Fenton-like chemistry in water:: Oxidation catalysis by Fe(III) and H2O2

The formation of active intermediates from the Fenton-like reagent (a mixture of iron(III) ions and hydrogen peroxide) in aqueous solution has been investigated using static DFT calculations and Car-Parrinello molecular dynamics simulations. We show the spontaneous formation of the iron(III) hydroperoxo intermediate in a first step. The Fenton-like reaction thus proceeds very differently compared to Fenton's reagent (i.e., the Fe-II/H2O2 mixture), for which we have recently shown that the first step is the spontaneous O-O lysis of hydrogen peroxide when coordinated to iron(II) in water. For the second step in the reaction mechanism of the Fenton-like reagent, we compare the possibilities of homolysis and heterolysis of the O-O bond and the Fe-O bond of the produced [(H2O)(5)(FeOOH)-O-III](2+) intermediate. We find that concomitant hydrolysis of the reacting species plays a crucial role and, taking this into account, that O-O homolysis ([(H2O)(4)(OH)Fe-III-OOH](+) --> [(H2O)(4)(OH)(FeO)-O-IV](+) + OH.) in vacuo is a likely second step with DeltaE(0k)double dagger = 26 kcal/mol. However, proper inclusion of the solvent effects is important, in particular, for the heterolysis reactions, in which case the large endothermicy of the charge separations can be compensated by the hydration energies from the ion solvation. In this work, we also calculate the free energy barrier for the O-O homolysis of the iron(III) hydroperoxo intermediate in aqueous solution at T = 300 K, using the method of constrained molecular dynamics and thermodynamic integration, resulting in DeltaA(300K)double dagger = 21 kcal/mol. Analysis of the vibrational spectra of the high-spin (S = 5/2) and low-spin (S = 1/2) Fe(III)OOH intermediates confirm the, in the literature, suggested effect of the spin state on the Fe-O and O-O bond strengths. In fact, we predict that with ligands inducing a low-spin iron(III) hydroperoxo intermediate, the barrier for the O-O homolysis will be even significantly lower.