SYNTHESES, X-RAY CRYSTAL-STRUCTURE ANALYSES, AND SOLID-STATE NMR-STUDIES OF SOME ZWITTERIONIC ORGANOFLUOROSILICATES

Reaction of dimethoxy(methyl)(pyrrolidinomethyl)silane [Me(MeO)2SiCH2NC4H8, NC4H8 = pyffolidinol and methoxy(methyl)phenyl(pyrrolidinomethyl)silane [MePh(MeO)SiCH2NC4H8] with hydrogen fluoride at 0-degrees-C in ethanol/hydrofluoric acid yielded trifluoro(methyl) (pyrrolidiniomethyl)silicate [F3Si(Me)CH2N(H)C4Hg (9), N(H)-C4H8 = pyrrolidinio] (yield 90% and 89%, respectively). The related trifluoro(phenyl)(pyrrolidiniomethyl)silicate [F3Si(Ph)CH2N(H)C4H8 (10)] was obtained by reaction of dimethoxy(phenyl)(pyrrolidinomethyl)silane [Ph-(MeO)2SiCH2NC4H8] with HF (yield 81%). 9 and 10 are the first zwitterionic (ammonioalkyl)organotrifluorosilicates. Crystal data for these two compounds are as follows. 9: space group P2(1)/n, a = 8.359(3) angstrom, b = 11.388(4) angstrom, c = 9.076(2) angstrom, beta = 93.40(3)-degrees, V = 862.4(5) angstrom3, T = -100-degrees-C, Z = 4, R = 0.037, R, = 0.037. 10: space group P2(1)/n, a = 9.542(4) angstrom, b = 12.536(4) angstrom, c = 10.996(3) angstrom, beta = 114.13(3)-degrees, V = 1200.4(7) angstrom3, T = -100-degrees-C, Z = 4, R = 0.043, R(w) = 0.043. The zwitterionic organofluorosilicates 9 and 10 contain a pentacoordinate silicon atom (formally negatively charged) and a tetracoordinate nitrogen atom (formally positively charged). In the crystal, the coordination polyhedrons around the silicon atoms can be described as distorted trigonal bipyramids: two of the fluorine atoms occupy the axial sites, whereas the third fluorine atom and the two carbon atoms are found in the equatorial positions. The zwitterionic species also exist in solution (9, CD3CN, CDCl3; 10, CD3CN; H-1, C-13, and F-19 NMR), displaying a rapid ligand exchange at room temperature (one 19F resonance). Solid-state NMR studies (N-15, Si-29) on authentic tetrafluoro(pyrrolidiniomethyl)silicate [F4SiCH2N(H)C4H8 (7)] and on (3-ammoniopropyl)tetrafluorosilicate [F4Si(CH2)3NH3 (8)] revealed evidence that 8 is indeed the earlier postulated product formed in the reaction of (3-aminopropyl)triethoxysilane [(EtO)3Si(CH2)3NH2] with HF in ethanol/hydrofluoric acid (7,delta(N-15) = -318.5 ppm, delta(Si-29) = -121.1 ppm; 8, delta(N-15) = -345.3 ppM, delta(Si-29) = -112.4 ppm).