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Origin of Regioselectivity in the Dehydrogenation of Alkanes by Pincer–Iridium Complexes: A Combined Experimental and Computational Study

  • Soumik Biswas
    Soumik Biswas
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
  • Michael J. Blessent
    Michael J. Blessent
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
  • Benjamin M. Gordon
    Benjamin M. Gordon
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
  • Tian Zhou
    Tian Zhou
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
    More by Tian Zhou
  • Santanu Malakar
    Santanu Malakar
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
  • David Y. Wang
    David Y. Wang
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
  • Karsten Krogh-Jespersen
    Karsten Krogh-Jespersen
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
  • , and 
  • Alan S. Goldman*
    Alan S. Goldman
    Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08854, United States
    *Email: [email protected]
Cite this: ACS Catal. 2021, 11, 19, 12038–12051
Publication Date (Web):September 14, 2021
https://doi.org/10.1021/acscatal.1c02872
Copyright © 2021 American Chemical Society

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    Abstract

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    PCP-pincer (κ3-2,6-C6H3(CH2PR2)2) iridium complexes have been reported to catalyze the transfer dehydrogenation of n-alkanes with high regioselectivity for the terminal position. We find that the very closely related PCOP (κ3-2,6-C6H3(CH2PR2)(OPR2)) and POCOP (κ3-2,6-C6H3(OPR2)2) complexes, in contrast, afford no such regioselectivity. The difference is a true kinetic phenomenon, i.e., it is not a result of isomerization subsequent to the formation of free α-olefin. In addition to direct observation of the distribution of n-alkane dehydrogenation products over time, the pronounced difference in regioselectivity is confirmed through intermolecular competition studies of the reverse reaction (olefin transfer hydrogenation) and of the dehydrogenation of cycloalkane vs n-alkane. Electronic structure (DFT) calculations indicate that the rate- and selectivity-determining step for dehydrogenation by the (PCP)Ir complexes is β-H transfer. C–H activation at the primary position is much more favorable than at secondary positions, but this is not responsible for the terminal regioselectivity; indeed, the formation of α-olefin via C2–H addition and transfer of the C1–H bond is calculated to be slightly more favorable than dehydrogenation proceeding via C1–H addition. For both PCP and POCOP complexes, the formation of the α-olefin iridium dihydride complex is more facile than the formation of internal-olefin complexes. The next step in the catalytic pathway, loss of olefin, is calculated to have an activation energy that is significantly greater than the metal–ligand (thermodynamic) bond energy. In the case of POCOP complexes, the loss of olefin, rather than β-H transfer, is the rate- and selectivity-determining step. The hydrocarbon moiety in the transition state for olefin loss has the character of a fully formed olefin; this favors the formation of internal olefin. The different regioselectivity of (POCOP)Ir vs (PCP)Ir catalysts is thus attributable to the different rate-determining steps of their respective catalytic cycles; this in turn can be explained in terms of different electronic effects of O versus CH2 linker exerted through the pincer aromatic ring.

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    Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.1c02872.

    • Full characterization data, synthesis of (tBu2PCPiPr2)IrH4, experimental procedures, and data for catalytic runs; computational details, including tables of thermodynamic quantities and optimized geometries, absolute energies (PDF)

    • mol files of relevant species (ZIP)

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    Cited By

    This article is cited by 6 publications.

    1. Alexander A. Kolganov, A. Sreenithya, Evgeny A. Pidko. Homogeneous Catalysis in Plastic Waste Upcycling: A DFT Study on the Role of Imperfections in Polymer Chains. ACS Catalysis 2023, 13 (20) , 13310-13318. https://doi.org/10.1021/acscatal.3c03269
    2. Naoto Uno, Ryo Nakano, Makoto Yamashita. Low-Temperature Dehydrogenation of Cyclooctane by Using Pincer Iridium Complexes Bearing an N,N′-Diarylated PNCNP Ligand. ACS Catalysis 2023, 13 (10) , 6956-6965. https://doi.org/10.1021/acscatal.3c01739
    3. Benjamin M. Gordon, Ashish Parihar, Faraj Hasanayn, Alan S. Goldman. High Activity and Selectivity for Catalytic Alkane–Alkene Transfer (De)hydrogenation by (tBuPPP)Ir and the Importance of Choice of a Sacrificial Hydrogen Acceptor. Organometallics 2022, 41 (22) , 3426-3434. https://doi.org/10.1021/acs.organomet.2c00401
    4. Benjamin M. Gordon, Nicholas Lease, Thomas J. Emge, Faraj Hasanayn, Alan S. Goldman. Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity. Journal of the American Chemical Society 2022, 144 (9) , 4133-4146. https://doi.org/10.1021/jacs.1c13309
    5. Kuan Wang, Lan Gan, Yuheng Wu, Min-Jie Zhou, Guixia Liu, Zheng Huang. Selective dehydrogenation of small and large molecules by a chloroiridium catalyst. Science Advances 2022, 8 (38) https://doi.org/10.1126/sciadv.abo6586
    6. Chuanyong Wang, Zhongqiu Xing, Qiangqiang Ge, Yangyang Yu, Minyan Wang, Wei-Liang Duan. Site-selective desaturation of C(sp 3 )–C(sp 3 ) bonds via photoinduced ruthenium catalysis. Organic Chemistry Frontiers 2022, 9 (16) , 4316-4327. https://doi.org/10.1039/D2QO00332E

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