Synthesis, characterization, and oxygenation of bis-fenced porphyrinato iron(II) and cobalt(II) complexes

The both-faces-hindered and highly symmetric complexes 5,10,15,20- tetra(2′,6′-dipivaloyl-oxyphenyl)porphyrinato-iron(II) and -cobalt(II), which have no amide residues but ester ones in the fence groups, have been synthesized. The iron(II) complex formed stable and reversible dioxygen adducts in toluene at 25 °C. The half-life for irreversible oxidation is longer than 1 d. The ligation properties for this complex with imidazole derivatives or dioxygen have been determined and compared to those of one-face-hindered porphyrinatoiron complexes. The bulky groups on both sides of the macrocycle decrease the binding constants for imidazole and for dioxygen. The binding characteristics of an imidazole derivative, which is sterically restrained by the bulky groups, have been clarified by Mössbauer and i.r. spectroscopic measurements. A kinetic study of dioxygen association and dissociation suggests that the low oxygen affinity is brought about primarily by the high rate of ligand dissociation.