Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes

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][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η:η-C6H12)][BArF4] 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 [kH/kD = 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 kH/kD for the second dehydrogenation results from a pre-equilibrium involving C–H oxidative cleavage and a subsequent rate-limiting β-H transfer step.


S.1. Experimental details S.1.1. General Methods
All manipulations (unless otherwise stated) were performed under an atmosphere of argon, using standard Schlenk techniques on a dual vacuum/inlet manifold or by employment of an MBraun glovebox. Glassware was dried in an oven at 130°C overnight prior to use. Pentane, benzene and dichloromethane (abbreviated as CH 2 Cl 2 ) were dried using an MBraun SPS-800 solvent purification system and degassed by three freeze-pump-thaw cycles. 1,2-F 2 C 6 H 4 (abbreviated as F 2 C 6 H 4 ) was stirred over Al 2 O 3 for two hours then over CaH 2 overnight before being vacuum distilled and subsequently degassed by three freeze-pump-thaw cycles. d 2dichloromethane (abbreviated to CD 2 Cl 2 ) and d 3 -acetonitrile (abbreviated to d 3 -MeCN) was dried by stirring over CaH 2 overnight before being vacuum distilled onto 3 Å molecular sieves and subsequently degassed by three freeze-pump-thaw cycles. d 6 -Benzene (abbreviated as C 6 D 6 ) was stirred over Na pieces overnight before being vacuum distilled onto 3 Å molecular sieves and subsequently degassed by three freeze-pump-thaw cycles. Isobutene and Isobutane was purchased from CK Gases and used as received. All other chemicals were purchased from commercial vendors and used as received.
[Rh(Cy 2 PCH 2 CH 2 PCy 2 )(C 6 H 4  Solution NMR data were collected on either a Bruker AVD 500 MHz or a Bruker Ascend 400 MHz spectrometer at room temperature unless otherwise started. Nondeuterated solvents were locked to standard external CD 2 Cl 2 solutions. Residual protio solvent resonances were used as a reference for 1 H NMR spectra. 2 H NMR spectra were referenced to CD 2 Cl 2 (δ 5.32). 31 P{ 1 H} NMR spectra were referenced externally to 85 % H 3 PO 4 (D 2 O). All chemical shifts (δ) are quoted in ppm and coupling constants in Hz.
Solid state NMR (SSNMR) samples were prepared packing powdered microcrystalline sample into either a 3.2 mm or 4 mm zirconia solid state rotor inside an argon filled glove box. SSNMR spectra were obtained on a Bruker Avance III HD S6 spectrometer equipped with a 9.4 Tesla magnet, operating at 100.6 MHz for 13 C and 162 MHz for 31 P, respectively, and a MAS rate of 10 kHz. Relaxation time for 1 H and contact time for 31 P{ 1 H} CP/MAS, 13 C{ 1 H} CP/MAS, 13 C{ 1 H} NQS and FSLG-HETCOR NMR experiments were optimized for each compound as appropriate. All 13 C{ 1 H} CP/MAS spectra were referenced to adamantane (up field methine resonance, δ 29.5) S3 on a scale where δ (TMS) = 0 as a secondary reference. The temperature for Variable Temperature (VT) NMR experiments at low temperatures was externally calibrated using lead nitrate (PbNO 3 ). The 1 H chemical shifts obtained from 1 H/ 13 C Frequency Switched Lee-Goldburg HETCOR SSNMR spectra were referenced internally to the p-ArH resonance of the [BAr F 4 ] -(δ = 7.12). S4 Gas phase 1 H NMR spectroscopy was carried out using a Bruker Ascend 400 MHz spectrometer. The spectrometer was pre-locked and shimmed to a separate CD 2 Cl 2 sample in a similar bore tube. The T1 delay was set to 1 s, and this has been previously shown to allow for the accurate comparison of integrals. S5 Electrospray ionization mass spectrometry (ESI-MS) was carried out using a Bruker MicrOTOF instrument directly connected to a modified Innovative Technology glovebox. S6 Typical acquisition parameters were used (sample flow rate: 4 μL min -1 , nebulizer gas pressure: 0.4 bar, drying gas: Argon at 333 K flowing at 4 L min -1 , capillary voltage: 4.5 kV, exit voltage: 60 V). The spectrometer was calibrated using a mixture of tetraalkyl ammonium bromides [N(C n H 2n+1 ) 4 ]Br (n = 2-8, 12, 16 and 18).
Samples were diluted to a concentration of 1 × 10 -6 M in the appropriate solvent before sampling by ESI-MS.

