Preparations And Magnetocaloric Properties Of Gd-based Molecular Materials Derived From Carboxylate Ligands

Relying on refrigeration technology,modern society can easily preserve food and have comfortable living space.Magnetic refrigeration is expected to replace the traditional refrigeration technology such as gas compression and liquid helium,which is a potential refrigeration technology.Isotropic gadolinium ion possesses a relatively large spin ground state and weak magnetic interactions.The molecule-based magnetorefrigerant materials assembled by Gd^III ions and organic carboxylate ligands with various coordination modes may exhibit relatively large magnetic densities,which is helpful for the construction of rare earth molecular magnetorefrigerant materials with excellent performance.In this thesis,Gd-based molecular magnetorefrigerant materials with large magnetic entropy changes have been prepared by choosing organic carboxylic acids as the primary liagnds and Gd^III ion sources as the main reactants under hydro/solvothermal conditions.The effects of reaction conditions on the structures as well as the magneto-structural relationships were studied in detail,and the general rules of the construction of Gd-based molecular magnetorefrigerant materials derived from organic carboxylic acids were clarified.In addition,the main influencing factors toward magnetocaloric effect（MCE）were analyzed.The main contents of this thesis are summarized as follows.（1）Five kinds of sterically hindered monocarboxylate ligands including pivalic acid（Hpiv）,o-methoxybenzoic acid（Homba）and salicylic acid（H₂sa）,3,5-dinitrobenzoic acid（Hdnba）and 5-nitrosalicylic acid（H₂nsa）were selected as primary ligands and phen as auxiliary terminal ligands.Five Gd^III cluster complexes were successfully prepared under hydrothermal conditions via the use of different metal ion sources.Among them,complex 3 is trinuclear Gd（III）clusters,and complexes 1,2,4 and 5 take binuclear cluster structures.Structural analysis shows that the coordination modes are syn-syn-μ₂-η¹:η¹ and/or syn-syn-μ₂-η¹:η² and the use of sterically hindered monocarboxylate ligands and chelating auxiliary ligands facilitates the construction of Gd^IIIII clusters.Magnetic studies indicate that weak magnetic interactions between the adjacent Gd^III ions exist in all these complexes,with weak antiferromagnetic coupling in 1-4 and weak ferromagnetic coupling in 5,respectively.The maximum magnetic entropy changes(-ΔS_m^max)of complexes 1-5are larger than 16.8 J kg^-1 k^-1,suggesting that they are potential low-temperature magnetorefrigerant materials.Although their magnetic densities are smaller due to the use of heavier organic ligands,the weak magnetic interactions still promote the appearance of relatively MCE.（2）Eight light multidentate bi-and tricarboxylic acids including malic acid（H₂ma）,oxydiacetic acid（H₂oda）,glutaric acid（H₂glu）,succinic acid（H₂suc）,fumaric acid（H₂fum）,oxalic acid（H₂ox）,citric acid（H₄cit）and nitrilotriacetic acid（H₃nta）were used as primary ligands to form a series of gadolinium complexes（6-15）by the coordination with Gd^III ions.Complexes 6-13 are constructed from dicarboxylic acids.Complex 6 is a mononuclear Gd^III cluster,while complexes 7and 11 take three-dimensional layer-pillar structure.Complex 8 exhibits a one-dimensional double-chain ladder structure connected by oxalates,while complexes 9 and 12 are three-dimensional coordination polymers composed of one-dimensional chains.Complex 10 is a one-dimensional（1D）chain structure based on the Gd₂O₂ core connected by hydroxy bridge.Complex 13 presents a Zigzag chain based two-dimensional（2D）structure.Complexes 14 and 15 are constructed from tricarboxylic acids.Complex 14 is a three-dimensional structure containing carboxylate-bridged layers,and complex 15 displays a double-layer structure.Magnetic investigations show that there are weak antiferromagnetic（8,10,13,14,15）or ferromagnetic（6,7,9,11,12）interactions between the neighboring Gd^III ions.They exhibit large MCEs(-ΔS_m^max>32 J kg^-1 K^-1 and 75 mJ cm^-3 K^-1).（3）A Gd-based metal-organic framework[Gd（pda）（ox）_0.5（H₂O）₂]_n（16）was successfully synthesized by using the two smallest dicarboxylic acid[propandioic acid（H₂pda）and oxalic acid（H₂ox）].After heating at different temperatures,complexes[Gd（pda）（ox）_0.5（H₂O）]_n（17）and[Gd（pda）（ox）_0.5]_n（18）were obtained through the single-crystal-to-single-crystal（SC-SC）transformation.Interestingly,they can not only realize the reversible SC-SC transformation induced by temperature and water vapor,but also dispaly weak ferromagnetic interactions and large magnetic entropy changes(ΔH=7 T,-ΔS_m^max>45.0 J kg^-1 K^-1).The stepwise removal of coordinated water molecules occurs under the heating conditions,while the stepwise recoordination of water molecules occurs in the steam environment.The results show that complex18 exhibits the largest magnetic entropy change and the highest thermal stability,which is an ideal candidate as rare earth low-temperature magnetorefrigerant material.