6TH-ORDER KINETICS IN A CATALYZED AUTOXIDATION OF A MACROCYCLIC COBALT(II) COMPLEX

The 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 was studied in the presence of Br-. This otherwise very slow reaction is strongly catalyzed by Co(dmgBF2)2 (dmgBF2 = (difluoroboryl)dimethylglyoximate). The rate law shows a first-order dependence on Co(tim)2+, O2, Br-, and H+, and is second-order with respect to the catalyst. The catalytic autoxidation follows a clean 4:1 Co(tim)2+:O2 stoichiometry and yields the corresponding halocobalt(III) macrocycle. The reduction of oxygen to water clearly bypasses the hydrogen peroxide stage. The transition state is proposed and the mechanism of the sixth-order catalytic reaction is considered and discussed. It features O2 binding and halide-bridged electron transfer. There is no evidence for involvement of the hydroxyl radical.