Multisite Proton-Coupled Electron Transfer Facilitates Oxidative Photocatalysis in a Molecular Zr-Based Coordination Compound.

The development of mediators that harness visible light to drive proton-coupled electron transfer (PCET) offers a promising pathway to achieving challenging redox transformations in a more sustainable manner and with enhanced thermochemical efficiency. However, designing photocatalytic systems based on earth-abundant metals while gaining precise control over their excited-state reactivity remains a significant challenge. Here, deprotonation of the hydroxy ligands in the Zr3(O)(OH)3 nodes of a photoactive coordination cage is shown to unlock the photocatalytic oxidation of strong O-H and C-H bonds (70-100 kcal·mol-1). Mechanistic investigations reveal that this oxidative process proceeds via a multisite PCET pathway involving ground-state, pre-association followed by a static quenching mechanism. This contrasts with the dynamic quenching mechanism governing the reductive PCET previously reported for the same system. Collectively, these findings establish an unprecedent ambipolar PCET mechanism with a new class of photocatalytic mediators based on an earth abundant metal.