Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction.

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn III 3 Mn IV O 3 (CH 3 COO) 3 (A-α-SiW 9 O 34 )] 6- . Theoretical results indicated that catalytic active species 1 (Mn 4 III,III,IV,IV ) was formed after O 2 formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn IV -oxyl radical 4 (Mn 4 IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn 2 O 2 core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn 4 III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol -1 . Subsequent PCET oxidation of Int1 to Int2 (Mn 4 III,IV,IV,IV -O 2 ⋅) enabled the O 2 release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.