Synthesis and coordination behaviors of new double-sided porphyrinatoiron(II) complexes: Effect of the pocket-size for imidazole on dioxygen binding.

The synthesis and characteristic ligation behaviors of new "double-sided porphyrinatoiron(II) complexes" having two pockets of different spacing-size on each side of a ring plane and a larger sized one on both sides of a macrocycle, are described. The equilibrium constants for the imidazole derivatives to the four-coordinated porphyrinatoiron(II)s were increased in proportion to the size of the pocket-space, which is reflected by the DELTA-H term. The five-coordinated complexes of the porphyrinatoirons, obtained by the addition of 1,2-dimethyl-imidazole (1,2-dmin) in a toluene solution, formed stable and reversible dioxygen adducts at 25-degrees-C. The oxygen affinities for the iron(II) complexes increased as the steric bulk on the rear side of the porphyrin plane was relieved. These changes in the oxygen affinity are attributed to the strength of the pi-electron donation from the imidazole to the central iron ion.