Direct observation of supramolecular binding of light hydrocarbons in vanadium(iii) and (iv) metal–organic framework materials

Binding of C2H2 in MFM-300(VIII) showing interactions to O–H, carboxylate O-centres and intermolecular packing.


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obtained as a green powder and stored in glovebox under N 2 . An important issue in desolvating MFM-300 (V III ) is to avoid oxidation. Elemental analysis for MFM-300(V III ), C 16

Hydrocarbon adsorption
CH 4 , C 2 H 2 , C 2 H 4 and C 2 H 6 sorption isotherms were recorded at 273-303 K on an IGA-003 system under ultrahigh vacuum. The CH 4 , C 2 H 4 and C 2 H 6 gases used were of ultrapure research grade (99.99%). C 2 H 2 was purified by dual-stage cold-trap systems operated at 195 K (dry ice) before introduction to the IGA system.

Neutron Powder Diffraction (NPD) Experiments and Structure Determination.
Neutron powder diffraction experiments were carried out at WISH, a long wavelength powder and single crystal neutron diffractometer at the ISIS Facility at the Rutherford Appleton Laboratory (UK). The instrument views a solid methane moderator providing a high flux of cold neutrons with a large bandwidth, transported to the sample via an elliptical guide. The WISH divergent jaws system allows tuning the resolution according to the need of the experiment; in this case, it was set up in high resolution mode. The WISH detectors are 1m long, 8mm diameter pixellated 3He tubes positioned at 2.2m from the sample and arranged on a cylindrical locus covering 10-170 degrees in 2θ scattering angle. To reduce the background from sample environment, it is equipped with an oscillating radial collimator that defines a cylinder of radius about 22 mm at 90 degrees scattering.
A sample of desolvated MFM-300(V III ) was loaded into a cylindrical vanadium sample container with an S4 indium vacuum seal and connected to a gas handling system. The sample was degassed at 10 -7 mbar and 100 °C for 1 day to remove any remaining trace guest water molecules. The temperature during data collection was controlled using a helium cryostat (7 ± 0.2 K). The loadings of hydrocarbons (for example, described for C 2 D 2 hereafter) were performed by the volumetric method at ambient temperature to ensure that C 2 H 2 was present in the gas phase when not adsorbed and also to ensure sufficient mobility of C 2 D 2 inside the crystalline structure of MFM-300(V III ). NPD data were collected for the bare material, and the material dosed with 1.0 and 2.0 C 2 D 2 per vanadium. The sample was then slowly cooled down to 7 K to ensure C 2 D 2 was completely adsorbed with no condensation in the cell. Sufficient time was allowed to achieve thermal equilibrium before data collection. Rietveld refinements on the NPD patterns of the bare MOF and the samples at various C 2 D 2 loadings were performed using the TOPAS software package. The initial Fourier difference maps were used to find the isosurfaces of the three-dimensional difference scattering-length density distribution for C 2 D 2 molecules. In this treatment the C 2 D 2 molecules were treated as rigid bodies; we first refined the centers of mass, orientations, and occupancies of the adsorbed C 2 D 2 , followed by full profile Rietveld refinement including the positions of metals and linkers, together with their corresponding lattice parameters, resulting in satisfactory R-factors. The final refinements on all the parameters including fractional coordinates, thermal parameters, occupancies for both host lattice and adsorbed C 2 D 2 molecules, and background/profile coefficients yielded very good agreement factors. No restriction of the molecule position was used in the refinement. The total occupancies of C 2 D 2 molecules obtained from the refinement are also in good agreement with the experimental values for the C 2 D 2 loading.

Inelastic Neutron Scattering (INS)
INS experiments were undertaken using the TOSCA spectrometer at the ISIS facility. MFM-300(V) was loaded into an 11 mm diameter vanadium sample can and outgassed at 1 x 10 -7 mbar and 100 °C for 1 day.
The sample was loaded into a helium closed cycle refrigerator (CCR) cryostat and cooled to 11 K for data S5 collection. The hydrocarbon gas was introduced by warming the samples to 290 K, and the gas dosed volumetrically from a calibrated volume. The gas-loaded sample was then cooled to 11 K over a period of 2 h to ensure good mobility of adsorbed hydrocarbon within the crystalline structure of MFM-300(V). The sample was kept at 11 K for an additional 30 mins before date collection to ensure the thermal equilibrium (see Supplementary method for more details of DFT Calculation for INS spectra)

DFT Calculation for INS spectra
Modelling by Density Functional Theory (DFT) of the bare and C 2 H 2 -loaded MOFs was performed using the Vienna Ab initio Simulation Package (VASP). 1 The calculation used Projector Augmented Wave (PAW) method 2,3 to describe the effects of core electrons, and Perdew-Burke-Ernzerhof (PBE) 4 implementation of the Generalized Gradient Approximation (GGA) for the exchange-correlation functional. Energy cutoff was 900eV for the plane-wave basis of the valence electrons. The lattice parameters and atomic coordinates determined by NPD in this work were used as the initial structure. Some of the C 2 H 2 sites have partial occupancy, and to account for this properly a supercell calculation would be desirable, but too costly in practice. Instead, a single unit cell was used and the partially occupied sites were modified to be either occupied or unoccupied, according to their local environment and symmetry (there needs to be either a complete C 2 H 2 molecule or no molecule, and the overall probability of being occupied needs to be proportional to the actual occupancy). The total energy tolerance for electronic energy minimization was 10 -8 eV, and for structure optimization it is 10 -7 eV. The maximum interatomic force after relaxation was below 0.005 eV/Å. The optB86b-vdW functional 5 for dispersion corrections was applied. The vibrational eigen-frequencies and modes were then calculated by solving the force constants and dynamical matrix using Phonopy. 6 The O'Climax software 7 was used to convert the DFT-calculated phonon results to the simulated INS spectra.
The structures relaxed by the DFT calculation slightly deviate from the ones obtained from NPD experiments (e.g., the orientation of the C 2 H 2 molecule in Fig. 5a). This is not unusual, and in this case, the S6 main reason could be associated with how "partial occupancy" is handled in the simulation, as described above.
To be more specific, DFT phonon calculations are usually computationally intensive and one cannot afford very large supercells with no symmetry. The above approach was used in order to best capture the local structure while still having the simulation feasible. But, the "randomness" associated with the partial occupancy cannot be reproduced fully, and the periodic boundary conditions inevitably results in certain symmetry constraints that are not present in the real material or the NPD data refinement. These discrepancies can lead to the different orientation of the C 2 H 2 molecules, and it appears to be more prominent in the case of MFM-300(VIII). It should be noted that other factors may also play a role, such as insufficient treatment of the van-der-Waals interactions in DFT. Pressure (bar) Gas uptake (mmol g -1 ) C 2 H 2 273K C 2 H 2 303K C 2 H 2 323K Pressure (bar) Gas uptake (mmol g -1 ) C 2 H 6 273K C 2 H 6 303K C 2 H 6 323K