Copper-(II) and -(I) co-ordination by hexa-amine ligands of different rigidities. A thermodynamic, structural and electrochemical investigation

Two new polyamine ligands 2,9-bis(2-methylaminoethylaminomethyl)phenanthrol ine (L3) and 6,6'-bis(2-methylaminoethylaminomethyl)-2,2'-bipyridyl (L4) ha ve been synthesized. L3 and L4 contain respectively a phenanthroline and a bipyridyl unit bearing two N-methylethylenediamine side-arms. Their co-ordi nation properties toward Cu-II were studied by means of potentiometric, cal orimetric, UV-vis spectroscopic and electrochemical measurements and compar ed with those of macrocyclic ligands with similar molecular architecture (L 1 and L2). The macrocyclic ligands give only mononuclear complexes, while t he acyclic ligands form mono- and bi-nuclear complexes in aqueous solution, due to their higher flexibility. Considering the mononuclear complexes, th e analysis of the thermodynamic parameters for copper(II) co-ordination sho ws that the complexes with the acyclic ligands are mainly stabilized by the entropic change. On the contrary, the enthalpic term gives a more favourab le contribution to the formation of the complexes with the macrocyclic liga nds. This different behaviour is discussed in terms of ligand rigidity and ligand and metal desolvation. The crystal structure of the [CuL3](2+) compl ex shows the metal to be six-co-ordinated, with a rather unusual geometry, enveloped inside the ligand cleft and almost co-ordinatively saturated and shielded from solvent molecules. At the same time the two heteroaromatic ni trogens are weakly bound to the metal.