Two Plus Four Equals Three–Iron(II)/Iron(IV) Comproportionation as an Additional Pathway for Iron(IV)-Oxido Reactions

Iron enzymes are ubiquitous in nature. In particular, enzymes with iron-oxygen cofactors as active sites perform a vast variety of reactions. Both iron(III)-hydroxido and iron(IV)-oxido species have been observed to play a catalytically active role. In order to complement biochemical investigations, a large variety of synthetic compounds using these motifs were synthesized in past decades to study and understand their inherent reactivity. One such synthetic model complex is [Fe^IV(O)(Py₅Me₂)]²⁺, (Py₅Me₂ = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine, henceforth labeled L1), which was used as a model complex for epigenetically relevant iron(II)/α-ketoglutarate-dependent ten-eleven translocation 5-methylcytosine dioxygenases (TET). Additionally, [Fe^III(OH)(Py₅(OH)₂)]²⁺ (Py₅(OH)₂ = pyridine-2,6-diylbis [di(pyridin-2-yl)methanol, henceforth labeled L2) was tested as a lipoxygenase model. We have complemented the available complexes of these related pentapyridyl complexes to include all oxidation states II-IV and performed detailed spectroscopic and spectrometric investigations. We found that iron(II) and iron(IV)-oxido compounds (cross-)comproportionate readily to form iron(III)-hydroxido species, which represents a major side reaction for model complex investigations. We also investigated the oxidative reactivity of a new iron(IV)-oxido complex.