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

Abstract

Activation of hydrogen peroxide by Fe^II salts (Fenton systems) leads to a myriad of oxidizing agents whose nature, Fe^IVO, 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(L₅²)(CH₃CN)]²⁺ (1) (where L₅² 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)(L₅²)]²⁺ and [Fe^III(OOH)(L₅²)]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)(L₅²)]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)(L₅²)]²⁺ is obtained via heterolytic O–O cleavage of the oxidant assisted by the base in a peroxidase-like mechanism whereas [Fe^III(OOH)(L₅²)]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.