Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates

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


Introduction
The transition metal promoted isomerization of alkenes is an atom efficient process that has many applications in industry and ne-chemicals synthesis; 1-3 such as the Shell Higher Olen Process, 4 "on purpose" olen conversion technologies that produce propene from butene/ethene mixtures, [5][6][7][8] and the isomerization of functionalized alkenes. 9 Homogeneous processes are well-studied for a wide range of transition metal catalysts 1,[9][10][11] and commonly, although by no means exclusively, use catalysts based upon later transition metals such as Co, 12 Ni, 13,14 Ru, [15][16][17] Rh, [18][19][20] Pd, 21 and Ir, [22][23][24] which operate at relatively low temperatures, sometimes at room temperature. 17,[19][20][21][25][26][27] Process based upon heterogeneous catalysts or surface organometallic chemistry (SOMC) are also well established. 28,29 Alkene isomerization also plays a key role in alkane dehydrogenation, 30 and subsequent tandem upgrading processes such as metathesis 31 or dimerization, 32,33 where the kinetic product of dehydrogenation is a terminal alkene that can then undergo isomerization (Scheme 1). 34 The dehydrogenation of light alkanes such as butane and pentane, and their subsequent isomerization is particularly interesting, as while these alkanes are unsuitable as transportation fuels or feedstock chemicals, their corresponding alkenes have myriad uses. 32,33,35 The discovery of abundant sources of light alkanes in shale and offshore gas elds provides additional motivation to study their conversion into fuels and commodity chemicals. 36 As light alkanes are gaseous at, or close to, room temperature and pressure the opportunity for solid/gas catalytic processes under these conditions is presented. Such conditions are also attractive due to the physical separation of catalyst and substrates/products that they offer, as well as opportunities to reduce thermally-induced catalyst decomposition processes.
Although heterogeneous solid/gas systems for alkane dehydrogenation and alkene isomerization are well known, 29,37-40 they oen require high temperatures for their operation which lead to reductions in selectivity as well as catalyst deactivation through processes such as coking. As is oen the case 41,42 well-dened supported or molecular systems can offer lower barriers, albeit still having to overcome the endergonic dehydrogenation reaction when run without a sacricial acceptor. 43,44 As far as we are aware there are only a handful of examples of purely molecular, i.e. not supported, solid-phase catalysts for alkane dehydrogenation or alkene isomerization. The Ir-pincer catalysts, such as Ir(PCP iPr )(C 2 H 4 ) [PCP iPr ¼ k 3 -C 6 H 3 -2,6-(CH 2 P i Pr 2 ) 2 ], recently reported by Goldman and coworkers, promote the transfer dehydrogenation, and subsequent double-bond isomerization, of butane, pentane and octane using acceptors such as tert-butylethene, ethene or propene. 45,46 These operate at temperatures of 200 C or above in sealed-tube conditions in which all the alkane is expected to be in the gas phase, and can actually outperform homogeneous systems in terms of activity. Experimental evidence points towards a presumed molecular species as the active catalyst, although the precise details have not been disclosed. Siedle & Newmark reported the room temperature solid/gas isomerization of simple alkenes using iridium or rhodium phosphine cations partnered with Keggin-type trianions, such as [Ir(H) 2 (PPh 3 ) 2 ] 3 [PW 12 O 40 ], [47][48][49] however the precise molecular structure of the catalyst was not determined.
