Rpyr(2)en)](.)CH(3)OH(.)3H(2)O determined by X-ray diffraction. The Rpyr(2)en(2-) ligand coordinates through both the N-amine and O-phenolate moieties in a symmetrical alpha-cis type coordination mode, i.e. with both O-phenolato donors cis to the O-oxo atoms and trans to each other. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.