Water OxidationCatalysis by an Iridium Complex Stabilizedwith an N,N,O-Donor Tripodal Ligand

The facial tridentate N,N,O-donor ligand bpeH [1,1-di(pyridin-2-yl)ethanoate, <b>L2</b>] is based on the successful pyalkH [2-(2′-pyridyl)-2-propanoate, <b>L1</b>] ligand. <b>L1</b> yields a precatalyst [Cp*Ir(pyalk)Cl] (<b>1</b>) that we have used extensively for water oxidation catalysis. We now find that <b>L2</b> readily forms an Ir(III) water oxidation precatalyst, [Cp*Ir(bpe)]Cl (<b>2</b>) that can subsequently be chemically activated with sodium periodate to form a novel Ir(IV) water oxidation catalyst in the form of a blue solution species (<b>BS2</b>) analogous to the blue solution species (<b>BS1</b>) formed from <b>1</b>. By optimizing the NaIO₄ stoichiometry in the activation process, a O₂ yield for water oxidation of 84% was achieved. A comparison of the activation of <b>1</b> and <b>2</b> showed that <b>2</b> yields a water oxidation catalyst with a higher O₂ yield. However, <b>BS2</b> exhibited a 10-fold lower turnover frequency and reaction rate compared to <b>BS1</b>, likely because water molecules cannot access the positions trans to the μ-oxo ligand. This limitation causes <b>BS2</b> to evolve into a more stable but less catalytically active molecular configuration. After O₂ evolution following the addition of NaIO₄ has ceased, <b>BS2</b> reaches a quiescent state able to maintain its molecular integrity such that it can be reactivated with periodate even after 10 days under ambient conditions, restoring approximately 80% of the initial O₂ yield. Notably, minimal periodate addition was sufficient to reactivate the catalytic species. <b>L2</b> further allowed the acquisition of the first clearly identifiable ¹H NMR spectrum of a blue solution. While the formation of paramagnetic species complicated complete NMR spectroscopic characterization, ongoing efforts are focused on elucidating the molecular structure of both the active and dormant species.