Cooperative Co-Activation of Water and Hypochlorite by a Non-Heme Diiron(III) Complex

Aqueous solutions of the iron(III) complex of N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate (tpena) react with hypochlorite (ClO^-) to produce the reactive high-valent [Fe^IV(O)(tpena)]⁺. Under catalytic conditions, in bicarbonate-buffered media (pH 8) with a set ionic strength (10 mM NaCl), kinetic analysis shows that two equivalents of [Fe^IV(O)(tpena)]⁺ per one ClO^- are produced, with benign chloride ions the only byproduct. An unprecedented supramolecular activation of ClO^- by {(HCO₃)⊂[(tpena)Fe^III(μ-O)Fe^III(Htpena)]}²⁺ is proposed. This mode of activation has great advantage for use in the catalytic oxidation of C-H bonds in water since: (i) the catalyst scaffold is protected from oxidative degradation and (ii) undesirable radical side reactions which produce toxic chlorinated compounds are circumvented by this novel coactivation of water and ClO^-. The unique activation mechanism by the Fe-tpena system makes possible the destruction of organic contaminants as an add-on technology to water disinfection by chlorination, demonstrated here through (i) the catalytic oxidation of micropollutant metaldehyde, and (ii) mineralization of the model substrate formate. The resting-state speciation at pH 3, 5, 7, and 9, as well as the catalytically active iron speciation are characterized with Mössbauer and EPR spectroscopy and supported by DFT calculations. Our study provides fundamentally new insights into the design and activation mode of iron-based catalysts relevant to applications in water remediation.