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

Abstract

AbstractThe 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 VIVO systems and 51V NMR for the VVO2 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 VOLH2 (only with 3 and 4), VOLH, VOL, and VOLH–1. Dinuclear species (VOL)2H 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 VIVO 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.