Gas Chromatography Electron Ionization-Mass Spectrometry (GC EI-MS) analyses
were performed on an Agilent 7200 quadrupole time of flight (Q-ToF) instrument equipped with a 7890B gas chromatograph and a PAL auto-sampler fitted with a 2.5 mL headspaces syringe. Instrument control and data processing were performed using Agilent MassHunter software. The system was calibrated within 1 hour prior to the analysis and its mass accuracy with external calibration (as used for these experiments) is better than 5 ppm for 2 hours following calibration. Samples were prepared in 20 mL headspace vials. Vials were incubated at 35 °C for 30 seconds before a 2.5 mL sample of the headspace was taken and injected into the GC inlet (headspace syringe was held at 40°C). The GC-inlet was operated in split mode held S7 at 300 °C with a 2:1 split. The column was a Restek RT Q-Bond 30 m x 320 µm with at 10 µm film thickness. The oven was held at 40 °C for 4 min then ramped at 20 °C/min to 300 and held for 3 min. Column flow rate was 1 mL/min. Mass spectrometer was operated in EI mode and with the ionization energy set to 20 eV.  F 4 ]. After this time, working under an atmosphere of isobutene (1 bar) and at -78 °C in an acetone/dry ice bath, the sample was dissolved in CH 2 Cl 2 (3 mL), filtered via cannula and layered with pentane (25 mL) at the same temperature. The solution was warmed to ambient temperature and after 3 days yielded orange block like crystals suitable for single crystal x-ray diffraction. Yield: 169 mg (86 %). The crystalline material is stored in a glove box in a freezer operating at -25 °C.

S.2.4.6. Solid state decomposition products of [1-C 6 H 12 ][BAr F 4 ] under H 2
A powdered microcrystalline sample of [1-C 6 H 8 ][BAr F 4 ] (35 mg) was packed in a 3.2 mm SSNMR rotor, inside an argon filled glove box. The rotor was then placed in a custom built glass J Young flask S7 and the sample was then exposed to H 2 (1 bar, 298 K). After 80 minutes, the rotor cap was fitted under a flush of H 2 . The sample was immediately transferred to the bore of a pre-cooled (158 K) SSNMR spectrometer (~ 10 mins, total time = 90 mins) and analyzed 31 P{ 1 H} and 13 C{ 1 H} solid state NMR spectroscopy.

H 10 ][BAr F 4 ]; [1-C 4 D x -alkane][BAr F 4 ].
The reaction was left for 90 minutes; after which the crystal color changed to a dark orange. After this time, it was found isobutane-d x (x = 10 -7) had been expelled from the crystal lattice, shown by gas phase 1 H and 2 H NMR spectroscopy of the NMR tube headspace. S6 The volatiles were then distilled into a clean NMR tube with CD 2 Cl 2 and analyzed by 1 H, 2 H, 13 C { 1 H} NMR spectroscopy and GC EI-MS.
The remaining solid material was dissolved in C 6 F 2 H 4 to quantitatively yield [1- No isobutane could be identified in the solution 1 H or 2 H NMR spectrum, suggesting total expulsion of isobutane from the lattice by D 2 is complete after 90 minutes.  Although approx. 10 % C 4 H 10 + , m/z = 58.074 (calc. 58.078) can be located, in the experimental condition presented above, the percentage of C 4 D 10 + in the isobutaned x sample is below the detection intensity of the equipment, and hence cannot be unequivocally identified.

