Vanadium(IV) and ‐(V) Complexes of Reduced Schiff Bases Derived from Aromatic o ‐Hydroxyaldehydes and Tyrosine Derivatives

The reduced Schiff bases of salicylaldehyde and pyridoxal (and o ‐vaniline) with L ‐tyrosine (Tyr) and D , L ‐ o ‐tyrosine ( o ‐Tyr), designated as sal‐Tyr ( 1 ), sal‐ o ‐Tyr ( 2 ), pyr‐Tyr ( 3 ), pyr‐ o ‐Tyr ( 4 ), and o ‐van‐ L ‐Tyr ( 5 ), as well as the oxidovanadium(IV) complex VO(sal‐ o ‐Tyr) ( 6 ) have been prepared. The compounds have been characterized in the solid state and in solution. The structure of 3 has been determined by X‐ray diffraction. Complexation of these ligands with vanadium in aqueous solution has been studied by pH potentiometry, UV/Vis, and circular dichroism (for the L ‐Tyr derivatives), as well as by EPR for the V IV O systems and 51 V NMR for the V V O 2 systems. Stoichiometries and complex formation constants have been determined by pH potentiometry (25 °C, I = 0.2 M KCl) 1:1 complexes are formed in most systems with variable protonation states VOLH 2 (only with 3 and 4 ), VOLH, VOL, and VOLH –1 . Dinuclear species (VOL) 2 H and (VOL) 2 were identified only in the case of 3 . Spectroscopic data provided information about the most probable binding modes for each stoichiometry. The V IV O complexes formed with the o ‐Tyr‐derived ligands are more stable than those with L ‐Tyr, the more adequate coordination position of the phenolate in the o ‐Tyr ligands shows a more significant stabilizing effect compared with 3 and 4 .