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(IV)O2+ 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(IV)O2+ 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(IV)O2+ 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(IV)O2+ 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.