A Mössbauer study on the (protoporphyrinato IX)iron(II) complexes of imidazole and substituted imidazoles as axial ligands in frozen aqueous solutions

Abstract

A Mössbauer study on (protoporphyrinato IX)- iron(II) complexes of the type [(PPIX)Fe(II)(ImR) n] where ImR=imidazole (ImH), 1-methylimidazole (1-MeIm) or 2-methylimidazole (2-MeImH) indicates that the ImH and 1 MeIm complexes are low spin S = O and monomeric in 1:1 ethanol:water whereas the 2 MeImH complex in this solvent is present as a five-coordinated high spin ( S = 2) aggregate. The quadrupole splitting (Δ E Q) of the bis(1-MeIm) compound was found to be similar to that of reduced cytochrome b 5·. In a frozen aqueous solution of a detergent, ethyltrimethylammonium bromide (CTAB), the five-coordinated complex [(PPIX)Fe(II)- (2-MeImH)] was obtained as a monomeric high spin compound with a Δ E Q similar to that of deoxy myoglobin. A low spin bis(2-MeImH) complex was also found in the frozen (78 K) solution of the detergent with Δ E Q = 1.26 which is similar to that found for the low spin [(PPIXFe(II)] complexes with sterically hindered amines. Analysis of the isomer shifts and the quadrupole splittings of the low spin bis(ImR) complexes indicate that the dominant mechanism in the covalent bonding is the σ donation of imidazole → iron and the observed trend of σ basicity of the coordinated ligands found is: 2-MeImH < 1-MeIm < ImH. Hydrogen bonding of coordinated imidazoles and steric hindrance of axial ligands are shown to influence the trend in basicity of the free ligands.