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

Abstract

AbstractThe catalytic disproportionation of by non‐heme Fe(II) complexes of H2O2 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, 1O2 trapping and DFT methods. Earlier DFT studies indicated that disproportionation of H2O2 catalysed by Fe(II)‐N4Py complexes produce only 3O2, 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 H2O2, both of which yield 3O2/H2O2 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 3O2 only is confirmed experimentally in the present study through 1O2 trapping and NIR luminescence spectroscopy. However, attempts to use the 1O2 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 3O2. 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 H2O2 and homolysis to form Fe(IV)=O and hydroxyl radical. Notably, after all H2O2 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.