Thermodynamic and spectrophotometric investigation of complex formation between hydrogen, cobalt(II), nickel(II), and copper(II) ions and 2-amino-N-hydroxypropanamide in aqueous solution

Abstract

The overall formation constants of the complexes formed between H⁺, Co²⁺, Ni²⁺, and Cu²⁺, and 2-amino-N-hydroxypropanamide (ahpr) have been measured at 25 °C and l= 0.5 mol dm^–3(KCl) using potentiometric and spectrophotometric techniques. Complexes of monoaminohydroxamic acids and Cu^II, Ni^II and Co^II are shown to involve chelation via the hydroxamate moiety NHO^– and the amino group NH₂, which are pH sensitive. The protonaton constants of the ligand and the formation constants of several metal complexes have been calculated from potentiometric and spectrophotometric data with the aid of the programs SUPERQUAD and SQUAD, respectively. The complexes [CuL]⁺, [Cu₂(OH)L₂]⁺, [CuL₂], [Cu(OH)L₂]^–, [NiL]⁺, [NiL₂], [Ni(OH)L₂]^–, and [CoL]⁺, [CoL₂], [Co(OH)L₂]^–, and [Co₂L]³⁺ are formed where L^–= CH₃CH(NH₂)CONHO^–. The solution electronic spectra are reported. The u.v.-visible investigations provide important evidence for the formation of different metal(II) complexes with 2-amino-N-hydroxypropanamide, depending on the pH; moreover they can be used to estimate the co-ordination sphere around the metal ions and to observe the equilibria between different complexes. The experimental curves [ε= f(λ)] for some complexes, obtained from refinement of absorbance data with the program SQUAD, have been resolved into precisely positioned absorption bands by Gaussian analysis using a non-linear least-squares computer program NLIN. The best data resulting for the metal(II)–ahpr systems have been employed in a weak tetragonal [Cu(OH)L₂, CuL₂] or a square-planar (NiL₂) ligand-field model to calculate as far as possible ligand-field parameters. The specific tendencies of aminohydroxamic acids towards different metal ions have been tested as models for substratum-metal bonding in biological reactions, and the probable structures and stabilities of the chelated compounds formed in aqueous solution are discussed in terms of their biological importance and potential usefulness as therapeutic agents.