PYRIDINES WITH AN APPENDED METALLOCYCLAM SUBUNIT - VERSATILE BUILDING-BLOCKS TO SUPRAMOLECULAR MULTIELECTRON REDOX SYSTEMS

A pyridine subunit can be appended to a metallocyclam fragment through a template reaction which involves the condensation of a [M(II)(2,3,2-tet)]2+ compleX (M = Ni, Cu) with formaldehyde and pyridine-3-carboxamide or pyridine-4-carboxamide, in the presence of base. The crystal and molecular structure has been determined for the low-spin [3-(4-pyridiniumylcarbonyl)-1,3,5,8,12-pentaazacyclotetradecane]nickel(II), in which the nitrogen heteroatom of the pendant pyridine subunit is protonated. Single-crystal X-ray diffraction data were collected with the use of Cu Kalpha radiation: space group P1BAR with a = 15.038 (2) angstrom, b = 13.576 (2) angstrom, c = 8.171 (2) angstrom, alpha = 108.12 (2)-degrees, beta = 84.66 (2)-degrees, gamma = 109.02 (2)-degrees, V = 1498.8 (5) angstrom3, and Z - 2 (R - 0.075 1, R(w) - 0.0751). Reaction of the metallocyclam-pyridine conjugates 3a,b and 4a,b (M(II)L2+) with [PtIICl4]2- and cis-[RuII(bipy)2Cl2], in a 2:1 ratio, gives the supercomplexes cis-[(MIIL)2PtIICl2]4+ and cis-[(M(II)L)2Ru(II)(bipy)2]6+, respectively. Cyclic voltammetry and controlled potential coulometry studies on nonaqueous solutions have shown that the cis-[(NiIIL)2PtIICl2]4+ supercomplex undergoes a simultaneous two-electron oxidation to the cis-[(NiIIIL)2PtIICl2]6+ species, i.e. according to two one-electron processes, whose potentials are separated by the statistical term (36 mV). The cis-[(M(II)L)2Ru(II)(bipy)2]6+ supercomplex, in acetonitrile solution 0.1 mol dm-3 in Bu4NClO4, undergoes a three-electron oxidation process: first, the one-electron oxidation of the Ru(II) center, followed, at a potential 300 mV more positive, by the two-electron process involving the oxidation of the Ni(II) ions of the macrocyclic subunits. On the other hand, in a 0.1 mol dm-3 Bu3BzCl solution, the (1 + 2) sequence of the three-electron release of the cis-[(M(II)L)2Ru(II)(bipy)2]6+ supercomplex is changed to (2 + 1): the oxidation of the two nickel centers occurs at a much lower potential, due to the stabilizing effect on Ni(III) exerted by the axially bound Cl- ions. Such a favorable effect is not experienced by the coordinatively saturated Ru(II) center, which is oxidized at a potential 250 mV more positive.