Synthesis,Structures And Magnetic Properties Of The Ln-SMMs Based On Bisphenol-Containing Schiff Base Ligands

An important feature of Single Molecule Magnets（SMMs）is that they can maintain magnetic bi-stable state at the molecular level within a certain temperature range,and display characteristics of both classical and quantum mechanics.This property makes SMMs to be potentially applied in a wide range of fields,such as high density information storage,magnetic refrigeration,quantum computation and molecular spintronics.With the more attention being paid to the SMMs,many new materials with enhanced magnetic properties were discovered and many theoretical methodologies for studying SMMs were developed.Lanthanide-based SMMs,especially dysprosium-based SMMs,have received a great deal of attention.The strong magnetic anisotropy and magnetic bi-stability of dysprosium make it the most promising element in the field of SMMs.In this dissertation,twenty complexes were synthesized under solvothermal conditions by using four bisphenol-containing multi-dentate ligands.Complexes were obtained by adjusting the size of the terminal coordination solvent molecules,changing auxiliary ligands,introducing transition metal ions with high spin ground state,and varying the solvent polarity of the reaction system.The complexes were characterized by single crystal diffraction.The magnetic properties were studied.CASSCF calculations were employed to rationalize the relaxation mechanisms and the relationship between the magnetic properties and structures of these complexes.The research contents are as follows.1.Five binuclear rare earth complexes with bisphenol-containing Schiff-base ligand 2-{[（2-hydroxy-3-methoxybenzyl）imino]methyl}-4-methoxyphenol（H₂L¹）were obtained under solvothermal conditions.They are formulated as[Ln₂（L¹）₂（NO₃）₂（MeOH）₂]（Ln₌Dy（1）,Gd（4））,[Ln₂（L¹）₂（NO₃）)₂（EtOH）₂]（Ln₌Dy（2）,Gd（5））,and[Dy₂（L¹）₂（NO₃）₂（DMF）₂]·0.5EtOH（3）.The coordination unit structures of these five complexes are similar.Compounds 1-3 were obtained by changing the size of the coordinated terminal solvent molecules.The results of AC magnetic susceptibility studies showed that 1-3 were all single-molecule magnets.Complexes 1 and 2 showed a single relaxation process with relatively high energy barriers,while 3 showed a two-step relaxation process.The appearance of two-step relaxation process of 3 was explained by ab initio calculation method.It is possible that the asymmetry of the configuration of central metal ions and the small angle（θ）between the easy magnetic axis and the two DyⅢ ion vectors resulted in the two-step relaxation process.2.Reactions of Ln_NO33·6H₂O（Ln₌Dy,Er）with the Schiff base ligand H₂L¹ and auxiliary ligands HO₂CPh,（2-NO₂）HO₂CPh,（4-OCH₃）HO₂CPh and HPYFA（HPYFA=4-pyridine formic acid）,respectively,under solvothermal conditions resulted in the formations of five Ln₄ clusters and a one-dimensional chain complex with the formulae[Dy₄L¹₄（O₂CPh）₂（NO₃）₂（EtOH）₂]（6）.[Dy₄L¹₄（（2-NO₂）O₂ CPh）₂（NO₃）₂（EtOH）₂]（7）,[Dy₄L¹₄（（4-OCH₃）O₂CPh）₂（NO₃）₂（EtOH）₂]（8）,[Ln₄L¹4（NO₃）₂（PYFA）₃（EtOH）₃]·EtOH（Ln₌Dy（9）,Er（11））and[Dy₂L¹3HPYFA)（EtOH）]n（10）.They were structurally and magnetically characterized.Strikingly,complexes 9 and 10 were obtained in the same reaction system by varying the amount of EtOH solvent.The single crystal X-ray diffraction studies revealed that 6-9 and 11 possess linear tetranuclear structures,while 10 displays a one-dimensional chain configuration.All of six complexes are central symmetric,and constructed by employing Dy₂ units as the building blocks.Magnetic dynamics studies revealed that complexes 6-10 are SMMs with relatively high energy barriers.CASSCF calculations showed that the change in steric hindrance of auxiliary ligands restrains zero field of quantum tunneling,further affecting magnetic exchange interactions of central metal.As a result,five complexes with different energy barriers were given.This work demonstrates a new concept for extending the structure from 0D to 1D with enhanced energy barrier via employing different