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

A facile synthetic method of heavy alkali metal octahydrotriborates (MB3H8, M = K, Rb, and Cs) has been developed simply through reactions of pure alkali metals with THF·BH3 without using electron carriers or other reaction media, such as mercury, silca gel or inert salts as done previously, at room temperature with very high yields. However, no discernable reactions were observed when pure Li and Na were used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. It is found that the low sublimation energies of K, Rb, and Cs are the key factor for the initiation of the reactions. The syntheses can be carried out at room temperature because all the elementary reaction steps have low energy barriers, which are completely different from the reactions of Li/NaBH4 with THF·BH3 under reflux. The high stability and solubility of KB3H8 are examined and its crystal structure is obtained for the first time.

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