Dehydrogenation of n -Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

Akshai Kumar, Tian Zhou, Thomas J. Emge, Oleg Mironov, Robert J. Saxton, Karsten Krogh-Jespersen, Alan S Goldman

Research output: Contribution to journalArticle

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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 iPr4PCP, 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 (iPr4PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of (iPr4PCP)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 (tBu4PCP)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. (Figure Presented).

Original languageEnglish
Pages (from-to)9894-9911
Number of pages18
JournalJournal of the American Chemical Society
Volume137
Issue number31
DOIs
Publication statusPublished - Aug 12 2015

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Iridium
Alkanes
Alkenes
Dehydrogenation
Paraffins
Olefins
Catalysts
Isomerization
Hydrogen
Propylene
Ethylene
Hydrogenation
Discrete Fourier transforms
Gases
Ligands
Pressure

ASJC Scopus subject areas

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

Cite this

Dehydrogenation of n -Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product. / Kumar, Akshai; Zhou, Tian; Emge, Thomas J.; Mironov, Oleg; Saxton, Robert J.; Krogh-Jespersen, Karsten; Goldman, Alan S.

In: Journal of the American Chemical Society, Vol. 137, No. 31, 12.08.2015, p. 9894-9911.

Research output: Contribution to journalArticle

Kumar, Akshai ; Zhou, Tian ; Emge, Thomas J. ; Mironov, Oleg ; Saxton, Robert J. ; Krogh-Jespersen, Karsten ; Goldman, Alan S. / Dehydrogenation of n -Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product. In: Journal of the American Chemical Society. 2015 ; Vol. 137, No. 31. pp. 9894-9911.
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AU - Krogh-Jespersen, Karsten

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N2 - 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 iPr4PCP, 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 (iPr4PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of (iPr4PCP)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 (tBu4PCP)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. (Figure Presented).

AB - 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 iPr4PCP, 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 (iPr4PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of (iPr4PCP)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 (tBu4PCP)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. (Figure Presented).

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