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

Non-heme iron(IV) oxido complexes are proposed as intermediates in organic oxidations with non-heme N4-ligated iron catalysts. However, they are not observed during catalyzed oxidations at room temperature using H₂O₂. Their absence can be attributed to rapid reactions with H₂O₂ or organic substrates and rapid conversion to inactive Fe(III) species. [(MeN3Py)Fe(II)(CH₃CN)₂]²⁺ (<b>1</b>, where MeN3Py is 1,1-di(pyridin-2-yl)-<i>N</i>-methyl-<i>N</i>-(pyridin-2-ylmethyl)methanamine) is a typical example with two solvent molecules coordinated to the Fe(II) center in a cis-arrangement. Here, the Fe(IV)═O complex [(MeN3Py)(CH₃CN)Fe(IV)═O]²⁺ (<b>4</b>) is generated in moderate yield at room temperature by in situ oxidation of <b>1</b> with a water/H₂O₂-free peroxy acid, phenyl peracetic acid (PhPAA). We show that oxygen atom transfer (OAT) from <b>4</b> to styrene is accompanied by recovery of <b>1</b>, but it competes with comproportionation to form inactive Fe(III) complexes. Water and acid facilitate the exchange of the solvent ligand with PhPAA needed to generate Fe(IV)═O, but water also accelerates comproportionation. Water-accelerated comproportionation accounts for the absence of observable Fe(IV)═O species during catalytic oxidations with H₂O₂ and increases the apparent rate of reaction with styrene.