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 [FeIV(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, [FeIII(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.