Selectivity of Coordination Sites in Ni(II), Zn(II), and Cd(II) Complexes with 7-[(3,5-Dichloro(or bromo)-2-pyridyl)azo]-8-hydroxyquinoline-5-sulfonic Acid

Abstract

A new multidentate ligand, 7-[(3,5-dichloro(or bromo)-2-pyridyl)azo]-8-hydroxyquinoline-5-sulfonic acid (sodium salt, abbr. as 3,5-di-X-PAHQS, X=Cl or Br) which contains pyridylazonaphthol (PAN) and 8-quinolinol skeletons, was prepared for the first time in order to investigate its coordination behavior toward metal(II) ions. For comparative studies, the ligands (2-[(3,5-dichloro(or bromo)-2-pyridyl)azo]-1-hydroxynaphthalene-4-sulfonic acid (sodium salt, abbr. as 3,5-di-X-αPANS, X=Cl or Br), 1-[(3,5-dichloro(or bromo)-2-pyridyl)azo]-2-hydroxynaphthalene-6-sulfonic acid (sodium salt, abbr. as 3,5-di-X-βPANS, X=Cl or Br), and 7-[(4-chloro or unsubstituted phenyl)azo]-8-hydroxyquinoline-5-sulfonic acid(sodium salt, abbr. as 4-Cl-BAHQS for chloro substituted phenyl and BAHQS for unsubstituted phenyl, respectively)) were also prepared. The coordination modes of the synthesized multidentate ligands with Ni(II), Zn(II), and Cd(II) ions were investigated both spectrophotometrically and polarographically, in aqueous solution. Judging from the variations of the reduction potentials of the azo group with metal complexations, in the case of the ligands of BAHQS systems, the azo group did not participate in complexation. On the contrary, for the ligands of the PAN and PAHQS systems, the azo group in these ligands clearly participated in metal-complexation. On the basis of the results of polarographic studies and of spectral data, the complexing equilibria of these complexes were analyzed and the stability constants calculated. The dissociation constants and the reduction potentials of these new ligands are also discussed.