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© 2016 American Chemical Society. We previously reported a dimeric IrIV-oxo species as the active water oxidation catalyst formed from a Cp∗Ir(pyalc)Cl {pyalc = 2-(2′-pyridyl)-2-propanoate} precursor, where the Cp∗ is lost to oxidative degradation during catalyst activation; this system can also oxidize unactivated CH bonds. We now show that the same Cp∗Ir(pyalc)Cl precursor leads to two distinct active catalysts for CH oxidation. In the presence of external CH substrate, the Cp∗ remains ligated to the Ir center during catalysis; the active species-likely a highvalent Cp∗Ir(pyalc) species-will oxidize the substrate instead of its own Cp∗. If there is no external CH substrate in the reaction mixture, the Cp∗ will be oxidized and lost, and the active species is then an iridium-μ-oxo dimer. Additionally, the recently reported Ir(CO)2(pyalc) water oxidation precatalyst is now found to be an efficient, stereoretentive CH oxidation precursor. We compare the reactivity of Ir(CO)2(pyalc) and Cp∗Ir(pyalc)Cl precursors and show that both can lose their placeholder ligands, CO or Cp∗, to form substantially similar dimeric IrIV-oxo catalyst resting states. The more efficient activation of the bis-carbonyl precursor makes it less inhibited by obligatory byproducts formed from Cp∗ degradation, and therefore the dicarbonyl is our preferred precatalyst for oxidation catalysis.

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