Syntheses And Magnetodynamics Of Dysprosium Single-Ion Magnets With Octahedral/Pentagonal Bipyramid Coordination Geometries

Single-Molecule Magnets?SMMs?that exhibit superparamagnetic slow relaxation at the molecular scale represent ideal candidates for the next generation of high density information storage and molecular spin electronics devices.Among the SMMs family,the Single-Ion Magnets?SIMs?where only one paramagnetic metal ions present in the molecule are attractive as they represent the smallest size possible and the ligand field environment is easy to be fine-tuned at will.In this dissertation,for the purpose of constructing SIMs with different coordination geometries,we select Dy^III ion with strong magnetic anisotropy and magnetic bistability as the spin center,tetradentate nitrogen ligands,halogen anions as weak ligand field,and phenol oxygen and phosphine oxygen ligands as strong ligand field,taking advantage of the steric hindrance and charge-driving effects,to design and synthesize a series of pentagonal bipyramidal?PBP?and octahedral Dy^III SIMs.Through the study of coordination structure and magnetic properties,combined with ab initio calculation,the internal magneto-structural correlations and relaxation mechanism are explored.The specific research contents are as follows:1.Four PBP Dy^? complexes were synthesized with the tetradentate Schiff base imine-based ligand N,N?-bis?2-methylenepyridinyl?ethylenediamine?Bpen?and the phenol oxygen ligands with different substituents.Complexes realize the orderly regulation of the ligand field while maintaining the geometry of the lanthanide ion.The structures,magnetic properties and ab initio calculations of four complexes are studied in detail,and the differences of their magnetic anisotropy and relaxation paths are elucidated.The studies of the electronic and magnetic structure of the relatively good 2 and the reported excellent 2'which have the very similar coordination environments,the key structural factors that significantly affect the magnetic properties are explored,which provides a reference for the design of Dy^?SIM with high-performance PBP geometry.2.In order to further improve the SMMs properties,based on the enlightenment from the first part,the imine ligand Bpen was reduced to the H-substituted amine ligand Hbpen,and the H-substituted amine-based complexes 5-7 corresponding to imine-based complexes1-3 were prepared by using Hbpen.It is found that the H-substituted amine have stronger ligand field than the unreduced imine Schiff base ligands due to easily protonated H atoms.To further illustrate this effect,the H atoms of the ligand was substituted with-CH₃ to obtain the methylated ligand Mbpen,and CH₃-substituted amine-based complex 8 similar to 1 and5 was prepared.Based on the well-defined axial and equatorial configurational distribution of PBP systems,the relationship between the magnetic anisotropy and the ligand field strength of different type of nitrogen atoms is systematically elucidated through the detailed magnetic and theoretical studies,which provides a ligand design guidance for the construction of SMMs with significant magnetic anisotropy.3.In order to further study the importance of weakening the transverse ligand field in improving magnetic anisotropy and relaxation behavior,a pseudo-linear Dy^?SIM?10?was constructed in an octahedral framework by using large volume and low surface charge density iodine ions as weak donor atoms combined with the neutral Cy₃PO ligands.Compared with the PBP complexes constructed by phosphine oxide ligands,complex 10 not only reduces the coordination number of equational donors,but also reduces the transverse ligand field strength,thus realizing the pseudo-linear ligand field environment.10 shows an effective barrier of 1062 K and hysteresis loops open up to 9 K.Moreover,the profound influence of strong equatorial ligand substitution on the electronic structure and relaxation pathway is clearly explored in complexes 9 and 10.4.In order to further understand the importance of weakening the transverse ligand field,and considering that the electronegativity and bonding distance of halogen ions change regularly with the increase of the ion radius,three Dy^?SIMs 11-13 with similar octahedral geometry were obtained by combining Cy₃PO ligands and different chloride ions,bromine ions and iodine ions to crystallize in tetrahydrofuran.Through the magnetic investigation and theoretical calculation,the SIM behavior of complexes changes from the field-induced Raman relaxation?11?to the zero field thermally activated relaxation?12 and 13?,and the climbing relaxation path increases gradually,with complex 12 as the first excited state and complex 13 as the second excited state.The results show that the coordination of heavy halogens can reduce quantum tunneling of magnetization directly to a large extent and therefore increase the axial magnetic anisotropy of lanthanide ions.