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

The design and synthesis of a Gd(III) metal-organic framework with the formula [Gd-4(BTDI)(3)(DMF)(4)](n) (<bold>JXUST-40</bold>, 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 <bold>JXUST-40</bold> and {[Gd-4(BTDI)(3)(H2O)(4)]<middle dot>6H(2)O}(n) (<bold>JXUST-40a</bold>) was achieved under the stimulation of heat and solvents. Both <bold>JXUST-40</bold> and <bold>JXUST-40a</bold> exhibited good stability when soaked in common solvents and aqueous solutions with pH values of 1-12. Magnetic studies showed that <bold>JXUST-40a</bold> has a larger magnetocaloric effect with -Delta S-m(max) = 26.65 J kg(-1) K-1 at 2 K and 7 T than <bold>JXUST-40</bold> 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 <bold>JXUST-40</bold> and <bold>JXUST-40a</bold>. In addition, both isomorphic [Dy-4(BTDI)(3)(DMF)(4)](n) (<bold>JXUST-41</bold>) and {[Dy-4(BTDI)(3)(H2O)(4)]<middle dot>6H(2)O}(n) (<bold>JXUST-41a</bold>) displayed slow magnetic relaxation behaviour.