SYNTHESIS AND CHARACTERIZATION OF STERICALLY HINDERED DIARYLSILANES CONTAINING 2,4,6-TRIMETHYLPHENYL AND 2,4,6-TRIS(TRIFLUOROMETHYL)PHENYL SUBSTITUENTS, X-RAY CRYSTAL-STRUCTURE OF BIS[2,4,6-TRIS(TRIFLUOROMETHYLPHENYL)]FLUOROSILANE

Sterically hindered diarylsilanes have been prepared by two synthetic routes. Dimesitylsilane, Mes(2)SiH(2) (1), (Mes = 2,4,6-trimethylpheny l) was synthesized by reaction of mesityl magnesium bromide with HSiCl 3 followed by reduction with LiAlH4, or by reaction of mesityl magnesi um bromide with (TfO)(2)SiH2. The mixed diaryl system, MesPhSiH(2) (2) , was prepared by reaction of (TfO)PhSiH(2), with one equivalent of Me sMgBr. Diarylsilanes containing the 2,4,6-tris(trifluoromethyl)phenyl substituent, R(F), were prepared by reaction of HSiCl3 with 2 equivale nts of R(F)Li to give (R(F))(2)SiHF (3) through a Cl/F halogen exchang e. Reduction of 3 with LiAlH4 afforded (R(F))(2)SiH2 (4) which can als o be prepared from R(F)Li and (TfO)(2)SiH2. All compounds have been ch aracterized by multinuclear NMR, IR, mass spectrometry and chemical an alyses. An X-ray crystallographic study of 3 shows that the immediate geometry about silicon is approximately tetrahedral with a C-Si-C angl e of 115.8(1)degrees. There are unusual intramolecular interactions in 3 with four short Si...F contacts with the ortho-CF3 substituents on the aromatic ring which results in an overall tetracapped tetrahedral geometry about silicon. Crystal data for 3 are as follows: monoclinic, P2(1)/c, with a = 9.827(2) Angstrom, b = 15.938(3) Angstrom, c = 13.2 39(3) Angstrom, V = 2072.6(8) Angstrom(3), Z = 4; and R = 0.0492 (R(w) = 0.0535). The short intramolecular Si...F contacts are observed in s olution for both 3 and 4 by Si-29 NMR spectroscopy.