Mixed-ligand complex formation equilibria of CuII with glycylglycinate and guanylurea

Abstract

Combined spectrophotometric and computer based pH metric investigation on the mixed ligand complex formation equilibria of Cu II with glycylglycine (H 3 N + CH 2 CONHCH 2 COO - , HGG ± ) and guanylurea [H 2 1 NC-(=O) 2 NHC(= 3 NH) 4 NH 2 , HGu] in aqueous solution at 25 ± 1 °C, at a fixed ionic strength, 1 = 0.1 mol dm -3 (NaNO 3 ), indicate variety of binary and ternary complexes: [Cu(GG) + aq], [Cu(GG-H)(H 2 O)], [Cu(GG-H)(OH)-], [Cu(Gu) + aq], [Cu(Gu)(OH)], [Cu(Gu-H)(OH) - ], [Cu(GG-H)(Gu) - ], [Cu(GG-H)(Gu-H) 2- ] and [Cu(GG-H)(Gu-2H) 3 -]. At pH ≤4.5, GG-ion provides neutral (NH 2 , C=O) bidentate chelation to Cu II and above pH 4.5 ~ 5, coordinated GG- is transformed to amide deprotonated glycylglycinate dianion, (GG-H) 2- , which provides terdentate equatorial (NH 2 , N-, COO-) chelation to Cu II . Consequently in the electronic spectra λ max of Cu II is blue shifted. λ max of the ternary complex, [Cu(GG-H)(Gu)-], indicates a square pyramidal geometry existing in a isomeric structural equilibrium, involving bidentate axial (= 3 N) and equatorial (O=C)/(H 2 1 N)/(H 1 N=C) chelation by Gu- ion. Above pH 7, [Cu(GG-H)(Gu) - ] undergoes deprotonation of the (H 1 N=) and HO-(enolic) moieties of coordinated Gu- in successive steps.