Coordination equilibria of mixed ligand complexes of Cuˡˡ with glycylglycine and biguanide

Abstract

Spectrophotometric and computer based pH metric investigations on the mixed ligand complex formation equilibria of Cuˡˡ with glycylglycine [(H₃⁺NCH₂CONHCH₂COO⁻), HGG±] and biguanide [(H₂¹N C(=²NH) ³NH C(=⁴NH) ⁵NH₂), Bg] in aqueous solution at 25±1ºC at a fixed ionic strength, I = 0.1 mol dm⁻³ (NaNO₃) in different molar proportions, indicate the formation of [Cu(GG)(H₂O)₂], [Cu(GG-H)(H₂O)], [Cu(Bg-H)(OH)], [Cu(GG-H)(Bg)], [Cu(GG-H)(Bg-H)-], [Cu₂ (GG-H)₂(Bg)], [Cu₂(GG-H)₂(Bg-H)⁻], [Cu₂(GG-H)₂(Bg-2H)²⁻] complexes. Shifting of electronic spectral λmax of Cuˡˡ with increase in pH correlates with a meridonial-facial isomeric equilibrium in the coordination modes of amide deprotonated glycylglycinate dianion, GG-H²⁻, in [Cu(GG-H)(Bg)]. Existence of the binuclear mixed ligand complexes indicates basket-like bridging double bidentate [(H₂¹N, =⁴NH), (=²NH, ⁵NH₂)] mode of chelation by the biguanide species.