Synthesis And Characterization Of Fe(Ⅱ) Spin Crossover Complexes Based On Bromopyridine Multidentate Ligand

Among many metal based functional materials,so-called spin crossover(SCO)complexes attract attention by their fascinating ability to be reversibly changed between high spin(HS)and low spin(LS)states,and this consequently changes all their associated physical properties,such as magnetic properties,crystalline structure,colour and soon,making it potential for molecular sensors and information storage devices.Based on the previous research of our group about tetradentate ligands of spin-crossover complexes,we introduced-Br group into the pyridine ring of the selected ligand aimed to change and research its spin-crossover.Four chapters are included in the dissertation.In chapter 1,the research progress of spin crossover complexes were included,which highlighted the uncommon test in other research named Mossbauer spectrum.What follows is the introduction of influencing factors of spin crossover temperature and the polymorphism of spin crossover complexes.Chapter 2 introduces the research of the polymorphism of the spin crossover complexes with the same formula[FeLBr(dca)2](LBr =N,N’-2(5-Br2-Pyridine methyl)-1,2-ethylenediamine)with a-form as thermodynamic product,while γ-form as kinetic product.Tests distinguishing and researching three polymorphs have been finished including elemental analysis,infrared spectroscopy,single-crystalline test,PXRD,DSC,magnetic and Mossbauer spectrum.In three polymorphs,strong hydrogen bonds formed by dca and NH group dominate the molecules with forming strongly coupled chains of complexes along the c and b axis in α-form and β-form.In case of γ-form,those NH…NC hydrogen bonds connect the complexes into strongly coupled 2D layers parallel to the ab plane.Symmetry breaking observed for the α-form is rare among complexes showing one-step SCO:a single Fe(Ⅱ)site in the HS state converts to two crystallographically distinct Fe(Ⅱ)sites in the LS state.The two-step transition for the β-form is related to a re-entrant symmetry change,while the Mossbauer spectroscopy reveals an unusual co-existence of HS,HS-LS and LS phases during the two-step SCO.On the contrary,the γ-form stay HS.In the third chapter,we get the single crystals of LBr with KSCN and KSeCN.The characteristics of the two complexes were explored from IR,TG,PXRD,single crystal,magnetic.From the compare of PXRD of two complexes,they are allomerism.The[FeLBr(NCS)2]in HS state were connected with intermolecular bonds formed by Br atoms and S atoms respectively while only formed by S atoms in LS state.Both two complexes have gradual spin crossover with the spin crossover temperature differing about 25 K.In chapter 4,the whole thesis work is summarized.