Three microporous metal-organic frameworks assembled from dodecanuclear {NiII6LnIII6} subunits: synthesis, structure, gas adsorption and magnetism

摘要

Three isostructural heterometallic metal-organic frameworks (MOFs) {[Ln(2)Ni(OAc)(5)(HL)(L)]solvent molecules}(n) (H2L = 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 {Ni(II)Ln(III)(HL)}(6) metallomacrocycles alternately connected by {Ln(III)(L)} connectors, in which an empty one-dimensional channel decorated by the basic hydrazone interior is generated. Due to their Ln(III)-independent microporous nature, the activated sample of 1 as a representative example has a significant CO2 uptake up to 42.2 cm(3) g(-1) and an unusually high CO2/N-2 and CO2/CH4 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-III spin shows slow magnetization relaxation under zero dc field, whereas 3 possessing isotropic Gd-III ions displays a significant cryogenic magnetocaloric effect with a maximum entropy change of 26.6 J kg(-1) K-1 at 3.0 K and 70 kOe. These interesting results can provide valuable information on gas separation-based multifunctional 3d-4f MOF materials.