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

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• 作者：Daria L. Huang ; David J. Vinyard ; James D. Blakemore ; Sara M. Hashmi ; Robert H. Crabtree
• 刊名：Organometallics
• 出版年：2017
• 出版时间：January 9, 2017
• 年：2017
• 卷：36
• 期：1
• 页码：199-206
• 全文大小：464K
• ISSN：1520-6041

文摘

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-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 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.