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.