Supramolecular engineering of functional mimics for Nature’s oxygen evolving center in Photosystem II (OEC-PSII) has been widely applied in the design of molecular architectures for artificial water splitting. Recently, we have reported the benefit of cooperative effects (e.g. hydrogen bonding) within the cavity of multinuclear Ru(bda) macrocycles (bda: 2,2’ bipyridine-6,6’-dicarboxylate) as promising strategy to mechanistically mimic the natural blueprint. Herein, we introduce bio-inspired cyclic mononuclear Ru catalysts M1 (X = CH) and M2 (X = N) to probe the effect of a proximal base on the catalytic performance under acidic and neutral conditions. Our detailed investigations by single crystal X-ray and pH-dependent NMR analysis revealed that the catalytically active Ru centers are significantly affected by the relative spatial orientation of the respective axial ligand framework. Kinetic investigations, including pH dependent kinetic isotope effect studies, disclosed a mechanistic switch from the bimolecular I2M (interaction of two M-O units) mechanism to unimolecular water nucleophilic attack (WNA) pathway resulting from the pH-induced ligand rotation at pH 1 for Monomer M2. Accordingly, the cyclic architecture leads to the formation of a confined space with the catalytic center, which accelerates the rate of water oxidation (TOF = 140 s-1) and thus makes M2 to one of the best performing homogenous Ru water oxidation catalysts in chemical water oxidation to date.

Enzyme-like water oxidation catalysis within pH-switchable cyclic mononuclear ruthenium complexes

ACS Spring 2022

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