Yixian Xiang

• Polyoxometalates (POMs) are a large class of discrete, polynuclear metal-oxo anions with an enormous structural and compositional diversity and interesting properties such as thermal and oxidative stability, tunable acidity, redox and magnetic properties, and tunable solubility in aqueous and organic media. The research performed here deals with the synthesis and structural characterization of novel POMs and the investigation of their physicochemical properties, e.g. catalytic, magnetic, electrochemical, medicinal, spectroscopic. Besides 3d transition metal-substituted POMs also the very novel class of polyoxo-noble-metalates was investigated. In the former project, lacunary heteropolytungstate precursors were reacted with different 3d transition metal ions under various reaction conditions. In the latter project, the focus was on noble metal ions with d8 configuration (e.g. Pd2+, Au3+) as addenda. Overall, this work has resulted in the discovery of 16 novel compounds. This thesis comprises five chapters: Chapter I provides a comprehensive introduction to the field of POM chemistry, including isopolyanions and heteropolyanions, the classic plenary, lacunary and transition metal-substituted polyanions, and the very novel class of polyoxo-noble-metalates. Chapter II describes the synthetic procedures for the different POM precursors which were used in this work, and also presents the various analytical techniques used to characterize the products. Chapter III discusses eight novel 3d transition metal-substituted POMs, which were synthesized and characterized during the course of this work. We have obtained three NiII-substituted POMs, the Ni7-containing, dimeric, sandwich-type [Ni7O6(H2O)6(P2W15O56)2]22- (1), the Ni14-containing, tetrameric [Ni14(OH)6(H2O)10(HPO4)4(P2W15O56)4]34- (2), and the Ni5-containing, dimeric, asymmetric, sandwich-type [Ni5(OH)4(H2O)4(β-GeW9O34){β-GeW8O30(OH)}]13- (3), Polyanion 2 contains the largest nickel-oxo aggregate known in polyoxotungstate chemistry. The 14-FeIII-containing [Fe14O6(OH)13{P2W15O56}4]31- (4) and the 18-FeIII-containing [Fe18O12(OH)8(H2O)3W2O8(CH3COO)2(P2W15O56)4]32- (5) are composed of four trilacunary Wells-Dawson type {P2W15} fragments stabilizing fourteen and eighteen, respectively, iron(III)-oxo units in a tetrahedral fashion. The magnetic properties of 1 - 5 have also been investigated. Three Keggin-type polyanions functionalized by 4,4’-bipyridine were also prepared, [(4,4’-C10H8N2)(ZnSiW11O39)]6- (6), [(C10H8N2)(ZnSiW11O39)2]12- (7), and [(4,4’-C10H8N2)(CoSiW11O39)2]12- (8). Chapter IV presents the synthesis and structural characterization of eight novel polyoxo-noble-metalates. This includes the 3d transition metal-centered polyoxopalladate(II) cages [(ScO8)Pd12As8O32]13- (9), [(CoO8)Pd12As8O32]14- (10), and [(CuO8)Pd12As8O32]14- (11), the alkaline earth metal-centered polyoxopalladates(II) [SrPd12O6(OH)3(C6H5AsO3)6(CH3COO)3]4- (12) and [BaPd15O10(C6H5AsO3)10]8- (13), the two silver-capped polyoxopalladates(II) {Ag4[Pd13As8O40]}10- (14) and {Ag2[Pd13As8O40]}12- (15) containing heterometallic Ag-Pd interactions, as well as the polyoxoaurate(III) [Au4O4(SeO3)4]4- (16). All compounds described above were characterized in the solid state by IR spectroscopy, single-crystal X-ray diffraction and thermogravimetric analysis. The solution stability of the diamagnetic compounds was confirmed by multinuclear NMR spectroscopy (e.g. 1H, 13C, 45Sc, 77Se, 183W). Chapter V describes studies on the above compounds performed by our collaborators, including electrochemistry, magnetism, and mass spectrometry. Several of the above compounds were also investigated by chemical industry partners for catalytic applications.