Synthesis And Properties Of Molecule-based Magnetic Materials With Slow Magnetic Relaxation Behavior

Molecule-based magnetic materials are gradually becoming the next generationof functional materials, thanks to their small size, light weight, high transparency,good solubility, good plasticity, easy processing and chemical tunability. Particularly,single molecule magnets (SMMs), single ion magnets (SIMs) and single chainmagnets (SCMs) showing slow magnetic relaxation have been a very active academicresearch focus. In these molecular nanomagnets there exists not only classical effectof macroscopic magnets but also quantum effect. They could potentially findapplications in high-density information storage, quantum computing or molecularspintronics. Although a considerable amount of such magnets have been reported, themechanism of magnetic relaxation and the structure-magnetic property relationshipare still not well understood. Additionally, the pursuit of higher energy barrier to spininversion and higher blocking temperature will be scientists’ long-term objective.This dissertation, mainly focusing on molecular nanomagnets with slowmagnetic relaxation behavior, consists of five chapters.Chapter one mainly reviews the development of molecule-based magneticmaterials and gives a short account of the research background and purposes.In the second chapter, five cobalt-based compounds and two dysprosium-basedcompounds [Co4(meq)4Cl4] MeCN (1),[Co3(meq)5(Hmeq)]ClO4(2),[Co2(meq)2Cl4][H2meq]2(3),[Co3(hmp)6(MeOH)2](ClO4)2(4),[Co2(hmp)4(N3)2]3MeOH (5),[Dy4O(OH)2(meq)8](6), and [Dy2(meq)2(Hmeq)4(NCS)4](7) have been prepared. Compounds1-3consist of CoII4, CoII3, and CoII2units, respectively, and in which theinteractions are antiferromagnetic in nature. They don’t show SMM behavior becauseof their near-zero ground-state spin or low anisotropy. Compound4contains lineartrinuclear CoIII-CoII-CoIIIunits, and it shows slow magnetic relaxation at lowtemperature. As the terminal CoIIIions are diamagnetic, the magnetism of compound4arises from the single CoIIcenter. Compound5is diamagnetic since it presents abinuclear CoIII2structure. Compound6consists of DyIII4units in which the DyIIIionsare ferromagnetic coupled, and it shows SMM behavior. Compound7consists ofDyIII2units where the DyIIIions are antiferromagnetic coupled. It doesn’t show SMMbehavior.In the third chapter, firstly, we study the magnetic properties of threemononuclear cobalt-based compounds,[Co(PPh3)2Cl2](8),[Co(DPEphos)Cl2](9),and [Co(Xantphos)Cl2](10). All of these compounds exhibit significant uniaxialanisotropy, and slow magnetic relaxation behaviour could be observed under anapplied field. Their energy barriers are between29.9and37.1K. To study theinfluence of ligand field on the magnetic properties, we synthesize another twomononuclear compounds [Co(DPEphos)(NCS)2](11) and [Co(DPEphos)(OCN)2](12)with different pseudohalide ions. Magnetic measurements demonstrate that bothcompounds show SIM behavior, but the energy barrier of12is significant higher thanthat of11. The difference in the thermal-active energy barrier could be attributed tothe difference in the magnetic anisotropy, which most likely arises from the ligandfield strength of the psedohalogen ligand since the geometry of CoIIion is quitesimilar in compounds11and12. Compared to isothiocyanate anion, cyanate ligandhave weaker field, thereby yielding a higher anisotropy.In the fourth chapter, six three-dimensional coordination frameworks[Co2(4-ptz)2(bpp)(N3)2]n(13), Ni2(4-ptz)2(bpp)(N3)2]n(14),[Co2(4-ptz)2(bpe)(N3)2]n(15),[Ni2(bdt)(bpp)2(N3)2]n(16),[Co3(OH)2(bdt)2(bpp)2(H2O)2]n(17), and[Co3(OH)2(bdt)2(bpe)(H2O)2]n2H2O (18) have been hydrothermally synthesized byemploying5-substituted tetrazoles as the main organic ligands and different assistantligands. All the compounds consist of well-isolated one-dimensional magnetic chains which are further linked by diamagnetic organic ligands. Compounds13-16showsimilar chain structure where the metal ions are connected by alternating singleμ-EE-N3and triple (μ-EO-N3)(μ-tetrazole)2bridges. It was demonstrated that thesecompounds exhibits intrachain alternating antiferromagnetic/ferromagnetic coupling,but the antiferromagnetic one is dominant. Compounds13-16don’t show SCMbehavior. Compounds17and18contain similar Co3(μ3-OH)2chains while theirinter-chain distances are different. Magnetic measurements and analysis demonstratethat the chains in these two compounds are ferrimagnetic with alternatingantiferromagnetic/antiferromagnetic/ferromagnetic interaction. Spin canting behaviorare observed due to the single-ion magnetic anisotropy of CoIIions and theantisymmetric exchange coupling. Wide hysteresis loops are also observed at lowtemperature. Their alternating-current (AC) susceptibility plots demonstrate SCM-likemagnetic relaxation and another antiferromagnetic transition. Because of theshortened inter-chain distances in compound18, it possesses stronger inter-chainantiferromagnetic interaction, and the characteristic of antiferromagnetic long-rangeordering is more significant.The fifth chapter makes a summary of this dissertation. Our research involves thedesign, synthesis and magnetic properties of molecular nanomagnets. The influence oftheir structures on the magnetic behavior has been carefully studied. Our results cansupply some guiding significance for the exploration of molecular nanomagnets withslow magnetic relaxation as well as afford academic bases for the enhancement oftheir energy barrier and blocking temperature.