| Single molecule that exhibits slow magnetic relaxation behavior and hysteresis are known as SingleMolecule Magnets(SMMs)and are promising candidates for high-density information storage,molecular spin and quantum computing.Increasing the blocking temperature and the spin-reversal barrier is of paramount importance to make SMMs practically applicable.Research over the last three decades has shown that increasing uniaxial magnetic anisotropy is a key factor in the construction of high-performance SMMs.Understanding and applying magnetic anisotropy is of great importance for the rational design of high performance SMMs.Among them,Co(Ⅱ)and Dy(Ⅲ),which have strong magnetic anisotropy and unquench orbital angular momentum,are considered excellent candidates for the construction of highperformance SMMs.As SMMs have been increasingly studied,researchers have found that the crystal fields,structural distortions,uncoordinated layers and coupling effects all have a significant effect on their magnetic anisotropy.In this thesis,divalent cobalt ions and trivalent dysprosium ions were chosen as suitable center metal to design and synthesize complexes with a pentagonal bipyramidal geometry,and their structures and magnetic properties were tested and characterized.The aim of present work is to investigate the self-assembly mechanism of SMMs and to obtain the method of governing magnetic anisotropy,easy-axis orientation and magnetic interactions,and further to achieve a precise and controlled preparation of these materials,as follows:1、Four mononuclear cobalt(Ⅱ)complexes,with the formulas[Co(dapbh)(H2O)(CH3OH)](1),[Co(Hdapbh)(N3)(CH3OH)]·(CH3OH)(2),[Co(H2aapbh)(CH3OH)2]·(NO3)2(3)and[Co(H2bapbh)(H2O)(NO3)]·(NO3)(4),have been synthesized by combining Co(Ⅱ)ions with the planar open-chain N3O2 ligand H2LR(R=Ph,CH3,H),the discretely seven-coordinated Co(Ⅱ)cores in all complexes display distorted pentagonal bipyramid geometries.The combined analyses of magnetic data and ab initio calculations unveil large easy-plane magnetic anisotropies for these complexes(D=+37.338,+37.273,+41.138,+41.139 cm-1 for 1-4,respectively),which indicate that the chemical alterations of the equatorial ligand and the ligand field strength in the axial positions synergistically fine-tune the magnitude of the D values.Magnetic investigations demonstrate the field-induced single-molecule magnet behavior in complexes 1,2 and 4 with diverse energy barriers(Ueff)of 12.25 for 1,44.15 for 2 and 48.72 K for 4,corresponding to the geometrical distortion of heptacoordinated Co(Ⅱ)ion.That is,the most deviation from the ideal D5h symmetry in 4 is responsible for the highest barrier.2、Two mononuclear Dy(Ⅲ)complexes,in which each Dy(Ⅲ)is seven-coordinated,namely[Dy(bcbpen)Cl](5)and[Dy(bcbpen)Br](6),have been prepared using a multichelating Schiff base ligand H2bcbpen.X-ray crystallography showed that both complexes have similar coordination geometry environment with distorted pentagonal bipyramid geometries around Dy(Ⅲ)ions.Magnetic investigations demonstrate the excellent single-molecule magnet behavior with Ueff of 907.0 for 5 and 1270.4 K for 6.The open hysteresis loops at zero field are visible up to 8 K for 5 and 10 K for 6 at the sweep rate of 0.0045 T s-1.The ab initio calculations point out that calculated KDs are more axial in 6 than 5,supporting the more prominent SMMs behavior for the former complex.The charges on the axial O atoms are discovered to be nearly three times larger compared to the N atoms of the bcbpen ligand.The charge on the Cl atom in 5 is more negative than that on the Br atom in 6.Compared to 5,proving the weaker QTM and the more significant relaxation barrier for 6. |
