Catalytic Oxidation of Water with High-Spin Iron(IV)-Oxo Species: Role of the Water Solvent

We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion of H₂O into H₂O₂ in water solution by the Fe^IVO²⁺ group under room-temperature and -pressure conditions. We compute the free energy of formation of an O(water)-O(oxo) bond using thermodynamic integration with explicit solvent and we examine the subsequent generation of H₂O₂ by proton transfer. We show that the O-O bond formation follows the standard reactivity pattern observed in hydroxylation reactions catalyzed by high-spin (S = 2) iron(IV)-oxo species, which is initiated by the transfer of one electron from the highest occupied molecular orbital of the moiety attacking the Fe^IVO²⁺ group, either a -C-H bonding orbital (hydroxylation) or a lone pair of a water molecule (water oxidation). The highly electrophilic character exhibited by the Fe^IVO²⁺ ion, which is related to the presence of an acceptor 3σ∗ orbital at low energy with a large contribution on the O end of the Fe^IVO²⁺ ion, is the crucial factor promoting the electron transfer. The electron transfer occurs at an O(water)-O(oxo) distance of ca. 1.6 Å, and the free energy required to favorably orient a solvent H₂O molecule for the O(oxo) attack and to bring it to the transition state amounts to only 35 kJ mol^-1. The ensuing exoergonic O-O bond formation is accompanied by the progressive weakening of one of the O-H bonds of the attacking H₂O assisted by a second solvent molecule and leads to the formation of an incipient Fe²⁺-[O-O-H]^-[H₃O⁺] group. Simultaneously, three additional solvent molecules correlate their motion and form a hydrogen-bonded string, which closes to form a loop within 5 ps. The migration of the H⁺ ion in this loop via a Grotthuss mechanism leads to the eventual protonation of the [O-O-H]^- moiety, its progressive removal from the Fe²⁺ coordination sphere, and the formation of free H₂O₂ in solution.