Magnetic Relaxation And Spin Transition Control Of Mononuclear Fe~Ⅱ And Co~Ⅱ Complexes

The arrival of the information age raises higher requirements for high-density information storage.Traditional magnetic materials have faced the limitation of their magnetic storage density due to the size effect.The emergence of magnetic bistable molecules as a new type of magnetic storage material at the molecular level has aroused the research interest of researchers,including molecular nanomagnets,spin-crossover and charge-transfer complexes.3d transition metal ions are abundant in the earth’s crust,and 3d electrons have a variety of arrangements in the five d orbitals.3d transition metal ions can show very rich magnetic behaviors in different coordination environments,which are ideal research objects for the construction of magnetic bistable molecules.The first part of the work is to construct mononuclear octacoordinate complexes with slow magnetic relaxation by designing suitable ligands and transition metals.The effects of high coordination ligand field and metal center local symmetry on the magnetic bistability behavior of SIMs complexes were investigated through understanding the relationship between the structure and magnetism of the complexes.In the second part of the work,firstly,spin-crossover and fluorescent bifunctional complexes are constructed,and then these complexes are formed into complexes with cyclobis（4,4′-（1,4-phenylene）bispyridine-p-phenylene）tetrakis（hexafluorophosphate）（Ex Box·4PF₆）macrocycles through supramolecular interaction,and the effect of host-guest interaction on the properties of spin-crossover bistable is studied.The main research results of this paper are as follows:（1）Complexes 1-7 were obtained by coordination of 6,6’-bis（4,5-dihydrooxazol-2-yl）-2,2’-bipyridine（L¹）and 6,6’-bis（5,6-dihydro-4H-1,3-oxazin-2-yl）-2,2’-bipyridine（L²）with Fe^II,Co^II,Mn^II,and Ni^II,respectively.Crystal structure tests show that 1-3 and 6 are all eight-coordination structures,while 4,5 and 7 are six-coordination structures.Magnetic tests show that slow magnetic relaxation was not observed for 1-7 under zero field,which may be caused by severe ground state mixing and fast quantum tunneling at low temperature.Compounds 1-2 exhibit slow magnetic relaxation under the applied dc field,but the magnetic anisotropy of them has completely different sources.The two Fe^II centers in 1 exhibit easy-axial anisotropy with relatively small negative D values-4.897 and-4.825 cm^-1,respectively,while the center of Co^II in 2 is easy-plane anisotropy with a large D value of+46.42 cm^-1.Compounds 3-7 also don’t show slow magnetic relaxation under an applied dc field.（2）Using（E）-N’-（（5-（Pyrene-1-yl）pyridin-2-yl）methylene）benzohydrazide（L³）and（E）-4-（tert-Butyl）-N’-（（5-（pyrene-1-yl）pyridin-2-yl）methylene）benzohydrazide（L⁴）as ligands,the complexes 8-12 were obtained by directional coordination with Fe^Ⅱand Zn^Ⅱ,respectively.8and 9 are spin crossover and fluorescence bifunctional complexes,and diamagnetic 10-12 are used as a control complexes to discuss the influence of host-guest interaction on spin-crossover properties.The pyrene ring of the 8-12 can be nested into the macrocyclic molecule Ex Box⁴⁺through supramolecular interaction,and the combination of 8-12 and Ex Box⁴⁺is strong.After the addition of the Ex Box⁴⁺macrocycle,the difference between the residual peaks of the deuterated reagents in the inner and outer tubes of 8 increases,indicating that the spin state of8 has changed,which further indicates that the host-guest interaction can affect the spin state of the central metal ion.Evans magnetic moment test shows that the host-guest interaction in the current system has no significant effect on the spin state of Fe^II,which may be caused by the mismatch between the intensity of host-guest interaction and the intensity of coordination field.In the following work,the ligand field will be further optimized to achieve the reversible regulation of the spin states of the central metal ions by the host-guest interaction.