We have recently reported the synthesis, using single-crystal to single-crystal solid/gas techniques, 41,50-53 of well-dened sigma-alkane complexes, 54 [56][57][58][59] The key to these complexes' relative stability in the solid-state is the arrangement of [BAr F 4 ] À anions that provide a well-dened cavity (i.e. they are "crystalline molecular asks", 60,61 Scheme 2b) that results in very small changes in unit cell volumes and retention of crystallinity during the transformations of the organometallic cation. This allows for the characterization of products directly by singlecrystal X-ray crystallography and solid-state NMR spectroscopy (SSNMR). These complexes, some of which are stable at room temperature (e.g. R ¼ Cy, [BAr F 4 ]), allow for the reaction chemistry of sigma-alkane complexes to be probed using solid/ gas experimental techniques, for example C-H activation processes. 62 Of relevance to this paper is the use of the alkane as a labile ligand that can be readily displaced in solid/gas reactivity and catalysis. We have recently reported that [Rh( i Bu 2 -PCH 2 CH 2 P i Bu 2 )(h 2 h 2 -NBA)][BAr F 4 ] reacts with ethene to form [Rh( i Bu 2 PCH 2 CH 2 P i Bu 2 )(ethene) 2 ][BAr F 4 ], that will catalyze ethene hydrogenation using solid/gas techniques, and also briey commented on 1-butene isomerization. 63 This  F 4 ] may provide the ideal platform for studying solid/gas reactivity and catalysis in exceptionally well-dened molecular systems, providing a highly reactive {Rh(bisphosphine)} + fragment with cis vacant (or at least very weakly stabilized) sites, Scheme 3. We report here that this is the case, and show that the alkane ligand in [BAr F 4 ] can be substituted for ethene, propene and 1-butene to give well-dened alkene complexes, some of which can only be prepared using such solid/gas routes. For propene and butene complexes rapid double-bond isomerization processes occur in the solid-state, which have been probed using variable temperature solid-state NMR spectroscopy, D-labelling studies and periodic DFT calculations. These exceptionally well-dened crystalline systems, which we term solid-state molecular organometallic catalysts (SMOM-Cat), are also active precatalysts for the solid/gas isomerization of 1-butene, demonstrating structure/activity relationships between the extended molecular structure and the measured catalytic activity. They also catalyze the transfer dehydrogenation/isomerization of butane to 2-butene. ]-Oct results in the generation of one equivalent of free NBA, which manifests itself by a thin coating on the resulting single-crystals. SSNMR spectroscopy at 298 K provided diagnostic data indicative of reaction in the bulk. A single, but broad, environment was observed in the 31 F 4 ]. Remarkably, given that NBA is being expelled, this transformation is also a single-crystal to single-crystal one in the solid-state, as shown by an X-ray structure determination at 150 K. Starting from [BAr F 4 ] (Scheme 2) this represents a rare example of a sequential reaction sequence for such processes. 50 We suggest that the CF 3 groups on the anions result in some plasticity of the solid-state lattice, which allows for the movement of the NBA, 65 given that there are no signicant channels in the crystal lattice. There is a space group change from to P2 1 /n (Z ¼ 4) to C2/c (Z ¼ 4) on substitution, and we, 56,59 and others, 52,66-68 have commented upon similar changes previously in solid/gas reactions. The nal rened structural model (Fig. 1a) has a signicant R-factor (10.7%) which we attribute to an increase in mosaicity on the single-crystal to single-crystal transformation and the loss of some high-angle data.

Results and discussion
Nevertheless the renement is unambiguous and shows a [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(h 2 -C 2 H 4 ) 2 ] + cation encapsulated by an almost perfect octahedron of [BAr F 4 ] À anions in the extended lattice (Fig. 1b). There is crystallographically imposed C 2 symmetry. The ethene ligands on the Rh-center are disordered over two sites, with a C]C distance of 1.36(1)Å, consistent with    (Fig. 2b). This results in ethene ligands that sit in a well-dened pocket of [BAr F 4 ] À anions (Fig. 2c). When inspected down the crystallographic c-axis the cations and anions are arranged under 3-fold symmetry so that they form a hexagonal structure of three ion pairs (Fig. 2d), resulting in cylindrical pores that run through the crystalline lattice (Fig. 2e). Moreover, these pores are decorated with the inward pointing {Rh(ethene) 2 } + fragments, so that the ethene ligands are potentially accessible from the pore channels (Fig. 2f). Taking into account the van der Waals radii 74 this pore-width is just less than 1 nm, and the calculated (PLATON  4 ]-Oct in which there are no solventaccessible voids. These pores are presumably lled with solvent, but we nd no denitive regions of electron density that we could assign to pentane (the most likely candidate) or CH 2 Cl 2 , and the calculated solvent-accessible volume likely represents the upper limit. There are other smaller trigonal prismatic pores, but these are formed from the CF 3 groups of the [BAr F 4 ] À anion and do not contain any {Rh(ethene) 2 } + fragments. Crystals of [1-(ethene) 2 ][BAr F 4 ]-Hex lose long range order when isolated in bulk by removal of solvent and rapid drying under vacuum, as measured by X-ray crystallography. We suggest this is due to loss of the disordered solvent in the pores, as 1 H and 31 P{ 1 H} solution NMR spectroscopy of this material shows essentially identical signals to [1-(ethene) 2 ][BAr F 4 ]-Oct indicating that ethene has not been lost; while elemental analysis is consistent with the formulation. Material that retains its pore structure, as measured by a unit-cell determination of an isolated crystal, and that is useful for catalysis (vide infra) is prepared by rapid transfer of single-crystals from the cold mother liquor and drying using a ush of ethene. 77 Using this methodology an acceptable yield of 62% (72 mg) is achieved. Material that has been exposed to an extended vacuum is not active for butene isomerization (see later).
