Lanthanide Metal–Organic Frameworks with High Chemical Stability as Multifunctional Materials: Cryogenic Magnetic Cooler and Luminescent Probe

A series of neutral lanthanide metal–organic frameworks (Ln-MOFs) with the formulas {[Ln4(L)3.5(μ3-OH)3(μ4-O)(H2O)2]·H2O}n (Ln = Sm for 1, Ln = Eu for 2, Ln = Gd for 3, H2L = 2,5-pyrazine dicarboxylic acid) were prepared by using the multicarboxylic acid 2,5-pyrazine dicarboxylic acid to control the hydrolysis of Ln3+ ions. The coordination sites of Ln3+ ions were partially occupied by L2–, leaving only a limited number of sites available for aqua coordination, which inhibited the further hydrolysis of Ln3+ ions to form the precipitates of lanthanide oxides/hydroxides. Finally, the three-dimensional metal frameworks of compounds 1–3 were formed by {Ln4O4} as secondary building units (SBUs) and organic ligand L2– as linkers. These three compounds showed high stability, and especially compound 2 exhibited high chemically stability. The solid-state fluorescence emission spectra showed that compound 2 exhibited the red characteristic emission of Eu3+ ions. The studies of the luminescence recognition of different organic solvents indicated that compound 2 had an obvious fluorescence recognition effect on n-hexane featuring luminescent quenching. Meanwhile, compound 2 showed a sensitive detection effect and good cyclic effect on CrO42– in water solution, suggesting that compound 2 might be used as a highly stable and sensitive fluorescent probe for CrO42– ions in aqueous medium. Besides, magnetic studies showed that compound 3 had large magnetocaloric effects with a −ΔSm value of 42.4 J kg–1 K–1 at 2 K and 7 T, which might be a potential cryogenic magnetic cooler.