Syntheses And Properties Of New Lanthanide-Based Single-Molecule Magnets

Single-molecule magnets (SMMs), which possess a large overall ground-state spin quantum number and a significant magnetic anisotropy, have attracted intensive interest owing to the intriguing magnetic behavior associated with the quantum tunneling of the magnetization (QTM) and slow relaxation process. They are potentially useful in quantum computing, high density data storage and molecular spintronics. With the development of experimental techniques and magnetic theories, many SMMs with various structures and magnetic properties have been reported. Recently, the studies reveal that some paramagnetic lanthanide ions with the larger and unquenched orbital angular momentum can be used to construct SMMs containing higher effective energy barriers, and further provide important implication that the interaction between the anisotropy of the paramagnetic ion and the crystal field environment plays a crucial role in tuning their magnetic properties. On the other hand, the need for novel multifunctional molecule-based materials is continuously increasing. One of the goals from scientists is to prepare SMMs that possess one or more physical/chemical properties, such as optical activity, conductivity, porosity or ferroelectricity, in one molecule. In particular, the design and synthesis of multifunctional molecular magnetic semiconductors with high critical temperature has been one of the most important challenges. This combination involving both conductivity and magnetism establishes a coupling between mobile and localized electrons, which mainly arise from organic moieties assembled in networks and paramagnetic metal ion, and opens up the road for the synthesis of nano-magnetic materials with application values. In this thesis, we are working on the design, syntheses and magneto-structural studies of new lanthanide-based SMMs with novel structures and functional properties.1. A series of seven-coordinated single lanthanide(III)-based complexes,[(TPP)Dy(LoEt)](1, H2TPP=5,10,15,20-tetraphenylporphyrine; LoEt-=(η5-cyclopentadienyl)tris(diethylphosphito-p)cobaltate(III)),[(TPP)Tb(LoEt)](2),[(TPP)Ho(LoEt)](3),[(TPP)Gd(LoEt)](4),[(Pc)Dy(LoEt)](5, H2Pc=phthalocyanine),[(Pc)Tb(LoEt)](6),[(Pc)Ho(LoEt)](7),[(Pc)Gd(LoEt)](8) and [(Pc)Y(LoEt)](9) are synthesized based on tripodal Klaui’s ligand NaLoEt and porphyrin H2TPP or phthalocyanine H2PC ligands. Single crystal X-ray diffraction analysis reveals that the coordination geometry of the Ln(III) ion can be described as a "4:3piano stool". Magnetic measurements reveal that the Dy(III) and Tb(Ⅲ) complexes1,2,5and6exhibit field-induced slow relaxation of magnetization, and they are a class of seven-coordinated single lanthanide-based SMMs. All of the results confirm the magnetic relaxation properties of these single paramagnetic lanthanide complexes are closely related to the magnetic anisotropy and the crystal field environment of the central lanthanide ion.2. Three seven-coordinate single lanthanide(Ⅲ) complexes with electrochemically active TTF-Schiff base ligand H2L1and the tripodal Klaui’s ligand NaLOEt,[(LoEt)Dy(L1)](10, H2L1=2,2’-((2-(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)-1,3-benzodithiole-5,6-diyl)bis(nitrilomethylidyne)bis(4-chlorophenol); LOEt-=(η5-cyclopentadienyl)tris(diethyphosphito-p)cobaltate(Ⅲ)),[(LoEt)Tb(Li)](11) and [(LoEt)Ho(Li)](12), are synthesized and structurally characterized. All of the complexes are also characterized by the absorption spectra, electrochemical/spectroelectrochemical techniques and magnetic measurements. The results suggest their reversible redox properties for classical TTF units, and the Dy(Ⅲ) complex10exhibits the field-induced slow relaxation of magnetization with an energy barrier of41.6K. 3. Three new single paramagnetic lanthanide-based complexes,[(LOEt)Dy(L2)](13, H2L2=5,11,17,23-tetrakis(1,1-dimethylethyl)-25,27-dihydroxy-26,28-dimethoxy-calix[4]arene; LOEt-=(η5-cyclopentadienyl)tris(diethylphosphito-p)cobaltate(III)),[(LOEt)Tb(L2)](14) and [(LOEt)Ho(L2)](15), are synthesized with the multidentate calix[4]arene ligand H2L2and tripodal Klaui’s ligand NaLOEt. All of the complexes have been characterized by single crystal X-ray diffraction analysis, thermal stability, absorption spectra, and magnetic measurements. Single crystal X-ray diffraction analysis reveals that the two sterically bulky ligands provide a compact environment around the lanthanide core, resulting into the effective isolation of entire molecule. The magnetic properties and magneto-structural correlation in this seven-coordinated system are investigated. The dysprosium complex13shows typical single-molecule magnetic behavior with characteristic magnetic hysteresis loops and the slow relaxation of magnetization.4. A series of new triple-decker dinuclear sandwich-type lanthanide complexes,[(Pc)2Dy2(L3)(H2O)]·2CHCl3·H2O (16, H2L3=(1S,2S)-N,N’-bis(5-bromo-2-hydro-xyl-phenylmethene)-1,2-diphenylethylenediimine; H2PC=phthalocyanine),[(Pc)2Tb2(L3)(H2O)]·2CHCl3·H2O (17),[(Pc)2Ho2(L3)(H2O)]·2CHCl3·H2O (18) and [(Pc)2Gd2(L3)(H2O)]·2CHCl3·H2O (19), based on both phthalocyanine ligand H2Pc and chiral tetradentate Schiff base ligand H2L3are successfully synthesized. The corresponding characterization has been performed by elemental analysis, IR, UV-vis, X-ray single crystals diffraction analysis and magnetic measurements. According to the molecular structure, there are two kinds of distinct coordination environment around Ln(Ⅲ) ions, probably associated with the different easy axis magnetic anisotropy. Magnetic studies reveal that complex16exhibits the weak ferromagnetic coupling interactions between Dy(Ⅲ) ions at the low temperature and filed-induced SMM behavior with double relaxation processes.5. Two new triple-decker binuclear sandwich-type lanthanide complexes,[(Pc)Dy2(Li)2(CH3OH)]·5ClCH2CH2Cl (20, H2L1=2,2’-((2-(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)-1,3-benzodithiole-5,6-diyl)bis(nitrilomethylidyne)bis(4-chlor-ophenol); H2Pc=phthalocyanine) and [(Pc)Tb2(L1)2(CH3OH)]·5ClCH2CH2Cl (21) based on both phthalocyanine ligand H2Pc and redox-active Schiff base ligand H2L1are successfully synthesized. The corresponding characterization has been accomplished by X-ray crystallography, absorption spectra, electrochemical techniques and magnetic measurements. Interestingly, the non-centrosymmetric target complexes display typical electrochemical activity, and magnetic studies confirm that complex20exhibits the weak ferromagnetic f-f coupling interactions at the low temperature and filed-induced SMM behavior with stronger QTM.