Acid–base properties of N–methylhistamine [4-(2-methylaminoethyl)imidazole] and NN-dimethylhistamine [4-(2-dimethylaminoethyl)imidazole] and their complexing capacity with cobalt(II), nickel(II), copper(II), and zinc(II)

Abstract

Equilibria of N-methylhistamine [4-(2-methylaminoethyl)imidazole] and NN-dimethylhistamine [4-(2-dimethylaminoethyl)imidazole] with protons and bivalent metals (Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺), in aqueous solutions at 25·0 °C and ionic strength 0·1M-KCl, have been investigated potentiometrically. The ligands behave as bivalent bases with log K₁= 9·904(4), log K₂= 5·874(6) for N-methylhistamine and log K₁= 9·334(2), log K₂= 5·821(4) for NN-dimethylhistamine, corresponding to the basic sites amino-group and tertiary imidazole nitrogen atom, respectively. The stabilities of the metal complexes are in accord with the Irving–Williams series. With respect to variation of the ligand with the same metal ion, the stability decreases in the series histamine > N-methylhistamine > NN-dimethylhistamine. The copper(II) ion forms hydrogen complexes with the N-methylated ligands, whose stabilities follow the order N-methylhistamine < NN-dimethylhistamine.