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]­[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.