Porous materials made from metal-organic frameworks (MOFs) are well known and can be used for a wide range of applications including gas separation and catalysis, and can oen incorporate reactive metal sites as part of the framework, 52,[78][79][80][81][82][83] or as an encapsulated cation in an anionic porous network. 84,85 However porous organometallic materials that are principally constructed from non-covalent interactions are less common. 51,86-89 As far as we are aware [1-(ethene) 2 ][BAr F 4 ]-Hex represents a rare example where the likely site of any potential catalytic activity, that is labile ethene ligands, are focused directly into the pore, being similar to that reported by Brookhart for Ir(POCOP)( 51 The contrast between the extended structure of  , which we show to have one propene ligand h 2 -bound with a supporting g-agostic Rh/H 3 C interaction, and the $O h arrangement of anions retained. This is also a single-crystal to single-crystal transformation that allows for the molecular structure to be determined directly by single-crystal X-ray diffraction. Shorter reaction times (1 hour) led to incomplete reaction as evidenced by the formation of [1-BAr F 4  . These data differ slightlybut signicantlyfrom those which come from material prepared by the solid/gas route (vide infra). Crystalline material of this bis-propene adduct can be obtained by recrystallization of [1-(propene)][BAr F 4 ], as prepared by solid/gas routes, from CH 2 Cl 2 /pentane saturated with propene at low temperature (À20 C). A single-crystal X-ray determination conrms the formulation (see ESI †) showing a similar structure to the cation [1-(ethene) 2 ][BAr F 4 ], with the two propene ligands canted from the square plane by 29 Fig. 3. The C 3 -hydrocarbon is disordered over two positions, related by a non-crystallographic two-fold rotation, Fig. 3b; while the octahedral arrangement of anions is retained, Fig. 3c. As for [1-(ethene) 2 ][BAr F 4 ]-Oct the solid/gas reaction led to loss in high-angle data and a reduction in the quality of the renement (R ¼ 12.7%). This, alongside the disordered organic fragment, means that a detailed discussion of the bond lengths and angles is not appropriate, and the hydrogen atoms were placed in calculated positions. Although the two C-C distances appear to show differentiation between C-C and C]C bonds [e.g. C100-C200, 1.361(9); C200-C300, 1.239(9)Å], both measure shorter than might be expected (and calculated, vide infra) 91,92 which likely is a consequence of the poor structure and rotational disorder. All three Rh-C distances reect Rh-C bonding interactions, but within error are the same [e.g. 2.15(2)-2.29 (3) A]. Thus, although the gross structure is unambiguous in showing a single C 3 fragment bound to the metal center, whether it is an h 2 -bound propene with a supporting g-agostic 93 interaction (I, Scheme 7) or the isomeric allyl-hydride, that arises from g-C-H activation of propene (II), 22,49,94-97 cannot be determined due to the quality of the data. We thus turned to variable temperature SSNMR and solution NMR spectroscopy, as well as periodic DFT calculations, to determine the precise structure. These studies show that at low temperatures the alkene/agostic tautomer is favored, which at higher temperatures accesses the allyl-hydride in both solid-state and solution.
The 158 K 31 P{ 1 H} SSNMR spectrum of [1-(propene)][BAr F 4 ], prepared in situ, shows two broad environments at d 101.3 and 90.4, consistent with the two different phosphorus environments in the single-crystal X-ray structure. At this low temperature in the 13 C{ 1 H} SSNMR spectrum two, approximately equal intensity, signals are observed in the region associated with bound alkene ligands, 98 at d 94.2 and 78.8, alongside a high-eld signal at d 6.5 in the region indicative of an agostic M/ H 3 C interaction. 99 Warming to 298 K results in a broad, but asymmetric, 31 P signal in the SSNMR spectrum at d 95.6; while in the 13 C{ 1 H} SSNMR spectrum a broad signal was observed at d 93.7, and the high-eld signal present at 158 K was absent. These data suggest a uxional process is occurring in the solidstate at room temperature, 100 that is slowed at 158 K. Low temperature 1 H/ 13 C HETCOR experiments, that we 56,62 and others 101 have previously shown to be useful in determining 1 H NMR chemical shis for sigma interactions in the solid-state, were not successful. The variable temperature 31 P{ 1 H} NMR data have been modelled using rate-constants derived from a line-shaped analysis, and a resulting Eyring analysis gives DG ‡ ¼ 10(1) kcal mol À1 and DS ‡ ¼ À7(3) cal K À1 suggesting a slightly ordered transition-state.
