Equilibrium and solution structural study of the proton, copper(II), nickel(II) and zinc(II) complexes of 1-(2-aminoethylamino)-1-deoxy-D-galactitol

Abstract

Protonation and copper(II), nickel(II) and zinc(II) complex-formation equilibria of 1-(2-aminoethylamino)-1-deoxy-D-galactitol and the solution co-ordination structure of the complexes were investigated by potentiometric titrations, UV/VIS absorption, CD, EPR and ¹³C NMR spectroscopies in aqueous solution (I = 0.1 mol dm^-3, NaClO₄; T = 298 K). In acidic media the ethane-1,2-diamine (en) residue dominates the co-ordination for all the metal ions studied, but the spectroscopic results are consistent with weak co-ordination of alcoholic hydroxy group(s). In equimolar solution and at neutral pH, dialkoxide-bridged dimeric species were formed with nickel(II) and copper(II). In case of an excess of galactitol bis- and tris-complexes were also detected. In the latter system metal-promoted deprotonation occured above pH 9 and in this way the second ligand is successively displaced from the co-ordination sphere of copper(II). In the finally formed MLH_-2 species the ligand is co-ordinated by two amino and two deprotonated alcoholic hydroxy groups. Nickel(II) formed a ML₂H_-2 complex in the alkaline pH region. The deprotonation processes of the ZnL₂ complex leading also to ML₂H_-2 took place at almost the same pH as for NiL₂. Evidence of the deprotonation of the alcoholic hydroxy groups is available only above pH ≈ 11 in the zinc(II)-containing system.

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