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.

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.

The synthesis of new Schrock-Osborn Rh(I) pre-catalysts with ortho-substituted DPEphos ligands, [Rh(DPEphos-R)(NBD)]-[BArF4] [R = Me, OMe, iPr; ArF = 3,5-(CF3)2C6H3], is described. Along with the previously reported R = H...

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.

The preparation and reactivity with H 2 of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C 6 H 4 PPh 2 ) 2 ) are described. Ru(ZnPhos)(CO) 3 (2) and Ru(ZnPhos)(IMe 4 ) 2 (4; IMe 4 = 1,3,4,5tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 (ZnMe) 2 (1) or Ru(PPh 3 ) 3 HCl/LiCH 2 TMS/ ZnMe 2 with CO and IMe 4 , respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO) 2 that then adds H 2 over the Ru−Zn bond to form Ru(ZnPhos)(CO) 2 (μ-H) 2 (3). In contrast, 4 reacts directly with H 2 to set up an equilibrium with Ru(ZnPhos)(IMe 4 ) 2 H 2 (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L 2 intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H 2 to add across the Ru−Zn bond, but for L = IMe 4 , this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe 4 . Steric effects associated with the larger IMe 4 ligands are not significant. Species 4 can be considered as a Ru(0)L 4 species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L 4 species devoid of strong π-acceptor ligands.