Solid-state molecular organometallic catalysis (SMOM-cat): synthetic routes, unique structural motifs, mobility in the solid-state and very active gas/solid isomerization catalysts.

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.

The regioselectivity of deprotonation reactions between arene substrates and basic metalating agents is usually governed by the electronic and/or coordinative characteristics of a directing group attached to the benzene ring. Generally, the reaction takes place in the ortho position, adjacent to the substituent. Here, we introduce a protocol by which the metalating agent, a disodium-monomagnesium alkyl-amide, forms a template that extends regioselectivity to more distant arene sites. Depending on the nature of the directing group, ortho-meta' or meta-meta' dimetalation is observed, in the latter case breaking the dogma of ortho metalation. This concept is elaborated through the characterization of both organometallic intermediates and electrophilically quenched products.

The covalent attachment of electron deficient perfluoroaryl substituents to a bis‐iodotriazole pyridinium group produces a remarkably potent halogen bonding donor motif for anion recognition in aqueous media. Such a motif also establishes halogen bonding anion templation as a highly efficient method for constructing a mechanically interlocked molecule in unprecedented near quantitative yield. The resulting bis‐perfluoroaryl substituted iodotriazole pyridinium axle containing halogen bonding [2]rotaxane host exhibits exceptionally strong halide binding affinities in competitive 50 % water containing aqueous media, by a factor of at least three orders of magnitude greater in comparison to a hydrogen bonding rotaxane host analogue. These observations further champion and advance halogen bonding as a powerful tool for recognizing anions in aqueous media.

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.