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-trimethylphenyl) was synthesized by reaction of mesityl magnesium bromide with HSiCl3 followed by reduction with LiAlH4, or by reaction of mesityl magnesium bromide with (TfO)(2)SiH2. The mixed diaryl system, MesPhSiH(2) (2), was prepared by reaction of (TfO)PhSiH(2), with one equivalent of MesMgBr. Diarylsilanes containing the 2,4,6-tris(trifluoromethyl)phenyl substituent, R(F), were prepared by reaction of HSiCl3 with 2 equivalents of R(F)Li to give (R(F))(2)SiHF (3) through a Cl/F halogen exchange. Reduction of 3 with LiAlH4 afforded (R(F))(2)SiH2 (4) which can also be prepared from R(F)Li and (TfO)(2)SiH2. All compounds have been characterized by multinuclear NMR, IR, mass spectrometry and chemical analyses. An X-ray crystallographic study of 3 shows that the immediate geometry about silicon is approximately tetrahedral with a C-Si-C angle 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 = 13.239(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 solution for both 3 and 4 by Si-29 NMR spectroscopy.