Studies On The Syntheses, Structures And Magnetic Properties Of Azido-Metal Complexes And Polynuclear Metal Cluster Compounds

The investigation of molecule-based magnetic materials with fascinating structures has become one of the most active fields in materials and inorganic chemistry for their interesting properties and potential application. Azido-metal compounds in which azido bridges provide the magnetic exchange pathways have evoked researchers' considerable interest because of their unusual magnetic properties. So far, over a thousand azido-metal complexes, with novel structures and diverse magnetic properties, have been reported. However, azido/carboxylate compounds which consist of azido and carboxylate bridges have seldom been reported, probably due to competition of charge from synthesis and offset of couplings from magnetism. So, to construct azido/carboxylate compounds, especially ferromagnet, is a challenging task. Because their unique phenomena of slow relaxation of magnetization, quantum tunnel effects and phase interference make them very promising for applications in high density information storage, quantum computing and molecular spintronics, polynuclear cluster compounds with single molecule magnet (SMM) behaviors attract researchers' great attentions. Up to till, over two hundred SMMs and a thousand polynuclear metal clusters have been reported, but there are still lots of shortages in width and depth. It is urgent to carry out more profound investigation from several aspects: increasing nuclearity, heightening magnetic anisotropy and spin ground state, enhancing the blocking temperature, apprehending mechanism of magnetic exchange, improving magnetic properties through tune of structure. This thesis, containing two parts, has studied the syntheses, structures and magnetic properties of four azido-metal magnetic complexes and twenty polynuclear cluster compounds. The concrete contents consisting of five chapters are as follows:In chapter 1, a brief introduction of the background and relative concepts of this work are presented. The progresses of azido magnetic compound and polynuclear metal cluster are concisely reviewed, and the research significance and main results of this thesis are also summarized.In chapter 2, four azido-metal complexes are described. Under hydrothermal condition, four azido-metal complexes (1-4) have been constructed from rigid organic carboxylate ligands. Their structural characterizations show that azido anions mainly takes on ferromagnetic bridging modes in all the compounds, and their overall strutures are 3D, 1D, 0D and 2D, respectively. The studies of magnetic properties indicate that global ferromagnetic, ferromagnetic, ferromagnetic and antiferromagnetic behaviors are observed, respectively, in which complexes 1 and 3 are long-range ordered ferromagnets, moreover, for 2 and 3, DFT calculations have perfectly explained the alteration of structures effecting on the magnetic properties. Complex 1 is the first ferromagnet in azido/carboxylate system.In chapter 3, the syntheses, structures and magnetic properties of Fe-Ln clusters are studied. By incorporation of Ln^â…¢into Fe^â…¢system, ten Fe-Ln clusters (5-14) are obtained under solvothermal conditions. X-ray single crystal diffractions indicate that they have hexadeca-nuclear Fe₁₂Ln₄ topology. Magnetic measurement demonstrate that competitive antiferromagnetic and ferromagnetic couplings in Fe-O core result in spin ground state of S = 8. Moreover, molecular magnetic anisotropy has been induced from single-ion magnetic anisotropy of Sm^â…¢, and exhibited slow relaxation of magnetization. These Fe₁₂Ln₄ clusters are the largest cluster in Fe-Ln system.In chapter 4, the syntheses, structures and magnetic properties of Fe^â…¢clusters are investigated. Seven Fe^â…¢compounds (15-21) are prepared solvothermally, and which are structurally characterized by X-ray single crystal diffraction. Their crystal structures can be described as Fe₈ rod, Fe₈ wheel, Fe₈ cluster, Fe₈ cluster, 1D alternating [Fe₃] and [FeNa₂] chain, 1D [Fe₆] wheel chain and 2D sheet based on [Fe₂₀] cluster, respectively. Magnetic measurement of 21 indicate that competitive antiferromagnetic couplings result in spin frustration in [Fe₂₀] unit, which produce a spin ground state of S = 5. The [Fe₂₀] unit in 21 is the largest iron unit.In chapter 5, the syntheses, structures and magnetic properties of Mn clusters are discussed. Three Mn complexes (22-24) are obtained from solvothermal conditions, and their structures are Mn₁₂ cluster, Mn₁₂ cluster and 2D network based on [Mn₁₂] unit, respectively. For 24, the study of magnetic properties demonstrate that couplings in [Mn₁₂] unit are strongly antiferromagnetic, and ac signals can be detected below 2.4 K. Structural modifications in 22-24 have made the easy-axial directions of Mn^â…¢ions shifted from parallel in cluster/vertical in crystal lattice (22) to parallel in cluster/parallel in crystal lattice (23 and 24).