Hetero-Homometallic Cluster: Synthesis And Mangnetism

One of the principal synthetic challenges in the area of polynuclear metal complexes is control of molecular topology. In recent years this has become of particular importance in the field of Single Molecule Magnets, in which a combination of high spin ground states and uniaxial anisotropy leads to an energy barrier to the reversal of the magnetization. Heterometallic aggregates are also of particular interest in the understanding of magnetic interactions between different spin carriers. For the construction of such aggregates, the "metal complex as ligand" approach has often proved to be fruitful in the designed synthesis of compounds. Such a strategy has also been successfully employed in the synthesis of framework compounds constructed from secondary building units (SBUs) in which codrdination compound units are connected through rigid ligands such as oxalate or 4,4'-bipyridyl. In the present work it has been shown how magnetic building-blocks, often designated "bricks" according to the nomenclature of Kahn, can be linked together in a variety of ways ultimately to produce hierarchical structures.In chapter 2, the synthesis, structure and magnetic properties of compound 2 and 4 are described. They are made up of two different building-blocks, [Mn^â…¡(bdoa)(H₂O)₃] and [Mn^â…¢(sal)Cl], which possess coordinated functional groups, such as carboxylates that can form magnetically-active bridges to other paramagnetic metal ions, and/or has exchangeable terminal ligands (e.g. H₂O, MeOH) which can be replaced by bridging units from other metal complexes. The attractive feature of compound 2, [Ni₁₂Mn₆(bdoa)₆(O₂CMe)₁₂(μ₃-OMe)₁₂(MeOH)₁₂·21MeOH], is that it contains three tetranuclear nickel cubes connected by a Mn₆(bdoa)₆(O₂CMe)₃ unit, which allow us to probe the effect on magnetic interaction between metal cubes with high-spin ground states and different transition metal ions. Compound 4, [Cu^â…¡₂(sal)₂Mn^â…¡Cl₂·1.5 MeOH], is a tri-nuclear cluster which comes from the build-block [Mn^â…¢(sal)Cl]. It also shows a very rare and interesting ferromagnetic intercluster interaction, which creates a chain of Cu₂Mn units along the c axis.Chapter 3 describes the strategy of using bifunctional ligand to construct 3d-4f heterometallic clusters. It was also found that by using pre-prepared tri-nuclear ion cluster as the starting material, 3d-4f bimetallic clusters could be obtained as well. By using a pre-designed Schiff-base tripodol ligand, Compound 5, [Gd₂Cu₅(OH)₄(Br)₂(H₃L)₄(NO₃)(OH₂)₄](NO₃)_1.78(Br)_0.22·7H₂O], is successfully synthesized. Magnetic measurements indicate that this compound has a spin ground state of S=17/2. Due to the very weak magnetic anisotropy of Cu^â…¡and Gd^â…¢ions, this compound doesn't show any SMM behavior. Compound 7, which contains 4 Fe^â…¢and 2 Ln^â…¢, comes from the mixing of a tri-nuclear ion cluster and lanthanide ions combined with a co-ligand, has a rather small spin ground state. Howerver, the introduce of Dy^â…¢ions makes this compound show magnetic anisoatropy.Chapter 4 describes the synthesis, structure and magnetic properties of compound 8, which contain 10 Mn centers. Magnetic measurements indicate that all the Mn ferromagnetically couples, resulting a spin ground of S=22. Furthermore, the super-tetrahedral Mn₁₀ unit could be used to build bigger aggregates with very large spin ground states.Chapter 5 shows the synthesis and structures of three 1-D chains, which come from the same build-blocks used in Chapter 2. The high dimensional magnetic property is not the main goal of this thesis, so the magnetism of these compounds are not mentioned in this chapter.