Reversible single-crystal-to-single-crystal transition in Gd(iii) metal–organic frameworks induced by heat and solvents with a significant magnetocaloric effect

Abstract

The design and synthesis of a Gd(III) metal–organic framework with the formula [Gd₄(BTDI)₃(DMF)₄]_n (JXUST-40, H₄BTDI = 5,5′-(benzo[c][1,2,5]thiadiazole-4,7-diyl)diisophthalic acid) are reported hererin. Interestingly, a reversible single-crystal-to-single-crystal transition between JXUST-40 and {[Gd₄(BTDI)₃(H₂O)₄]·6H₂O}_n (JXUST-40a) was achieved under the stimulation of heat and solvents. Both JXUST-40 and JXUST-40a exhibited good stability when soaked in common solvents and aqueous solutions with pH values of 1–12. Magnetic studies showed that JXUST-40a has a larger magnetocaloric effect with –ΔS^max_m = 26.65 J kg⁻¹ K⁻¹ at 2 K and 7 T than JXUST-40 due to its larger magnetic density. Structural analyses indicated that the coordinated solvent molecules play a crucial role in the coordination environment around the Gd(III) ions and the change in the framework, ultimately leading to the changes in the pore size and magnetism between JXUST-40 and JXUST-40a. In addition, both isomorphic [Dy₄(BTDI)₃(DMF)₄]_n (JXUST-41) and {[Dy₄(BTDI)₃(H₂O)₄]·6H₂O}_n (JXUST-41a) displayed slow magnetic relaxation behaviour.