Similar behavior is observed using solution NMR spectroscopy when [1-(propene)][BAr F 4 ], prepared by the solid/gas route, is dissolved in CD 2 Cl 2 . Although rapid decomposition (less than 30 minutes) occurs at 298 K to give unidentied products, immediate data collection led to reliable solution NMR data. At 298 K the 31 P{ 1 H} NMR spectrum shows a single environment d 95.2 [J(RhP) ¼ 181 Hz], while the 1 H NMR spectrum shows a very broad signal at d 5.07 of relative integral $1 H in addition to signals in the aliphatic and aryl regions. Such a chemical shi is characteristic of the methine proton in an h 3 -allyl ligand. 98,102 The hydride region was featureless. These data suggest a uxional process is also occurring in solution at 298 K. Cooling to 193 K in CD 2 Cl 2 slows both decomposition and the uxional process. assigned to the methyl group that includes the agostic C-H/Rh interaction that is undergoing rapid rotation. The signals at d 4.54, 3.55 and À0.02 become broad on warming, and disappear into the baseline at 253 K suggesting that they are mutually exchanging. In contrast the signal assigned to the methine proton remains essentially unchanged in chemical shi, and can be tracked to the broad signal observed at d 5.07 at 298 K.
Insight into the detailed structure of the propene adduct [1-(propene)][BAr F 4 ] was obtained via periodic density functional theory (DFT) calculations at the PBE-D3 level, where this approach has previously been shown to reproduce the solidstate structures and uxionality of related sigma-alkane complexes very effectively. 54  was computed to be endergonic by 4.7 kcal mol À1 . In contrast, in solution, molecular calculations indicate the formation of [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(h 2 -C 3 H 6 ) 2 ] + from free propene and [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(h 2 -C 3 H 6 )] + is exergonic by 9.5 kcal mol À1 , and this bis-propene adduct is accessible experimentally in solution (see above). Fig. 4b provides calculated 13 C and 1 H chemical shis associated with the [1-(propene)] + cation in the solid-state, based on GIPAW calculations on the extended [1-(propene)] [BAr F 4 ] structure. Excellent agreement is found with the experimental low temperature 13 C SSNMR data for the propene ligand, providing further support for the formulation of an h 2propene/g-agostic complex. The calculations assign a high-eld 1 H resonance of d À3.9 to the agostic proton in the 1 H NMR spectrum of the static structure, while the average chemical shi computed for all three methyl protons is d À1.0 that reects a dynamic CH 3 group. This is to the high eld of the observed value of d À0.02 in solution and may reect environment effects in the solid-state. Thus when the model used in the calculation is changed to the isolated cation an average value of d À0.1 is computed, with the geminal protons in particular shiing to lower eld (d calc ( 1 H) +2.3, +0.9). In contrast, the agostic proton is less sensitive to the model employed, shiing by only 0.4 ppm to d calc À3.5 in the isolated cation. Local ring current effects arising from proximal aryl groups of the [BAr F 4 ] À anion have previously been shown to be signicant for  [BAr F 4 ] in the solid-state. 55 One possible mechanism for the uxional process observed experimentally at room temperature in solution and the solidstate is a 1,3-hydrogen shi involving C-H activation of the bound propene in I to give the allyl-hydride II (Scheme 7), followed by reinsertion, either degenerate or onto the distal carbon atom. A similar process has been suggested for the double bond shi in Ir-pincer systems such as Ir(POCOP tBu )(h 2 -propene) [POCOP tBu ¼ k 3 -C 6 H 3 -2,6-(OP t Bu 2 ) 2 ]. 22 The 1,3-hydrogen shi would result in a formal double bond isomerization in propene, but proceeds with no overall chemical change to the complex. If this was happening rapidly 94 at room temperature, and such an equilibrium favored the propene tautomer, then a hydride signal would likely not be observed in the 1 H NMR spectrum. In contrast, as the proton associated with the central carbon (C2 in Fig. 4) does not undergo rapid exchange, it should be observed, and we propose that this corresponds to the signal at d 5.07 in the 298 K solution 1 H NMR spectrum.
