Studies on Benzothiazole Derivatives as Chelating Agents. III. Acid-base Equilibria and Chelate Formation with Metal Ions

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The acid dissociation constants of 2-, 4-and 7-substituted benzothiazole derivatives and the formation constants of the complexes formed by these ligands with Mg^<2+>, Cu^<2+>, Ni^<2+>, Co^<2+>, Zn^<2+>, Mn^<2+> and Cd^<2+> were determined potentiometrically at 25°in 2/1 (v/v) dioxane/water solution. The order of the basicity of the ligands is 4-substituted > 7-substituted > 2-substituted benzothiazole derivatives. Also the basicity of the benzothiazole derivatives, substituted at the same position, is in the order : aminomethyl > azomethine > azo compounds. Except for the azonaphthol derivatives, a linear relationship exists between the basicity and the π electron density of the hydroxyl group of the ligands, which may suggest that hydrazone form is predominant in the azo-hydrazone tautomerism of the benzothiazolylazonaphthol derivatives. The stability of the metal complexes corresponds to the basicity of the ligands and the log K_<ML> values approximately follow the Mellor-Maley stability sequence. It is expected that the benzothiazolylazo derivatives would be good metallochromic indicators in chelatometric titrations with ethylenediaminetetraacetic acid as titrant.
公益社団法人日本薬学会の論文
1977-11-25