The Study On The Structure Regulation And Single-molecule Magnetic Properties Of Carbonyl Urea-coordinated Rare Earth Complexes

In recent years,a variety of novel single-molecule magnets（SMMs）have been reported.It has been found that rare earth ions such as Tb³⁺,Dy³⁺,Er³⁺have larger spin value and higher magnetic anisotropy,which are very suitable for the construction of SMMs with excellent performance.At the same time,the molecular structure with high symmetry and low coordination number is conducive to achieving SMMs with high energy barrier and/or blocking temperature.Based on this,we have chosen anion ligands such as metallocyanate precursor and hexafluorophosphate to coordinate or balance the charge with rare earth ions.And the local coordination geometry was adjusted by adding several urea ligands with high symmetry and large steric hindrance as auxiliary ligands,so as to obtain SMMs with novel structure and good magnetism.The main research of this thesis is as follows:1.Three isostructural cyano-bridged two-dimensional brick-wall 3d-4f complexes have been synthesized using K₃[Fe（CN）₆]and K₃[Co（CN）₆]as bridging ligands,and1,3-Dimethyl-2-imidazolidinone（DMI）as ancillary ligand with rare earth metal ions(Ln=Y³⁺,Dy³⁺).The magnetic properties of compounds 1YFe,2Dy Fe and 3Dy Co were characterized and analyzed.The static magnetic studies show that 2Dy Fe and 3Dy Co exhibit paramagnetic behavior and 1YFe exhibits weak ferromagnetic interaction.In ac magnetic measurements,2Dy Fe and 3Dy Co both generate significant frequency-dependence under a 1 k Oe dc field.The effective energy barrier of 2Dy Fe is 22.2 K,while that of 3Dy Co reaches 53.1 K.2.Three isostructural cyano-bridged heterodinuclear 3d-4f complexes have been synthesized using K₃[Fe（CN）₆]and K₃[Co（CN）₆]as bridging ligands,and propylene urea（PU）as ancillary ligand with rare earth metal ions(Ln=Y³⁺,Dy³⁺).The static magnetic studies show that 4YFe,5Dy Fe and 6Dy Co exhibits paramagnetic behavior.In ac magnetic measurements,4YFe shows no magnetic relaxation behavior,while 5Dy Fe and 6Dy Co only exhibit obvious frequency-dependence in the imaginary part of ac susceptibilities at low temperature under a zero dc field.It indicates that the magnetic relaxation behavior of this system is greatly affected by quantum tunneling effect in the absence of a static field.Under an applied dc field,4YFe,5Dy Fe and 6Dy Co show typical slow magnetic relaxation with suppressed quantum tunneling effect.The effective energy barrier of 6Dy Co reaches 80.77 K.3.Two 4f mononuclear complex with D_4dcoordination symmetry have been synthesized using PF₆^–as a counter anion,and propylene urea（PU）as ancillary ligand with rare earth metal ions(Ln=Dy³⁺,Tb³⁺).The magnetic properties of complexes 7Dy and 8Tb were studied.Ac susceptibility measurements indicates that the complex 7Dy has a strong frequency-dependence and a high effective energy barrier when no external dc field is applied.On the contrary,the complex 8Tb with a similar structure shows no magnetic relaxation behavior with overwhelming quantum tunneling effect both in the absence and presence of an applied dc field,probably because Tb ^Ⅲions only exhibit single-ion magnetic properties under more strict conditions of magnetic anisotropy compared to Dy^Ⅲ ions.