Complex formation of ferric protoporphyrin IX from the reaction of hemin with ammonia and small aliphatic amines

Complexes of Fe ^III protoporphyrin IX (Fe ^IIIPPIX) with the amido anion were obtained from the reaction of Fe ^IIIPPIX 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-NH ₂, plus NH ₄Cl, 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 EtNH ₂ and Et ₂NH. These reactions were monitored using X-ray diffraction (XRD), and i.r. and Mössbauer 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 Mössbauer parameters for hemin complexes with arginine and 2-aminoguanidine, which also have pure σ interaction with the porphyrin iron, were included and discussed.