Creation, Structure, and Reactivity of Novel Hypercoordinate Organosilicon Compounds

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Fluoride-ion exchange reactions between a tetracoordinate silane and the corresponding pentaccordinate fluorosilicate constitute a class of atom-transfer selfexchange reactions at a main-group atom center, the mechanism has been studied very scarcely. The exchange has been supposed to proceed via a transition state having linear arrangement of [Si-F-Si]. A theoretical study have shown that SiH_nF_<4-n> (n=1-3) forms a stable linear Si-F-Si bridge with [SiH_nF_<5-n>]^- in the gas phase. We have developed the synthesis and structure of novel intramolecular fluoride-ion donor-acceptor systems and related bissilicates. A dynamic ^<13>C NMR study has shown exchange of the fluoride-ions in the fluoride-ion donor-acceptor systems in both unimolecular and bimolecular processes. The large negative activation entropies for the bimolecular exchange suggest that the reaction proceeds via a rather unusual fluorine-bridged cyclophane-like transition states.Crystal structures of the intramolecular fluoride-ion donor-acceptor systems have been analyzed by X-ray crystallography. These systems are expected to give a novel concept to design the ion conducting materials.Novel neutral hexacoordinate silicates having tropolonate ligands were synthesized. X-ray crystallographic analysis for diphenyl- and dimethylbis (tropolonato)- silicates showed that these silicates have octahedral structure with the two alkyl and phenyl groups at the cis positions. Essentially no bond alternation in the C-C bonds of tropolonate rings suggests that the tropolonates serve as intramolecular countercations in the silicates. Analysis of the dynamic ^<13>C NMR spectra of these silicates revealed that there exists an isomerization process through a bond-rupture-recombination mechanism rather than a concerted mechanism.