Folding‐Induced Promotion of Proton‐Coupled Electron Transfers via Proximal Base for Light‐Driven Water Oxidation

Abstract

Proton-coupled electron-transfer (PCET) processes play a key role in biocatalytic energy conversion and storage, for example, photosynthesis or nitrogen fixation. Here, we report a series of bipyridinecontaining di-to tetranuclear Ru(bda) macrocycles 2 C-4 C (bda: 2,2'-bipyridine-6,6'-dicarboxylate) to promote OÀ O bond formation. In photocatalytic water oxidation under neutral conditions, all complexes 2 C-4 C prevail in a folded conformation that support the water nucleophilic attack (WNA) pathway with remarkable turnover frequencies of up to 15.5 s À 1 per Ru unit respectively. Single-crystal X-ray analysis revealed an increased tendency for intramolecular π-π stacking and preorganization of the proximal bases close to the active centers for the larger macrocycles. H/D kinetic isotope effect studies and electrochemical data demonstrate the key role of the proximal bipyridines as proton acceptors in lowering the activation barrier for the crucial nucleophilic attack of H 2 O in the WNA mechanism.