Wenjing Liu

• Discrete metal-oxides, also known as polyoxometalates (POMs), possess a large structural and compositional variety, and a unique range of associated physicochemical properties, leading to applications in many different areas such as catalysis, materials science, and medicine. Transition metal-substituted POMs are an important subclass, and many derivatives of 3d metal ions are known, but to date only a handful of Cr-containing POMs has been reported. In the course of this work a novel synthetic strategy has been developed which has led to the discovery of six novel CrIII-containing POMs 1a – 6a. The two polyanions [CrIII(HXVW7O28)2]13− (X = P (1a), As (2a)) are the first examples of mononuclear Cr-containing heteropolytungstates, and they exhibit an exceptionally large magnetic anisotropy. Systematic evolution of this structural family by heteroatom substitution has resulted in the two di-CrIII-containing heteropolytungstates, [CrIII2(B-β-SiW8O31)2]14- (3a) and [CrIII2(B-β-GeW8O31)2]14- (4a). Polyanions 3a and 4a represent the first examples of sandwich-type POMs comprising two {XW8O31} (X = Si, Ge) units, and they are stable in solution from pH 1 – 7. Biological studies on 3a and 4a demonstrated interesting antidiabetic and anticancer activity. The polyanion [CrIII2(AsIIIW6O23)2(AsIIIO3)2]14- (5a) exhibits the first example of a di-CrIII-containing tungstoarsenate(III) and comprises two {(AsW6O21)(AsO3)2} units linked by two CrIII centers. The preparation of 1a - 5a demonstrates a remarkable heteroatom template effect in the formation of new POM archetypes. The CrIII-containing hexatungstate [H3CrIIIW6O24]6- (6a) reveals the first chromium derivative of this large Anderson-Evans-type structural heteropolytungstate family. Following the newly developed synthetic strategy it was possible to isolate another 14 novel polyanions 7a – 20a comprising hexa-, hepta-, and octa-heteropolytungstates, which are all described in Chapter 4. For example, the iron(III)-containing [FeIII(HPW7O28)2]13- (7a) and the high-valent manganese(III)-containing [MnIII(HPW7O28)2]13- (8a) were prepared, and shown to possess interesting magnetic properties. The di-manganese(II)-containing [H3MnII2(AsIIIW6O21)2(AsIIIO3)4]11- (14a) is composed of two equivalent {(AsW6O21)(AsO3)2} units with four terminal arsenate groups. By a slightly modified synthetic procedure the nickel(II)- and manganese(II)-containing polyanions [{Na(H2O)MII(P4W6O34)}2]18- (M = NiII (19a), MnII (20a)) could be prepared, which have a sandwich-type structure capped by three phosphate groups on either side.