Kinetic and mechanistic studies of the gas-phase thermolysis of hexaborane(12), and of its cothermolysis with other binary boranes and with CO, BH3·CO and H2

Abstract

The kinetics of thermal decomposition of hexaborane(12) have been studied by a mass-spectrometric technique in the pressure range 1–10 mmHg and temperature range 60–127 °C. In conditioned Pyrex vessels B₆H₁₂ decomposes in a homogeneous, first-order, gas-phase process which affords B₅H₉ and B₂H₆ in a molar ratio of 2:1 as the main volatile products. In marked contrast with the known thermolyses of other small boranes, only minor amounts of non-volatile solid hydride are produced. The Arrhenius activation energy (81.3 ± 2.6 kJ mol^–1) and the unusually low pre-exponential factor (3.1 × 10⁸ s^–1) are very similar to those associated with decomposition of the structurally related B₅H₁₁, consistent with a mechanism involving elimination of {BH₃} as the rate-determining step in both cases. Small amounts of B₆H₁₀ and H₂ are also produced, in what is believed to be a competing but minor reaction channel involving the direct elimination of dihydrogen from B₆H₁₂. Detailed comparisons of the initial rates of consumption of B₆H₁₂ in its thermolysis alone and in the presence of either CO or BH₃·CO suggest a mechanism in which the B₂H₆ arises from direct association of two {BH₃} groups, and not from the interaction of {BH₃} with a second molecule of B₆H₁₂. Separate cothermolysis reactions with H₂, B₂H₆, B₄H₁₀, B₅H₉, B₅H₁₁ and B₆H₁₀ indicate that there is no direct reaction between the B₆H₁₂ molecule and any of these binary boranes under the conditions used, implying that B₆H₁₂ itself is not an intermediate in the stepwise build-up to B₁₀H₁₄.