Fenton-like chemistry in water: oxidation catalysis by Fe(III) and H$_2$O$_2$

Abstract:

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$^\mathrm{II}$/H$_2$O$_2$ 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 [(H$_2$O)$_5$Fe$^\mathrm{III}$OOH]$^{2+}$ intermediate. We find that concomitant hydrolysis of the reacting species plays a crucial role and, taking this into account, that O-O homolysis ([(H$_2$O)$_4$(OH)Fe$^\mathrm{III}$OOH]$^{+}$ $\rightarrow$ [(H$_2$O)$_4$(OH)Fe$^\mathrm{IV}$O]$^{+}$ + OH.) in vacuo is most favorable with $\Delta E^\ddagger_\mathrm{0k}=26$ kcal/mol. However, also the proper inclusion of the solvent effects is important. We have therefore calculated 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 $\Delta A^\ddagger_\mathrm{300k}=21$ kcal/mol. Analysis of the vibrational spectra of the high-spin ($S=5/2$) and the low-spin ($S=1/2$) Fe(III)OOH intermediate confirms 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.