Spectrophotometric study of binary and ternary systems involving metal ions and benzylidenepyruvates: equilibria in aqueous solutions

Abstract

The protonation of 4-dimethylaminobenzylidenepyruvate (DMBP) and 2-chloro-4-dimethylaminobenzylidenepyruvate (2-Cl-DMBP) and their complex formation with Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II) and Al(III) have been studied by potentiometric and spectrophotometric methods at 25 °C and ionic strength 0.500 M, held with sodium perchlorate. The stability order found for 1 : 1 complexes of both ligands is Al(III) > Cu(II) > Pb(II) > Ni(II) > Zn(II) > Co(II) > Cd(II) > Mn(II). The stability changes move in the same direction as the pK_a of the ligands. The results are compared with literature values reported for metal ion-pyruvate systems. Thermodynamic stabilities of ternary complexes formed in Cu(II)-B-L^– systems, where B = 2,2′-bipyridyl (bipy), ethylenediamine or glycinate and L^–= DMBP or 2-Cl-DMBP, were also determined. The Cu(bipy)L⁺ species are more stable than would be expected on purely statistical grounds. The importance of the π system associated with bipy on the enhanced stability of its mixed ligand complexes is stressed. Analytical applications of the investigated ligands are outlined.