Synthesis And Magnetic Properties Of Fe~?/Co~? Spin-crossover Complexes

Due to their potential applications in the areas of sensors,switches and memory devices,bistable materials have attracted a great deal of research interest.As one of the main families of bistable materials,spin crossover(SCO)compounds can change their physical properties such as magnetism,optics and structure features under a spin transition triggered by external stimuli such as temperature,light,pressure and so on.Up to now,most of the reported spin crossover compounds are iron(?)complexes,while the reported air-stable iron(?)compounds are relatively limitted.In addition,only a few cobalt(?)complexes with thermal hysteresis loop have been reported.Because the cobalt(?)SCO compounds exhibit a smaller entropy change during the spin transition,the SCO properties of the Co(?)compounds are more sensitive to the modification of the ligand field strength and the environments of the cobalt center.In this thesis was focused on the spin crossover compounds of iron(?)and cobalt(?)with the main results as following:1.Iron(?)spin crossover compounds based on tetradentate schiff base ligands.Compounds 1-8 were synthesized from a tetradentate schiff base ligand 4-Me Ohapen.Different anions were used to modify their SCO properties,hoping to increase their cooperativity.Compounds 6-8 exhibit SCO behavior with different SCO properties.In compound 6,one-dimensional chain structures were formed by the hydrogen bonds between the cations and the anions,and weak N-H…?and C-H…?interactions between these chains were also noticed.Meanwhile,no hydrogen-bonded 1D chains were found for compounds 7 and 8.This structural difference might be the reason that compound 6 have the most abrupt spin crossover property with a small thermal hysteresis loop while the SCO transitions in compounds 7 and 8 are gradual.2.Cobalt(?)spin crossover compounds based on Terpy ligand.Using different anions,five very similar cobalt(?)spin crossover compounds were synthesized and characterized.The only different for these compounds are the different anions.Compounds 9 and 10 are spin crossover compounds with thermal hysteresis loops.In addition,the transition temperature of compound 9 is the highest among all the reported cobalt(?)spin crossover compounds with thermal hysteresis loops.Through structural characterization and analysis of the above mentioned cobalt(?)spin crossover compounds,we concluded that?–?interaction and hydrogen bond can influence the properties of the spin crossover compounds,and change of the anions can effectively regulate the properties of the cobalt(?)spin crossover compounds.3.Cobalt(?)spin crossover compounds containing crystallization solvents.Two cobalt(?)spin crossover compounds containing the same crystallization solvent and different counter anions were syntheized and characterized.Magnetic characterization indicates that the two compounds'properties are very similar with incomplete spin crossover.We infer that in this kind of spin crossover system,anion effect on its properties is limited,while the crystallization solvent molecules can block the cobalt(?)center in a low spin state.