CRYSTAL-STRUCTURE AND EFFECT OF HIGH EXTERNAL PRESSURES ON THE VIBRATIONAL-SPECTRA OF ALL-TRANS-HEXABENZYLCYCLOHEXASILANE, [SI(H)CH2PH]6

Hexabenzylcyclohexasilane, [Si(H)CH2Ph]6 (1), has been synthesized in moderate yield (approximately 35%) by dehydrocoupling benzylsilane in the presence of dimethyltitanocene (Cp2TiMe2,CP = eta5-C5H5) catalyst. Compound 1 crystallizes in the trigonal R3BAR (C3i2) space group [a = 25.077-(2), c = 5.7031(6) angstrom; V = 3105.9(4) angstrom3; D(calcd) = 1.157 g cm-3 for Z = 3; R(w) = 0.040]. It has the flattest Si6 ring thus far reported for a cyclohexasilane [Si-Si-Si = 117.02(7)-degrees; dihedral Si-Si-Si-Si angle = 33.58(7)-degrees-], despite the absence of any transannular interactions. The IR and Raman spectra of 1 have been recorded at various pressures up to approximately 40 kbar with the aid of diamond-anvil cells. The pressure dependences (dnu/dP) and relative pressure sensitivities (d In nu/dP) of the major IR and Raman bands have been determined. From the X-ray crystallographic and vibrational spectroscopic results, there is evidence for rotation of the benzyl groups in the solid state and a pressure-induced phase transition at 20 +/- 1 kbar. Moreover, the larger dnu/dP values for nu(SiH) and nu(SiSi), compared to those for v(CH) and v(CC) in cyclohexane, suggest that, at least in this particular case, SiH and SiSi bonds are more anharmonic than are CH and CC bonds.