To probe this uxional process further, solid/gas catalysis using [1-NBA][BAr F 4 ] and 3,3,3-d 3 -propene was performed using $6 equivalents of alkene at 298 K and the headspace gas interrogated using 2 H NMR spectroscopy (Scheme 8). Aer 5 minutes deuterium was now observed in both the C1 alkene (cis and trans positions relative to the methyl) and the methyl positions, and aer 1 hour D-incorporation approached that expected for a statistical distribution at the C1 and C3 positions [0.39 : 0.57 ratio]. A very small amount of D-incorporation into the C2 methine position (4%) was also measured at this time. Aer 16 hours all positions were deuterated to a level close to that predicted from a simple statistical distribution between all three positions. The rapid D-scrambling at the C1 and C3 positions is fully consistent with a mechanism for uxionality that invokes facile C-H activation via an allyl-intermediate. 24 This rapid catalytic solid/gas H/D scrambling in 3,3,3-d 3 -propene using [1-NBA][BAr F 4 ] can be compared to that measured in solution phase under stoichiometric conditions for Ir(PO-COP tBu )(h 2 -d 3 -propene) that requires heating ($40 h at 333 K), 22 or the slow (greater than 16 h) solid/gas reactivity of [(Ph 3 P) 3 -IrH 2 ] 3 [PW 12 O 40 ] with 3,3,3-d 3 -propene that results in intramolecular scrambling in the nal allyl-hydride product. 49 Interestingly, that this solid/gas catalysis is much faster ($5 minutes) compared with bulk-scale synthesis of [ Further evidence for both the agostic interaction and an exchange process occurring in [1-(propene)][BAr F 4 ] comes from interrogation of a number of samples prepared using 3,3,3-d 3propene aer 16 hours, in which the D-label would be expected to be in all three C-positions (i.e. Scheme 8). 103 The corresponding g-agostic signal in the 193 K solution 1 H NMR spectrum integrates to a total of 1.5 protons, as expected for the statistical distribution of deuterium, and comes from an ensemble combination of CH 3 and CDH 2 and CD 2 H groups (A, B and C, Scheme 9). A signicant isotopic perturbation of equilibrium (IPE) would be thus expected to be observed in the 1 H NMR spectrum for the three CH 3 , CH 2 D and CHD 2 isotopomers, as, due to zero-point energy differences between C-H and C-D, agostic Rh/H-C interactions are favored. 104 However four signals are observed at d 0.08, À0.11, À0.28 and À0.58 in the 500 MHz 1 H NMR spectrum at 193 K. 105 We suggest that the extra signal comes from the diastereomeric pair in the CH 2 D isotopomer that arises from the relative orientation of the C-D, agostic and alkene bonds so that the two hydrogen atoms cannot become equivalent by a simple rotation (H a and H b in structure B). These data fully support the presence of an agostic interaction in [1-(propene)][BAr F 4 ]. Chemical shi calculations on the isolated cation of these isotopomers, taking into account the respective Boltzmann weighting factors recreate the observed relative chemical shis well (d À0.1, À0.24, À0.41 and À0.62, ESI †). The lowest-eld signal for the agostic C-H, experimentally observed at d 0.08, is assigned to isotopomer A and would be expected to have a very similar chemical shi to that observed in per-protio [1-(propene)] [BAr F 4 ], d À0.02, assuming any intrinsic chemical shi change is small. 106 We speculate that this difference in chemical shi may be due to a small, but signicant, equilibrium concentration of the (close in energy) allyl-hydride being present on isotopic substitution at low temperature, that is not observed in the 1 H NMR spectrum at low temperature due to a combination of low abundance and broad signals. Three signals are observed for the alkene protons, that are slightly shied from the perprotio complex: d 4.87, 4.52 and 3.54 each integrating to 0.5 H. The isotopomers are not resolved in these signals.
Periodic DFT calculations have also been used to explore the uxionality and related H/D exchange processes associated with the propene ligand in the solid-state (Scheme 10). Starting from cation I (0.0 kcal mol À1 ), oxidative cleavage of the agostic C3-H bond proceeds with a barrier of 9.8 kcal mol À1 to give allylhydride II at +3.4 kcal mol À1 . C1-H reductive coupling then proceeds via a transition state at +10.9 kcal mol À1 to reform the propene complex as I 0 (+1.2 kcal mol À1 ) in which the alkene and agostic moieties have swapped positions compared to I. This exchange process renders the two phosphorus centers nearequivalent with a modest overall barrier of 10.9 kcal mol À1 , consistent with it being readily accessible at room temperature. 107 The slightly different energies of I and I 0 (and the transition states linking these structures to II) reect the different orientations of the propene ligand within the crystal lattice. 108 In addition to this net 1,3-H shi, rotation of the propene ligand is also readily accessible, with a barrier of 10.4 kcal mol À1 interconverting I and its rotated form I rot , while I 0 and I 0 rot are linked via a transition state at 9.9 kcal mol À1 . These rotated forms correspond to the alternative orientation of the propene ligand seen crystallographically (dened by positions C101, C201 and C301, Fig. 3) and their similar energies (in particular I and I rot are within 0.1 kcal mol À1 ) are consistent with the approximately 50 : 50 occupation of these two components in the solid-state structure. These calculated barriers to uxionality compare very well with that derived experimentally (DG ‡ (exp) ¼ 10(1) kcal mol À1 ). An alternative rearrangement via rotation of the h 3 -allyl ligand (II to II rot ) involves a transition state at +20.6 kcal mol À1 and so is not competitive.
