Modulating supramolecular binding of carbon dioxide in a redox-active porous metal-organic framework

Hydrogen bonds dominate many chemical and biological processes, and chemical modification enables control and modulation of host–guest systems. Here we report a targeted modification of hydrogen bonding and its effect on guest binding in redox-active materials. MFM-300(VIII) {[VIII2(OH)2(L)], LH4=biphenyl-3,3′,5,5′-tetracarboxylic acid} can be oxidized to isostructural MFM-300(VIV), [VIV2O2(L)], in which deprotonation of the bridging hydroxyl groups occurs. MFM-300(VIII) shows the second highest CO2 uptake capacity in metal-organic framework materials at 298 K and 1 bar (6.0 mmol g−1) and involves hydrogen bonding between the OH group of the host and the O-donor of CO2, which binds in an end-on manner, =1.863(1) Å. In contrast, CO2-loaded MFM-300(VIV) shows CO2 bound side-on to the oxy group and sandwiched between two phenyl groups involving a unique ···c.g.phenyl interaction [3.069(2), 3.146(3) Å]. The macroscopic packing of CO2 in the pores is directly influenced by these primary binding sites.


Materials and other measurements.
All reagents were used as received from commercial suppliers without further purification. Analyses for C, H and N were carried out on a CE-440 elemental analyzer (EAI Company). Thermal gravimetric analyses (TGA) were performed under N 2 flow (100 ml/min) with a heating rate of 2 °C/min using a TASDT-600 thermogravimetric analyser (TA Company). Powder X-ray diffraction data (PXRD) were collected over the 2θ range 4-50 o on a Bruker Advance D8 diffractometer using Cu-Kα 1 radiation (λ = 1.54056 Å, 40 kV/40mA). XPS spectra were collected on a Kratos Axis Ultra X-ray photoelectron spectrometer equipped with an aluminium/magnesium dual anode and a monochromated aluminium X-ray source.

Single crystal Data and Details of the Structure Determination for MFM-300(V)
Single crystal X-ray structures were solved by direct methods and refined by full matrix least-squares on F 2 using the SHELXL software package. 1  The sample of desolvated MFM-300(V III ) was loaded into a cylindrical vanadium sample container with an 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 CO 2 were performed by volumetric method at ambient temperature, in order to ensure that CO 2 was present in the gas phase when not adsorbed and also to ensure sufficient mobility of CO 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 CO 2 per vanadium.
The sample was then slowly cooled down to 7 K to ensure CO 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 with various CO 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 CO 2 molecules. In this treatment the CO 2 molecules were treated as rigid bodies; we first refined the centers of mass, orientations, and occupancies of the adsorbed CO 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 CO 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 CO 2 molecules obtained from the refinement are also in good agreement with the experimental values for the CO 2 loading.

DFT Calculation for INS spectra
Modelling by Density Functional Theory (DFT) of the bare and CO 2 -loaded MOFs was performed using the Vienna Ab initio Simulation Package (VASP). 2 The calculation used Projector Augmented Wave (PAW) method 3,4 to describe the effects of core electrons, and Perdew-Burke-Ernzerhof (PBE) 5 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 CO 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 CO 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 6 for dispersion corrections was applied. The vibrational