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

Activation of hydrogen peroxide by Fe salts (Fenton systems) leads to a myriad of oxidizing agents whose nature, FeO, or hydroxyl radicals and Fe species, is dictated by the reaction conditions, in particular the pH value. Using the non heme Fe complex [Fe(L)(CHCN)] (1) (where L 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(O)(L)] and [Fe(OOH)(L)], 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(O)(L)] 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(O)(L)] is obtained via heterolytic O-O cleavage of the oxidant assisted by the base in a peroxidase-like mechanism whereas [Fe(OOH)(L)] 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.