RELATIVE STABILITIES OF XSiH2B5H8 (X = H, SiH3, Cl) SILYLPENTABORANE(9)S: SYNTHESIS OF CHLOROSILYL- AND DISILANYL-PENTABORANES

Abstract

Reaction of LiB5H8 with Si2H5Cl in Et2O at −45°C yields μ-Si2H5B5H8 (10), which in Et2O at 25°C isomerizes to 2-Si2H5B5H8 (11). At 120°C in the gas phase, 11 undergoes conversion to an 11/1-Si2H5B5H8 (12) mixture. The three disilanylpentaborane(9) isomers have been separated by high-vacuum fractional distillation. Reaction of 1- SiH3B5H8 with BCI3 yields 1-(ClSiH2)B5H8 (9). The new silylpentaboranes have been characterized by spectral data. Thermal equilibration of the μ-, 2- and 1-isomers of SiH3B5H8, (ClSiH2)B5H8 and Si2H5B5H8 in a static reactor at 120°C and in a flow reactor at 350°C and 425°C results in 1- and 2-isomer mixtures. The 1-isomer for all compounds studied is favored at all temperatures. The measured equilibrium constants (K) range from; 8.8 ± 0.4 (SiH3B5H8) and 7.3 ± 0.4 (Si2H5B5H8) at 120°C to 1.6 ± 0.4 (SiH3B5H8), 1.3 ± 0.2 (Si2H5B5H8) and 2.2 ± 0.6 [(ClSiH2)B5H8] at 425°C. These are discussed in terms of substituent conjugative effects.