Challenges in the synthetic routes to Mn(BH4)2: insight into intermediate compounds

Abstract

We have studied the reaction of MnCl₂ with MBH₄ (M = Li⁺, Na⁺, K⁺) in Et₂O. Crystal structures of two new intermediates, named [{M(Et₂O)₂}Mn₂(BH₄)₅] (M = Li⁺, Na⁺), were elucidated by X-ray diffraction. Mn(BH₄)₂ in a mixture with LiBH₄ or NaBH₄ forms upon the solvent removal in a vacuum. [{M(Et₂O)₂}Mn₂(BH₄)₅] contains 2D layers formed by Mn and BH₄ groups, linked through the alkali metal atoms coordinated to Et₂O. The loss of the solvent molecules leads to the segregation of the partially amorphous or nanocrystalline LiBH₄/NaBH₄ and a creation of the 3D framework of the crystalline Mn(BH₄)₂. While using LiBH₄ led to Mn(BH₄)₂ contaminated with LiCl, presumably due to an efficient trapping of the latter salt by the [Mn(BH₄)₂–Et₂O] system, the reaction with NaBH₄ produced chlorine-free Mn(BH₄)₂ accompanied with NaBH₄. Using KBH₄ led to the formation of K₂Mn(BH₄)₄ as a second phase. Two pyridine-containing solvomorphs, [Mn(py)₃(BH₄)₂] and [Mn(py)₄(BH₄)₂]·2py, were isolated in pure form. However, Mn(BH₄)₂ partly decomposes upon removal of pyridine molecules.