UC San Diego

The assembly of metal oxide clusters with metal linkers offers a route to new metal oxide framework materials that bridge the gap between metal organic frameworks and zeolites. Such materials offer advantages over traditional bulk or nanocrystalline doped metal oxides because the atom placement is precisely defined. The use of these all inorganic frameworks as functional materials, however, remains largely unexplored. Recently, a new type of all-inorganic network material has been developed by using polyoxometalates (POMs). POMs exhibit great properties in the field of redox chemistry, hydrolyzable protons and anionic and oxygen-rich nature. Among all POMs, Preyssler cluster is one of the most attractive metal oxide clusters because of its easy preparation, stability in a wide pH range (0–11) and great electroactive properties.In Chapter2, the general experimental methods have been introduced. The basic synthetic design are shown by figures and words. In Chapter 3, I present the synthesis of metal oxide frameworks comprised of [NaP5W30O110]14− assembled with Mn, Fe, Co, Ni, Cu or Zn bridging metal ions. X-ray diffraction shows that the frameworks adopt the same assembly regardless of bridging metal ion. Furthermore, our synthesis allows for the assembly of isostructural frameworks with mixed-metal ion bridges, or with clusters that have been doped with Mo, providing a high degree of compositional diversity. This consistent assembly enables investigation into the role of the building blocks in the properties of the metal oxide frameworks. The presence of bridging metal ions leads to increased conductivity compared to unbridged frameworks, and frameworks bridged with Fe have the highest conductivity. Additionally, Mo-doping can be used to enhance the conductivities of the frameworks. Similar structures can be obtained from clusters in which the central Na+ has been replaced with trivalent cations Bi3+ or Sm3+. Overall, the optical and electronic properties are tunable via choice of bridging metal ion and cluster building block and reveal emergent properties in these cluster-based frameworks. These results demonstrate the promise of using polyoxometalate clusters as building blocks for tunable complex metal oxide materials with emergent properties. In Chapter 4, I present the use of the Preyssler polyoxoanion, [NaP5W30O110]14− ({P5W30}), as a platform for evaluating the role of cations on the assembly of transition-metal-bridged polyoxometalates (POMs) into coordinatively linked frameworks. Specifically, we show that the assembly architecture of Co2+-bridged {P5W30} frameworks is dependent on the identity and amount of alkali or alkaline earth cations present during crystallization. The inclusion of Li+, Na+, K+, Mg2+ or Ca2+ in framework synthesis is used to selectively synthesize five different Co2+-bridged {P5W30} structures. We evaluate the role of the competition between K+ and Co2+ for binding to {P5W30}. Furthermore, we show that anions that are not incorporated into the framework structure indirectly impact this balance by competing for K+ during crystallization. The role of ion-pairing on framework assembly structure and available porosity are discussed. Overall, these results provide insight into factors governing the ability to achieve controlled assembly of POM-based coordination networks. In Chapter 5, I present the use of a modified Preyssler anion, [NaP5MoW29O110]14− ({P5MoW29}), as a platform for understanding the process of photoreduction in the metal oxide. The frameworks can be photoreduced by sunlight, allowing the storage of electrons under mild conditions. Titration with the molecular redox species shows that this reduction is reversible without changing the crystal structure. Remarkably, the Zn2+-bridged {P5MoW29} can maintain their reduction status over six months, suggesting the unique structure stability. These results demonstrate that the polyoxometalate self-assembly frameworks can be an excellent material to understand the process of photo-electron transportation and develop new metal oxides with emergent properties. In Chapter 6, Single-crystal-to-single-crystal (SCSC) transformations have received considerable interest in crystal engineering, providing a key platform for creating new materials and understanding chemical reactions and mechanisms. We synthesize a new metal oxide framework based on a modified Preyssler ion, linked with Ln3+, for understanding the process of SCSC transformations with an atomic characterization from the initial cluster and transition state to the final extended network.in the metal oxide. By tracing the labeled center ions of crown-type Polyoxometalate, Preyssler ion, in the crystal-grown process, a direct, comprehensive, visible SCSC transformation is confirmed. The post-synthetic method was used to synthesize sites precisely controlled Bi/Tri-Ln coordination networks. Overall, these results provide insight into factors influencing SCSC transformations during the crystallization process and offer a new pathway to new precise solid-state materials.