Synthesis,Structures And Magnetic Properties Of Single Molecule Magnets Based On Small Organic Molecules With Hydroxyl Groups

In recent years,the design and synthesis of single molecule magnets have become a hot research topic because of their potential applications in the new high density information storage materials,magnetic refrigeration materials,spintronics,etc.In the process of building the single molecule magnets,the selection of the propriate ligand which could chelate or bridge metal ions is one of the key factors.Among all the organic ligands,the polyhydroxy Schiff-base and thiazole derivatives are the most used ligands.In this thesis,some polyhydroxy Schiff-bases and 2-（benzo[d]thiazol-2-yl）-6-methoxyphenol have been used as ligands,and 26 complexes were synthesized by solvothermal method.The structures of these complexes were characterized by X-ray single crystal diffraction,IR,elemental analysis and X-ray powder diffraction.Magnetic properties of some of these complexes were also studied.The main results are summarized as follows.1.A series of tetranuclear lanthanide compounds,[Ln₄（μ₃-OH）₂L₄（NO₃）₂（DMF）₂]（H₂L=2-（2-hydroxy-3-methoxybenzylideneamino）phenol;Ln=Dy（1）;Tb（2）;Er（3）;and Gd（4））,have been prepared under hydrothermal conditions.X-ray crystal structure analysis reveals that 1–4 are polymorphs.The cores of the structures can be described as co-planar defect-dicubane.The solid-state luminescent property investigations revealed that the Ln^III ions have very deep influence on the luminescence of H₂L.The magnetic properties for 1-4 were studied.The Dy₄ complex exhibits single-molecule magnet behavior.2.Fivehexanuclearlanthanideclustersofcompositions[Ln₆（μ₄-O）₂（HCOO）₂L₄（HL’）₂（DMF）₂]（Ln=Dy（5）,Er（6）,Ho（7）,Tb（8）,Gd（9）;H₂L=2-（（2-（hydroxymethyl）phenylimino）methyl）-6-methoxy-phenol;H₃L’=3-（（2-（hydroxy-methyl）phenylimino）methyl）benzene-1,2-diol,and H₃L’was derived from the hydrolysis of the H₂L ligand）have been prepared under solvothermal conditions.The[Ln₆]cores of5-9 possess an unprecedented motif,namely,two tetrahedron Ln₄ units sharing an edge and two vertices.The six Ln^III ions of 5-9 are connected via twoμ₄-O anions.The magnetic susceptibility studies reveal that 5 exhibits frequency dependence of the ac-susceptibility typical of single-molecule magnets and the energy barrier of 5 is 85 K.3.TheemploymentofaSchiffbaseligand2-（（2-（hydroxymethyl）phenylimino）methyl）phenol（H₂L）in the synthesis of Ln^III-Co^III clustersisstudied.Fourhexanuclearclustersofcompositions[Ln₄Co₂（μ₃-OH）₂（NO₃）₄（CH₃COO）₄L₄（DMF）₂]·2C₂H₅OH[Ln=Dy（10）,Eu（11）,Gd（12）,Tb（13）]have been prepared under solvothermal conditions.The centrosymmetric hexanuclear metallic core comprises of a defect-dicubane Ln₄ moiety,and two Co^III ions are linked to the defect-dicubane unit through two alkoxo groups and two phenoxido groups.This structure gives a chair-like topology.The magnetic properties for 10-13 were studied.The Dy₄ unit of 10 exhibits slow magnetic relaxation behavior.The U_eff for 10 of41.9 K is a relatively high value among the reported Dy₄ SMMs with defect-dicubane cores.4.Sevenheteronuclear3d-4fclustersofcompositions[LnML₂（NO₃）₂（CH₃COO）（MeOH）]（HL=2-（benzo[d]thiazol-2-yl）-6-methoxyphenol;M=Co,Ni;Ln=Dy,Er,Ho,Y）have been prepared under hydrothermal conditions.X-ray crystal structure analysis reveals that Ln^III and M^II ions were bridged by two phenolate oxygens from two L^-ligands.The magnetic properties for 14-20 were studied.Complexes14 and 18 are potential single-molecule magnets.5.Six transition metal clusters of compositions[Mn₇L₆]（NO₃）₂·2CH₃OH（21）,[Fe₇L₆]（NO₃）₂·2CH₃OH（22）,[Co₇L₆]（NO₃）₂·2CH₃OH（23）,[Cu₇L₆]（NO₃）₂（24）,[Zn₇L₆]（NO₃）₂·2CH₃OH（25）,and[Cu₆Na₂L₆]（NO₃）₂·4CH₃OH（26）（H₂L=2-（2-hydroxy-3-methoxybenzyli-deneamino）phenol）have been synthesized under solvothermal conditions.The structures of 21-26 have been established by single-crystal x-ray diffraction studies.Complexes 21-25 possess disc-shaped{M₇^II}cores,with the central M^II ion and the six peripheral M^II ions being linked by sixμ₃-O atoms.The six M^II ions of the rim of the disc are pairly bridged by sixμ₂-O atoms.Complex 22 represents the second example of disc-shaped{Fe₇}cluster in which all of the Fe atoms exhibit+2oxidation state.And this is the first time that the five isostructural heptanuclear clusters were constructed by using an identical Schiff base ligand.The molecule of complex 26contains a capsule-like{Cu₆^IINa₂^I}core.In this core,two sodium ions reside on the two vertices of the capsule and the 12 vertices of the{Cu₆O₆}hexagonal prism are occupied by alternately arrayed six Cu^II ions and six O atoms.Photoluminescence studies suggest strong emission for 25 in the solid state at room temperature.Variable-temperature dc magnetic susceptibility studies were carried out on polycrystalline samples of 21-24 and 26.Ferromagnetic M^II···M^II interactions were determined for 22 and 23,while the antiferromagnetic interactions dominate in 21,24 and 26.The strong out-of-phase,c’’,signals were observed for 23 at zero field,suggesting that 23 might be an SMM.