Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ‑Alkane Complexes

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]­[BArF 4] and [Rh­(Cy2PCH2CH2PCy2)­(η:η-C6H12)]­[BArF 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 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 [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.