Coordination equilibria in the complex formation of guanylurea with CuII: Formation and stability of binary CuII-guanylurea and ternary CuII-guanylurea-glycinate complexes

TLDR: Combined pHmetric and spectrophotometric investigations on the complex formation equilibria of CuII with guanylurea (H21NC(=O) 2NH) in aqueous solution indicates variety of binary and mixed-ligand complexes.

Abstract: Combined pH-metric and spectrophotometric investigations on the complex formation equilibria of CuII with guanylurea (H21NC(=O) 2NH.C(=3NH) 4NH2), hereafter, GuH, in the absence and in the presence of glycine (GlyH), in aqueous solution indicates variety of binary and mixed-ligand complexes: Cu(Gu)+, Cu(Gu)(OH); Cu(Gu)2, Cu(Gu-H)(Gu)−, Cu(Gu-H)22−, Cu(Gu-H)(Gu-2H)3−; Cu(Gly)+, Cu(Gly)(OH); Cu(Gly)(Gu); Cu(Gly)(Gu-H)−, Cu(Gly)(Gu-2H)2−; (Gly)Cu(Gu)Cu(Gly)+, (Gly)Cu(Gu-H)Cu(Gly) and (Gly)Cu(Gu-2H)Cu(Gly)−. At pH < 6, guanylurea anion (Gu−) acts as a [(C=O), 3N−] or [=]NH, 3N−] bidentate ligand and above pH 7 it is transformed through a coordination equilibrium into a (=1N−, =3N−) bidentate ligand, similar to biguanide dianion. Occurrence of dinuclear complex species, (Gly) Cu(Gu)Cu(Gly)+, in the complexation equilibria, indicates bridging double bidentate [(1NH2, 3N−), (C=O, 4NH2)] and/or [(1NH2, 4NH2), (C=O, 3N−)] chelation by Gu− ion in an isomeric equilibrium. Above pH 6·5, the dinuclear complex decomposes mostly to the mononuclear species, Cu(Gly)(OH) and Cu(Gu)(OH) and only partly deprotonates to (Gly)Cu(Gu-H)Cu(Gly) and (Gly)Cu(Gu-2H)Cu(Gly)−. Electronic spectral shifts, with change of pH have been correlated with the possible modes of coordination of guanylurea species.