The influence of phenyl substitutions on the dimerization of highly substituted iron(III) porphyrin hydroxo complexes

The modeling of iron porphyrins is essential in the study of heme proteins. These complexes readily undergo autooxidation and dimerization; however, this dimerization process can be inhibited by introducing phenyl groups. Herein, the dimerization of highly substituted iron(III) porphyrins is investigated. The stability of water-insoluble iron(III) hydroxo complexes was monitored via thin-layer chromatography and was found to increase with the number of phenyl groups. Dodecaphenyl-substituted complexes formed hydroxo complexes, tetraphenyl and hexaphenyl complexes formed mu-oxo dimers, and octaphenyl and decaphenyl complexes existed as hydroxo complex-mu-oxo dimer mixtures. Crystals of the dodecaphenyl-substituted iron(III) hydroxo complexes were successfully isolated. The decaphenyl-substituted complex was unique in that the respective crystals of both the hydroxo complexes and mu-oxo dimer could be isolated from the chloride complexes. Furthermore, the synthesis and proton equilibria of the new porphyrin iron(III) water-soluble complexes were investigated. Under neutral to basic conditions, at equilibrium, dodecaphenyl-substituted complexes exclusively formed hydroxo complexes, whereas octaphenyl, hexaphenyl, and tetraphenyl complexes exclusively formed mu-oxo dimers, and decaphenyl complexes existed as hydroxo complex-mu-oxo dimer mixtures. Additionally, the order of the dimerization reaction was examined using the initial-rate and half-life methods, which confirmed that the dimerization is a second-order reaction that follows a mechanism wherein two hydroxo complexes form a dihydroxo-bridged intermediate. This result was supported by the dimerization of water-soluble iron(III) hydroxo complexes. The new hydroxo complexes with non-planar porphyrin cores synthesized in this study are expected to have a significant impact on heme protein reaction modeling.

Automated summary

The modeling of iron porphyrins is important in studying heme proteins. In this study, the dimerization of highly substituted iron(III) porphyrins was investigated. The introduction of phenyl groups inhibited the dimerization process, and the stability of iron(III) hydroxo complexes increased with the number of phenyl groups. The synthesis and proton equilibria of water-soluble porphyrin iron(III) complexes were also studied. The order of the dimerization reaction was determined, and it was found to follow a mechanism where two hydroxo complexes form a dihydroxo-bridged intermediate. These findings have significant implications for modeling heme protein reactions.