INTRAMOLECULAR LIGAND EXCHANGE OF PENTACOORDINATED ANIONIC SILICATES, RSiF4 −, VIA 29Si AND 19F NMR SPECTROSCOPY, SOLUTION AND SOLID-STATE STRUCTURES

New pentacoordinated anionic silicates were synthesized as potassium 18-crown-6 salts, [RsiF4][K-18-crown-6], where R = 2,4,6-tri-tert-butylphenyl (TTBP) (1), mesityl (2), n-propyl (3), 1,2-ethanediylbis(tetrafluorosilicate) (4), tert-butyl (5), cyclohexyl (6), benzyl (7), p-chlorophenyl (8), p-tolyl (9), m-tolyl (10), and o-tolyl (11), by fluoride addition to the respective fluorosilane, RsiF3. 19F and 29Si variable-temperature NMR studies provide the first example of an anionic RsiF4 −; derivative whose intramolecular ligand exchange is “stopped” in solution. This occurs for 1 and indicates the magnitude of steric effects required to accomplish this. A comparison is made with substituent effects introduced in isoelectronic phosphoranes, RPF4 to stop ligand exchange on the NMR time scale. NMR data of all other silicates indicate rapid intramolecular exchange down to the lowest temperatures studied. A barrier energy for fluorine exchange in 1 of 12.8 kcal/mol was obtained. The molecular structures of 1 and 2 are reported and indicate the effects of steric crowding on distortion of the resulting trigonal bipyramidal geometries, more so with 1 than 2. The latter agrees with trends in 19F and 29Si chemical shifts comparisons within the series RsiF4 − and RsiF3. Silicate 1 crystallizes in the orthorhombic space group P212121 with a = 17.563 (4) Å, b = 22.528 (5) Å, c = 9.800 (2) Å, and Z = 4. The meshyyl derivative 2 crystallizes in the monoclinic space group P21/ m with a = 8.158 (3) Å, b = 14.847 (7) Å, c = 10.664 (1) Å, β = 91.24 (2) °, and Z = 2. The final conventional unweighted residuals are 0.041 (1) and 0.040 (2).