Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes

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]­[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.

Reversible Encapsulation of Xenon and CH2Cl2 in a Solid‐State Molecular Organometallic Framework (Guest@SMOM)

“…[7] Thesimilarities with processes that occur in metalloenzymes,asprobed by structural biology techniques,a re particularly interesting. [7] Thesimilarities with processes that occur in metalloenzymes,asprobed by structural biology techniques,a re particularly interesting.…”

“…[2] Forexample,the isolation and characterization of s-alkane complexes in the solid state is achieved by as imple solid/gas SC-SC hydrogenation reaction of an alkene precursor such as [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(NBD)][BAr F 4 ]( NBD = norbornadiene,A r F = 3,5-(CF 3 ) 2 C 6 H 3 )t of orm the corresponding salkane complex, [BAr F 4 ] (Figure 1; norbornane = NBA). [3] Further examples of NBA, [4] pentane, [5] cyclooctane, [6] isobutane and cyclohexane [7] s-complexes have all been reported with av ariety of [Rh(chelating phosphine)] + ligand sets,and some of these show remarkable stability in the solid state (months at 298 K). Thes tability of these SMOM systems allows for these s-complexes to undergo further SC-SC transformations.For example,reaction with D 2 (leading to CÀH/CÀDe xchange at the alkane), [7,8] H 2 loss (acceptorless alkane dehydrogenation) [7] or substitution of the NBAligand, e.g.,F igure 1B.…”

“…[3] Further examples of NBA, [4] pentane, [5] cyclooctane, [6] isobutane and cyclohexane [7] s-complexes have all been reported with av ariety of [Rh(chelating phosphine)] + ligand sets,and some of these show remarkable stability in the solid state (months at 298 K). Thes tability of these SMOM systems allows for these s-complexes to undergo further SC-SC transformations.For example,reaction with D 2 (leading to CÀH/CÀDe xchange at the alkane), [7,8] H 2 loss (acceptorless alkane dehydrogenation) [7] or substitution of the NBAligand, e.g.,F igure 1B. [1,7] Key to this reactivity is the well-defined confined microenvironment provided in the solid state by the [BAr F 4 ] À anions that provide arelatively robust encapsulating framework-related to MOFs [9] or supramolecular catalysts.…”

“…Thes tability of these SMOM systems allows for these s-complexes to undergo further SC-SC transformations.For example,reaction with D 2 (leading to CÀH/CÀDe xchange at the alkane), [7,8] H 2 loss (acceptorless alkane dehydrogenation) [7] or substitution of the NBAligand, e.g.,F igure 1B. [1,7] Key to this reactivity is the well-defined confined microenvironment provided in the solid state by the [BAr F 4 ] À anions that provide arelatively robust encapsulating framework-related to MOFs [9] or supramolecular catalysts. [10] This supports the structural reorganization associated with the reactive metal center and also allows reversible access for gases and small organic compounds,albeit in what is essentially an on-porous material.…”

A Structurally Characterized Cobalt(I) σ‐Alkane Complex

“…1 g) of σ‐alkane complexes such as [Rh( L1 )‐NBA][BAr F 4 ] (Figure B; NBA=norbornane, Ar F =3,5‐(CF 3 ) 2 C 6 H 3 , L1 =Cy 2 PCH 2 CH 2 PCy 2 ) can be formed by simple hydrogenation of a cationic diene precursor (norbornadiene, NBD). These complexes can show remarkable stabilities (months, 25 °C), allowing for their full characterization (single‐crystal X‐ray diffraction, solid‐state NMR spectroscopy) and studies into alkane mobility and reactivity . Their isolation comes, in large part, from the stabilizing [BAr F 4 ] − anion microenvironment, which often forms an approximately octahedral cage surrounding the metal cation.…”