| When the crystal field splitting energy(?)is close to the electron pairing energy(P),spin-crossover(SCO)or spin-transition(ST)between high and low spin state(LS(?)HS)could be achieved by some external perturbations,such as temperature,pressure,light and so on.In most cases,SCO happens in transition metal compounds with a d4?d7 electronic configuration,under an appropriate ligand field.As the occurrence of the SCO is usually accompanied with changes in certain physical and chemical properties,such as magnetism,color,volume,and electrical conductivity,SCO compounds are ideal materials for information storage components,sensors,and optical switches and so onAs many Fe-based SCO compounds show broad thermal hysteresis around room temperature,and Fe(?)SCO complexes can be stable in air,Fe?/? SCO complexes have been the most widely studied SCO materials.In this thesis,we focus on the Fe?/?SCO complexes containing tridentate Schiff base ligands.The main structure of this thesis consists of the following contents:?.Series of[Fe?(qsal)2]+SCO compounds with different organosulfonate anions were studied.Using the[Fe?(qsal)2]+ and several monovalent organosulfonates as counter anions,complexes of the general formula of[Fe(qsal)2]Aˇsolv(1-4)have been prepared and studied structurally and magnetically.Compounds 1-3 exhibit different SCO behaviors while compound 4 remains in the HS state.Interestingly,despite of the existence of two-dimensional(2D)?-? interactions and three-dimensional(3D)hydrogen-bond interactions in 1,compound 2 shows more interesting SCO property,which is related to the more compact 2D ?-? stacking interactions.Similar to 2,compound 3 also exhibits a more complete and abrupt SCO behavior than compound 1 due to the tighter cationic stacking.In addition,the different sizes of above organosulfonates also infiuence the solid state stacking of the cations,result in different magnetic behaviors to some extent.?.Series of[Fe?(qsal-X)2]+ SCO compounds with the substituted Hqsal ligands were investigated.Following the above studies,substituents were introduced to the Hqsal ligand to modify the ligand fields and also the SCO properties.Using monovalent and divalent organosulfonates,compounds 5-9([Fe?(qsal-X)2]Aˇsolv(X?5-OCH3,5-CH3))were prepared and characterized.All of them undergo SCO transitions.Compound 8 shows a complete and abrupt SCO feature with the strongest ?-?interactions among compounds 5-8,while compound 6 has similar magnetic behavior due to the existence of strong hydrogen-bond interactions.Magneto-structural relationship studies on compounds 5-8 reveal that the synergistic effect in above SCO systems is mostly affected by intermolecular interactions including ?-? interactions and hydrogen-bond interactions.Moreover,both interactions are highly responsible for more abrupt SCO behaviors.As for the anions,they can slightly affect the SCO behaviors of compounds 5-8 through their various sizes and shapes.In addition,the strategy of ligand modification was also discussed in the magneto-structural relationship of compounds 1,5 and 9.The introduction of 5-substituents on the Hqsal ligand in above three complexes increases the steric hindrance and also reduces the hydrogen-bond interactions between the ionic layers to some degree.At the same time,this modification in compounds 5 and 9 also changes the solid-state packing mode of the cation layers,and greatly enhances the ?-? synergy.Therefore,a more abrupt and complete SCO behavior with higher transition temperature was observed in compounds 5 and 9 under the 5-substitutional groups.?.Series of ligand-modified[Fe?(pphmp)2]2+ SCO compounds were studied.A new ligand and their Fe? complexes 10-13 with the general formula of[FeL2](ClO4)2ˇsolv(L?4-Cl-pphmp,2-Cl-pphmp,2-CH3-pphep and 2-Br-pphep)were prepared.While compounds 10 and 11 both exhibit a low spin state,compounds 12 and 13 show incomplete and gradual SCO behaviors. |
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