Construction And Characterization Of Low Dimensional Fe~Ⅱ Spin Crossover Compounds

Spin crossover（SCO）compounds,which can exhibit fast response to external stimuli and switching property between low spin and high spin states in the molecular level,have attracted widespread attention from researchers because of their potential applications in sensor and memory devices.However,the controllable assembly and synergetic modulation of spin crossover compounds are still challenging.This dissertation focuses on the construction of novel cyanide-bridged spin crossover compounds by using tricyanometalate building blocks due to its abilities in directional assembly and electron transimission.Meanwhile,the strategies of modifying intramolecular metal-metal charge transfer（MMCT）,auxiliary ligands and intermolecular cooperativity are systematically investigated to manipulate the cyanide-bridged SCO compounds.Moreover,the coupling of magnetism and dielectricity properties are further studied,which can be switched by the stimuli-tuned conversion between high spin and low spin states.The results have been summarized as follows:（1）Tricyanometalate building blocks[Bu₄N][Fe^III（Tp）（CN）₃],ligand pyridine-4-carboxaldehyde and metal ions M^II（M=Fe,Ni,Mn）were assembled into three neutral nanocages 1-3 with the core structure of Archimedean polyhedra.Structural and spectroscopic analysis revealed that the oxidation and deprotonation of precursor ligands played an essential role during the assembling process,resulting in versatile electronic configurations of the clusters.In addition,magnetic studies demonstrateed that the ferromagnetic interactions spanned over the nanocages and compound 3 exhibited large magnatic entropy changes.（2）To obtain polynuclear SCO-active clusters,we intend to introduce the ferrous-triazole units,which are benchmark of SCO compounds owing to their outstanding and robust SCO properties.In addition,the metallocyanate building block[Ph₃PMe][Fe^III(Tp^4-Me)（CN）₃]was adopted to cut the M^II-triazole chain(M^II=Ni,Co,Fe)into cluster fragment.In this way,compounds 4-6 composed of 13 metal centers were successfully synthesized and compound 6(M^II=Fe)exhibited SCO behavior.The introduction of metallocyante building block resulted in the formation of Prussian blue analogues unit(Fe^III-CN-Fe^II)in Fe-triazole systems,which made the intramolecular metal-metal charge transfer（MMCT）become possible and could be further used to manipulate the SCO property of compound 6.These results provide a new alternative for manipulating SCO behaviors.（3）In order to study the influence of rigid connection of interchain cooperativity on the SCO properties units,rigid monodentate ligand pep（4-（2-phenylethynyl）pyridine）and bidentate ligand dpa（1,2-bis（4-pyridyl）ethyne）,tricyanometalate building blocks[Bu₄N][Fe^III（Tp）（CN）₃],[Ph₃PMe][Fe^III(Tp^4-Me)（CN）₃]and[Bu₄N][Fe^III（pz Tp）（CN）₃]with different pyrazol substituent groups,as well as Fe^II were chosen to construct cyanide-bridged double zig-zag chain compounds 7-10.The influences of different substituent groups from building blocks on SCO properties were discussed.The abrupt spin transition with hysteresis was observed in 10.This can be attributed to the enhanced cooperativity between SCO centers through the introduction of bidentate bridging ligand.Dielectric measurements showed step-like changes of dielectric constants indicating the synergetic modulation of magnetism and polarity via spin state conversion.