Hydroxide anion attack governs O-O bond formation in water oxidation catalysed by a mononuclear manganese complex

Electrocatalytic water oxidation by the mononuclear Mn(II) pyridinophane complex [{Py₂N(tBu)₂}Mn(H₂O)₂]²⁺ has been experimentally observed to occur under strongly basic conditions (pH = 12.2). In this work, the mechanism of this reaction has been re-examined using density functional theory (DFT) calculations at the SMD/M06-L-D3/def2-TZVP and SMD/B3LYP*-D3/def2-TZVP//SMD/M06-L-D3/def2-TZVP levels of theory. Previous studies have proposed that the active species responsible for O-O bond formation is the bis-oxo Mn(V) complex [{Py₂N(tBu)₂}Mn(O)₂]²⁺, and that this step proceeds either through an oxo-oxo coupling (OOC) mechanism or via a water nucleophilic attack (WNA) mechanism. However, we found that O-O coupling through these two mechanisms requires activation barriers of approximately 26 kcal mol^-1, which are too high to be accessible under the employed reaction conditions. Based on our calculations, this crucial step proceeds via the free hydroxide nucleophilic attack (FHNA) mechanism, in which hydroxide attacks the bis-oxo Mn(V) complex, with a calculated activation free energy of approximately 19 kcal mol^-1. Further analysis reveals that in the O-O bond-forming step, both the nucleophilicity of the attacking species and the energy of the Mn d_z² acceptor orbital play key roles in lowering the activation barrier. Our calculations show that the energy of this Mn d_z² acceptor orbital can be tuned through modifications to the ligand backbone of the Mn pyridinophane complex. These computational findings may offer useful insights for the rational design of more effective catalysts for water oxidation.