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.

A Combinatorial Approach To Discover New Chelators for Optical Metal Ion Sensing

“…(C 16 H 11 N 4 O 7 SNa)Na: calcd, 5.39, found, 4.03. The sodium content of the sample indicated that the monosodium salt (sulfonate) was the major component in agreement with literature data for structurally related compounds. , The IR spectrum showed the presence of water; similar compounds were reported to crystallize with various amounts of water. , The water content contributed to the difference between the sodium content of the sample and the calculated amount for the anhydrous monosodium salt. Thus, the general formula C 16 H 12 - x N 4 O 7 SNa x ·(H 2 O) y was proposed for the components of this product.…”

“…Consistent with this observation, Hirohata et al found that a multidentate azo dye formed chelates with more metal ions than a structurally related bidentate azo compound 13b. The structure and stability of the chelate(s) formed between an azo dye and a metal ion depend on a number of factors including the size of the chelate ring(s), the number of chelate rings per molecule, steric restrictions, the basicity of the ligand, and the nature of the metal. 7a,13a The hard and soft acid and base (HSAB) properties of the chelating sites and metal ion strongly influence the coordination mode of a multidentate ligand toward a metal ion, when multiple types of chelates can be formed. ,,

5 Absorbance spectra of chelates of dye 4aak with selected metal ions.

6 Absorbance spectra of chelates of dye 4iab with selected metal ions.

7 Absorbance spectra of chelates of dye 4aah with selected metal ions.

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“…Solvent molecules can be incorporated in azo dye−metal complexes as ligands. ,, The character of the solvent and the pH of the chelate solution often influence the structure and stability of the dissolved metal chelate. ,− In our experiments, buffers and pH values were selected in order to minimize solvent interferences with metal ion chelation. Thus, metal complexes were formed in either a neutral MOPS or a slightly acidic acetate buffer 2d.…”

“…Multidentate azo dyes usually form complexes with a variety of heavy metal ions. ,, The UV−visible absorbance spectra of such complexes often show spectral diversity among the metal ions bound to the same chelator . Therefore, most of our lead structures were designed to be multidentate dyes ( 4 ).…”

“…Therefore, most of our lead structures were designed to be multidentate dyes ( 4 ). The structures were chosen following literature data about similar azo chelators; 7a, in particular, o -hydroxy-azo and 8-quinolyl-azo derivatives are known multidentate ligands. ,,, For instance, 7-[(3,5-dihalo-2-pyridyl)azo]-8-hydroxyquinoline-5-sulfonic acids (halo: chloro or bromo) were reported to form highly stable complexes with soft or borderline Lewis acids such as Ni(II), Zn(II), and Cd(II). , In these chelates, the metal ions were coordinated selectively to the N−N−O terdentate moiety, containing the azo group. , On the other hand, V IV O, a hard Lewis acid, bonded selectively to the N−O skeleton in the 8-hydroxyquinoline moiety of the same ligand . This example illustrates that multidentate azo dyes can form various types of chelates depending on the nature of the metal ion.…”