Competition between Oxygen Atom Transfer from a Non-Heme N4-Coordinated Fe(IV) ═ O to Styrene and Water-Accelerated Comproportionation

Biomimetic non-heme iron(IV) oxido complexes coordinated by N4-ligands are important models for the catalytic sites of many non-heme iron dependent enzymes. Oxidations with non-heme N4 ligated iron catalysts using H2O2 as terminal oxidant are proposed to involve intermediate formation of high-valent Fe(IV)=O and Fe(V)=O species. However, these species are not observed during catalyzed oxidations using H2O2, despite that such species can be generated with other oxidants. Instead these complexes convert rapidly to the Fe(III) state (e.g., Fe(III)-OH, Fe(III)-OOH, etc) upon addition of H2O2 and it is therefore to understand why. Coordination by N4-ligands leaves two coordination sites free, one of which is occupied, e.g., by the oxido ligand and a sixth coordination site for occupation by solvent or other monodentate ligands. The impact of this sixth ligand on the reactivity of these catalysts, e.g., in oxygen atom transfer (OAT) to alkenes, is of general interest as a potential handle to tune activity. However, it can impact the reaction pathways available also. [(MeN3Py)Fe(II)(CH3CN)2]2+ (1, where MeN3Py is 1,1-di(pyridin-2-yl)-N-methyl-N-(pyridin-2-ylmethyl)meyhanamine)) is 1typical of this class of complex with two solvent molecules coordinated to the Fe(II) center in a cis-arrangement. Here, [(MeN3Py)(S)Fe(IV)–O]2+ (4, where S = CH3CN, H2O, or carboxylic acid) is generated at room temperature by oxidation of 1 with a water/H2O2-free peroxy acid, phenyl peracetic acid (PhPAA). OAT from 4 to styrene is shown to be accompanied by recovery of 1, but competes with the formation of inactive Fe(III) complexes (e.g., [((MeN3Py)Fe(III))2(µ-O)(µ-PhCH2CO2)]4+). Water and acid facilitate the exchange of the solvent ligand with PhPAA needed to generate Fe(IV)–O, but water also accelerates comproportionation between Fe(IV)=O and 1. Importantly, comproportionation increases the observed rate of decay of Fe(IV)=O when styrene is added, however it results in reduced efficiency. These data rationalize the absence of high valent Fe=O species during catalysis with H2O2 as terminal oxidant.