Structure-property Correlations In Mn~Ⅲ Spin-crossover Materials

The fourth-period transition metal octahedral complexes with electronic configurations from d⁴to d⁷are ideal systems for preparing magnetic bistable spin crossover（SCO）materials.Among them,Schiff-base ligands and their metal complexes have attracted people’s attention for a long time because of their excellent mouldability.The design and preparation of spin crossover materials with thermal hysteresis effect near room temperature is a research hotspot at present.Such complexes are ideal materials for the preparation of novel molecular switches,sensors and magnetic memories.In this paper,a series of（sal-N-1,5,8,12）^2-ligands are are introduced to conduct an in-depth study on the spin cross-behaviour of Mn（III）hexadentate Schiff-base complex by changing the types and position of substituents on the ligand benzene ring,the types of anions and the existence of solvent molecules reported.In the first system,in order to explore the effect of introducing hydroxyl ligands on supramolecular interactions on Schiff base ligands,the complex[Mn（4-OH-sal-N-1,5,8,12）]NO₃（1）and[Mn（4-OH-sal-N-1,5,8,12）]Cl O₄（2）were designed and prepared.The number of hydrogen bonds is increased by hydroxyl groups to strengthen the connection between anions and cations and improve the cooperation of the system,thus increasing the driving force of the spin crossover complex.However,the magnetic susceptibility measurement results show that the above two complexes are in the high spin（HS）state in the test temperature range.Although Hirshfeld surface analysis confirms that the introduction of hydroxyl groups enhances intermolecular cooperation,hydroxyl groups also form stable dimers in adjacent cations,making the Mn（III）centre less prone to deformation.There are five novel compounds studied in the second system:[Mn（3-OMe-sal-N-1,5,8,12）]Sb F₆·H₂O（3）,[Mn（4-OMe-sal-N-1,5,8,12）]Sb F₆·H₂O（4）,[Mn（4-OMe-sal-N-1,5,8,12）]Cl O₄·H₂O（5）,[Mn（3-OMe-sal-N-1,5,8,12）]Cl·1.5H₂O（6）,[Mn（3-OMe-sal-N-1,5,8,12）]Br（7）.Different from the previous system,most of the structures of these complexes are water solvates.Among them,complex 4 is in the low spin（LS）state at 100 K and exhibits gradual spin crossover behavior,while the Mn center of complex 7 exhibits intermediate state（IS）at 300 K.The third system focuses on the influence of the positional effect of ligand substituents on the spin crossover properties of Mn（III）hexadentate Schiff base complexes when controlling the anions are AsF₆^–.The subjects of this system are[Mn（3-OMe-sal-N-1,5,8,12）]AsF₆·H₂O（8）,[Mn（4-OMe-sal-N-1,5,8,12）]AsF₆·H₂O（9）and[Mn（5-OMe-sal-N-1,5,8,12）]AsF₆（10）.The hexahedral AsF₆^–anion ensures that there is enough space between the Mn（III）cations for deformation,in addition,the effect of H₂O molecules can be further investigated.