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Higher coordination in organosilicon compounds
Nucleophilic substitution reactions at silicon are frequently thought to occur through the formation of pentacoordinated or hexacoordinated intermediates or transition states. Pentacoordinated compounds have geometries ranging from trigonal bipyramidal to square or rectangular pyramidal. Interconversion from one geometry to another occurs via Berry pseudorotation. Hexacoordinated compounds have geometries ranging from octahedral to bicapped tetrahedral. Silicon compounds which achieve higher coordination as a result of intramolecular coordination are useful model compounds for the intermediates in nucleophilic substitution reactions. Group 14 and 15 elements are the donor atoms commonly found to behave in this regard. Six compounds which incorporate both sulfur and silicon atoms into an eight-membered ring system are synthesized. In addition, a disiloxane with two eight-membered ring systems is obtained as a hydrolysis product. Si-S coordination is observed in the five crystal structures that are obtained. Steric and electronic effects on the degree of coordination are compared. The degree of coordination is measured by calculating the distortion of the geometry around the silicon atom from a tetrahedral to a trigonal bipyramidal geometry. Four other organosilanes that have the sulfur atom as part of a dangling moiety attached to the eight-membered ring are also synthesized. No coordination was observed in this type of system, with the silicon atom retaining a tetrahedral geometry. The 29Si NMR spectra of all 11 organosilanes are indicative of the nature of the substituents on the silicon atom. A total of seven zwitterionic silicates are synthesized. Four of these zwitterions are obtained from an organosilane that is thought to be either penta- or hexacoordinated as a result of intramolecular nitrogen donation. A pentacoordinated organosilane, which models a proposed intermediate for this reaction, is shown to form one of the zwitterions. Cleavage of a Si-C bond occurs in the formation of these zwitterions, and a mechanism for their formation is proposed. Three other zwitterions have two nitrogen atoms on the alkyl chain, with one of the nitrogen atoms capable of coordination. No further coordination is observed, and the 29Si NMR spectra for all the zwitterions. are consistent with the retention of a pentacoordinate structure. Hydrogen-bonding is present in all of these zwitterions. Crystal packing effects are responsible for the observed geometries of these compounds, which are found to span the Berry coordinate from a TBP to an RP.
Mercado, Ramil-Marcelo Lantican, "Higher coordination in organosilicon compounds" (1999). Doctoral Dissertations Available from Proquest. AAI9920631.