Synthesis, Crystal Structures And Magnetic Properties Of The Lanthanide-based Coordination Complexes

Recently, studies on the synthesis of polynuclear lanthanide-based single-moleculemagnets have received considerable attention, especially for the Dy³⁺, Tb³⁺, Ho³⁺, Er³⁺and Yb³⁺ions. To obtain lanthanide clusters with novel topologies, the selection ofligand and bridging ligand is one of the key factors. In this thesis, the polydentate Schiffbase2-{[（2-hydroxy-3-methoxyphenyl）methylidene]amino}benzoic acid （H₂L） hasbeen used as the ligand, because it has four different kinds of oxygen atoms and onenitrogen atom, and shows various coordination modes. Meanwhile, by changing thereaction conditions, the ligand may be decomposed to the two reactants, anthranilic acidand o-vanillin. Therefore the coexistence of three ligands may lead to a great number ofcomplexes with novel topologies and superior properties.Based on the considerations mentioned above, we have prepared three types ofhigher-nuclearity homometallic lanthanoid clusters （1-13）, one type of3d-4f/4f singlechain complexes （14-18） and two types of lower-nuclearity lanthanide clusters （19-25）by alternating the bridging ligands, pH values of the reaction system or combining withthe3d transition metals. The crystal structures and magnetic properties of them havebeen studied. The results showed that among of them, three complexes showfield-induced slow relaxation of magnetization while1complex may show single chainmagnet properties. The results of this thesis are briefly summarized as follows.1. A family of new octanuclear lanthanide compounds[Na₂Ln₆L₈（C₆H₄NH₂COO）₂（C₂H₅OH）₂（H₂O）₄（NO₃）]·NO₃·C₂H₅OH·nH₂O [H₂L=2-{[（2-hydroxy-3-methoxyphenyl）methylidene]amino}benzoic acid; Ln=Dy （1）（n=15）; Tb （2）, Gd （3）（n=14）; Sm （4）（n=16）] have been prepared under hydrothermalconditions. The X-ray crystal structure analysis reveals that a nitrate ion, which displaysan unusual μ8-η2:η2:η4tridentate bridging mode and bridges all the eight metal ions, islocated in the center of the cluster. The core of the structure is a [4+2] combination of anLn4cubane and an Ln₂unit, which is quite different from previously reported Ln6cases. Alternating current susceptibility measurements reveal that the DyIII–containing cluster[Na2Dy6] exhibits slow relaxation of magnetization.2. By increasing the pH value of the reaction system, three higher nuclearityhomometallic lanthanoid clusters,[Dy₈L₄L’₂L’’₂（μ3-OH）₆（NO₃）₆]·8（H₂O）（5），[Ln₁₀L₆L’₂L’’₂（μ3-OH）₆（μ2-OH）₂（H₂O）₂（NO₃）₄]·2（NO₃）·8（H₂O）·（C₂H₅OH）（Ln=Er （6）;Ho（7））, have been successfully obtained. The crystal structures and magnetic propertiesof them have been studied. All the3complexes have four typical mono-, bi-cappedtriangles and then was linked to each other by carboxylate oxygen atoms, leading tosystematic cubic （5） or bi-capped （6,7） square prism core geometry. Alternating currentsusceptibility measurements revealed that the DyIII–containing cluster [Dy8] exhibitsslow relaxation of magnetization.3. The first examples of silicate bridging lanthanide clusters,[Ln₈L₈（C₆H₄NH₂COO）₄SiO₄] Ln=Dy（8）, Tb（9）, Gd（10）, Nd（11）, Pr（12）, Sm（13）] havebeen obtained via hydrothermal reaction by using sodium silicate as bridging ligand.Crystal structural analysis shows that one silicate ion located in the center of the clusteradopts a μ8:η³:η³:η³:η³tetradentate bridging mode, and this [SiO₄] core binds to all theLn（III） ions generating the [Ln8SiO₄] cluster. Within the cluster eight Ln（III） ions arearranged in an unprecedented three-layer structure. The short Ln…Ln distance revealsunnegligible magnetic interaction between them. Magnetic studies indicate that the[Dy8SiO₄] complex exhibits slow relaxation of magnetization. The [Ln8SiO₄] complexrepresents the first lanthanide cluster based on the silicate building block.4. Five single chain complexes with three types of structures, namely, twoisostructural3d4f heterometallic single chaincomplexes{[Dy₂M₂L₄（NO₃）（H₂O）₂][Dy₂M₂L₄（NO₃）（CH₃COO）（H₂O）]·（NO₃）}n（M=Co（14）, Ni（15）），two pure4f single chain complexes [LnL’（C₂H₅OH）（NO₃）₂]n（Ln=Pr（16）, Nd（17）） and one4f single chain complex [PrL’3]n, have been reported. Crystalstructural analysis showed that the3d4f heterometallic complexes have an interestingbi-ladder single chain structures similar to the DNA molecule structure, while thecomplexes16and17have the ladder single chain structures and18only shows singlechain motif. Furthermore, the metal ions in complexes16-18were connected via nitrateions or three O atoms from the carboxylate groups, indicating strong interactionsbetween them. Alternating current susceptibility measurements reveal that the DyIII-NiII containing complex shows a strong frequency dependency, thus may exhibit slowrelaxation of magnetization as a SCM.5. Two hexanuclear complexes [Ln₆L₆（μ4-H₂O）（NO₃）₆（H₂O）₂]·C₂H₅OH（Ln=Pr（19）, Nd（20）） and five isomorphic dinuclear complexes[Ln₂（HL）₂（NO₃）₄（C₂H₅OH）₂（H₂O）₂]（Ln=Tb（21）, Eu（22）,Gd（23）, Er（24）, Nd（25）） havebeen prepared with or without the addition of NaOH to the reaction system. Resultsshowed that complexes19-20have new type of [4+2] topology, in which the Ln4unitshows a typical cubane motif with a μ4-H₂O being located in its center. The distance ofμ4-O…Ln³⁺is2.36and the angles of Ln-μ4-O-Ln are in the range of100.2-114.2,indicating efficient interactions between the metal ions. The dinuclear complexes arecentrosymmetric which may lead to the low magnetic axiality and thus being not goodfor magnetic property. Magnetic studies indicate the [Tb2] complex exhibitsferromagnetic interactions without an out-of-phase signal.