Structural Study of Mg-Based Metal-Organic Frameworks by X-ray Diffraction, 1H, 13C, and 25Mg Solid-State NMR Spectroscopy, and First-Principles Calculations

Understanding the formation principles, structure, and stability of magnesium-based metal-organic frameworks is a difficult task that requires application of a set of experimental and theoretical methods. In this work, three polycrystalline Mg-benzene-1,3,5-tricarboxylates, differing in the dimensionality of their frameworks, were studied by X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles calculations based on the density functional theory (DFT). XRD provided information on the average periodic structures of the three materials. Natural abundance Mg-25, C-13, and H-1 magic-angle spinning NMR spectra confirmed the proposed structural models, elucidated hydrogen bonds within two of the materials, and detected the strong effect of the anisotropy of the bulk magnetic susceptibility in one. The calculations of C-13 and Mg-25 chemical shifts and Mg-25 quadrupolar coupling parameters with the gauge-including projector-augmented wave approach enabled assignment of carbon and magnesium NMR signals to carbon and magnesium sites within the three metal-organic framework structures. The detected C-13 and Mg-25 NMR parameters could not be simply related to the geometry of the environments of these nuclei. DFT-based calculation of formation energies of the materials enabled the prediction of the thermodynamical stability of their structures.