Complex Formation of Ferric Protoporphyrin IX From the Reaction of Hemin with Ammonia and Small Aliphatic Amines

Complexes of FeIII protoporphyrin IX (FeIIIPPIX) with the amido anion were obtained from the reaction of FeIIIPPIX chloride (hemin) with ammonia and small aliphatic amines under solvent free conditions. The reaction of hemin with gaseous ammonia leads to a pentacoordinated complex at the iron site, PPIX–Fe–NH2, plus NH4Cl, while at the peripheral propionic acidic groups ammonium carboxylate is formed. The corresponding stoichiometry (1:4 molar ratio of hemin to ammonia) was confirmed by the adsorption isotherm. Analogous reactions and complex formation were observed with EtNH2 and Et2NH. These reactions were monitored using X-ray diffraction (XRD), and i.r. and Mossbauer spectroscopies. The isomer shift and quadrupole splitting values of the resulting complexes are in correspondence with the strong σ-donor character of the amido anion linked to the iron atom. For comparison, the Mossbauer parameters for hemin complexes with arginine and 2-aminoguanidine, which also have pure σ interaction with the porphyrin iron, were included and discussed.