Base-controlled mechanistic divergence between iron(iv)-oxo and iron(iii)-hydroperoxo in the H2O2 activation by a nonheme iron(ii) complex

Activation of hydrogen peroxide by Fe II salts (Fenton systems) leads to a myriad of oxidizing agents whose nature, Fe IV O, or hydroxyl radicals and Fe III species, is dictated by the reaction conditions, in particular the pH value. Using the non heme Fe II complex [Fe II (L 5 2 )(CH 3 CN)] 2+ (1) (where L 5 2 is the pentadentate ligand N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) we have observed the simultaneous formation of two reaction intermediates, [Fe IV (O)(L 5 2 )] 2+ and [Fe III (OOH)(L 5 2 )] 2+ , in its reaction with excess hydrogen peroxide in the presence of sub-stoichiometric amounts of triethylamine. Kinetic and spectroscopic monitoring of the reaction mixture and of independently prepared [Fe IV (O)(L 5 2 )] 2+ in the presence of the different constituents of the reaction mixture allows drawing a mechanistic scheme. These two reactive species are formed simultaneously following two independent and competitive pathways. [Fe IV (O)(L 5 2 )] 2+ is obtained via heterolytic O-O cleavage of the oxidant assisted by the base in a peroxidase-like mechanism whereas [Fe III (OOH)(L 5 2 )] 2+ is generated upon homolytic O-O cleavage of hydrogen peroxide. The relative contribution of these two pathways can be tuned by adjusting the amount of base used.