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

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.