Spectroscopic Characterization of Alumina-Supported Bis(allyl)iridium Complexes: Site-Isolation, Reactivity, and Decomposition Studies

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• 作者：Ryan J. Trovitch ; Neng Guo ; Michael T. Janicke ; Hongbo Li ; Christopher L. Marshall ; Jeffrey T. Miller ; Alfred P. Sattelberger ; Kevin D. John ; R. Thomas Baker
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：March 1, 2010
• 年：2010
• 卷：49
• 期：5
• 页码：2247-2258
• 全文大小：1015K
• 年卷期：v.49,no.5(March 1, 2010)
• ISSN：1520-510X

文摘

The covalent attachment of tris(allyl)iridium to partially dehydroxylated γ-alumina is found to proceed via surface hydroxyl group protonation of one allyl ligand to form an immobilized bis(allyl)iridium moiety, (═AlO)Ir(allyl)₂, as characterized by CP-MAS ¹³C NMR, inductively coupled plasma-mass spectrometry, and Ir L₃ edge X-ray absorption spectroscopy. Extended X-ray absorption fine-structure (EXAFS) measurements taken on unsupported Ir(allyl)₃ and several associated tertiary phosphine addition complexes suggest that the η³-allyl ligands generally account for an Ir−C coordination number of 2 rather than 3, with an average Ir−C distance of 2.16 Å. Using this knowledge, combined EXAFS and X-ray absorption near-edge structure studies reveal that a small amount of Ir⁰ is also formed upon reaction of Ir(allyl)₃ with the surface. It was found that the addition of either 2,6-dimethylphenyl isocyanide or carbon monoxide to the supported complex allows spectroscopic identification of the supported bis(allyl)iridium complexes, (═AlO)Ir(allyl)₂(CNAr) [Ar = 2,6-(CH₃)₂C₆H₄] and (═AlO)Ir(allyl)₂(CO)₂, respectively. Although samples of the supported bis(allyl)iridium complex are active for the dehydrogenation of cyclohexane to benzene at temperatures between 180 and 220 °C, in situ temperature-programmed reaction XAFS and continuous-flow reactor studies suggest that Ir⁰ nanoparticles, rather than a well-defined Ir³⁺ complex, are responsible for the observed activity.