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 (FeO2+)-O-IV group under room-temperature and-pressure conditions. We compute the 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 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 (FeO2+)-O-IV 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 (FeO2+)-O-IV ion, which is related to the presence of an acceptor 3 sigma* orbital at low energy with a large contribution on the 0 end of the (FeO2+)-O-IV ion, is the crucial factor promoting the electron transfer. The electron transfer occurs at an 0(water)0(oxo) distance of ca. 1.6 angstrom, and the five energy required to favorably orient a solvent H2O molecule for the 0(oxo) attack and to bring it to the transition state amounts to only 35 kJ more. 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.