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 β_pqr=[M_pH_qL_r]/[M]^p[H]^q[L]^r were obtained: ahhe, log β₀₁₁= 9.172(1), log β₀₂₁= 16.547(2); Co^II–ahhe, log β₁₀₁= 6.423(25), log β₁₀₂= 10.964(35), log β_1–12= 1.77(7); Ni^II–ahhe, log β₁₀₁= 6.571(35), log β₁₀₂= 13.902(8), log β_1–12= 6.17(9); Cu^II–ahhe, log β₁₀₁= 10.29(14), log β₁₀₂= 19.695(26), log β_2–12= 20.767(18), log β_1–12= 9.877(43); aihp, log β₀₁₁= 9.088(5), log β₀₂₁= 16.130(7); Ni^II-aihp, log β₁₀₂= 14.383(19), log β_1–12= 5.53(7); Cu^II–aihp, log β₁₀₂= 20.31(6), log β_2–12= 21.469(38), log β_1–12= 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 Cu^II–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.