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

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⁺ (1, where MeN3Py is 1,1-di(pyridin-2-yl)-N-methyl-N-(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⁺ (4) is generated in moderate yield at room temperature by in situ oxidation of 1 with a water/H₂O₂-free peroxy acid, phenyl peracetic acid (PhPAA). We show that oxygen atom transfer (OAT) from 4 to styrene is accompanied by recovery of 1, 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.