amount of solvent.3.Three hexanuclear heterometallic clusters（12-14）were synthesized by employing the ligand H₂L¹ and CoⅡ,ZnⅡ,and DyⅢ/YⅢ salts under solvothermal conditions.The structures of three complexes consist of four stacked defect tetracubane cores.The effects of 3d and 4f metal ions on the magnetic properties of these complexes were explored.Magnetic studies revealed that complexes 12 and 13 behave as SMMs with different energy barriers,but complex 14 does not display behaviors like SMMs.The results demonstrate that complexes which are not SMMs can be changed into SMMs by adjusting the central metal purposely.4.Two bisphenol-containing Schiff-base ligands 4-chloro-2-{[（2-hydroxy-3ethoxybenzyl）imino]methyl}phenol（H₂L₂）and 2-{[（2-hydroxy-3-ethoxybenzyl）imino]methyl}-4-methoxyphenol（H₂L₃）were synthesized.Two binuclear dysprosium complexes were obtained by employing H₂L₂ and H₂L₃ as ligands under solvothermal conditions.They are of compositions[Dy₂（L=）₂（NO₃）₂（EtOH）（DMF）]·2EtOH（15）and[Dy₂（L3）₂（NO₃）₂（MeOH）₂]（16）.The two DyⅢ centers in 15 have D2d and D4d geometric symmetries respectively,and the DyⅢ centers in 16 all have D2d geometric symmetries.The magnetic property studies revealed that both complexes are SMMs.The energy barrier difference was rationalized by ab initio calculation.It is anticipated that the energy barrier of SMMs can be effectively enhanced by improving the symmetry of the center metals.5.A new bisphenol-containing Schiff-base ligand with bigger steric hindrance 5-chloro-2-{[（2-hydroxy-3-ethoxybenzyl）imino]methyl}phenol（H2L⁴）was synthesized.In order to explore the effect of the steric hindrance of the ligand on the structure and magnetic properties of the complexes,two binuclear dysprosium complexes of formulae[Dy₂（（P-OEt）L⁴）₂（NO₃）₂（DEF）₂]（17）and[Dy₂（（AP-OEt）L⁴）₂（NO₃）₂（DEF）₂]（18）were obtained under solvothermal conditions.Interestingly,17 and 18 display isomeric structures and "symbiotic" in one reaction system.They can be completely separated by regulating the polarities of solvents.Complex 17 with two ethoxy groups of the ligands exhibiting parallel arrangement（P-OEt）can be converted to 18 in which the two ethoxy groups of the ligands displaying antiparallel arrangement（AP-OEt）by varying the polarities of the solvents.Complex 18 is more stable in dynamic environments than 17.In 17,one DyⅢ center adopts D2d symmetry and displays triangular dodecahedral configuration.The other one exhibits a square antiprism spatial configuration（D4d geometric symmetry）.In 18,the two DyⅢ centers adopt D2d symmetry and show triangular dodecahedral configuration.Complexes 17 and 18 both are single-molecular magnets with different energy barriers and relaxation mechanisms.These magnetic dynamics results are further explained by CASSCF calculation.It is found that the tiny structural changes caused by isomerism in SMMs can be the fundamental reason for the magnetic differences.This result opens a new prospective for designing single-magnetic molecules with isomeric structures.6.Two binuclear dysprosium complexes[Dy₂（L⁴）₂（NO₃）₂（MeOH）₂]（19）and[Dy₂（L⁴）₂（HL⁴）（NO₃）（MeOH）0.5]·MeOH（20）were prepared by using H2L⁴ as ligand under solvothermal conditions.They are coexisted in one reaction system.Complex 20 can be isolated by adjusting the polarities of the solvents of the reaction system.The difference between the two binuclear complexes lies in the number of ligand sets.The geometric symmetry of the coordination configurations of DyⅢ ions of the two complexes are also different.The magnetic property studies showed that two complexes both are single-molecule magnets with relatively high energy barriers.Interestingly,complex 19 exhibits a single relaxation process,while complex 20 displays a two-step relaxation process.It revealed that complexes with different configurations can be separated by adjusting the polarities of the solvents of the reaction system.It is also an effective strategy to synthesize single-molecule magnets with two-step relaxation process by adjusting the polarity of the reaction system.