Chelate formation of certain heterocyclic azo phenanthols with metal ions

Abstract

The compounds, 1-(5-nitro-2-pyridylazo)-, 1-(2-quinolylazo)-, 1-(2-benzothiazolylazo)-, 1-(2-benzox-azolylazo)-9-phenanthols were synthesized as reagents for the determination of small amounts of metals and their metal complexing properties were studied. The influence of the heterocyclic azo groups on the analyticl properties of the reagents were studied. The sensitivity of the reagents for the metal ions was found to be higher than that of parent compound 1-(2-pyridylazo)-2-naphthol. Extension of the π-system of the molecule should lead to sensitive reactions. The effect of nitro group in 5-position of the pyridine ring on sensitivity was large. The acid dissociation constants of the reagents and the formation constants of 1 : 1 chelates in aqueous dioxane were determined spectrophotometrically. Replacement of pyridine ring by benzothiazole or benzoxazole resulted in a decrease in pK_a2 values of the reagents. The effect of these groups on the acidity of the hydroxyl group was practically the same and the nitro group in 5-position of the pyridine ring had a great effect on the basicity of N atom of the pyridine. The higher acidity of these reagents was accompanied by decrease in metal chelate stabilities, but proton displacement constants of 1-(2-benzoxazolylazo)-9-phenanthol (BOAP) chelates were larger than those of the other reagents, so that the pH values at which BOAP formed metal chelates were 1 to 3 units higher. The distribution behavior of the reagents and their chelates formed were studied. The composition of the extractable copper chelates was found to be neutral 1 : 2 chelates. An analytical advantage of these reagents was lower aqueous solubilities and higher distribution constants of the chelates.