The g-agostic interaction observed in the ground state structure of [1-(propene)][BAr F 4 ] is directly related to C-H activation transition states calculated for the isomerization of h 2bound alkenes via allyl-hydride intermediates, 22 and closely related to those calculated for b-methyl migration from alkyl groupsthe microscope reverse of the chain propagation step in olen polymerization. 109 Scheme 9 (a) Isotopomers [1-(d 3  forms via an intermolecular process involving transfer dehydrogenation to exogenous butene. Consistent with this butane was observed to be formed in the headspace. A likely mechanism is one of interception of an associated allyl-hydride (e.g. IV Scheme 12) by butene and subsequent b-elimination/reductive elimination of butane. 63 We suggest that in the solid-state such an intermolecular process is attenuated as it requires the coordination of two equivalents of the C4-alkene which is disfavored (as computed for the bis-propene analogue) by the local crystalline environment around the metal cation due to intermolecular steric effects within the anion-cage. Despite we have not been able to determine its solid-state structure by single-crystal X-ray diffraction.
The single-crystal X-ray structure of [1-(butene)][BAr F 4 ] prepared by the solid/gas route shows a {Rh(Cy 2 PCH 2 CH 2 -PCy 2 )} + moiety on which a cis-2-butene ligand can be successfully modelled (150 K, space group C2/c, Z ¼ 4, Fig. 5). Unfortunately, the alkene ligand bound with the metal is disordered over two sites (crystallographically imposed) which when coupled with the loss in high-angle data on the solid/gas transformation means that bond metrics have an associated signicant error, and the hydrogen atoms associated with the butene fragment were not located. Nevertheless the structure is clear, and very closely related to that of [1-(propene)][BAr F 4 ].

More structural detail is provided by periodic DFT calculations on [1-(butene)][BAr F
4 ] that provide rm evidence for an h 2binding mode supported by a g-agostic interaction from one methyl (see Fig. 6a). Although butene is introduced as the 1-isomer it is 2-butene that is predominately bound to the metal center, in its cis-form. This is veried by vacuum transfer of CD 3     solution and SSNMR spectra, and DFT calculations that show 2butene to be bound as the cis-isomer. 3 is also present. These data are consistent with the 150 K singlecrystal X-ray structure (Fig. 5) and the structure computed in the solid-state by DFT [ Fig. 6a], and point to a uxional process in the solid-state at 298 K, that is slowed at lower temperatures, while retaining one agostic Rh/H 3 C interaction (i.e. , indicate a weakly bound ligand trans to one phosphorus environmentlikely the agostic interaction observed in the solid-state structure. The 13 C{ 1 H} NMR solution spectrum shows a single environment in the alkene region, d 90.2, and a single high-eld signal, d 10.9, assigned to the methyl groups. Both these signals are at approximately the frequency average of the corresponding signals in the 158 K 13 C { 1 H} SSNMR spectrum, which we suggest reects the lowtemperature limiting structure. The 1 H NMR spectrum displays a single alkene environment (2 H relative integral) at d 5.08, and an integral 6 H high eld signal at d 0.56. A 31 P/ 1 H HMBC experiment shows that this high eld signal correlates strongly with the 31 P environment that shows the large coupling with 103 Rh; and DEPT experiments indicate it to be a CH 3 group. These data suggest time-averaged C s symmetry at 193 K in solution. On warming rapid decomposition starts that eventually forms [1-(butadiene)][BAr F 4 ] in ca. 50% yield alongside other uncharacterized products, 111 that mean we have not been able to study this process at higher temperatures in solution.
