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 cycle

Chen Cheng, Bong Gon Kim, Damien Guironnet, Maurice Brookhart, Changjian Guan, David Y. Wang, Karsten Krogh-Jespersen, Alan S. Goldman

Research output: Contribution to journalArticle

41 Citations (Scopus)

Abstract

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 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)(C2H 4) 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/H 2), 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.

Original languageEnglish
Pages (from-to)6672-6683
Number of pages12
JournalJournal of the American Chemical Society
Volume136
Issue number18
DOIs
Publication statusPublished - May 7 2014

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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