IRON(II)-CATALYZED AUTOXIDATION OF A MACROCYCLIC COBALT(II) COMPLEX

The otherwise very slow reduction of O2 by Co(tim)2+ (tim = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) in acidic aqueous media is efficiently catalyzed by iron(II) ions. The rate law shows first-order dependences on Co(tim)2+, O2, and Fe2+ but remains independent of halide and hydrogen ion concentrations. The catalytic autoxidation yields Co(tim)3+ and involves the four-electron reduction of O2 as indicated by the stoichiometry, which is independent of the reagent in excess. Moreover, the reduction of oxygen to water clearly bypasses the stage in which Co(tim)2+ would reduce hydrogen peroxide, since this independently known reaction leads to other products. The small deviations from the exact model proposed, more evident at the 340-nm isosbestic point for Co(tim)2+ and Co(tim)3+, arise from a minor side reaction. The chemistry of the system is consistent with the formation of [(tim)CoOOFe4+] and [(tim)CoOOH2+] in succession.