(CU-II[N,N'-BIS(2-AMINOETHYL)-2-(2-(4-PYRIDYL)ETHYL)MALONDIAMIDO(2-)]) - A CONVENIENT BUILDING-BLOCK FOR THE CONSTRUCTION OF SUPRAMOLECULAR COORDINATION-COMPOUNDS CONTAINING EXCHANGEABLE PERIPHERAL CU-II CATIONS

The ethylpyridino functionalized N,N'-bis(2-aminoethyl)-2-(2-(4-pyridil)ethyl)malondiamide ligand 1b has been prepared, its protonation and complexation properties (toward Cu-II) have been studied in aqueous solution, and the pertinent constants have been determined. The ligand is able to coordinate one Cu-II ion with the diamino-diamido binding unit, after the release of the two amido protons, thus forming complex 2, which still presents a free, outward-pointing pyridine unit, capable of coordinating a second metal center. Two 2 units have been appended, through their pyridine nitrogen atom, to the kinetically inert cis-Cl2PtII < fragment. The obtained supramolecular species 3 is able to release the peripheral Cu-II ions when the pH of the solution becomes acidic and to re-form the original complex when basicity is restored (the pertinent complexation constants have been determined). This system thus represents the first example of a supramolecular coordination compound which contains exchangeable peripheral transition metal cations. The crystal and molecular structure of 2 has been determined, disclosing the formation of ''head to tail'' dimers in the solid state, with an unusual coordination geometry of the Cu-II ions. Single-crystal X-ray diffraction data were collected with the use of Mo K alpha radiation: trigonal, space group P (3) over bar with a = b = 20.558(6) Angstrom, c = 7.562(3) Angstrom, V = 2768(2) Angstrom(3), and Z = 6 (R = 0.054, R(w) = 0.058). Finally, electrochemical studies revealed that both in the separate component and in the assembled system the Cu-II centers are reversibly oxidizable to Cu-III. In the supramolecular coordination compound, the two Cu-II centers weakly influence each other by means of electrostatic interactions.