Mixed-ligand complexes involving sulfur-containing ligands. Part 1. Nickel(II) ternary complexes of L-cysteine, D-penicillamine and L-cysteic acid with imidazoles

Abstract

Computer-based analysis of the pH-titration data obtained in aqueous perchlorate medium at 37 °C and I= 0.15 mol dm^–3(NaClO₄) for Ni^II–A–B ternary systems [A =L-cysteine (Cys), D-penicillamine (pen) or L-cysteic acid (Cya); B = imidazole (Him), histamine (Hist) or L-histidine (His)] showed the presence of the ternary species NiABH₂, NiABH, NiAB and NiAB₂ in addition to binary species with A or B. Under the same experimental conditions, binary systems of Ni^II with Cys, pen, Cya, Him, Hist and His have also been studied. For the NiABH species in the Ni^II–Cys–Him, –His and –Hist and Ni^II–pen–Him mixed-ligand systems, the site of protonation is on the primary ligand (A), while in the Ni^II–A–His and –Hist systems (A = pen or Cya) protonation occurs at the secondary ligand. In the NiABH₂ species, one proton is attached to the primary ligand and the other to the secondary ligand. For the NiAB and NiAB₂ species the Cys and pen primary ligands bind the metal through the mercapto and amino groups, while Cya binds Ni^II in a glycine-like mode. In these species, the Him, Hist and His secondary ligands are respectively mono-, bi-and ter-dentate. The probable reason for the higher stability of NiAB₂ species compared to NiAB species in the Ni^II–A–Him systems (A = Cys, pen or Cya) is discussed.