Potentiometric study on the complex-formation equilibria between 2-amino-N-hydroxyhexanamide and α-amino-1H-indole-3-N-hydroxypropanamide and cobalt(II), nickel(II), copper(II), and hydrogen ions in aqueous solution

Abstract

The equilibria occurring in aqueous solutions of 2-amino-N-hydroxyhexanamide (ahhe) and α-amino-1H-indole-3-N-hydroxypropanamide (aihp) with protons and bivalent metals have been studied by a potentiometric method. The protonation and complex-formation constants have been determined at 25°C and I= 0.5 mol dm–3 KCl. The following overall formation constants βpqr=[MpHqLr]/[M]p[H]q[L]r were obtained: ahhe, log β011= 9.172(1), log β021= 16.547(2); CoII–ahhe, log β101= 6.423(25), log β102= 10.964(35), log β1–12= 1.77(7); NiII–ahhe, log β101= 6.571(35), log β102= 13.902(8), log β1–12= 6.17(9); CuII–ahhe, log β101= 10.29(14), log β102= 19.695(26), log β2–12= 20.767(18), log β1–12= 9.877(43); aihp, log β011= 9.088(5), log β021= 16.130(7); NiII-aihp, log β102= 14.383(19), log β1–12= 5.53(7); CuII–aihp, log β102= 20.31(6), log β2–12= 21.469(38), log β1–12= 10.52(7). The ligands are bound to the metals through the N atom of the α-amino group and the deprotonated NHO– group. Electronic spectra of the CuII–ahhe chelates formed have been recorded. All the above metals form mononuclear ([ML]+ and/or [ML2]) and hydroxo complexes with each ligand; with copper(II), polynuclear species are also present.