title: Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product.
abstract: We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.
authors.affiliation: †Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States.|§Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati - 781039, Assam, India.
authors.forename: Akshai
authors.initials: A
authors.lastname: Kumar
authors.affiliation: †Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States.
authors.forename: Tian
authors.initials: T
authors.lastname: Zhou
authors.affiliation: †Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Emge
authors.affiliation: ‡Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States.
authors.forename: Oleg
authors.initials: O
authors.lastname: Mironov
authors.affiliation: ‡Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Saxton
authors.affiliation: †Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States.
authors.forename: Karsten
authors.initials: K
authors.lastname: Krogh-Jespersen
authors.affiliation: †Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States.
authors.forename: Alan S
authors.initials: AS
authors.lastname: Goldman