Syntheses, Crystal Structures And Magnetic Properties Of The Transition Metal/Transition-Rare Earth Mixed Metal Clusters

Since 1993, the complex [Mn12O12(O2CMe)16(H2O)4]·2(CH3COOH)·4H2O was found possessing single molecule magnets (SMMs) properties, this research field has received a wide attention of chemists, physicists and material scientists from home and abroad. The main reason for this interest is that SMMs possess not only the hysteresis loops like classical magnets in magnetization vs dc field, but also the quantum properties, which cannot be observed in the classical magnets, such as quantum tunneling of the magnetization (QTM) and quantum phase interference. Therefore, SMMs have been proposed as potential candidates for magnetic information storage, quantum computation and spintronic devices. The SMMs behavior is closely associated with their intrinsic properties of a high-spin ground state (ST) and a high uniaxial anisotropy (negative axial zero-field splitting, D), which result in an anisotropy-induced energy barrier to thermal relaxation of the magnetization. Thus, when a SMMs compounds is located in a magnetic field, then cooling the temperature below its blocking temperature (TB), spin polarization remains trapped, even after removal of the magnetic field, with the relaxation decay obeying Arrhenius law.Up to date, the vast majority of SMMs are polynuclear mixed-valence clusters containing MnⅢions, and other research systems such as other paramagnetic 3d transition metal clusters, lanthanide- or lanthanide-transition metal-based clusters and cyanide-based compounds are also reported. However, the blocking temperature (TB) of the present SMMs is too low for application. To improve the TB of SMM, people have employed multifarious synthetic methods to obtain high nuclear Mn clusters by using different multidentate chelating ligands, and then investigated the relationship between coordination modes of multidentate chelating ligands and these crystal structures and between crystal structures and magnetic properties by experimental and theoretical methods. However, to our knowledge, polynuclear clusters containing two or more kinds of multidentate chelating ligands have not been reported.In this paper, employing two kinds of different multidentate chelating ligands with paramagnetic 3d metal salts or with Mn clusters precursor in the reaction, a series of clusters containing these chelating ligands have been obtained and characterized by X-ray crystallography and SQUID magnetometer. The relationship between coordination models of different chelating ligands and crystal structures and between crystal structures and their magnetic properties are detailedly described. Fortunately, some of these clusters possess well single-molecule magnets properties. Moreover, some 3d-4f mixed metal clusters have also been obtained and their crystal structures and magnetic properties have also been characterized.The content includes two parts as follow:Part I. The reaction of one of the following multidentate chelating ligands (2, 6-pyridinedimethanol (pdm),2-hydroxymethylpyridine (Hhmp), methyl 2-pyridyl ketone oxime (mpkoH)) with 2,2'-dipyridyl ketone (dpk) in the solution containing Mn, Co or Ni or Mn clusters precursor gave complexes 1-8, which have been studied by structural and magnetic characterization. The structural topologies of complexes 6-8 are interesting and have not been reported by other research groups. Magnetic studies indicate that, for all complexes containing mixed multidentate chelating ligands, the magnetic coupling interactions between metal ions are antiferromagnetic. We also analyze the reason for antiferromagnetic coupling of a cubic [Ni4O4] cluster is the different distances between the hydroxyl group O atoms of different multidentate chelating ligands and the plane defined by its coordinated metal ions. Ac susceptibility studies indicate that complexes 7 and 8 show frequency dependence signals ofχ", especially, and complex 8 represents the third example in the [Mnn] (n> 12) clusters which show the peak ofχm" above 2.0 K in the AC magnetic susceptibility.Part II. Seven mixed metal clusters 9-15 have been obtained by differently synthetic methods and characterized by structural and magnetic analysis. While the complex 11 possess an interesting [Fe6Ce2] core in the Fe-Ln mixed metal clusters, which can be viewed as a hexagonal bipyramid, complex 12 can be viewed as the dimmer of 11. In this part, we mainly investigate the magnetic properties of 11 and 12, indicating that the energy barrier of 12 is obviously higher than that of 11. The reason for this may be that the magnetic coupling between [Fe6Ce2] units in 12 is ferromagnetic, leading to higher ground-state spin compared to 1. Magnetic analysis shows that frequency dependence signals ofχ" for complexes 10,12 and 13 are present.