Silicon in unusual coordination

Hertha Firnber Position T 101 Silicon in unusual coordination Michaela FLOCK 27.06.2000 Mobilization as well as organized condensation of silicate building blocks requires to increase the valence of silicon thus forming penta- and even hexacoordinate structures. Unusual silicon coordinations also occur as transition structures or reactive intermediates in the process of substituent exchange and in condensation reactions leading to polysilanes with chain, ring, or cage structures. While compounds with high coordinated silicon are experimentally difficult to synthesize due to their high reactivity, computer simulation at the ab initio level allows to obtain reliable data, such as thermodynamic stabilities or characteristic NMR 29Si chemical shifts, delta29Si. In the first part of this project basic relations, e.g. the apicophilicity rule, will be investigated for a comprehensive set of molecules with three, five, and six substituents at the silicon center. The next part is devoted to the study of the thermodynamic stability of complexes with free and preorganized donors (amino and phosphino groups). Accompanying computation of delta29Si allows to predict the `coordination shift` in these model compounds. The obtained theoretical knowledge about preorganized amino groups will be used and extended in the subsequent study of the active site of silicatein. These theoretical studies are guided by the search for mechanisms to transform mobilized silicate into nanostructures.

In this project we studied anionic reactive intermediates and transition structures of organosilicon compounds indispensible for the understanding of reactions applied in current synthetic research. Comparing non-empirically calculated molecular characteristics with measured spectroscopic data allowed an unambiguous structure assignment. The prediction of a comprehensive set of 29Si nuclear magnetic resonance data provides an informative basis for product identification of silylenes. The theoretical prediction of stabilities facilitated the systematic selection of suitable substances for syntheses. In the most stable compounds silicon forms four single bonds to neighboring atoms (coordination number 4, tetrahedral arrangement). The more the coordination number differs from four the higher the reactivity of the organosilicon molecules. Consequently these unusual species are difficult to be investigated by conventional chemical methods. Computer simulations at the ab initio level allow to obtain reliable data, such as structures, stabilities, and characteristic spectroscopic data. Comparing calculated with measured data facilitates the identification of reactive intermediates. For silylenes (coordination number 2) and disilenes (coordination number 3) the ab initio calculation of 29Si NMR chemical shifts made it possible to assign the measured data to yet unknown structures. Silyl anions (coordination number 3) are used as building blocks in the synthesis of oligo- and polysilanes. The results of studying the influence of the substituents on the configurational stability of the silyl anions were applied in reaction design. Anionic penta- and hexacoordinate organosilicon molecules have been studied with free and preorganised donors with rigid and flexible spacer groups. The results obtained for adduct formation gave first clues in connection with the enzymatic reaction mechanism for silicon polymerization.