S.3.1.4. Simulated solution 13 C{ 1 H} NMR spectra of isobutane-d x (x = 10 -7)
The solution 13 C{ 1 H} NMR spectrum of isolated isobutane-d x (x = 10 -7) ( Figure   S40) was de-convoluted using the Spin Simulation function on MestReNova, shown in Figure S43. The line width was kept at 6.0 Hz throughout, only changing the levels of population in each simulated environment. groups of isobutane. The signal marked by * is from a pentane impurity.
Three systems were used to recreate the multiplet at δ 23.85, modelling the 13 C methyl environments as -CD 3 , -CD 2 H and -CDH 2 . No evidence of -CH 3 could be seen in the experimental data so was not modelled. A separate fourth system was also modelled describing the triplet at δ 22.66; reported as the methine carbon of isobutane-d x , split by a single bound deuterium. The proto-methine group could not be modelled, as signals obscured by multiplet at δ 23.85.

S49
The simulation suggests that 62 % of the -CX 3 groups were of -CD 3 , 30 % are -CD 2 H and 8 % are -CDH 2 . This is consistent with the GC EI-MS shown in Figure   S41, which showed a high proportion of C 4 D 9 + and C 4 D 8 + from the signals at 66.127 and 65.121 respectively as well as the molecular ion of 50.098 + from C 3 D 7 The population ratio of each 13 C environment in the simulated spectra was calculated from the sum of each individual -CD x H Y component in each environment compared between the environments. These ratios suggest an approximate 1:4.2 ratio of carbon environments; albeit a little higher than predicted (1:3). This may be due to unmodelled -C-H methine group's signals obscured by the multiplet at δ 23.85, affecting this ratio.

S.3.3.1. General Experimental of H/D Exchange on [1-C 6 H 12 ][BAr F 4 ]
A thick walled NMR tube was charge with powdered microcrystalline [1-C 6 H 8 ][BAr F 4 ] (10 mg) in an argon filled glove box. The sample was then cooled to 77 K in liquid nitrogen, evacuated and backfilled with D 2 (1 bar, 298 K). After 30 min, sample was then cooled again to 77 K in liquid nitrogen, evacuated and backfilled with D 2 (1 bar, 298 K). This cycle was repeated once more, to give a total of 3 x 30 mins cycles,

S.3.4.1. Procedure for the solution-state deuteration of [1-C 6 H 8 ][BAr F 4 ]
A solution of [1-C 6 H 8 ][BAr F 4 ] (10 mg) in CD 2 Cl 2 in a thick walled NMR tube was freeze-pump-thaw degassed three times and backfilled with D 2 (1 bar, 298 K). Upon agitation the color of the solution changed from red to yellow. The volatile component was then isolated by trap-to-trap distillation. The product was identified as C 6 H 8 D 4 , which is the product of deuteration across the double bonds only.

S.4.3.2. Solid-state dehydrogenation of [1-C 6 H 12 ][BAr F 4 ] under vacuum to form [1-C 6 H 8 ][BAr F 4 ]
A J Young NMR tube was charged with powdered microcrystalline [1-C 6 H 8 ][BAr F 4 ] (10 mgs) in an argon filled glove box. The sample was then evacuated (2 × 10 -2 mbar) and backfilled with H 2 (1 bar, 298 K) for 15 minutes. The sample was then cooled to 77 K in liquid nitrogen and evacuated. After a steady vacuum was achieved (2 × 10 -2 mbar) the sample was rapidly warmed to 298 K and a timer was simultaneously started. After a set period of time (5 mins to 960 mins), the sample was cooled to 77 K in liquid nitrogen and CH 2 Cl 2 (0.40 mL) was condensed into the NMR tube. The sample was expediently thawed and rapidly transferred to the bore of a precooled (183 K) NMR spectrometer which was previously locked and shimmed to a sample of CD 2 Cl 2 . A 31 P{ 1 H} NMR spectrum was then acquired.
The above procedure was repeated on numerous occasions, varying the period of time the sample was exposed to vacuum at room temperature.

