Synthesis and Effectively Reversible Oxygenation of Some “Picket Fence” Type Porphyrinatocobalt(II) Complexes

Some “picket fence” type meso-tetrakis(2-substitu-ted phenyl)porphyrinatocobalt(II) complexes were newly synthesized, which had different peripheral substituents in their numbers and/or geometrical dispositions to form “cavity” on the porphyrin plane. Their O2 affinities were determined with 1-methylimidazole as an axial base at various temperatures in solution, by visible spectral measurements. On the basis of thermodynamic quantities, four neopentylcarbonylamino groups were the most effective as a picket fence, and the thermodynamic values of the complex were nearly comparable to those of cobalt(II)myoglobin. It was found that O2 affinity decreased with an increase in ΔH°, regardless of ΔS°. In the case of using various amines as an axial base, imidazoles were effective to enhance O2 affinity rather than pyridines.