Three microporous metal-organic frameworks assembled from dodecanuclear {NiLn} subunits: synthesis, structure, gas adsorption and magnetism.

Three isostructural heterometallic metal-organic frameworks (MOFs) {[LnNi(OAc)(HL)(L)]·solvent molecules} (HL = 2-hydroxyimino-N-[1-(2-pyrazinyl)ethylidene]-propanohydrazone, Ln = Dy for 1, Tb for 2 and Gd for 3) were solvothermally synthesized by varying rare-earth metal ions with different electron configurations. Their crystal structures, gas adsorption and magnetic behaviors were fully investigated. The three isomorphous MOFs exhibit three-dimensional microporous frameworks with two different orientated dodecane metallic {NiLn(HL)} metallomacrocycles alternately connected by {Ln(L)} connectors, in which an empty one-dimensional channel decorated by the basic hydrazone interior is generated. Due to their Ln-independent microporous nature, the activated sample of 1 as a representative example has a significant CO uptake up to 42.2 cm g and an unusually high CO/N and CO/CH adsorption selectivity of up to 98.8 and 16.8 at 298 K and 100 kPa. Magnetically, apparent antiferromagnetic interactions for both 1 and 2 as well as ferromagnetic coupling for 3 are respectively observed at low temperature resulting from the competition of magnetic anisotropy and intermetallic ferromagnetic superexchange. Additionally, 1 with highly anisotropic Dy spin shows slow magnetization relaxation under zero dc field, whereas 3 possessing isotropic Gd ions displays a significant cryogenic magnetocaloric effect with a maximum entropy change of 26.6 J kg K at 3.0 K and 70 kOe. These interesting results can provide valuable information on gas separation-based multifunctional 3d-4f MOF materials.