Computational mechanistic study on molecular catalysis of water oxidation by cyclam ligand-based iron complex
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Computational mechanistic study on molecular catalysis of water oxidation by cyclam ligand‑based iron complex Koteswara Rao Gorantla1 · Bhabani S. Mallik1 Received: 6 May 2020 / Accepted: 18 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The iron metal complexes containing cyclam-based macrocyclic ligands are active water oxidation catalysts, which can facilitate oxygen–oxygen bond formation during the water-splitting process. To understand the mechanism of the catalytic process, we explored this process by [Fe(cyclam)Cl2]Cl performing density functional theory-based first-principles calculations. We examined the energetics of the formation of the oxygen–oxygen bond through the investigation of complexes [FeV(cyclam)(O)2]+, and [ FeV(cyclam)(OH)(O)]+2. The process of water nucleophilic addition by this F eV–(oxo) complexes was explored in detail. Our computational study confirms the formation of these species, which were reported earlier in experimental conditions. The transition states for various reactions of the catalytic cycle were obtained within the implicit water model. From these calculations, we find that the proton transfers to cis–oxo or hydroxide moiety require high activation energy during the formation of the oxygen–oxygen bond. Our calculations reveal that the oxygen–oxygen bond formation by the transfer of a proton to the explicit water molecule requires more activation free energy than the transfer of a proton to the cis–oxo or hydroxide of the FeV–oxo species. Overall [FeV(cyclam)(O)2]+ and [FeV(cyclam)(OH)(O)]+2 species with one explicit water molecule requires similar free energy. The Mulliken spin density data confirm the formation of the superoxide complexes. The activation free energy for the release of the oxygen molecule is lesser than that of the oxygen–oxygen bond formation. The natural bond orbital analysis for the complexes before and after the formation of the oxygen–oxygen bond formation shows that this bond formation happens through the interaction of antibonding orbital, π*(dx2 2−y–2py) of Fe=O moiety, with the σ*–orbital of the hydroxide group of the water molecule. Keywords First-principles calculations · Water oxidation · Transition metal complex · Catalysis · Reaction mechanism
1 Introduction The generation of fuels from the artificial photosynthesis process is one of the alternatives to fossil fuels to overcome major problems like the greenhouse effect [1, 2] and energy crisis [3]. In this process, the solar energy is accumulated in the form of chemical bonds by splitting the water molecule [4–7] into oxygen and hydrogen. The formed hydrogen can be used directly as a fuel or converted into another Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00214-020-02664-2) contains supplementary material, which is available to authorized users. * Bhabani S. Mallik [email protected] 1
Department of Chemistry, Indian Institute of Technology Hyderabad, Sangareddy, Tel
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