| Autor: |
McKay AI; Chemistry Research Laboratories, University of Oxford , Oxford OX1 3TA , United Kingdom., Bukvic AJ; Chemistry Research Laboratories, University of Oxford , Oxford OX1 3TA , United Kingdom., Tegner BE; Institute of Chemical Sciences, Heriot Watt University , Edinburgh EH14 4AS , United Kingdom., Burnage AL; Institute of Chemical Sciences, Heriot Watt University , Edinburgh EH14 4AS , United Kingdom., Martı Nez-Martı Nez AJ; Chemistry Research Laboratories, University of Oxford , Oxford OX1 3TA , United Kingdom., Rees NH; Chemistry Research Laboratories, University of Oxford , Oxford OX1 3TA , United Kingdom., Macgregor SA; Institute of Chemical Sciences, Heriot Watt University , Edinburgh EH14 4AS , United Kingdom., Weller AS; Chemistry Research Laboratories, University of Oxford , Oxford OX1 3TA , United Kingdom. |
| Abstrakt: |
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(Cy 2 PCH 2 CH 2 PCy 2 )(η:η-(H 3 C)CH(CH 3 ) 2 ][BAr F 4 ] and [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(η:η-C 6 H 12 )][BAr F 4 ] 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 D 2 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 H 2 . 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 [ k H / k D = 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 k H / k D for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step. |