DFT calculations have explored the behavior of the [1-(butene)] + cation in the solid-state (see Scheme 12). Starting from the cis-2-butene isomer (III) oxidative cleavage of the agostic C1-H bond accesses an allyl-hydride species (IV) at +8.3 kcal mol À1 via a transition state at 14.5 kcal mol À1 . Both values are ca. 5 kcal mol À1 higher than the equivalent process with the propene analogue. Reductive coupling with the distal C3 carbon is no longer a near-degenerate process, but rather forms the 1-butene isomer (V) at +5.7 kcal mol À1 . An adduct such as V is presumably initially formed in the reaction of [BAr F 4 ] with 1-butene, however, the calculations suggest this would readily isomerize to the more stable cis-2-butene form with an overall barrier of only 8.8 kcal mol À1 . The calculations also indicate that V should be kinetically accessible at room temperature. Assessment of the energy of the [1-(trans-2-butene)] + cation within the solid-state lattice indicates it would lie 11.4 kcal mol À1 above III. This large energy difference again reects the environment imposed by the solid-state lattice, as calculations on the isolated cations indicate they lie within 0.3 kcal mol À1 of each other. The calculations also dene a libration of the cis-2-butene ligand in III that serves to interchange the source of the agostic interaction trans to P2, from the C1-H1 bond in III to the C4-H4 bond in III 0 (see Fig. 7). This process occurs with a computed barrier of 3.0 kcal mol À1 and would account for the uxionality observed in the SSNMR spectra, and the 193 K solution NMR spectra. Further rotation of the butene moiety produces a structure equivalent to the second component in the X-ray structure (i.e. based on positions C1 0 -C4 0 , Fig. 5). This second form has a computed energy of À0.4 kcal mol À1 and is accessible via an overall barrier of 22.7 kcal mol À1 .
These combined experimental and computational data suggest a low temperature limiting structure for [1-(butene)] [BAr F 4 ] that has 2-butene bound in the cis-form with a supporting agostic interaction from the methyl group (Scheme 12). In solution at 193 K a low energy libration of the 2-butene ligand provides time-averaged C s symmetry by exchanging the agostic methyl groups (i.e. C1 and C4, Fig. 7). This is slowed in the solidstate at 158 K. On warming in both solution and the solid-state there is evidence for further uxional processes occurring. While the NMR data do not allow us to discriminate between a simple full rotation of the alkene fragment or a reversible C-H activation to give an allyl-hydride, the calculations suggest that the latter is more accessible with a barrier of 14.5 kcal mol À1 (Scheme 12) compared to 22.5 kcal mol À1 for C 2 rotation.
Further evidence for the isomerization process in Scheme 12 being accessible in the solid-state comes from addition of D 2 to [1-(butene)][BAr F 4 ], which is shown to have cis-2-butene bound, but forms 1,2-d 2 -butane as the condensable volatile product: the product of D 2 addition to 1-butene (Scheme 13). This suggests that isomerization from 2-butene to 1-butene is fast (i.e. Scheme 12) and that hydrogenation of the terminal alkene is signicantly faster than the internal, a well-known observation for cationic Rh-based catalysts in solution. 112 2.5 Catalytic 1-butene isomerization: solid-state molecular organometallic catalysis (SMOM-Cat) The fast H/D exchange observed for [1-(propene)][BAr F 4 ], and isomerization of 1-butene to 2-butene in [1-(butene) -Hex were thus screened (but conditions not optimized) in the isomerization of 1-butene to 2-butene in solid/gas catalysis. Crystals of approximate edge length $0.2 mm were used for all (Fig. 8). We did not explicitly grade the samples, in the main due to the sensitivity of [1-(ethene) 2 ][BAr F 4 ]-Hex, and so the catalytic data presented should be viewed as indicative of the overall rate of isomerization rather than an absolute measure. This was performed on a small, but convenient, scale as we have described previously, 63 by taking a thick-walled NMR tube of volume 2.05 cm 3 tted with Teon stopcock that allows for the addition of gases, adding a crystalline sample of catalyst (2.5 mg, $1.7 mmol), brief evacuation, relling with 1-butene gas (1 atm, 86 mmol 113 ) and analysis by gas-phase 1 H NMR spectroscopy. This loading, assuming all sites in the crystalline material have the same activity, gives TON (bulk) of $51 for 100% conversion. This represents a minimum TON, as if only the most accessible sites, or those nearest to the surface, were kinetically competent then the actual number of active sites would be lower. The catalysts yield close to the thermodynamic equilibrium mixture of 1-butene : 2-butene of $3 : 97, 14,114,115 in a cis : trans ratio of 1 : 2 as measured by gas-phase infra-red and 1 H NMR spectroscopy (CDCl 3 ) of the dissolved gas. 110 Fig. 9 shows a time/conversion behavior for these three catalyst systems. [    similar overall temporal proles were observed compared to the rst addition of 1-butene (cf. Fig. 9).  F 4 ] is likely the resting state during catalysis. Although catalyzed double bond isomerizations in alkenes are common, those involving 1-butene and well-dened transition metal catalysts are less well represented. A notable homogenous example is Ni(h 6 -C 6 H 5 CH 3 )(SiCl 3 ) 2 that rapidly isomerizes 1-butene to 2-butene at 0 C in bromobenzene, at loadings as low as 0.1 mol% (TOF $8600 h À1 ). 