Single-Molecular Magnets Constructed By The Different Strategies

Since the beginning of the 1990s, single-molecular magnets ?SMMs? have been of great interest in the field of molecular magnetic materials because of their potential applications in high-density information storage and quantum computing. During the last twenty years, the studies on SMMs, including their synthetic strategies,experimental technique, and fundamental physics have been greatly improved and expanded. In this thesis, we devote to the preparation of new SMMs based on different design strategies. Though our research, we hope to understand the origin of the SMM behavior and the factors that influence the SMM properties. This dissertation is divided into six chapters. Following the Introduction to the SMM area and the summary of the current research progress in Chapter One, our results on SMMs based on different design strategies will be given in the subsequent Chapters ?2 to 6?. The abstract is given below.In Chapter Two, using the 3d-4f dinuclear heterometallic Schiff base compounds as the building blocks and the azide as the bridges, we reported the synthesis, structures and magnetic properties of four 3d-4f tetranuclear complexes ?[MLn]₂, M = Cu, Zn; Ln=Gd, Tb; 1-4? with the 4f centers bridged solely by end-on ?EO? azide. Ferromagnetic interactions were found between the 3d-4f center and Tb-Tb center. But the Gd-Gd interaction was found to be weak antiferromagnetic. Compound 1 ?[CuTb]₂? is a typical SMM with an effective energy barrier of 20.9 cm-1 ?30.1 K? with hysteresis loops observed at up to 2.4 K. Compound 2 ?[ZnTb]₂? is also a SMM that displayed slow magnetic relaxation.In Chapter Three, we successfully synthesized a series of EO azide bridged Cu???-4f compounds 5-18 and carefully characterized their structures and magnetic properties. We found that two different structure types for different lanthanide ions.Compounds 5-9 are one-dimensional chains with each heterometallic 3d-4f[Cu?LLn?] unit linked by three EO-N₃^- bridges. Compounds 10-18 are tetranuclear zero-dimensional structures with two heterometallic 3d-4f [Cu?LLn?] units linked by two EO-N₃^- bridges. Dc variable temperature dependent susceptibility measurements showed that the magnetic interaction between the metals centers in compounds 11, 12 and 13 were ferromagnetic. Ac magnetic measurements performed on 12, 13 and 14 demonstrated the presence of slow magnetic relaxation under zero applied dc field. The spin relaxation barrier Ueff are 19.54 cm-1 ?28.12 K? and 7.29 cm-1 ?10.49 K? for compounds 12 and 13. The SMM behavior of 12 was further confirmed by the presence of magnetic hysteresis loops. In compounds 13 and 14, fast quantum tunneling of the magnetization dominated the magnetic relaxation process at low temperature and no hysteresis loop was observed for them. Our results demonstrated that azido-bridged lanthanides could be synthesized successfully despite of the generally weak azido-lanthanide interaction.In Chapter Four, we synthesized and characterized five [CuLn]₂ ?Ln = Tb, Dy, Ho,Er and Tm? ?19-23? compounds with a chiral ligand. These structures and also magnetic properties of these compounds were very similar to the ones reported in Chapter Two.However, as they are crystalized in the triclinic space group and we were able to obtain large enough single crystals, detailed angular-resolved magnetic measurements were performed on the single crystals of these compounds. Based on these measurements, the susceptibility tensors of these compounds were obtained and the anisotropic magnetic axes of these compounds were obtained. Compounds 19, 20 and 21 were SMMs with the energy barriers being Ueff = 19.2 cm-1 ?27.6 K? and Ueff = 13.1 cm-1 ?18.9 K? for 19 and 20. Also, magnetic hysteresis loops were obtained for compound 19. From our studies, we were able to rationalize the origin of the magnetic anisotropy of the lanthanide centers in these compounds. The relationship between the magnetic anisotropy and the negative charge distribution of the coordination environments were established and discussed carefully.In Chapter Five, SMMs based on the [Mo?CN?7]₄- building block were studied.Using a series of nearly co-planar five-coordinated ligands, we synthesized a series of trinuclear M2Mo compounds ?24-29?. Their structures and magnetic properties were investigated. The [Mo??CN?7]₄- unit in all these compounds all had slightly distorted pentagonal pyramidal structure ?D5h? with two axial CN- ligands bridging the M?centers ?M = Mn and Fe?. Magnetic measurements confirmed the SMM behavior for all these compounds. For 24, 26, and 29, hysteresis loops were observed above 1.8 K.Compound 24 shows the highest anisotropy barriers of Ueff = 27.0 cm-1 ?38.8 K?.Anisotropic magnetic interaction of easy axis type is believed to be the origin of the SMM behavior of these compounds.In Chapter Six, we designed and synthesized seven mononuclear Co? complexes and three mononuclear Fe? complexes ?30-39?. They all have a metal center with a pentagonal pyramidal environment ?Dsh?. The magnetic measurements on these compounds revealed the slow magnetic relaxation behavior under applied dc magnetic field in these seven mononuclear Co? complexes and two mononuclear Fe? complexes?except compound 37?. In all these complexes, complex 36 had the highest anisotropy barriers of Ueff = 62.3cm-1 ?89.6 K?. Analyses of the Dc data of these Co? complexes give positive D values ?D > 0?, suggesting that even a positive D value could lead to slow magnetic relaxation. Up to now, these seven-coordinated mononuclear complexes were the first reported examples of the 3d SMMs with coordination number larger than 6.