Hydrophobic/Hydrophilic Directionality Affects the Mechanism of Ru-Catalyzed Water Oxidation Reaction

From the study of supramolecular dimers of [RuVO(pda)]+ (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [RuVO(bda)]+ (bda = 2,2′-bipyridine-6,6′-dicarboxylate) complexes, the O–O bond-forming intermediates in water oxidation, we found orientational distinction induced by the pda and bda ligands. The bda complex prefers the front-to-front geometry, while the pda complex favors the front-to-back geometry in the formation of prereactive geometry. In the bda complex, the hydrophobic oxo will point at another oxo with the bda directed toward water, which favors the I2M mechanism. In the pda complex, the hydrophobic oxo instead is directed toward a more hydrophobic phenanthroline moiety of the pda of another species, which disfavors I2M. The binding free energy of the nonproductive front-to-back of pda is 3 kcal mol–1 more stable than that of the prereactive dimer. This incorrect orientation leads to an additional rearrangement required before the O–O bond can be formed. Estimation of the rate constant shows 2 orders of magnitude lower reactivity for the I2M mechanism of the pda complex relative to the bda complex, which makes the water nucleophilic attack mechanism competitive.