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.

Solid/gas single-crystal to single-crystal (SC−SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy 2 P(CH 2 ) n PCy 2 )(L)]-[BAr F 4 ] (n = 3, 4) and [RhH(Cy 2 P(CH 2 ) 2 (CH)-(CH 2 ) 2 PCy 2 )(L)][BAr F 4 ] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by singlecrystal X-ray diffraction, have cations exhibiting Rh•••H−C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η 2 ,η 2 Rh•••H−C (e.g., [Rh(Cy 2 P(CH 2 ) n PCy 2 )(η 2 η 2 -NBA)][BAr F 4 ], n = 3 and 4), through to more weakly bound η 1 Rh•••H−C in which C−H activation of the chelate backbone has also occurred (e.g., [RhH(Cy 2 P(CH 2 ) 2 (CH)(CH 2 ) 2 PCy 2 )(η 1 -COA)]-[BAr F 4 ]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy 2 P(CH 2 ) 3 PCy 2 )][COA⊂BAr F 4 ], in which the metal center instead forms two intramolecular agostic η 1 Rh•••H−C interactions with the phosphine cyclohexyl groups. CH 2 Cl 2 adducts formed by displacement of the η 1 -alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy 2 P(CH 2 ) 2 (CH)(CH 2 ) 2 PCy 2 )(κ 1 -ClCH 2 Cl)][BAr F 4 ], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh•••alkane interactions along a C−H activation trajectory. These are negligible in [Rh(Cy 2 P(CH 2 ) 3 PCy 2 )][COA⊂BAr F 4 ]; in [RhH(Cy 2 P(CH 2 ) 2 (CH)(CH 2 ) 2 PCy 2 )(η 1 -COA)][BAr F 4 ], σ C−H → Rh σ-donation is supported by Rh → σ* C−H "pregostic" donation, and in [Rh(Cy 2 P(CH 2 ) n PCy 2 )(η 2 η 2 -NBA)][BAr F 4 ] (n = 2−4), σ-donation dominates, supported by classical Rh(dπ) → σ* C−H π-back-donation. Dispersive interactions with the [BAr F 4 ] − anions and Cy substituents further stabilize the alkanes within the binding pocket.

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(η n :η m -alkane)][BAr F 4 ] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H–C binding motifs at the metal coordination site: 1,2-η 2 :η 2 (2-methylbutane), 1,3-η 2 :η 2 (propane), 2,4-η 2 :η 2 (hexane), and 1,4-η 1 :η 2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H–C interactions with the geminal C–H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy 2 PCH 2 CH 2 PCy 2 )} + fragment and the surrounding array of [BAr F 4 ] − anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.

Single‐crystal to single‐crystal solid‐state molecular organometallic (SMOM) techniques are used for the synthesis and structural characterization of the σ‐alkane complex [Rh(tBu2PCH2CH2CH2PtBu2)(η2,η2‐C7H12)][BArF4] (ArF=3,5‐(CF3)2C6H3), in which the alkane (norbornane) binds through two exo‐C−H⋅⋅⋅Rh interactions. In contrast, the bis‐cyclohexyl phosphine analogue shows endo‐alkane binding. A comparison of the two systems, supported by periodic DFT calculations, NCI plots and Hirshfeld surface analyses, traces this different regioselectivity to subtle changes in the local microenvironment surrounding the alkane ligand. A tertiary periodic structure supporting a secondary microenvironment that controls binding at the metal site has parallels with enzymes. The new σ‐alkane complex is also a catalyst for solid/gas 1‐butene isomerization, and catalyst resting states are identified for this.