S.4.4.2. Isolation and solution-state characterization data for [1-C 6 H 10 ][BAr F 4 ]
Attempts to trap [1-C 6 H 10 ][BAr F 4 ] in solution were conducted using the following method: A J Young NMR tube was charge with powdered microcrystalline [1-C 6 H 8 ][BAr F 4 ] (10 mg) in a glove box. The sample was then evacuated and backfilled with H 2 (2 bar, 298 K) for 15 minutes. The sample was then cooled to 77 K in liquid nitrogen and evacuated. After a steady vacuum was achieved (2 × 10 -2 mbar) the sample was rapidly warmed to 298 K and a timer was simultaneously started. After 15 minutes, the sample was cooled to 77 K in liquid nitrogen and CH 2 Cl 2 (0.40 mL) was condensed into the NMR tube. The sample was expediently thawed and rapidly transferred to the bore of a precooled (183 K) NMR spectrometer which was previously locked and shimmed to a sample of CD 2 Cl 2 . A 31 P{ 1 H} NMR spectrum was then acquired.  S2 The resonances marked + could not be assigned.

S.4.4.4. Liberation of the bound cyclohexene from [1-C 6 H 10 ][BAr F 4 ].
A J Young NMR tube was charge with powdered microcrystalline [1-C 6 H 8 ][BAr F 4 ] (10 mg) in a glove box. The sample was then evacuated and backfilled with H 2 (2 bar, 298 K). The sample was then cooled to 77 K in liquid nitrogen and evacuated.
After a steady vacuum was achieved (2 × 10 -2 mbar) the sample was rapidly warmed to 298 K and a timer was simultaneously started. After 15 mins, the sample was cooled to 77 K in liquid nitrogen and CH 2 Cl 2 (0.40 mL) was condensed into the NMR tube. When thawing, care was taken such that the sample was not warmed above

S.4.5. Solid state dedeuteration of [1-C 6 D 12 ][BAr F 4 ] under vacuum to form [1-C 6 D 8 ][BAr F 4 ]
A thick walled NMR tube was charge with powdered microcrystalline [1-C 6 H 8 ][BAr F 4 ] (10 mg) and sample was cooled to 77 K in liquid nitrogen, evacuated and backfilled with D 2 (1 bar, 298 K). After 30 min, sample was then cooled to 77 K in liquid nitrogen, evacuated and backfilled with D 2 (1 bar, 298 K). This cycle was repeated once more, to give a total of 3 x 30 minute cycles, totaling 90 minutes under D 2 . The sample was placed under a steady vacuum (2 × 10 -2 mbar), was rapidly warmed to 298 K and a timer was simultaneously started. After set periods of time (5 mins to approx. 3 days) the sample was cooled to 77 K in liquid nitrogen and CH 2 Cl 2 (0.40 mL) was condensed into the NMR tube. The sample was expediently thawed and rapidly transferred to the bore of a precooled (183 K) NMR spectrometer which was previously locked and shimmed to a sample of CD 2 Cl 2 . A 31 P{ 1 H} NMR spectrum was then acquired.

S.5. Crystallographic and refinement data S.5.1. Crystal structure determinations
Single crystal X-ray diffraction data for all samples were collected as follows: a typical crystal was mounted on a MiTeGen Micromounts using perfluoropolyether oil and cooled rapidly to the collection temperature in a stream of nitrogen gas using an Oxford Cryosystems Cryostream unit. S11  Raw frame data were reduced using CrysAlisPro. S12 The structures were solved using SHELXT S13 and refined using full-matrix least squares refinement on all F 2 data using the SHELXL-18 S14 using the interface OLEX2. S15 All hydrogen atoms were placed in calculated positions (riding model). Disorder of the -CF 3 groups was treated by introducing a split site model and restraining geometries and displacement parameters.

