Organoiridium Pyridonates and Their Role in the Dehydrogenation of Alcohols

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• 作者：Aaron M. Royer ; Thomas B. Rauchfuss ; Danielle L. Gray
• 刊名：Organometallics
• 出版年：2010
• 出版时间：December 27, 2010
• 年：2010
• 卷：29
• 期：24
• 页码：6763-6768
• 全文大小：814K
• 年卷期：v.29,no.24(December 27, 2010)
• ISSN：1520-6041

文摘

New derivatives of 2-hydroxypyridine (2-hpH) and Cp*Ir(III) are described. Under conditions for catalytic dehydrogenation of 1-phenylethanol catalyzed by Cp*IrCl(κ²-2-hp) (1), the main species observed are [Cp*₂Ir₂H₂(2-hp)]Cl ([2]Cl) and Cp*IrHCl(κ¹-2-hpH) (3). Crystallographic analysis confirms that the cation in [2]PF₆ consists of a Cp*₂Ir₂(μ-H)_x²⁺ core complemented by a pyridonate ligand that bridges via O and N centers. Although [2]Cl is catalytically highly active, the related salt [2]PF₆ is not. Addition of chloride sources reactivates [2]PF₆. Collectively, our experiments indicate that [2]Cl is a resting state that reverts to a more active species, which we propose is 1 itself. In situ NMR observations and PPh₃ trapping experiments show that under catalytically relevant conditions 1 rapidly converts to 3, which can be observed spectroscopically. Compound 3 was independently generated by transfer hydrogenation of 1. In other experiments, 1 was found to ring-open upon treatment with PPh₃ to give Cp*IrCl(κ¹-2-hp)(PPh₃), which in turn was found to react with AgPF₆ to give [Cp*Ir(κ²-2-hp)(PPh₃)]PF₆. Both PPh₃ derivatives proved catalytically inactive for dehydrogenation. Cp*IrCl(κ²-2-hp-6-Me) was also prepared but could not be converted to κ¹-2-hpH-6-Me derivatives. The complex Cp*IrCl(C₅H₃O₂NH), nominally derived from the conjugate base of 2,6-dihydroxypyridine, features the novel ligand η³-C₃H₃(CO)₂NH.