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We report that the cationic iridium complex (iPrPCP)IrH+ catalyzes the transfer-dehydrogenation of alkanes to give alkenes, and hydrogen isotope exchange (HIE) of alkanes and arenes. Contrary to established selectivity trends found for C-H activation by transition metal complexes, strained cycloalkanes, including cyclopentane, cycloheptane, and cyclooctane, undergo C-H addition much more readily than n-alkanes, which in turn are much more reactive than cyclohexane. Aromatic C-H bonds also undergo H/D exchange much less rapidly than those of the strained cycloalkanes, but much more favorably than cyclohexane. The order of reactivity toward dehydrogenation correlates qualitatively with the reaction thermodynamics, but the magnitude is much greater than can be explained by thermodynamics. Accordingly, the cycloalkenes corresponding to the strained cycloalkanes undergo hydrogenation much more readily than cyclohexene, despite the less favorable thermodynamics of such hydrogenations. Computational (DFT) studies allow rationalization of the origin of reactivity and the unusual selectivity. Specifically, the initial C-H addition is strongly assisted by Ī²-agostic interactions, which are particularly favorable for the strained cycloalkanes. Subsequent to Ī²-C-H addition, the H atom of the Ī²-agostic C-H bond is transferred directly to the hydride ligand of (iPrPCP)IrH+, to give a dihydrogen ligand. The overall processes, C-H addition and Ī²-H-transfer to hydride, are calculated to generally have minima on the IRC surface although not necessarily on the enthalpy or free energy surfaces; these minima are extremely shallow, such that the 1,2-dehydrogenations are effectively concerted although asynchronous.
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Alkane Dehydrogenation and H/D Exchange by a Cationic Pincer-Ir(III) Hydride: A Novel Mechanism of C-H Addition and š½-H Elimination Leads to Unprecedented Selectivity
We report that the cationic iridium complex (iPrPCP)IrH+ undergoes addition of alkane C-H bonds, which is manifested by catalytic alkane transfer-dehydrogenation to give alkenes and by hydrogen isotope (H/D) exchange (HIE). Contrary to established selectivity trends found for C-H activation by transition metal complexes, strained cycloalkanes, including cyclopentane, cycloheptane, and cyclooctane, undergo C-H addition much more readily than n-alkanes which in turn are much more reactive than cyclohexane. Aromatic C-H bonds also undergo H/D exchange much less rapidly than those of the strained cycloalkanes, but much more favorably than cyclohexane. The order of reactivity toward dehydrogenation correlates qualitatively with the reaction thermodynamics, but the magnitude is much greater than can be explained by thermodynamics. Accordingly, the cycloalkenes corresponding to the strained cycloalkanes undergo hydrogenation much more readily than cyclohexene, despite the less favorable thermodynamics of such hydrogenations. Computational (DFT) studies allow rationalization of the origin of reactivity and the unusual selectivity. Specifically, the initial C-H addition is strongly assisted by š½-agostic interactions, which are particularly favorable for the strained cycloalkanes. Subsequent to š¼-C-H addition, the H atom of the š½-agostic C-H bond is transferred to the hydride ligand of (iPrPCP)IrH+, to give a dihydrogen ligand. The overall processes, C-H addition and š½-H-transfer to hydride, generally show intermediates on the IRC surface but they are extremely shallow, such that the 1,2-dehydrogenations are presumed to be effectively concerted although asynchronous.
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- Award ID(s):
- 2117792
- PAR ID:
- 10581500
- Publisher / Repository:
- ChemRxiv
- Date Published:
- Subject(s) / Keyword(s):
- C-H activation agostic bond dehydrogenation Hydrogen Isotope Exchange pincer iridium
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Posted Content:
- https://doi.org/10.26434/chemrxiv-2024-plh2j
