Cp* versus Bis-carbonyl Iridium Precursors as CH Oxidation Precatalysts

We previously reported a dimeric Ir-IV-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 high-valent 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-mu-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 Ir-IV-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.