Conclusion

We have performed static DFT calculations as well as ab initio (DFT) molecular dynamics simulations to study the ability of the ferryl ion [Fe$^{\rm IV}$O]$^{2+}$ to oxidize organic substrates in aqueous solution. The static DFT calculations show that the hydrated ferryl ion [H$_2$O)$_5$Fe$^{\rm IV}$O]$^{2+}$ is indeed very reactive and can oxidize methane to methanol, via two mechanisms.

Via the so-called methane coordination mechanism, with one water ligand replaced by methane, the weakly interacting reaction complex first forms a iron(IV)-hydroxo-methyl complex by hydrogen transfer from the methane to the oxo ligand. Then, methanol is formed in a second exothermic step by transfer of the CH$_3$ group from the metal to the hydroxo oxygen. The first step is the rate limiting step with a barrier of 22.8 kcal/mol. The energetics for the methane coordination mechanism in our aquairon complex are quite different from that of the methane oxidation by bare metal oxo species, for which this mechanism was found to be the most likely one. The water-methane ligand exchange reaction preceding the actual H-abstraction is endothermic by 23.4 kcal/mol.

Via an alternative mechanism, the so-called oxygen-rebound mechanism, the methane-to-methanol oxidation is practically barrierless in vacuo and also proceeds in two steps. In the first step, a hydrogen is abstracted from methane by the oxo ligand, forming an [Fe$^{\rm III}$OH$\cdots$.CH$_3$]$^{2+}$ complex. In the second step, the bound .CH$_3$ radical can transfer via a narrow channel to the hydroxo oxygen, forming again methanol. Overall, this mechanism shows strong similarities with methane oxidation by the enzyme methane monooxygenase and, to a lesser extent, with oxidation by cytochrome P450, although the calculated barriers in the cases of these bio-catalysts are significantly higher.

Solvent effects are important when dealing with charged reactants. We have therefore studied the solvent effects on the initial methane hydroxylation step in the oxygen-rebound mechanism in aqueous solution. The free energy barrier of the H-abstraction from methane in aqueous solution is estimated to be 22 kcal/mol, using the method of constrained molecular dynamics. The reaction barrier and the overall endothermicy of this first step are significantly higher in solution than they are in vacuo, which is due to the decreased hydration of the reacting oxo/hydroxo ligand in the transition state and the higher energy of solvation for the iron(IV)oxo ion compared to the iron(III)hydroxo ion.

In combination with our previous work on Fenton's reagent (a mixture of iron(II) ions and hydrogen peroxide in water), which showed the spontaneous formation of the much contested ferryl ion, this work strongly supports the mechanism proposed by Bray and Gorin for the oxidation of organic substrates by Fenton's reagent, with the ferryl ion as the active species.