Kinetics of The Reaction of Amine Complexes of Cobalt(II) Protoporphyrin IX Dimethyl Ester with Oxygen. Evidence for Hydrogen Bonding with Coordinated Oxygen

The kinetics are reported for reaction of amine complexes of cobalt(II) protoporphyrin IX dimethyl ester with oxygen in toluene to give products believed to be peroxo-bridged complexes. The reaction is first order in cobalt and oxygen and goes from first to zero order in amine (L) with increasing amine concentration. The determined rate law is consistent with the rate-determining step involving a unimolecular distortion of a 1:1 cobaltoxygen complex. Anomalously rapid rates for L = imidazole and benzimidazole are tentatively explained in terms of hydrogen-bonding effects involving the acidic proton on the imidazole ring. Similarities with hemoglobin and myoglobin are discussed. Thermodynamic data for 1:1 oxygen complexes are presented and discussed in terms of the rate law.