S.5.2. Additional comments crystal structures and refinement data [1-C 4 H 10 ][BAr F 4 ]: In reply to A-Level Alert [PLAT971_ALERT_2_A]:
The origin of this residual electron density relates to a positional disorder affecting a dicyclohexylphosphino group from the 1,2-Bis(dicyclohexylphosphino)ethane phosphine ligand from the asymmetric unit. This is too small contribution to be satisfactorily modelled. As such it was deemed better to retain this electron density with a low R-factor.

In reply to B-Level Alert 3 [PLAT910_ALERT_3_B]:
The sequential single-crystal to single-crystal transformation caused a loss in data quality at high angles. Because of S105 this data set was collected to satisfy a sine(theta_max)/wavelength 0.82 giving an acceptable R-factor.

In reply to B-Level Alerts 2 [PLAT971_ALERT_2_B], [PLAT971_ALERT_2_B] and
[PLAT973_ALERT_2_B]: The origin of this residual electron density relates to a positional disorder affecting a dicyclohexylphosphino group from the 1,2-Bis(dicyclohexylphosphino)ethane phosphine ligand from the asymmetric unit. This is too small contribution to be satisfactorily modelled. As such it was deemed better to retain this electron density with a low R-factor.

[1-C 4 H 8 ][BAr F 4 ]-SC-SC: In reply to A-Level Alert [PLAT029_ALERT_3_A]:
The sequential two single-crystal to single-crystal transformations caused a loss in data quality. Because of this, this data set was collected to satisfy an acceptable atom connectivity model only.

In reply to B-Level Alert [THETM01_ALERT_3_B]:
The sequential two single-crystal to single-crystal transformations caused a loss in data quality at high angles.
Because of this, this data set was collected to satisfy a sine(theta_max)/wavelength of 0.56 giving a model for atom connectivity with an acceptable R-factor.

In reply to B-Level Alert [PLAT023_ALERT_3_B]:
The sequential two single-crystal to single-crystal transformations caused a loss in data quality. Because of this, this data set was collected to satisfy an acceptable atom connectivity model only.

In reply to B-Level Alert [PLAT088_ALERT_3_B]:
The sequential two single-crystal to single-crystal transformations caused a loss in data quality. Because of this, the data/parameter ratio is acceptable.

In reply to B-Level Alert [PLAT911_ALERT_3_B]:
The sequential two single-crystal to single-crystal transformations caused a loss in data quality. Because of this, this data set was collected to satisfy an acceptable atom connectivity model only.

[1-C 6 H 8 ][BAr F 4 ]
Crystals of this material undergo a phase change upon slow cooling to 100 K. For this reason, the presented data was measured at 250 K. S106

[1-C 6 H 12 ][BAr F 4 ]
In reply to : The single-crystal to single-crystal transformation caused a loss in data quality. Because of this, the data/parameter ratio is acceptable.
In reply to : The origin of this residual electron density relates to a positional disorder affecting the Rh metal centre. This is too small contribution to be satisfactorily modelled. As such it was deemed better to retain this electron density with a low R-factor.

