Copper-(n) 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)phenanthroline (L3) and 6,6′-bis(2-methylaminoethylaminomethyl)-2,2′-bipyridyl (L4) have been synthesized. L3 and L4 contain respectively a phenanthroline and a bipyridyl unit bearing two Ar-methylethylenediamine side-arms. Their co-ordination properties toward Cu^II were studied by means of potentiometric, calorimetric, UV-vis spectroscopic and electrochemical measurements and compared with those of macrocyclic ligands with similar molecular architecture (LI and L2). The macrocyclic ligands give only mononuclear complexes, while the acyclic ligands form mono- and bi-nuclear complexes in aqueous solution, due to their higher flexibility. Considering the mononuclear complexes, the analysis of the thermodynamic parameters for copper(n) co-ordination shows that the complexes with the acyclic ligands are mainly stabilized by the entropie change. On the contrary, the enthalpic term gives a more favourable contribution to the formation of the complexes with the macrocyclic ligands. This different behaviour is discussed in terms of ligand rigidity and ligand and metal desolvation. The crystal structure of the [CuL3]²⁺ complex 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 nitrogens are weakly bound to the metal.