Ali Haider

• Polyoxometalates (POM) are a well-established class of discrete metal-oxides, with a large number of structures and compositions, and an unmatched combination of physicochemical properties. This work is focused on the deliberate synthesis of multinuclear metal-oxo clusters (e.g. Mn, Fe, Co) stabilized by POM fragments. Such compounds possess unique electronic, magnetic, optical, electrochemical, and catalytic properties. In the course of the synthetic work the influence of multiple parameters such as pH, ionic strength, temperature, counter cations, capping groups (e.g. phosphate, diphosphate), transition metal ions in different oxidation states, effect of halide ions, and POM precursors as different salts have been investigated systematically. Several multi-transition metal-containing heteropolytungstates comprising tetrameric, trimeric, and dimeric assemblies have been synthesized and structurally characterized by a multitude of analytical techniques in the solid state and in solution. This thesis is subdivided into six chapters. Chapter 1 is a general introduction to the research area, presenting the historical background, structural aspects, properties, and applications of polyoxometalates (POMs). Chapter 2 describes the background of transition metal-containing heteropolytungstates and the motivation to perform this work. Chapter 3 is an experimental section and describes the synthetic procedures for the POM precursors and the analytical techniques used to characterize the products. Chapter 4 is devoted to tetrameric compounds, and nine polyanions are presented belonging to three different structure types. Polyanions 1 – 6 are tetrameric assemblies of the {M16(XW9)4} (M = Co2+, Mn2+/3+, Fe3+) structure type, comprising a central M4O4 cubane unit, surrounded by four {M3XW9} Keggin units, ande this assembly is capped by four phosphate groups. Polyanion 7 is alsoa tetrameric assembly comprising four {Co2GeW10} units connected by two pyrophosphate, two tungstate, and two {Ge2O7} groups. Polyanions 8 and 9 comprise a 15-copper(II)-oxo cluster surrounded by four {SiW9} units and a halide ion (Cl- (8), Br- (9)) in the center. Chapter 5 presents the trimeric polyanion 10 comprising the assembly {Co9(GeW9)3} with two capping phosphate groups. Finally, Chapter 6 describes three dimeric polyanions comprising four Co2+ (11), Ni2+ (12), and Zn2+ (13) ions where the {M4(OH)2(H2O)2} core is sandwiched by two {SiW10} units.