The pentacoordinate anionic bis(siliconates) [o-C6H4(SiPhF2)2F]-,K+.18-crown-6 (1), [o-C6H4(SiF3)(SiPh2F)F]-,K+.18-crown-6 (2), and [o-C6H4(SiPhF2)(SiPh2F)F]-,K+.18-crown-6 (3) have been prepared from the corresponding o-disilylbenzene derivatives, by treatment with KF and 18-crown-6 in toluene at room temperature, as stable white crystals. The X-ray crystallography of 1-3 confirms the presence of a bent, unsymmetrical fluoride bridge between two silicon atoms. The geometry about each silicon atom is a deformed trigonal bipyramid (TBP) with two fluorine atoms occupying the apical positions and with the silicon atom being displaced slightly out of the equatorial plane, indicating residual tetrahedral character; the deviation depends on the number of fluorine ligands on the silicon atom. The apical Si-F bond lengths are correlated with pentacoordination characters, TBP(a) and TBP(e), defined by apical-to-equatorial and equatorial-to-equatorial bond angles, respectively. The results suggest that bis(siliconates) 1-3 can be regarded as the sequential models for the structural change from tetrahedral silane to pentacoordinate (TBP) siliconate in nucleophilic attack on a silicon atom. C-13, F-19, and Si-29 NMR spectra of three bis-(siliconates) have been examined to elucidate the solution structures. A solid-state MAS Si-29 NMR study has also been conducted for the first time on 2 and on the mono(siliconate) [PhSiF4]- to correlate with the solution structures. At room temperature or above, F-19 NMR spectra for 1 and 3 show a broad singlet each for all fluorine atoms owing to fast intramolecular exchange on the F-19 NMR time scale, while five fluorine atoms in 2 appear as two sharp singlets in the ratio of 4:1. The Si-29 NMR signal for 1 appears as a sextet due to coupling with five equivalent fluorine atoms at room temperature. C-13 NMR spectra for the ipso and ortho positions of 1 and 3 also exhibit sextets and quintets due to coupling with five and four equivalent fluorine atoms, respectively. These NMR studies demonstrate that 1 and 3 are the first examples of anionic siliconates in which all fluorine ligands undergo fast exchange over two silicon atoms. At the low-temperature limits, F-19 NMR spectra for 1-3 show ground-state structures: F(br), F(ap), and F(eq) atoms appear as individual signals. The dynamic variable-temperature F-19 NMR spectra have been interpreted by the following consecutive processes for intramolecular ligand exchange: (a) exchange of F(br) between tetracoordinate and pentacoordinate silicon atoms, (b) flipping of the F(br)-containing five-membered ring, (c) full rotation about the Si-C bond, and (d) pseudorotation at pentacoordinate silicon centers. The ease of these processes is correlates well with the pentacoordination character, % TBP, of silicon atoms estimated from the solid-state structures.
