Oxygen binding to cobalt(II) proto-, deutero- and meso-porphyrin IX dimethyl ester complexes in organic solvents

Abstract

The binding of oxygen to cobalt(II) meso, deutero- and proto-porphyrin IX dimethyl esters complexed with pyridine or 2-methylimidazole was investigated at −10°–−60°C in toluene or DMF solution, and the thermodynamic data related with the binding were presented. The oxygen affinity of cobalt meso-porphyrin complex was larger by the factor of 2.0–1.4 than those of the other complexes where oxygen affinities were not explained by a simple electron-withdrawing capability of 2,4-substituents of the porphyrin ring. The oxygen binding property was, generally, dependent on the solvent, suggesting that the solvation affects appreciably the oxygen binding to the complexes. The oxygen affinities of cobalt porphyrin complexes in various organic solvents were compared with those of their apomyoglobin complexes. The differences of oxygen affinities between both systems decreased with increasing the size of 2,4-substituents, and it was in the following order on 2,4-substituted porphyrins: Deutero ⪢ Proto ⪢ Meso. It was suggested that the 2,4-substitutent effect on the oxygen affinity of cobalt myoglobin complexes was not only caused by the direct electronic effect on the central cobalt atom, but also controlled by the stereochemical interaction between apomyoglobin and the porphyrin.