High-resolution solid-state NMR spectroscopy for the structural investigation of challenging metal organic frameworks
Résumé
Metal-organic frameworks (MOF) constitute an important class of crystalline porous materials. Since the introduction of the first porous MOF more than twenty years ago, more than 2000 three-dimensional MOF topologies have been described. The large surface areas and tunable pore sizes of MOFs makes them well suited for a variety of applications including gas storage, molecular sieving, or heterogeneous catalysis. As for other classes of solid catalysts, establishing structure- activity relationships is key for the rational design of MOFs with improved catalytic properties. Solid-state nuclear magnetic resonance (NMR) spectroscopy is well suited to characterize the molecular structure and dynamics of MOF materials, for example in cases where X-ray diffraction is insufficient to determine the topology of the framework or when the flexibility properties of the network needs to be investigated. We will first report the use of solid-state NMR experiments at ultra-fast magic angle spinning in combination with powder X-ray diffraction and first-principles calculations to resolve the crystal structure of a novel porous substituted imidazolate MOF. The validity of the method was confirmed by comparing DFT calculated proton chemical shifts with experimentally measured shifts, as well as by comparing the simulated powder diffraction pattern for the suggested crystal structure with the experimental one. We will then present the first application of DNP-enhanced solid-state NMR spectroscopy to MOF materials. We have recently shown (1), on a series of N-functionalized MOFs, that incipient wetness impregnation can readily be applied to impregnate these materials with a radical-containing solution. The DNP methodology provides reduction in experimental time of two orders of magnitude, even in a proline derivative material for which the radical does not enter into the pores. This approach enables the fast acquisition of 13C and 15N NMR spectra with high S/N ratio and therefore allows for the rapid characterization of intact MOF topologies.