3.4. Single-Crystal X-ray Crystallography

All the single-crystal X-ray diffraction data for the crystals were recorded on a supernova diffractometer with a multilayer mirror Cu Kα radiation (λ = 1.5418 Å) or Mo Kα (λ = 0.7107 Å). All the structures of compounds 1–4 were solved by a direct method of SHELX-97 and full-matrix least-squares refinement on F² was performed using SHELX-97 [50]. Except for some disordered water molecules, all non-hydrogen atoms were anisotropic refined on F² by the full-matrix least-squares technique using the SHELXL-97. Only the [Me₂NH₂]⁺ counterion in Gd³⁺ MOF can be solved from single crystal X-ray diffraction. The existences of [Me₂NH₂]⁺ in the other three MOFs and water molecules were further verified by the IR spectra, elemental analysis, and TGA data, obtaining the final chemical formulas of compounds 1–4. The CCDC (Cambridge Crystallographic Data Centre) numbers for compounds 1–4 are 2043852, 2043854, 2043855, and 2043853, respectively. See Tables S2–S5 for detailed crystallographic data.