Effect of Redox Potential on Diiron-Mediated Disproportionation of Hydrogen Peroxide

Heme and nonheme dimanganese catalases are widely distributed in living organisms to participate in antioxidant defenses that protect biological systems from oxidative stress. The key step in these processes is the disproportionation of H2O2 to O-2 and water, which can be interpreted via two different mechanisms, namely via the formation of high-valent oxoiron(IV) and peroxodimanganese(III) or diiron(III) intermediates. In order to better understand the mechanism of this important process, we have chosen such synthetic model compounds that can be used to map the nature of the catalytically active species and the factors influencing their activities. Our previously reported mu-1,2-peroxo-diiron(III)-containing biomimics are good candidates, as both proposed reactive intermediates ((FeO)-O-IV and Fe-2(III)(mu-O-2)) can be derived from them. Based on this, we have investigated and compared five heterobidentate-ligand-containing model systems including the previously reported and fully characterized [Fe-II(L1-4)(3)](2+) (L-1 = 2-(2'-pyridyl)-1H-benzimidazole, L-2 = 2-(2'-pyridyl)-N-methyl-benzimidazole, L-3 = 2-(4-thiazolyl)-1H-benzimidazole and L-4 = 2-(4'-methyl-2'-pyridyl)-H-1-benzimidazole) and the novel [Fe-II(L-5)(3)](2+) (L-5 = 2-(1H-1,2,4-triazol-3-yl)-pyridine) precursor complexes with their spectroscopically characterized mu-1,2-peroxo-diiron(III) intermediates. Based on the reaction kinetic measurements and previous computational studies, it can be said that the disproportionation reaction of H2O2 can be interpreted through the formation of an electrophilic oxoiron(IV) intermediate that can be derived from the homolysis of the O-O bond of the forming mu-1,2-peroxo-diiron(III) complexes. We also found that the disproportionation rate of the H2O2 shows a linear correlation with the Fe-III/Fe-II redox potential (in the range of 804 mV-1039 mV vs. SCE) of the catalysts controlled by the modification of the ligand environment. Furthermore, it is important to note that the two most active catalysts with L-3 and L-5 ligands have a high-spin electronic configuration.