Construction Of Dy Complexes With D_5h/D_4d Configuration And Study Of Slow Magnetic Relaxation Dynamics

Exploring the effects of different coordination configurations of dysprosium ions on the slow magnetic relaxation process is an important work in the study of dysprosium single-molecular magnets.Starting from the D_5h and D_4d coordination configurations of dysprosium ions,this thesis reveals the nature of slow magnetic relaxation by studying the configuration characteristics,and further illustrates the quantum tunneling of magnetization（QTM）relaxation of dysprosium single-ion magnets by the strength of transverse anisotropy.Based on polynuclear dysprosium single-molecule magnets with multiple spin centers,the influence of magnetic interaction on the slow magnetic relaxation behavior of polynuclear dysprosium single-molecule magnets is revealed by introducing suitable bridging ligands.The main content and conclusions of the paper are as follows:1.By selecting the tricyclohexylphosphine oxide ligand with larger steric hindrance as the strong axial ligand,and then the bidentate pyrrolidinedithiocarbamate acid anion（APDC）ligand on the equatorial plane as the weak ligand to construct the D_5h configuration single-molecule magnet,structural characterization shows,complex1 has two bidentate ligands in the cross section and a strong tricyclohexylphosphine oxide ligand.After magnetic testing,it is found complex 1 has serious quantum tunneling of magnetization under the zero fields.The main reason is that there is a strong lateral ligand field which leads to serious quantum tunneling.Therefore,based on the D_5hconfiguration,complex 2 with the same axial ligand but two bidentate ligands and a weakly coordinated bromide ion in the cross section was synthesized.According to the crystal field theory,when the same ligand is evenly distributed on the equatorial plane,the lateral anisotropy is reduced to a certain extent,QTM is inhibited,and the single-molecule magnet the magnetic properties have been significantly improved.Complex 2 satisfies this condition well,which well inhibits low-temperature quantum tunneling of magnetization,and significantly increases the time of the Orbach relaxation process.2.The Schiff base ligands with similar structures are combined with dysprosium ions and gadolinium ions to form zigzag tetranuclear dysprosium and gadolinium complexes.The configurations are mainly D_5h and D_4d.It is obvious that the complex3-6 has the same coordination mode,without changing the local coordination geometry,the introduction of one 2,6-dimethoxyphenol（DMOP）molecule in 5 in place of one bridging chloride anion and two acetonitrile molecules in 3 without the change of local coordination geometries,leading to the transformation of the magnetic interactions from antiferro-to ferro-magnetic coupling.Studies of the dynamic magnetic properties reveal that 3 behaves as a field-induced SMM,with a significant quantum tunneling of magnetization（QTM）at zero field,and complex 5 displays the enhanced SMM properties with an anisotropic barrier of U_eff=105 K at zero field.Ab initio calculations indicate that the Ising interaction alters the arrangement of the main magnetic axes surrounding four Dy^III centers in 5 and the different types of magnetic couplings generated between Dy^IIIions,ferromagnetic for 3 and antiferromagnetic for5,are mainly responsible for reducing the zero-field QTM.This study shows that the introduction of suitable bridging ligands can achieve the transformation of magnetic interaction,which can well achieve the suppression of quantum tunneling of magnetization（QTM）in polynuclear Dy^III single-molecule magnets,and make the relaxation time of the Orbach relaxation process obvious.The increase significantly improves the effective energy barrier of polynuclear dysprosium single-molecule magnets.