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

The catalytic disproportionation of by non‐heme Fe(II) complexes of H 2 O 2 the ligand N4Py (1,1‐bis(pyridin‐2‐yl)‐ N , N ‐bis(pyridin‐2‐ylmethyl)methanamine) and the formation and reactivity of Fe(III)‐OOH and Fe(IV)=O species is studied by UV/Vis absorption, NIR luminescence, (resonance) Raman and headspace Raman spectroscopy, 1 O 2 trapping and DFT methods. Earlier DFT studies indicated that disproportionation of H 2 O 2 catalysed by Fe(II)‐N4Py complexes produce only 3 O 2 , however, only the low‐spin state pathway was considered. In the present study, DFT calculations predict two pathways for the reaction between Fe(III)‐OOH and H 2 O 2 , both of which yield 3 O 2 /H 2 O 2 and involve either the S =1/2 or the S =3/2 spin state, with the latter being spin forbidden. The driving force for both pathways are similar, however, a minimal energy crossing point (MECP) provides a route for the formally spin forbidden reaction. The energy gap between the reaction intermediate and the MECP is lower than the barrier across the non‐adiabatic channel. The formation of 3 O 2 only is confirmed experimentally in the present study through 1 O 2 trapping and NIR luminescence spectroscopy. However, attempts to use the 1 O 2 probe ( ‐terpinene) resulted in initiation of auto‐oxidation rather than formation of the expected endoperoxide, which indicated formation of OH radicals from Fe(III)‐OOH, e. g ., through O−O bond homolysis together with saturation of methanol with 3 O 2 . Microkinetic modelling of spectroscopic data using rate constants determined earlier, reveal that there is another pathway for Fe(III)‐OOH decomposition in addition to competition between the reaction of Fe(III)‐OOH with H 2 O 2 and homolysis to form Fe(IV)=O and hydroxyl radical. Notably, after all H 2 O 2 is consumed the decay of the Fe(III)‐OOH species is 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.