Co-ordinative interactions in chelated complexes of silicon. Part 7. Dynamic nuclear magnetic resonance studies of fluorine exchange at five-co-ordinated silicon

Abstract

Recently, X-ray diffraction has revealed a trigonal-bipyramidal co-ordination for silicon in the chelated complex 1-(trifluorosilyl)-1,2,3,4-tetrahydro-1,10-phenanthroline, (C₁₂H₁₁N₂)SiF₃. At –90 °C in solution ¹⁹F n.m.r. data are in good agreement with the crystalline structure. At room temperature, rapid positional exchange of fluorine atoms is detected. The rearrangement occurs as an intramolecular reaction by an irregular mechanism involving the instability of co-ordinative bonding. An energy barrier of 7.5(2) kcal mol^–1 for the equilibration of fluorine nuclei has been derived from spectroscopic data by line-shape analysis. Intermolecular fluorine transfer between molecules is also observed, but this exchange remains very slow on the n.m.r. time-scale up to 130 °C.