Complex formation equilibria between the acetazolamide ((5-acetamido-1,3,4-thiadiazole)-2-sulphonamide), a potent inhibitor of carbonicanhydrase, and Zn(II),Co(II), Ni(II) and Cu(II) in aqueous and ethanol-aqueous solutions

Abstract

The stability constants for the equilibrium of complexation between acetazolamide and the divalent metal ions copper(II), nickel(II), zinc(II), and cobalt(II) have been determined by potentiometry both in water and in water-ethanol 50 vol. % solutions in 0.15 mol dm −3 NaNO 3 at 25°C. This mixed solvent has been used in order to obtain higher concentrations of acetazolamide in solution. For copper(II) and nickel(II), the binuclear species [Cu 2(Acm) 2] and [Ni 2(Acm) 3] 2− are detected in both solvents together with hydroxo species. The values of the stability constants are always higher in the mixed solvent than in water. For cobalt(II) and zinc(II), while in aqueous solution the hydrolysis of these metal ions prevented the formation of any complex, in water-ethanol 50 vol. % the metal complexes are stable enough to allow their formation. For zinc(II), the model system consists of the species [Zn(Acm)] and [Zn(Acm) 2] 2− , while for cobalt(II), of just [Co(Acm) 2] 2−. To be noticed, the inversion in the stepwise stability constant observed for zinc(II), which suggests changes in stereochemistry when [Zn(Acm) 2] 2− is formed.