O-O Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis

A deeper mechanistic understanding of the key O-O bond formation step of water oxidation by the [Ru(bda)(L)₂] (bdaH₂ = 2,2'-bipyridine-6,6'-dicarboxylic acid; L is a pyridine or isoquinoline derivative) family of catalysts is reached through harmonious experimental and computational studies of two series of modified catalysts with systematic variations in the axial ligands. The introduction of halogen and electron-donating substituents in [Ru(bda)(4-X-py)₂] and [Ru(bda)(6-X-isq)₂] (X is H, Cl, Br, and I for the pyridine series and H, F, Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent interactions between the axial ligands in the transition state for the bimolecular O-O coupling, resulting in a lower activation barrier and faster catalysis. From detailed transition state calculations in combination with experimental kinetic studies, we find that the main contributor to the free energy of activation is entropy due to the highly organized transition states, which is contrary to other reports. Previous work has considered only the electronic influence of the substituents, suggesting electron-withdrawing groups accelerate catalysis, but we show that a balance between polarizability and favorable π-π interactions is the key, leading to rationally devised improvements. Our calculations predict the catalysts with the lowest Δ G^⧧ for the O-O coupling step to be [Ru(bda)(4-I-py)₂] and [Ru(bda)(6,7-(OMe)₂-isq)₂] for the pyridine and isoquinoline families, respectively. Our experimental results corroborate these predictions: the turnover frequency for [Ru(bda)(4-I-py)₂] (330 s^-1) is a 10-fold enhancement with respect to that of [Ru(bda)(py)₂], and the turnover frequency for [Ru(bda)(6-OMe-isq)₂] reaches 1270 s^-1, two times faster than [Ru(bda)(isq)₂].