Studies on imidazole derivatives as chelating agents—IV Stability constants of the metal chelates of azoimidazoles

Abstract

Chelating abilities of several azoimidazoles containing a hydroxy and/or a dimethylanuno group, and the structures of their copper chelates, were investigated. 1-(5-Methyl-4-imidazolylazo)-2-naphthol(IAN) formed a brown-black chelate having the composition Cu(ligand)Br whereas 4-(5-methyl-4-imidazolylazo)dimethylaminobenzene (DAI) formed a reddish brown chelate Cu(ligand)(2)Br(2).2H(2)O. The contribution of the phenolic OH and the imino NH group to the binding with copper is discussed from the infrared spectra. The metal : ligand ratios of the Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) chelates were determined by the method of continuous variations and their stability constants were determined at 25 degrees in 50% v v dioxane-water by the Bjerrum-Calvin method. The OH-containing azoimidazoles were found to form stable chelates with logK(1)K(2) values 14.9-25.4 and the stabilities approximately followed the Mellor-Maley stability sequence. The compound DAI, which has no OH group, formed weak chelates with Co(II), Ni(II) and Cu(II).