Manganese Polyhydroxy Clusters: Synthesis And Studies Of Their Magnetic Properties

Polynuclear paramagnetic clusters (PMCs) have been the focus of considerable research efforts due to their potential to act as single-molecule magnets (SMMs), which represent nanoscale magnetic particles with a well-defined size. The studies on single-molecule magnets are in the interface between molecule?based magnets and nano scale magneticmaterials. Study of their unusual magnetic behavior will be not only beneficial for both physics and chemistry, but also potentially used in high?density information storage devices for quantum computing. So, the investigation has become one of the most active arenas in materials and inorganic chemistry for their interesting properties and potential application. Up to now, a great number of mixed-valence manganese clusters have been synthesized and their magnetic properties have been widely studied, mainly because they often exhibit large, and sometimes abnormally large, spin values in the ground state (S), and a negative uniaxial anisotropy (D) arises from the presence of Jahn-Teller distorted MnШions. Although manganese-carboxylate complexes with various sorts of capping/bridging ligands (oximes, amino-alcohol, tripodal alcohols, etc.) have been extensively developed affording various nuclearities, it is still a tremendous challenge for researchers to rationally design, precisely control, and effectively re-assemble novel PMCs not seen in manganese chemistry. As we know, different organic ligands play a crucial role in the successful synthesis of clusters with different properties, and the synthetic approaches towards new PMCs assemblies which usually involve the one-pot reaction of metal salts with different polydentate ligands or the ligand-substitution/modification of PMCs precursors would produce novel molecular architectures with fascinating magnetic behavior. Additionally, with pleasing structural aesthetics and interesting physical properties, Azides, in particular, is the most versatile ligand in terms of being an efficient magnetic coupler capable of constructing diverse structural topologies, and it has been found to bridge modes (μ1,1-(end-on, EO),μ1,3-(end-to-end, EE),μ1,1,3,μ1,1,1,μ1,1,1,1,μ1,1,3,3,μ1,1,1,3,3,3). In this dissertation, we have prepared 12 new polynuclear manganese clusters by using different organic ligands (tripodal Schiff base,tripodal alcohol and phosphonate) and analyzed their structures and magnetization. These results will be introduced from the following three issues:(1) Under the room temperature, we have shown that the versatile predesigned quinquedentate Schiff base ligand H4L (2-{[(2-hydroxy-3-methoxyphenyl)methylene]amino}-2-(hydroxymethyl)-1,3-propanediol) together with its coligand azides under different manganese metal salt conditions can provide access to two unusual polynuclear Mn8 1 and Mn16 2 clusters. Their structural characterizations show that the two compounds are the first examples to include two and four alternant tetrahedral MnIII3MnII cores bridged by quinquedentate Schiff base and versatile azides groups, and it is noteworthy that only three examples with triple symmetric EE and EO in binuclear Ni complexes have been reported while the combination of triple bridgingμ1,1-azides in manganese chemistry has never been explored until recently. The magnetic properties of the two compounds have been carefully studied solid-state dc and ac magnetic susceptibility measurements, which reveal dominant antiferromagnetic interactions between the both of magnetic centers, and the rapid increase of the frequency-dependence in the in-phase signal with decreasing temperature. Not adding the coligand azides, four oligomers manganese clusters have been constructed from the only rigid organic Schiff base ligand. Their structural characterizations show that the schiff base ligands can easily stabilize the high oxidation state of manganese but usually lead to low-dimensional MnШcompounds. In addition, azide ligands have been widely employed in polynuclear transition-metal compounds and as part of one-, two-, or three-dimensional extended networks, where the N3- ion has been found to exhibit a wide variety of coordination modes.(2) Two 2D coordination polymetic networks compounds of [Na3MnШ6(μ6-O)(thme)4(PhCO2)6(H2O)]·OH 7 and [NaMnШ4MnII8O2(thme)4(N3)- (OAc)8(AcOH)2(CH3O)4] 8 with 1,1,1-tris(hydroxymethyl)-methane (H3thme) as the ligand have been synthesized. Single crystal X-ray diffraction reveals that compound 7 reveals a 2D extended structure based on homovalent hexanuclear clusters similar to the first reported mixed-valent hexanuclear manganese clusters. The structure of the manganese-oxygen core of 7, which is the same as Lindqvist anion [M6O19]n–, can be described as an octahedron. Compound 7 is the first homo-valence repeating unit like Lindqvist anion [M6O19]n– reported in honeycomb-shaped or hexagonal networks for a manganese coordination polymer. Compound 8 is the mixed-valence dodecanuclear Mn cluster acted respectively as network nodes in the formation of rhombic grid-like layer structures, and it is only the second highest-nuclearity known where an azido has been used to bridge discrete large clusters in a stepwise manner to form a polymer. In addition, magnetic studies of 7 reveal that antiferromagnetically-coupled paramagnetic cluster behavior is operative within the hexameric Mn6 cluster and 8 exhibits antiferromagnetic coupling interactions and the interesting frequency dependant in-phase as well as out-of-phase susceptibility signals observed, respectively.(3) The syntheses, structures and magnetic properties of polynuclear manganese with benzylphosphonic acid ligand are investigated from simple manganese starting materials by manipulating reaction conditions. Four polynuclear manganese clusters (9 ?12) are prepared at the room temperature, and which are structurally chararcterized by X?ray single crystal diffraction. Their crystal structures can be described as Mn9,Mn10,Mn20 and Mn23 using the alternant triangle [Mn3]n units. Magnetic mearments of them indicate that competitive ferromagnetic and antiferromagnetic coupling in the manganese cores produce a spin ground state of S = 8,S = 10 and S = 22. The S = 22 of 11 and the core of 12 may be the largest spin ground state and largest manganese cluster based on phosphoate ligand in the reported examples.