Interaction of copper(II) and nickel(II) with a bis-amide ligand functionalized with pyridine moieties: Thermodynamic and spectroscopic studies in aqueous solution

We synthesized a new bis-amide ligand derived from the l(+)-tartaric acid. We then determined its protonation constants and the stability constants of the copper(II) and nickel(II) chelates by potentiometry as well as ESI-MS and UV–Vis spectroscopy. We found that both metal ions are able to induce the deprotonation and the coordination of an amide nitrogen donor atom. In the case of copper complexes, the data show the formation of two major species: Cu2(L2H−3)+ and Cu2(LH−4). EPR and XAS experiments led us to precise the relative structure of these compounds. In Cu2(L2H−3)+, each metal center is coordinated by pyridinic and amidic nitrogen atoms of one ligand and by nitrogen and oxygen atoms from pyridine and hydroxyl moieties from the other one. In Cu2(LH−4), the copper centers are coordinated by pyridinic and amidic nitrogen atoms, as well as a deprotonated hydroxyl group of the ligand. In this latter complex, the lower value of the Cu–Cu distance determined from EXAFS experiments and compared to the one of the solid species likely involve the formation of an exogeneous hydroxyl bridge between the two copper centers. With Ni(II) ions, the only one major species is the mononuclear Ni(LH−2) complex, in which Ni(II) is held in an octahedral environment with the metal center chelated by the two pyridinic and the two amidic nitrogen atoms, and two oxygen atoms from water molecules.

We investigate the formation in aqueous solution of complexes between the Ni(II) and Cu(II) and a bis-amide ligand containing pyridine moieties. The pyridine moiety allows the deprotonation of the amidic nitrogen atom and the binding of metal ions via the formation of a five-membered chelate ring. Cu(II) leads to the formation of dinuclear species, whereas only mononuclear complexes are formed with Ni(II).Figure optionsDownload as PowerPoint slide