Ionothermal Syntheses and Fundamental Properties of Selenido Metalates

Chalcogenido (semi)metalate clusters have been in the focus of inorganic chemistry and materials chemistry owing to their structural beauty and a variety of physical and chemical properties. Structurally, the tetrahedral coordination of their metal centers allows for the formation of zeolite-type framework compounds similar to zeolites, making them attractive from syntheses to application aspects. However, given these clusters are usually highly negatively charged, it is challenging to enable the formation of discrete cluster molecules in their solid-state. Understanding the molecular subunits and how they behave in solution has therefore been a hot topic in this field over the last decade. In addition, enriching the structural diversity of the chalcogenido metalate family, whether through variations in composition, connectivity between clusters, or the formation of new cluster types, provides valuable insights into the formation of these compounds and their potential applications.
To access discrete cluster molecules, and thus study their properties in solutions, strategies including a) incorporation of tetra-valent metal cations in the reaction system, b) charge reduction by netural ligands, and c) alkylation of chalcogenido metalate clusters have been employed. ... mehrIn this thesis, a new cluster oligomerization strategy is presented. By oligomerizing [Ge4Se10]4− clusters into dimeric [Ge8Se19]4− and tetrameric [Ge16Se36]8− in their corresponding salts, these compounds were made soluble in DMF owing to a reduced local cation-anion electrostatic interactions; their undecomposed transfer into solutions was evidenced by mass spectrometry. The optical properties and size distribution of particles in solutions were studied by means of UV-visible spectroscopy and dynamic light scattering, respectively. Another salt comprising [Cs@GeII4(GeIV4Se10)4]7− anions was also found soluble in DMF, and adds to the few examples of chalcogenido metalates capable of selectively capturing Cs+ during crystallization, as monitored by mass spectrometry, energy-dispersive X-ray spectroscopy, and single crystal X-ray diffraction. Quantum chemical studies were also carried out to understand its exclusive preference for inclusion of Cs+ compared to the lighter alkali metal ions.
For enriching the structural diversity of chalcogenido metalates, two strategies were introduced in this thesis, a) manipulating the anionic clusters via incorporation of Cu(I) atoms; and b) modifying the structure-directing agents during the formation of new cluster anions. The importance of Cu(I) atoms for tailoring the anionic clusters owing to the diverse coordination modes of Cu+ with chalcogen atoms was highlighted by the isolation of four new compounds. All of them are narrow band-gap semiconductors. In comparison to a copper-free compound, the presence of Cu(I) allowed access to directly manipulate the chalcogenido metalate anions. The modification of the ionic liquid cations led to varied structure-directing agents, which in turn afforded the formation of two new compounds, their anisotropic conductivity along horizontal and vertical directions were also further investigated. Additionally, further structural expansions within the chalcogenido metalate family have been realized upon using different types of ionic liquids, or modifying the precursor salts.
In summary, within this thesis, a new oligomerization strategy towards isolating finite cluster oligomer-based compounds was introduced, thereby improving the solubility of chalcogenido metalates in standard solvents. In addition, the significance of the incorporation of copper ions, of the modification of ionic liquid cations and anions, and of the variation of the precursor salts for expanding the chalcogenido metalate family members was demonstrated. This thesis will help the community to better understand the controllable synthesis of new chalcogenido metalate compounds.