Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes

Alasdair I. McKay, Alexander J. Bukvic, Bengt E. Tegner, Arron L. Burnage, Antonio J. Martı́nez-Martı́nez, Nicholas H. Rees, Stuart A. Macgregor, Andrew S. Weller

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Abstract

The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η:η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η:η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step.

Original languageEnglish
Pages (from-to)11700-11712
Number of pages13
JournalJournal of the American Chemical Society
Volume141
Issue number29
Early online date27 Jun 2019
DOIs
Publication statusPublished - 24 Jul 2019

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    McKay, A. I., Bukvic, A. J., Tegner, B. E., Burnage, A. L., Martı́nez-Martı́nez, A. J., Rees, N. H., Macgregor, S. A., & Weller, A. S. (2019). Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes. Journal of the American Chemical Society, 141(29), 11700-11712. https://doi.org/10.1021/jacs.9b05577