NEW ELECTROCATALYSTS FOR THE 4-ELECTRON REDUCTION OF DIOXYGEN BASED ON (5,10,15-TRIS(PENTAAMINERUTHENIUM(II)-4-CYANOPHENYL)-20-(1-METHYLPYRIDINIUM-4-YL)PORPHYRINATO)COBALT(II) IMMOBILIZED ON GRAPHITE-ELECTRODES

Coordination of Ru(NH3)(5)(2+) centers to the nitrile sites in (5,10,15-tris(4-cyanophenyl)-20-(1-methylpyridinium-4-yl)porphyrinato)cobalt(II) immobilized on pyrolytic graphite electrodes produces the triruthenated complex which acts as an electrocatalyst for the four-electron reduction of dioxygen to water. For comparison, RU(NH3)(5)(2+) centers were also coordinated to the nitrile sites in (5,10-bis(4-cyanophenyl)-15,20-bis(1-methylpyridinium-4-yl)porphyrinato)cobalt(II), (5,15-bis(4-cyanophenyl)-10,20-bis(1-methylpyridinium-4-yl)porphyrinato)cobalt(II) and (5-(4-cyanophenyl)-10,15,20-tris(1-methylpyridinium-4-yl)porphyrinato)cobalt(II) to produce the corresponding di- and monoruthenated complexes. The diruthenated complexes exhibit some electrocatalytic activity for the four-electron reduction of dioxygen, whereas the monoruthenated complex catalyzes only the two-electron reduction to hydrogen peroxide. None of the ruthenated cobalt porphyrins are catalysts for the reduction of hydrogen peroxide. The synthesis and the electrocatalytic behavior of the set of structurally related porphyrins are described. It is argued that the ruthenated porphyrins achieve their catalytic activity for the reduction of dioxygen to water by means of back-bonding interactions between the RU(NH3)(5)(2+) groups and the cobalt center of the porphyrin ring.