Abstract
Understanding how water oxidation to molecular oxygen proceeds in molecular metal-oxo catalysts is a challenging endeavor due to their structural complexity. In this report, we unravel the water oxidation mechanism of the highly active water oxidation catalyst [Mn4V4O17(OAc)3]3–, a polyoxometalate catalyst with a [Mn4O4]6+ cubane core reminiscent of the natural oxygen-evolving complex. Starting from the activated species [Mn44+V4O17(OAc)2(H2O)(OH)]1–, we scrutinized multiple pathways to find that water oxidation proceeds via a sequential proton-coupled electron transfer (PCET), O–O bond formation, another PCET, an intramolecular electron transfer, and another PCET resulting in O2 evolution, with a predicted thermodynamic overpotential of 0.71 V. An in-depth investigation of the O–O bond formation process revealed an essential interplay between redox isomerism and Jahn–Teller effects, responsible for enhancing reactivity in the catalytic cycle. This is achieved by redistributing electrons between metal centers and weakening relevant bonds through Jahn–Teller distortions, introducing flexibility to the otherwise rigid cubane core of the catalyst. These mechanistic insights are expected to advance the design of efficient bioinspired Mn cubane water-splitting catalysts.
Highlights
Should catalyze water oxidation at a low thermodynamic overpotential, that is, the overall reaction potential should be overcome in four equal steps;[6,14,15] (ii) the water oxidation catalysts (WOCs) should be stable under the oxidative conditions typically found in experimental photo- or electrocatalytic water-splitting setups;[6,16] (iii) earth-abundant elements should be used for the metal centers to minimize the cost and environmental impact of future industrial-scale usage;[16] and (iv) every synthetic WOC is judged by its activity, with the ultimate goal of approaching or even surpassing the natural oxygen-evolving complex (OEC).[17,18]
We propose a water oxidation mechanism for the bioinspired water oxidation catalyst [Mn4V4O17(OAc)3]3−, s(tOarHti)n]g1−fro(m1)t.hAe fatectrivaactteidvastpioenc,ietsh[eMcna4t4a+lVys4tOh17o(lOdsAfco)u2(r Hre2Odo)xequivalents in the form of four Mn4+ centers; the catalyst binds cofacial OH and H2O ligands that are positioned in close proximity, allowing them both to participate in water oxidation
We found that redox isomerism and JT effects play a prominent role in the water oxidation cycle of [Mn4V4O17(OAc)3]3−: In the O2 release step, JT distortions contribute to lowering the barrier of MnB3+−O bond cleavage in a straightforward manner
Summary
In the development of synthetic molecular WOCs, a number of central design criteria have emerged:[6,10] (i) the WOC should catalyze water oxidation at a low thermodynamic overpotential, that is, the overall reaction potential should be overcome in four equal steps;[6,14,15] (ii) the WOC should be stable under the oxidative conditions typically found in experimental photo- or electrocatalytic water-splitting setups;[6,16] (iii) earth-abundant elements should be used for the metal centers to minimize the cost and environmental impact of future industrial-scale usage;[16] and (iv) every synthetic WOC is judged by its activity, with the ultimate goal of approaching or even surpassing the natural oxygen-evolving complex (OEC).[17,18]. Co cubane WOCs have been extensively studied both experimentally and Received: August 7, 2021 Revised: September 28, 2021 Published: October 18, 2021
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