Synthesis, crystal structure and 71Ga solid state NMR of a MOF-type gallium trimesate (MIL-96) with μ3-oxo bridged trinuclear units and a hexagonal 18-ring network

A new gallium trimesate Ga12O(OH)12({OH}4, {H2O}5)[btc]6 · ≈24H2O, called MIL-96 (btc = 1,3,5- benzenetricarboxylate or trimesate species) was hydrothermally synthesized under mild conditions (210 °C, 5 h) in the presence of trimethyl 1,3,5-benzenetricarboxylate in water and characterized by single-crystal X-ray diffraction technique. The MIL-96 (Ga) structure exhibits a three-dimensional metal–organic framework containing isolated trinuclear μ3-oxo-bridged gallium clusters and infinite chains of GaO4(OH)2 and GaO2(OH)4 octahedra generating a hexagonal network based on 18-membered ring. The two types of gallium groups are connected to each other through the trimesate species which induce corrugated chains of gallium octahedra, linked via μ2-hydroxo bonds with the specific -cis–cis–trans- sequence. The 3D framework of MIL-96 reveals three kinds of cavities (two of them have estimated pore volumes of ≈480 and 860 Å3), in which are encapsulated free water molecules. The latter species are removed upon heating at 150 °C. The MIL-96 (Ga) compound was characterized by 71Ga solid state NMR at different magnetic fields (14.1 and 17.6 T). 71Ga spectra of MIL-96 (Ga) show the gallium exhibits relatively weak quadrupolar interactions compared to those observed in other MOF-type gallium carboxylates that have very strong quadrupolar interactions (MIL-61 and MIL-53).