Chelate Stabilities of certain oxine-type compounds

The acid dissociation constants for 4-hydroxybenzimidazole, 4-hydroxy-2-methylbenzimidazole. 4-hydroxy-2-ethylbenzimidazole, 4-hydroxybenzoxazole and 4-hydroxy-2-methyl benzoxazole have been determined in 50% v./v. 1,4-dioxane-water at 25 degrees and chelate stability constants of these ligands with Cu(II), Pb(II), Cd(II), Zn(II), Ni(II) and Co(II) (ligand to metal ratio 8: 1 and 4: 1) have been obtained by Calvin-Bjerrum potentiometric titration method.: The basicity of the ligands have been compared among themselves and with that of 8-hydroxyquinoline and the differences in values have been accounted for. 4-Hydroxvbenzimidazole chelates were found to be more stable than those of the 4-hydroxybenzoxazoles, the order of stability constants being Cu(II) > Pb(II) > Cd(II) > Zn(II) > Ni(II) > Co(II). The metal chelates of the 4-hydroxybenzimidazoles and the 4-hydroxybenzoxazoles were less stable than those of 8-hydroxyquinoline and 2-(o-hydroxyphenyl)-benzimidazole. The differences have been explained by the greater distance between the chelating atoms, by the orientation of the donor electrons on the nitrogen and by resonance contribution.