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 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, (1)O(2 )trapping and DFT methods. Earlier DFT studies indicated that disproportionation of H2O2 catalysed by Fe(II)-N4Py complexes produce only O-3(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 H2O2, both of which yield( 3)O(2)/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 (3)O(2 )only is confirmed experimentally in the present study through O-1(2) trapping and NIR luminescence spectroscopy. However, attempts to use the O-1(2) probe ( alpha -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 O-3(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 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.