Reaction of (N4Py)Fe with H₂O₂ and the relevance of its Fe(IV)=O species during and after H₂O₂ disproportionation

Abstract

The catalytic disproportionation of H2O2 by non‐heme Fe(II) complexes of the ligand N4Py and the formation and reactivity of Fe(III)‐OOH and Fe(IV)=O species is studied spectroscopically, 1O2 trapping and DFT methods. Two pathways for the reaction between Fe(III)‐OOH and H2O2 are predicted, both of which yield 3O2/H2O and involve either the S = 1/2 or the S = 3/2 spin state, with the latter being spin forbidden. A minimal energy crossing point (MECP) provides a route for the formally spin forbidden reaction. The formation of only 3O2 is confirmed experimentally in the present study through 1O2 trapping and NIR luminescence spectroscopy. Attempts to use the 1O2 probe (α–terpinene) resulted in auto‐oxidation, which indicated formation of OH radicals from Fe(III)‐OOH,. Microkinetic modelling reveals that another pathway for Fe(III)‐OOH decomposition competes with the reaction of Fe(III)‐OOH with H2O2 and homolysis to form Fe(IV)=O and hydroxyl radical. After all H2O2 is consumed the Fe(III)‐OOH species decays predominantly through a second order self reaction (with Fe(III)‐OOH). The conclusion reached is that the rate of O‐O bond homolysis in the Fe(III)‐OOH species to form Fe(IV)=O and an hydroxyl radical is too low to be responsible for the observed oxidation of organic substrates observed.