Water Oxidation Catalysis by an Iridium Complex Stabilized with an N,N,O-Donor Tripodal Ligand

Abstract

The facial tridentate N,N,O-donor ligand bpeH [1,1-di(pyridin-2-yl)ethanoate, L2] is based on the successful pyalkH [2-(2′-pyridyl)-2-propanoate, L1] ligand. L1 yields a precatalyst [Cp*Ir(pyalk)Cl] (1) that we have used extensively for water oxidation catalysis. We now find that L2 readily forms an Ir(III) water oxidation precatalyst, [Cp*Ir(bpe)]Cl (2) 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 (BS2) analogous to the blue solution species (BS1) formed from 1. 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 1 and 2 showed that 2 yields a water oxidation catalyst with a higher O₂ yield. However, BS2 exhibited a 10-fold lower turnover frequency and reaction rate compared to BS1, likely because water molecules cannot access the positions trans to the μ-oxo ligand. This limitation causes BS2 to evolve into a more stable but less catalytically active molecular configuration. After O₂ evolution following the addition of NaIO₄ has ceased, BS2 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. L2 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.