Mechanism for OO bond formation in a biomimetic tetranuclear manganese cluster – A density functional theory study

• Water oxidation mechanism catalyzed by Mn4O4L6 is studied by DFT calculations.
• O2 evolution mechanism is different in the gas phase and in a water solution.
• OO bond is formed via direct coupling of two bridging oxo groups in the gas phase.
• OO bond formation proceeds via water attack on the MnIV-oxyl radical in solution.

Density functional theory calculations have been used to study the reaction mechanism of water oxidation catalyzed by a tetranuclear Mn-oxo cluster Mn4O4L6 (L = (C6H4)2PO4−). It is proposed that the OO bond formation mechanism is different in the gas phase and in a water solution. In the gas phase, upon phosphate ligand dissociation triggered by light absorption, the OO bond formation starting with both the Mn4(III,III,IV,IV) and Mn4(III,IV,IV,IV) oxidation states has to take place via direct coupling of two bridging oxo groups. The calculated barriers are 42.3 and 37.1 kcal/mol, respectively, and there is an endergonicity of more than 10 kcal/mol. Additional photons are needed to overcome these large barriers. In water solution, water binding to the two vacant sites of the Mn ions, again after phosphate dissociation triggered by light absorption, is thermodynamically and kinetically very favorable. The catalytic cycle is suggested to start from the Mn4(III,III,III,IV) oxidation state. The removal of three electrons and three protons leads to the formation of a Mn4(III,IV,IV,IV)-oxyl radical complex. The OO bond formation then proceeds via a nucleophilic attack of water on the MnIV-oxyl radical assisted by a Mn-bound hydroxide that abstracts a proton during the attack. This step was calculated to be rate-limiting with a total barrier of 29.2 kcal/mol. This is followed by proton-coupled electron transfer, O2 release, and water binding to start the next catalytic cycle.

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