Facile Synthesis of Unsolvated Alkali Metal Octahydrotriborate Salts MB3H8 (M=K, Rb, and Cs), Mechanisms of Formation, and the Crystal Structure of KB3H8

Sublime reactivity: Unsolvated MB₃H₈ (M=K, Rb, and Cs) salts were prepared by reactions of the respective pure alkali metals with THF⋅BH₃ in high yield at room temperature, and the structure of unsolvated KB₃H₈ was elucidated. The proposed reaction mechanisms are based on experimental and theoretical results. It was found that the physical properties of the alkali metals play a crucial role.

Abstract

A facile synthesis of heavy alkali metal octahydrotriborates (MB₃H₈; M=K, Rb, and Cs) has been developed. It is simply based on reactions of the pure alkali metals with THF⋅BH₃, does not require the use of electron carriers or the addition of other reaction media such as mercury, silica gel, or inert salts as for previous procedures, and delivers the desired products at room temperature in very high yields. However, no reactions were observed when pure Li or Na was used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. The low sublimation energies of K, Rb, and Cs were found to be key for initiation of the reactions. The syntheses can be carried out at room temperature because all of the elementary reaction steps have low energy barriers, whereas reactions of LiBH₄/NaBH₄ with THF⋅BH₃ have to be carried out under reflux. The high stability and solubility of KB₃H₈ were examined, and a crystal structure thereof was obtained for the first time.

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