Complexing properties of [(glycylamino)methyl]phosphinic acids towards Co2+, Ni2+, Cu2+ and Zn2+ ions in aqueous solutions

Abstract

The complexing properties of the phosphinic acid analogues of glycylglycine ([(glycylamino)methyl]phosphinic acids) NH₂CH₂C(O)–NHCH₂P(O)(R)(OH) [GlyGly(P^R)] (R = Ph, Me and tert-Bu) towards Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ ions in aqueous solutions are investigated using potentiometry, and absorption and EPR spectroscopies. The potentiometric results indicate different properties of the Cu²⁺–GlyGly(P^R) systems in comparison with glycylglycine and its phosphonic acid analogue NH₂CH₂C(O)–NHCH₂P(O)(OH)₂ [GlyGly(P)]. The phosphinic dipeptides GlyGly(P^R) readily form complexes with Cu²⁺/ligand molar ratio 1 ∶ 2 exhibiting the (N_amine, N_amide)₂ coordination mode above pH 9 even in solutions with only a slight excess of the ligand. In the system with GlyGly, the Cu²⁺ complex is observed at pH 12–13 and a metal ∶ ligand ratio of 1 ∶ 500. The metal-induced deprotonation of the amide nitrogen and the extent of formation of the 1 ∶ 2 complexes depend on the acidity of the phosphinic acid group which is influenced by the substituent at the phosphorus atom. The models based on potentiometry are confirmed by spectroscopic measurements. Stability constants of the phosphinic dipeptides GlyGly(P^R) with the other metal ions are the same as for glycylglycine and, therefore, the same binding pattern through the amine group and peptide oxygen is proposed. The presence of protonated complexes is assumed in all the systems studied. In the protonated complexes, the peptide is bound through the phosphinic acid group whereas the amino group remains protonated. The presence of the species in the system containing the phosphonic acid analogue of glycylglycine [GlyGly(P)] with Cu²⁺, which was investigated by potentiometry previously, is confirmed by EPR spectroscopy.