The Synthesis And Magneto-structural Correlation Study Of Phenoxyl-O Bridged Dysprosium Single-molecule Magnets

Since the first rare earth single-molecule magnet was discovered in 2003,the rare earth ions are regarded as the ideal candidates for constructing single-molecule magnets due to they have significant magnetic anisotropy induced by the combination of large orbital angular momenta and spin-orbit coupling with coordination field.Since then,researches on rare earth single-molecule magnets have sprung up one after another,but the current theoretical understanding of molecular magnetic relaxation does not seem to be able to keep up with such a rapid pace.While obtaining the systems with significant anisotropy,people also try to establish the relationship between the magnetism of single molecule and the molecular structure,and some general conclusions have been concluded.However,these are far from enough.We need to build more research platforms that can help clarify and understand the source of relaxation and establish the magneto-structureal correlation,so as to provide theoretical basis and effective methods for obtaining molecular magnetic materials with high relaxation barrier and hysteresis temperature.The simple and easily modified phenoxyl-O bridged dinuclear rare earth single-molecule magnet systems provide such an ideal platform.On the one hand,molecular magnetic anisotropy can be fine-tuned by regulating local symmetry and coordination environment,and the influence of fine-tuning of anisotropy on the dunamic magnetic relaxation behavior of spin center can be clearly observed.On the other hand,through using the bridging ligands to transfer magnetic exchange interaction,the overall magnetic anisotropy can be optimized and the influence of magnetic interaction on molecular magnets can be studied in depth.In this paper,three types of phenoxyl-O bridging ligands,namely,8-hydroxyquinoline,N,N’-bis（salicylidene）-o-phenylenediamine schiff base and simple and modified 2,6-dimethoxyphenol compounds,were selected to work together with auxiliary ligands to design and synthesize several series of new dinuclear rare earth complexes which are stable in the air.The crystal structures of the complexes have been determined by single-crystal X-ray diffraction,and characterized by infrared absorption spectrum,thermogravimetric analysis,elemental analysis and X-ray powder diffraction.On the basis of introduction of magnetic exchange interaction,the coordination environment and local symmetry of the spin center,the nature of O-bridge and the guest solvent molecules were changed to realize the regulation of the ligand field,thereby adjusting the magnetic anisotropy of the whole molecule.Through the static,dynamic magnetic susceptibility measured by SQUID magnetometer combined with theoretical calculations,the magneto-structural correlations was systematically studied.Firstly,two series of rare earth single-molecule magnets were constructed with8-hydroxyquinoline organic ligands.The first series is 2-methyl-8-hydroxyquinoline self-assembled with different dysprosiumβ-diketonates to form four bridged dinuclear dysprosium complexes.By modifying the terminal substituents of peripheralβ-diketonates co-ligand,we successfully prevented the coordination of solvent molecules to Dy（III）ions utlizing theβ-diketonates with larger steric hindrance,and the local symmetries were effectively enhanced.Ab initio calculation illustrated that the fine-tuning of the terminal substituents cause Dy（III）ions to produce different single-ion anisotropy and intramolecular coupling,and the combination of high single-ion anisotropy and strong magnetic coupling interaction effectively improve the anisotropy energy barrier.In the second series,8-hydroxyquinoline was used as the bridging ligand,and introducing diamagnetic alkaline earth metals between rare earth ions Ln（III）to construct four linear heterometallic complexes[Ln-M-Ln],in which Dy-based complexes show significant relaxation behaviors at zero field.The dilution experiment showed that the magnetic interaction significantly suppress the QTM process in the absence of external magnetic field.Although the distance between the two f-electron centers separated by diamagnetic alkaline ions in Dy-based complexes is longer than 6.8?,there is still a long-range ferromagnetic interaction,which was successfully verified by ab initio theoretical calculation and magnetic dilution experiment.In addition,the magnetic axes of Dy（III）parallelly arrange and tend to the direction of two spin centers.This perfect linear figuration favors the communication between the longitudinally extending susceptibility tensors of two Dy（III）ions,which likely leads to the ferromagnetic interaction.Secondly,the third series of dinuclear dysprosium-based single-molecule magnets were constructed by using N,N’-bis（salicylidene）-o-phenylenediamine as bridging ligand.N,N’-bis（salicylidene）-o-phenylenediamine self-assembled with different dysprosiumβ-diketonates to form five inequitable dinuclear dysprosium-based complexes.The one-to-one correspondence between the two relaxation processes and the two spin centers was determined by fine-tuning the local coordination environment of one spin center.Moreover,the barrier was significantly increased by regulating the terminal ofβ-diketonate from electron donating to electron withdrawing substituent.Finally,three series of dinuclear rare earth single-molecule magnets were constructed using 2,6-dimethoxyphenol compounds as bridging ligands.The fourth series is the self-assembly of 2,6-dimethoxyphenol and rare earth halide to obtain six bridged dinuclear complexes.Moreover,the pentagonal bipyramidal geometry was successfully constructed by introducing phosphine oxide co-ligands.Among them,the highest effective barrier U_eff/k_B under zero field is 157.0 K and the blocking temperature is up to 5 K.The magnetic anisotropy of single ion was changed by fine-tuning crystal field and local symmetry,and the ab initio calculation illustrated that strong axial crystal field and weak transverse crystal field could effectively improve the magnetic anisotropy of single ion and consequently generate high energy barrier and blocking temperature.On this basis,it is conducive to improve the whole molecular magnetic anisotropy that the magnetic axes on two Dy（III）parallel and align on the vector connecting two Dy（III）ions.Furthermore,the construction of pentagonal bipyramidal geometry is not the only condition for obtaining high anisotropy.Only when the C5 rotation axis of the coordination polyhedron,pentagonal bipyramid,overlaps with the easy axis,the symmetry of crystal field can be considered as D_5h.In order to improve magnetic performance,D_5h symmetry should be with as less as possible distortion.The fifth series is the self-assembly of 2,6-dimethoxyphenol andβ-diketonates co-ligands with dysprosium chloride to obtain three bridged dinuclear Dy-based complexes with different guest solvents in the lattice,in which the highest effective barrier U_eff/k_B under zero field is 194.0 K and the blocking temperature reaches 3.5 K.The dilution experiment showed that the slow magnetic relaxation is derived from the single-ion anisotropy,and the intramolecular exchange coupling could effectively suppress the QTM process.In addition,the magnetic anisotropy was significantly affected by only removing or replacing the unbonded solvent molecules,and the effective barrier and hysteresis temperature were greatly increased.The sixth series is a self-assembly of dysprosium chloride with derivatives of 2,6-dimethoxyphenol andβ-diketonates co-ligands to obtain four bridged dinuclear Dy-based complexes.By introducing electron withdrawing substituent on the bridged ligands,the anisotropic energy barrier was greatly increased by five times.The highest effective energy barrier U_eff/k_B at zero field is 175.2 K,and the blocking temperature reaches 3.5 K.Additionally,the origin of two relaxation processes is attributed to the existence of two different units in lattice,which is illuminated via single-crystal to single-crystal transformation.In such studies,simple structural unit and easily modified ligand terminals make it possible to fine-tune the local symmetry and crystal field of spin center.They have enriched the theoretical basis that the single-ion magnetic anisotropy combine with appropriate coupling interaction to optimize the whole molecular magnetism,and the magneto-structural correlations about the attribution of relaxation source and the guest solvent effect have been established.