Structural Elucidation of Metal-Oxo Systems by X-Ray Scattering Methods

Metal-oxo clusters are discrete molecular metal-oxides. Most metals form numerous cluster compounds that can be ligated, unligated, charged or neutral, controlling their solubility in water and/or organic solvents. They are important species to study reaction pathways from the monomeric building units to the bulk metal oxides, and have many application potentials in several industrial and technological areas. To reveal their structure-function relationship requires obtaining direct structural information about these clusters. However, structural information in amorphous solid materials or in solution is challenging to obtain. X-rays and neutrons are widely utilized for structural analyses on crystalline samples. Small-angle scattering (SAS) and total scattering (TS) techniques are performed to observe important structural features on disordered systems by observing particle size, shape and atom-pair correlations in real space. Studies within this dissertation utilize in-house small-angle X-ray scattering (SAXS) and pair distribution function (PDF) analysis of X-ray total scattering to extract structural information on various metal-oxo systems. The thesis describes synthesis and characterization of metal-oxo systems both in solid and solution state, where a combination of these scattering methods along with other characterization techniques are utilized to obtain a complete structural description of metal-oxo cluster systems with metals from across the periodic table. The first study is a solid-state chemical reaction where crystalline NaBiO3·XH2O loses its long-range order upon contact with HNO3, and eventually it becomes a completely amorphous material. Sodium bismuthate is utilized for americium oxidation and separation in the nuclear fuel cycle, and also catalytic oxidation reactions. PDF analysis was performed to follow this reaction, considering the structural changes and describe the main features of the amorphous product. The following projects focus on solution characterizations. First, due to its novelty, the in-house total-scattering diffractometer was benchmarked for solution characterization of metal-oxo clusters. For these, we start with a simple system of water and standard solutions of {SiW12} Keggin ion and move forward to more complicated systems (uranyl-peroxide solutions) to benchmark the capability of the in-house instrumentation, as well as characterize an unknown cluster geometry. Following, PDF measurements were performed as a powerful complementary tool at different metal-oxo clusters in aqueous solutions. We proved that TiOSO4 (an important intermediate in industrial TiO2 production) dissolves and self-assembles as a {Ti18}-cluster in water, and eventually precipitates TiO2 anatase. Scattering and spectroscopy techniques were utilized to characterize the changes during these reactions. Further, new zirconium-peroxo oxalate clusters with 9-coordinate zirconium and face-sharing connectivity were prepared, where stability order of those clusters was established based on SAXS and PDF data. The observed structural features are in good agreement with the acquired single-crystal data, where several new zirconium peroxo oxalate cluster compounds and conventional oxalate coordination compounds of Zr and Hf were synthesized depending on the counter cation. We differentiated the chemistry of these two metals that can be important for Zr/Hf separation for nuclear industry. These latter examples of Ti and Zr show that a laboratory diffractometer is still a very powerful complementary tool to perform PDF analysis, even with weak scattering elements or low concentrations, to acquire important structural information at conditions far from ideal

Thorium Oxo‐Clusters as Building Blocks for Open Frameworks

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Discrete Hf18 Metal‐oxo Cluster as a Heterogeneous Nanozyme for Site‐Specific Proteolysis

The selective hydrolysis of proteins by non‐enzymatic catalysis is difficult to achieve, yet it is crucial for applications in biotechnology and proteomics. Herein, we report that discrete hafnium metal‐oxo cluster [Hf18O10(OH)26(SO4)13⋅(H2O)33] (Hf18), which is centred by the same hexamer motif found in many MOFs, acts as a heterogeneous catalyst for the efficient hydrolysis of horse heart myoglobin (HHM) in low buffer concentrations. Among 154 amino acids present in the sequence of HHM, strictly selective cleavage at only 6 solvent accessible aspartate residues was observed. Mechanistic experiments suggest that the hydrolytic activity is likely derived from the actuation of HfIV Lewis acidic sites and the Brønsted acidic surface of Hf18. X‐ray scattering and ESI‐MS revealed that Hf18 is completely insoluble in these conditions, confirming the HHM hydrolysis is caused by a heterogeneous reaction of the solid Hf18 cluster, and not from smaller, soluble Hf species that could leach into solution

Discrete Hf-18 Metal-oxo Cluster as a Heterogeneous Nanozyme for Site-Specific Proteolysis

The selective hydrolysis of proteins by non-enzymatic catalysis is difficult to achieve, yet it is crucial for applications in biotechnology and proteomics. Herein, we report that discrete hafnium metal-oxo cluster [Hf18 O10 (OH)26 (SO4 )13 ⋅(H2 O)33 ] (Hf18 ), which is centred by the same hexamer motif found in many MOFs, acts as a heterogeneous catalyst for the efficient hydrolysis of horse heart myoglobin (HHM) in low buffer concentrations. Among 154 amino acids present in the sequence of HHM, strictly selective cleavage at only 6 solvent accessible aspartate residues was observed. Mechanistic experiments suggest that the hydrolytic activity is likely derived from the actuation of HfIV Lewis acidic sites and the Brønsted acidic surface of Hf18 . X-ray scattering and ESI-MS revealed that Hf18 is completely insoluble in these conditions, confirming the HHM hydrolysis is caused by a heterogeneous reaction of the solid Hf18 cluster, and not from smaller, soluble Hf species that could leach into solution.status: publishe