In reply to CB-Level Alert [PLAT971_ALERT_2_C] and [PLAT972_ALERT_2_C]:
The origin of this residual electron density relates to a positional disorder affecting the Rh metal centre. This is too small contribution to be satisfactorily modelled. As such it was deemed better to retain this electron density with a low R-factor.
[   disorder is present on C2, and is shared between the two disorder components. Not shown is the disorder in the second molecule in the asymmetric unit, however is positions. This generated two possible structures depending on which C-C bond was dehydrogenated and the more stable form was used in the main paper.
For reactivity studies different reaction pathways were initially explored using an isolated rhodium molecular cation model with the Gaussian suite of programs (see details below). Transition states located in this way provided the basis for transition state searches in the solid state, with pre-optimisations in the solid state run by fixing the key reacting atoms at one of the Rh-centres. A partial vibrational analysis was then used to identify the corresponding imaginary mode. This pre-optimized TS structure was then refined using the dimer method S24 with the tighter convergence criteria detailed above. For challenging fluxional processes the climbing image nudged elastic band (CI-NEB) method, S25 using 8 or 12 images, was used to obtain candidate transition states that were then optimised using the dimer method as above. All optimized stationary points were characterized by analysis of their numerical second derivatives with a displacement of 0.01 Bohr. Minima and transition states have no or exactly one imaginary eigenvalue, respectively. All transition states were further analysed using an "IRC-like" approach, whereby transition state geometries were displaced along the negative mode in both directions and then fully optimising the two resulting structures. Further details on this protocol have been reported elsewhere. S26 A spurious imaginary frequency (i = 6 cm −1 ) was found in one of the stationary points (IX), which appears to be due to numerical inaccuracies in the vibrational analysis, which is a familiar issue in these type of calculations. S27-28 Gibbs free energies for structures computed in the solid state were calculated using the TAMkin software toolkit. S29 All computed structures are available as a separate file of Cartesian coordinates.

S.6.2. Molecular Calculations
Molecular calculations employed the GAUSSIAN 09 (revision D.01) program package S30 and employed the BP86 GGA functional. S31-32 Stuttgart-Dresden (SDD) S33 relativistic effective core potentials (ECP) in combination with the associated basis sets were utilized to describe Rh and P, with polarization functions added for P (ζ = 0.387). S34 The 6-31G(d,p) basis sets S35-36 were used on remaining atoms.

S115
Electronic structure analyses were performed on the geometries of the Rh cations extracted from the CP2K-optimised structures with the heavy atoms fixed at the experimental positions. An electron density file suitable for further analysis was generated from a single-point calculation.
The topology of the electron density was analysed by means of QTAIM (Quantum Theory of Atoms in Molecules), S37 as implemented in the AIMALL package. S38 Inner shell electrons on Rh and P modelled by ECPs were represented by core density functions (extended wavefunction format).
NBO calculations were performed using the NBO 6.0 program S39 , using the same geometries as for the QTAIM calculations above.
NCI calculations were performed using the NCIPLOT program S40-41 , using the nearest neighbour ion-pair molecular structures extracted from CP2K optimised geometries. The promolecular electron density was employed.
Orbital plots were created with Chemcraft S42 with an outer contour value of 0.0625.

S.6.3. NMR Calculations
Isotropic 13 C and 1 H magnetic shielding constants (σ iso ) were generated using the GIPAW method S43-44  These settings were shown to yield converged NMR parameters on related systems. S26 The single-point calculations were performed using the PBE GGA functional, S22 using Ultra-soft pseudopotentials generated on-the-fly to represent the core electron shells. Scalar-relativistic effects were incorporated through the zerothorder regular approximation (ZORA). S47-48 The ultrafine SCF convergence threshold of 10 -8 eV atom -1 was used throughout. Computed 13 C isotropic shielding constants were then converted into chemical shift values (δ) using the linear regression fit S49 S116 from the previously studied pentane system. S26 As previously relative isotropic proton ( 1 H) chemical shifts were obtained by referencing computed shielding constants against those of the standard tetramethylsilane (TMS) according to δiso ( Figure S111: Labelling scheme employed. computed isotropic chemical shifts (σ iso ) and relative chemical shifts (ppm) using the labelling scheme shown in Figure S108. a a Model 1 data are based on the fully optimised geometry of [1-C 6 H 12 ][BAr F 4 ] in the solid-state; Model 2: data are based on the experimental structure with only the positions of H and F atoms being optimised; ion-pair model based on the nearest neighbour ion-pair extracted from the fully optimised geometry in Model 1. Note: Analyses were performed for both crystallographically independent cations contained in the unit cell.

Distance (Å) Fully Optimized Heavy Atom Fixed a Experiment
Rh