Synthese,Structures,Magnetic And Iuminescent Properties Of Lanthanide Coordination Compounds Based On Aromatic Carboxylate Ligands

Lanthanide coordination compounds are one important class of functional coordination compounds,which have potential applications in photoluminescence and molecular magnets.Their aesthetically appealing structural topologies are associated with the high coordination number and flexible coordination geometry of the lanthanide ions.Lanthanide ions have large magnetic moments and remarkable anisotropy arising from the strong spin-orbit coupling,which make lanthanide-based compounds to be a class of magnets with diverse magnetic dynamic behavior.Thus the lanthanide compounds have attracted increasing interest in the field of lanthanide-based single molecule magnets（SMMs）recently.It has been reported that the magnetic anisotropy for a lanthanide SMM is sensitive to the local coordination geometry of the lanthanide ion and the ligand field effects.Thus the choice of the appropriate ligand system plays a pivotal role in the design of the lanthanide SMMs.The carboxylate-based ligands with diverse coordination modes have been successfully employed in the construction of lanthanide coordination compounds,which exhibit good thermal-and air-stability resulted from the strong coordination bonds between the lanthanide ions and carboxylate oxygen atoms.However,only a limited number of lanthanide-carboxylate SMMs have been reported due to the quantum tunneling of magnetization（QTM）effect of the lanthanide ion under zero magnetic field.The zero-field QTM is commonly observed for lanthanide compounds,especially for the lanthanide-carboxylate compounds,due to the carboxylate oxygen donor possessing a relatively lower negative charge than those of phenoxo-oxygen andmethoxyloxygendonors.Thereforetheconstructionofthe lanthanide-carboxylate SMMs is still a challenge for chemists.In this thesis,the aromatic carboxylic acid ligands were used for synthesis of the lanthanide compounds.Nine novel lanthanide compounds have been obtained herein.Firstly,2-quinolinecarboxylate（QLC-）with a quinoline backbone providing an N donor and two carboxylate O donors was selected for the construction of the lanthanide coordination compounds.The quinoline N and the carboxylate oxygen atoms can be synergistically coordinated to the lanthanide center,which will produce various structures.The large skeleton of the 2-quinolinecarboxylate can separate the molecules spatially,which avoids the magnetic interactions between the molecules in the crystal packing,facilitating the formation of SMMs.Three compounds,[Dy₄（QLC）₁₂（H₂O）₆]·4H₂O（1）,[Tb₄（QLC）₁₂（H₂O）₆]·4H₂O（2）and[Dy（QLC）₃（H₂O）₂]_n（3）were synthesized based on the single 2-quinolinecarboxylate ligand.Three other compounds,[Dy₂（μ₂-OH）₂（QLC）₄（1,10-phen）₂]·4H₂O（4）,[Dy（QLC）₂（BDC）_0.5（H₂O）₂]_n（5）and[Tb（QLC）₂（BDC）_0.5（H₂O）₂]_n（6）were constructed from the 2-quinolinecarboxylate and 1,10-phenanthroline orterephthalate mixed ligands.Finally,threecluster-basedcompoundsof[（CH₃）₂NH₂]₂[Ln₆（μ₃-OH）₈（O₂C-C₆H₄F）₁₂（H₂O）₄]（Ln=Gd（7）,Dy（8）and Tb（9））have been obtained using the 2-fluorobenzoic ligand.This thesis contains four chapters.The first chapter is the introduction,describing the concept,synthesis methods and potential applications of the functional coordination compounds.Moreover,the concept and development of single-molecule magnets（SMMs）including lanthanide SMMs,are also presented in this chapter.The research idea of this thesis is included here.In the second chapter,three coordination compounds based on the single2-quinolinecarboxylate ligand are described.The crystal structures and the physical properties of the three compounds are provided.Compound 1 is a linear tetranuclear Dy₄ structure and isostructural to compound 2.Compound 1 is a one-dimensional（1D）chain wherein the single Dy^IIIions are bridged by the 2-quinolinecarboxylate ligands.The detailed magnetic measurements on 1 and 3 showed that compounds 1 and 3exhibit slow magnetization relaxation in the absence of the direct-current（dc）field,typical for SMMs.Compound 1 displays two magnet relaxation processes that related to the two crystallographically independent in compound 1.Interestingly,structural evolution from linear Dy₄ cluster of 1 into 1D chain of 3,the anisotropy energy barrier(U_eff)are significantly enhanced from 45.4（2）to 144.2（1）K,which could be the result of a more favorable crystal field for Dy^IIIII ion in compound 3.The Tb ^IIIII compound 3 shows no slow magnetization relaxation but displays typical Tb^IIIII emissions.The introduction of 1,10-phenanthroline or terephthalate ligands into the Ln-2-quinolinecarboxylate system,three lanthanide compounds 4?6 were obtained.The Dy^IIIII compounds 4 and 5 didn’t show the slow magnetization relaxation but exhibit a field-induced slow magnetization relaxation.This because the presence of the quantum tunneling effect under zero dc field,and the applied field suppresses the quantum tunneling effect.Finally,the luminescent properties of compound 6 were investigated.Three hexanuclear cluster compounds,namely,[（CH₃）₂NH₂]₂[Ln₆（μ₃-OH）₈（O₂C-C₆H₄F）₁₂（H₂O）₄（Ln=Gd（7）,Dy（8）and Tb（9））,were synthesized from the2-fluorobenzoic ligand.Compound 7 exhibits a magnetic refrigeration effect due to its magnetic isotropy.While compound 8 displays no slow magnetization relaxation even in the presence 2 kOe dc field.Compound 9 with the Tb^IIIII emissions were observed.The last chapter of the thesis is the conclusion and outlook of this work.