Solvent effects on the protonation constants of N,N′-bis(2-carbamoylethyl)ethylenediamine and the equilibrium constants for its copper(II) complexes in methanol–water mixtures

Abstract

The interactions of N,N′-bis(2-carbamoylethyl)ethylenediamine (bcen) with copper(II) have been investigated as a function of solvent composition in methanol–water mixtures by a potentiometric technique. Each of the two protonation constants of the ligand passes through a minimum at 80%(w/w) methanol. The stability constant of the copper(II) complex exhibits an accelerating increase with increasing methanol content in the solvent, the net increase being 2 000-fold on going from water to 99%(w/w) methanol. Each of the deprotonation constants for [Cu(bcen)]²⁺ and [Cu(H_–1bcen)]⁺ passes through a maximum at 80%(w/w) methanol. These patterns are interpreted in terms of electrostatic effects, base solvation, and proton solvation as a function of the solvent composition.