Mechanism of the dehydrogenation of Mg(BH4)2·THF0.25 to Mg(B10H10): role of the THF solvate.

Abstract

Dehydrogenation of Mg(BH₄)₂·THF_x (1·THF) at 180 °C has been previously found to produce primarily a mixture of Mg(B₃H₈)₂ (2) and Mg(B₁₀H₁₀) (3). It has now been found that dehydrogenation under a H₂ back pressure of 20 bar suppresses the formation of 2 and results in very high (>95%) selectivity for 3. The production of 3 is not inhibited by hydrogen pressure. This demonstrates that the reaction pathway leading to its formation does not involve reversible, rate limiting H₂ elimination and is divergent from the pathway leading to 2. Unlike the pathway leading to the formation of 2, the dehydrogenation of 1·THF to 3 is irreversible under moderate conditions. Concurrent with the dehydrogenation reaction, THF is slowly converted to Mg(OBu_n)₂, curtailing the production 3. However, the dehydrogenation process is revived by addition of a fresh aloquate of THF to the reaction mixtures. It is hypothesized that THF enables the production of 3 by stabilizing the key intermediate, [B₂H₅]^- and by the low melting point (<100 °C) of 1·THF which allows long-range diffusion of borane anions at much lower energies than those required for their transport in the solid-state.