13 Although the long term stability and recyclability was not commented upon, in other solvents signicant decomposition was noted. Other homogenous systems are known, 14,114,118 as are heterogeneous systems that operate at room temperature. 119,120 However, we believe that catalysts such as [1-(ethene) 2 ][BAr F 4 ]-Hex are the rst well-dened molecular systems that operate at 298 K under, industrially appealing, solid/gas conditions. In addition, they offer ne control of the spatial environment in the solid-state (i.e. show structure/activity relationships), show TOF (min) that are competitive with the fast homogenous systems, and, moreover are recyclable. Although the solid/gas catalysts Ir(PCP iPr )(C 2 H 4 ) 45,46 or [Rh(PPh 3 ) 2 (CO)] 3 [PW 12 O 40 ] 47-49 promote the isomerization of alkenes, higher temperatures are reported for the former (125 C) while the latter is ill-dened on the molecular level and also not particularly active. A MOF-supported {Ir(ethene) 2 } fragment has been reported to dimerize ethene to butene, for which a mixture of isomers is reported but no further details were given regarding the isomerization process. 82

Transfer dehydrogenation of butane to 2-butene
The ability of [1-NBA][BAr F 4 ] to mediate the gas/solid transfer dehydrogenation of butane to butenes has been briey explored (Scheme 14), monitored by gas-phase NMR spectroscopy as measured by the temporal proles of butane and butene. As for the isomerization catalysis, a thick walled NMR tube was charged with nely-crushed crystalline [1-NBA][BAr F 4 ] ($6 mg, 4.1 mmol) and ethene and butane were admitted to the tube in a 1 : 2 ratio (total gas pressure 1 atm, 51 mmol butane). Periodic monitoring of the head space in the NMR tube showed that slow transfer dehydrogenation was occurring to form 2-butene, presumably by slow dehydrogenation to form 1-butene (not observed) and rapid isomerization. Aer 168 h at 298 K there was a 33% conversion, which equates to $4 turnovers. The catalysis was also shown to operate at 80 C with an excess of ethene (2 : 1), under which conditions 68% conversion of butane to butenes is observed (TON ¼ 4). Under these conditions the crystals lost denition and became wax-like. Although these turnover numbers are considerably smaller those reported for the best solid-phase molecular catalyst Ir(PCP iPr )(C 2 H 4 ) in the pentane/propene system at 240 C (e.g. TON greater than 1000), or related well-dened silica supported catalysts, 44 the observation of any catalytic activity at 298 K for this challenging reaction is encouraging. As far as we are aware this is the rst time solid/ gas transfer dehydrogenation has been reported using a well-dened molecular catalyst at room temperature and low pressures. The resting state observed in the bulk during dehydrogenation catalysis is [1-(ethene) 2 ][BAr F 4 ]. We cannot discount that a parallel slow ethene dimerization reaction also occurs under these conditions (Section 2.1).

Conclusions
Solid/gas organometallic chemistry performed on well-dened molecular species offers opportunities in synthesis and catalysis that complement solution routes. 41 We show here that the rhodium sigma-alkane complex [1-NBA][BAr F 4 ] (that is prepared by solid/gas routes itself) acts as a synthetic starting point for a variety of reactive and unusual alkene complexes that are challenging to prepare pure by solution routes. The alkane thus acts as "token" ligand, 121 and it is the unique environment provided by the solid-state packing of anions that allows for such single-crystal to single-crystal transformation to occur at such reactive but well-dened molecular cations. When experimental observations are combined with periodic DFT and chemical shi calculations a very complete picture of structure, mobility and reactivity in the solid-state can be formed, as we have also recently demonstrated in [BAr F 4 ] and other systems. 58,59,62 Such solid-state molecular organometallic systems thus provide a platform for synthesis and characterization equal in many ways to solution. Further demonstration of this comes from deployment of some of these systems in solid/gas butene isomerization and butane transfer dehydrogenation catalysis that shows catalysis in such an environment is certainly possible, and even rather efficient in some cases. Moreover, correlations between the solid-state structure (i.e. porosity) and high turnover frequency can be made. These results suggest that, more generally, solid-state molecular organometallic systems may offer opportunities and advantages in synthesis and catalysis and thus should be considered as a valuable additional approach to be used in the armoury of the organometallic chemist. That a relatively stable sigma-alkane complex (albeit not a methane, or related complex, which have been observed at very low temperatures in solution 122,123 ) acts as a precursor to this chemistry in the solid-state evokes Chatts prediction in 1976: "I believe that in twenty-ve years methane will be the most popular ligand in coordination chemistry". 124,125 It will be interesting to see if sigma-alkane complexes similar to those described here, more generally, are versatile enough to realize this prediction using solid/gas chemistry.