Secondary Coordination Sphere Effects in Macrocycle-Embedded Mononuclear Ru(bda) Water Oxidation Catalysts

Integration of Ru(bda) [bda = (2,2'-bipyridine)-6,6'-dicarboxylate] water oxidation catalysts together with dibenzofuran, dibenzothiophene, or carbazole heterocycles within macrocycles 1-3 affords structures with different conformational preferences. The latter are dictated by noncovalent interactions of the bda oxygens with NH (hydrogen bond) or S (chalcogen bond). The different orientations afford changes in the secondary coordination sphere, thereby influencing the rate and mechanism of photocatalytic water oxidation for the three mononuclear Ru(bda)-based catalysts, with the dibenzothiophene- and carbazole-based ones favoring the unimolecular pathway (first-order kinetics), typically related to water nucleophilic attack (WNA), and the dibenzofuran one operating via a bimolecular pathway (second-order kinetics), typically related to the interaction of two metal-oxo species (I2M). NMR and single crystal X-ray analyses provided insight into the conformational preferences of the precursor macrocycles in the Ru(II) state. Photocatalytic studies including the H/D kinetic isotope effect (KIE) in deuterated water together with theoretical studies on the orientation angle-dependent energy profile for macrocycles in the Ru(V) state afforded a structure-property relationship that explains the outcome of the water oxidation experiments.