71Ga Slow-CTMAS NMR and Crystal Structures of MOF-Type Gallium Carboxylates with Infinite Edge-Sharing Octahedra Chains (MIL-120 and MIL-124)

Two gallium carboxylates of MOF-type have been hydrothermally synthesized (210 °C) by using 1,2,4,5-benzenetetracarboxylate (Ga4(OH)8[C10O8H2]·5−5.6H2O or MIL-120) or 1,2,4-benzenetricarboxylate (Ga2(OH)4[C9O6H4] or MIL-124) as linkers. The structures of both compounds solved from XRD analysis, revealed two independent crystallographic sites for the gallium atoms, which are connected to each other through μ2-hydroxo edge, generating zigzag chains with a cis−trans mode. The inorganic ribbons are linked to each other via all the carboxylate functions of the pyromellitate ligand to form a 3D framework (MIL-120) or two of the carboxylate arms of the trimellitate ligand to form a 2D structure (MIL-124). For the latter, one of the carboxylate arms is not bonded to the gallium centers, resulting in the layered aspect of the final structure. Solid-state NMR using the slow-CTMAS method has been successfully implemented for the determination of the gallium site parameters for MIL-120, allowing more resolved spectra at moderate magnetic fileds than at very high field. Only one gallium site is visible for MIL-124, indicating a dynamic disorder resulting from the nonfully connected carboxylate species of the trimellitate ligand.