Hydroxylation Catalysis by Mononuclear and Dinuclear Iron Oxo Catalysts: a Methane Monooxygenase Model System versus the Fenton Reagent FeIVO(H2O)52+

Hydroxylation of aliphatic C-H bonds is a chemically and biologically important reaction, which is catalyzed by the oxidoiron group FeO2+ in both mononuclear (heme and nonheme) and dinuclear complexes. We investigate the similarities and dissimilarities of the action of the FeO2+ group in these two configurations, using the Fenton-type reagent [FeO2+ in a water solution, FeO(H2O)(5)(2+)] and a model System for the methane monooxygenase (MMO) enzyme as representatives. The high-valent iron oxo intermediate MMOHQ (compound Q) is regarded as the active species in methane oxidation. We show that the electronic structure of compound Q can be understood as a dimer of two (FeO2+)-O-IV units. This implies that the insights from the past years in the oxidative action of this ubiquitous moiety in oxidation catalysis can be applied immediately to MMOHQ. Electronically the dinuclear system is not fundamentally different from the mononuclear system. However, there is an important difference of MMOHQ from FeO(H2O)(5)(2+): the largest contribution to the transition state (TS) barrier in the case of MMOHQ is not the activation strain (which is in this case the energy for the C-H bond lengthening to the TS value), but it is the steric hindrance of the incoming CH4 with the ligands representing glutamate residues. The importance of the steric factor in the dinuclear system suggests that it may be exploited, through variation in the ligand framework, to build a synthetic oxidation catalyst with the desired selectivity for the methane substrate.