Determination of the molecular structure of amorphous Mg(B3H8)2(THF)2 through infrared spectroscopic and computational studies

Abstract

The infrared absorption spectrum of amorphous Mg(B 3 H 8) 2 (THF) 2 was measured in the mid-infrared region between 600 and 3300 cm-1. In the frequency range corresponding to B-H stretching vibrations, the observation of absorptions due to the movements of Mg-H-B bridging hydrides provides evidence of direct bond between octahydrotriborate ions and magnesium. Starting from this configuration, DFT and ab-initio calculations at the MP2, B3LYP and B97D level were utilized in order to find the lowest energy geometry of the Mg(B 3 H 8) 2 (THF) 2 complex. The results of all the computational studies indicate that the interaction of Mg with the [B 3 H 8 ]ions entails two B-Mg and two B-H-Mg bonds. The calculated infrared absorptions closely reproduce the experimental spectrum. Moreover, calculations provide evidence of the stabilization of the THF solvate of Mg(B 3 H 8) 2 and indicate that the THF complex has a greater stabilization energy than dialkyl ethers complexes by ~25 kJ/mol.

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

“…In the literature,pure alkali metals have been reported to react with THF·BH 3 to afford MB 3 H 8 and MBH 4 (M = Na, K) in moderate yields only when they were adsorbed on silica gel at 160 8 8C, finely dispersed in inert salts by ball milling, [13,14] or present in the form of aliquid K/Na alloy. [15] Compared with these previous methods,t he current synthetic strategy using pure potassium metal with THF·BH 3 has several advantages.…”

“…First, the reaction is carried out without using mercury or any other electron carriers,g reatly simplifying the subsequent workup.F urthermore,t he quantitative conversion and short reaction time make the reaction much more efficient. [26] Although NaB 3 H 8 is soluble in ethers, [12][13][14] strong solvation makes it difficult to be accurately weighed for further use. Unsolvated KB 3 H 8 is more stable than unsolvated NaB 3 H 8 in the solid state and in aqueous or THF solution ( Figure S16).…”

“…Unsolvated KB 3 H 8 is more stable than unsolvated NaB 3 H 8 in the solid state and in aqueous or THF solution ( Figure S16). [12][13][14] ForK B 3 H 8 ,t he coordination bond between the THF molecule and the potassium cation is so weak that unsolvated KB 3 H 8 can be conveniently prepared. [2,27] Unsolvated NaB 3 H 8 can be obtained by dissolving the oily THF-solvated product in diethyl ether and dichloromethane,w hich will help to break the coordination between THF and the Na + cations to completely remove THF.…”

“…[7,8] However,a lthough its synthesis and reactivity have been continually improved and explored, its mechanism of formation has not been fully understood. [8][9][10][11][12][13][14][15][16][17][18] Very recently,o ur group reported that MB 3 H 8 (M = Li and Na) could be synthesized by reacting the corresponding borohydrides with THF·BH 3 in THF under reflux. [19a] Thereaction mechanism of the formation of B 3 H 8 À was elucidated, and found to involve akey B 3 H 10 À intermediate,which undergoes H 2 elimination via ahigh barrier.…”

“…[19] Herein, we report afacile method for the synthesis of the heavier alkali metal salts of B 3 H 8 À ,M B 3 H 8 (M = K, Rb,a nd Cs), which is based on directly reacting the respective alkali metals with THF·BH 3 in THF at room temperature.I nt he current method, no electron carriers,s uch as naphthalene or triphenylboron, or other reaction media, such as mercury, silica gel, or inert salts,w ere used, as done in previous works. [8,[12][13][14][15][16][20][21][22] Thesimplified synthetic method enables the convenient preparation of the heavier MB 3 H 8 alkali salts on large scale in very high yields.W ealso report adetailed study of the mechanism of KB 3 H 8 formation and its crystal structure for the first time.…”