Adsorption on graphite and catalytic reduction of O2 by the macrocyclic complex of cobalt(II) with 2,3,9,10-tetraphenyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene

The complex of Co(III) with the macrocyclic ligand ph-tim (ph-tim = 2,3,9,10-tetraphenyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) adsorbs irreversibly on roughly polished pyrolitic graphite electrodes. The adsorbed complex exhibits two, reversible, one-electron voltammetric waves corresponding to Co(III)/Co(II) and Co(II)/Co(I) redox couples located at 0.32V and -0.07V, respectively. The adsorbed Co(II) complex generated at 0.32V catalyzes the electroreduction of O-2 to H2O2 in a separate voltammetric step at more negative potentials which vary with pH. Catalytic plateau currents for the reduction of O-2 at rotating pyrolytic graphite disk electrodes coated with [(ph-tim)Co](ads)(3+) are limited by the rate of coordination of O-2 to the reduced catalyst, [(ph-tim)Co](ads)(2+). An upper limit of 150 M-1 was placed on the equilibrium constant for the binding of O-2 to [(ph-tim)Co](ads)(2+). A catalytic mechanism compatible with the results is proposed and discussed. (C) 1997 Elsevier Science S.A.