Synthesis and Characterization of Carbazolide-Based Iridium PNP Pincer Complexes. Mechanistic and Computational Investigation of Alkene Hydrogenation: Evidence for an Ir(III)/Ir(V)/Ir(III) Catalytic C
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- 作者：Chen Cheng ; Bong Gon Kim ; Damien Guironnet ; Maurice Brookhart ; Changjian Guan ; David Y. Wang ; Karsten Krogh-Jespersen ; Alan S. Goldman
- 刊名：Journal of the American Chemical Society
- 出版年：2014
- 出版时间：May 7, 2014
- 年：2014
- 卷：136
- 期：18
- 页码：6672-6683
- 全文大小：656K
- 年卷期：v.136,no.18(May 7, 2014)
- ISSN：1520-5126
文摘
New carbazolide-based iridium pincer complexes (carbPNP)Ir(C2H4), 3a, and (carbPNP)Ir(H)2, 3b, have been prepared and characterized. The dihydride, 3b, reacts with ethylene to yield the cis-dihydride ethylene complex cis-(carbPNP)Ir(C2H4)(H)2. Under ethylene this complex reacts slowly at 70 掳C to yield ethane and the ethylene complex, 3a. Kinetic analysis establishes that the reaction rate is dependent on ethylene concentration and labeling studies show reversible migratory insertion to form an ethyl hydride complex prior to formation of 3a. Exposure of cis-(carbPNP)Ir(C2H4)(H)2 to hydrogen results in very rapid formation of ethane and dihydride, 3b. DFT analysis suggests that ethane elimination from the ethyl hydride complex is assisted by ethylene through formation of (carbPNP)Ir(H)(Et)(C2H4) and by H2 through formation of (carbPNP)Ir(H)(Et)(H2). Elimination of ethane from Ir(III) complex (carbPNP)Ir(H)(Et)(H2) is calculated to proceed through an Ir(V) complex (carbPNP)Ir(H)3(Et) which reductively eliminates ethane with a very low barrier to return to the Ir(III) dihydride, 3b. Under catalytic hydrogenation conditions (C2H4/H2), cis-(carbPNP)Ir(C2H4)(H)2 is the catalyst resting state, and the catalysis proceeds via an Ir(III)/Ir(V)/Ir(III) cycle. This is in sharp contrast to isoelectronic (PCP)Ir systems in which hydrogenation proceeds through an Ir(III)/Ir(I)/Ir(III) cycle. The basis for this remarkable difference is discussed.