Synthesis and Reactivity of molecular Silicas with and without Oxygen Defects and Their Chalcogen Analogues

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The fascinatingly diverse reactivity of silica and its heavy homologues towards various substrates (e.g., in order to immobilize metal sites or metal-based clusters on such surfaces) is strongly dependent on the chemical nature of the terminal groups and oxygen defects. In the case of silica, the presence of water and/or oxygen defects not only determines the concentration of terminal Si-OH groups but will also influence its morphological stability, that is, its tendency to undergo post-agglomerization. One way to unravel structure-reactivity relationships of such materials on the molecular level is provided by taking ad-vantage of molecular models featuring different sorts of terminations such as Si-O-Si, Si(OH)n, Si=O moieties, and even oxygen deficiency. However, the synthetic access to reliable model compounds is still very limited due to the lack of suitable molecular precursors. This is why the project is devoted to the development of unprecedented synthetic methods for the generation of well-defined molecular models of silica and its heavy homologues.Starting point of our investigation is the previously developed (‘water-free’) selective chalcogenation a bis-N-heterocyclic carbene stabilized zero-valent silicon complex (‘silylone’) which enabled the synthesis and isolation of corresponding compounds bearing mononuclear SiX, SiX2 (X = O, S, Se, Te) and Si(CO3)2 molecules. The donor-acceptor interaction of the latter with the bis-carbene ligands prevents them from polymerization. With this compounds in hand we will study the reactivity of these entities towards water and related OH containing compounds aiming at the synthesis (via consecutive selective hydrolysis) of the corresponding ligand supported mononuclear Si(OH)n compounds (n = 2,3,4), including attempts to isolate a complex of the elusive silic acid, Si(OH)4. Moreover, starting from acyclic and cyclic chlorosilanes of the type SinCl2n (n = 5,6) and their transformation to the corresponding siloxy- and hydroxo-substituted cyclosilanes, we will continue to investigate their ability to serve as molecular models of oxygen-deficient silicas and to act as molecular precursors to give oxygen-deficient silicas such as SiO by a non-hydrolytic pathway.