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

Leonardo Bernasconi*, Andranik Kazaryan, Paola Belanzoni, Evert Jan Baerends

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

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Abstract

We use density functional theory (DFT) and ab initio molecular dynamics to study the conversion of H2O into H2O2 in water solution by the FeIVO2+ 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 FeIVO2+ 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 FeIVO2+ 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 FeIVO2+ 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.

Original languageEnglish
Pages (from-to)4018-4025
Number of pages8
JournalACS Catalysis
Volume7
Issue number6
Early online date8 May 2017
DOIs
Publication statusPublished - 2 Jun 2017

Keywords

  • catalytic oxidation
  • density functional theory
  • molecular dynamics
  • solution
  • water

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