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 [(Mn3MnO3)-Mn-III-O-IV(CH3COO)(3)(A-alpha-SiW9O34)](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 center dot). Importantly, 4 had an unusual butterfly-shaped Mn2O2 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 center dot) 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.

Automated summary

Density functional theory calculations were used to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex. Theoretical results showed that sequential proton-coupled electron transfer oxidations led to the formation of an oxygen radical, facilitating the formation of O-O bond. Subsequent oxidation reactions enabled the release of oxygen.