Copper(II) and nickel(II) complexes of pyridylamido hexadentate ligands: chemical speciation and spectroscopic studies

Two novel potentially hexadentate ligands, 1,10-(2-bis picolinamide)-4,7-diazadecane (pycdpnen) and 1,8-bis(2-picolinamide)-3,6-dioxaoctane (pycdado) have been synthesised as their hydrochloride salt; its protonation constants and the stability constants of the copper(II) and nickel(II) chelates have been determined by potentiometry. Amide groups deprotonation permits the formation of [MLH−1]+ species in all cases, while only pycdado gives [MLH−2] species. The solid complexes of copper and nickel with the neutral and the deprotonated ligands have been synthesised and characterised by IR, UV–Vis and ESR spectroscopy. The amidic groups are coordinated through the oxygen atoms in all solid complexes [ML](ClO4)2 (M=Cu2+ and Ni2+). The complexes obtained with the deprotonated forms of the ligands imply the coordination through the nitrogen atoms of the amidic groups.