Reversible O–O Bond Scission and O₂ Evolution at MOF-supported Tetramanganese Clusters

Abstract

The scission of the O–O bond in O₂ and the formation of O–O bonds during the evolution of O₂ in photosynthesis are the engines of aerobic life as we know it. Likewise, the reduction of O2 and its reverse, the oxidation of reduced oxygen species to form O₂, are indispensable components of emerging renewable technologies for energy storage and conversion. Storing and unleashing the energy contained within the O₂ molecule requires control over the formation or scission of the four-electron double bond between two oxygen atoms. Nature performs these demanding multi-electron transformations by distributing the redox burden among multiple metal ions: evolution created metalloenzymes with polynuclear clusters wherein metal ions act in concert to deliver or accept multiple electrons. Most relevant in this sense are the tetramanganese-calcium (Mn₄CaO₅) cluster in the oxygen-evolving complex of photosystem II, responsible for O₂ formation, and the tricopper clusters in multi-copper oxidases, which mediate O₂ reduction. The critical step in both O₂ reduction and O₂ evolution is the interconversion between multinuclear metal-oxo species, where the O atoms carry formal –2 charges, and O–O bonded species where the oxygen atoms are more oxidized. Attempts to mimic the natural systems with synthetic analogs have led to a number of elegant molecular clusters that shed light on the electron transfer events and stepwise mechanism of O–O bond cleavage and formation. To our knowledge, however, the interconversion between molecular O₂ and metal-oxo species in either synthetic or enzymatic discrete systems has not been documented. Here, we report that a tetramangenese cluster formed by self-assembly within a metal–organic framework (MOF) spontaneously cleaves the oxygen-oxygen double bond and reduces O₂ by four electrons at room temperature. The ensuing tetranuclear manganese-oxo cluster engages in weak C–H bond activation and, more importantly, is competent for O–O bond formation and O₂ evolution at elevated temperature, enabled by the head-to-head orientation of two fully reduced oxo atoms that bridge neighboring Mn pairs. This study demonstrates the viability of four-electron interconversion between molecular O₂ and metal-oxo species and highlights the importance of site-isolation for achieving multi-electron chemistry at polynuclear metal clusters.