Manganese borohydride; synthesis and characterization

Solvent-based synthesis and characterization of alpha-Mn(BH4)(2) and a new nanoporous polymorph of manganese borohydride, gamma-Mn(BH4)(2), via a new solvate precursor, Mn(BH4)(2)center dot 1/2S(CH3)(2), is presented. Manganese chloride is reacted with lithium borohydride in a toluene/dimethylsulfide mixture at room temperature, which yields halide and solvent-free manganese borohydride after extraction with dimethylsulfide (DMS) and subsequent removal of residual solvent. This work constitutes the first example of establishing a successful, reproducible solvent-based synthesis route for a pure, crystalline, stable transition metal borohydride. The new polymorph, gamma-Mn(BH4)(2), is shown to be the manganese counter-part of the zeolite-like compound, gamma-Mg(BH4)(2) (cubic, a = 16.209(1) angstrom, space group Id (3) over bara). It is verified that large pores (diameter > 6.0 angstrom) exist in this structure. The solvate, Mn(BH4)(2)center dot 1/2S(CH3)(2), is subsequently shown to be the analogue of Mg(BH4)(2)center dot 1/2S(CH3)(2). As the structural analogies between Mg(BH4)(2) and Mn(BH4)(2) became evident a new polymorph of Mg(BH4)(2) was identified and termed zeta-Mg(BH4)(2). zeta-Mg(BH4)(2) is the structural counterpart of alpha-Mn(BH4)(2). All synthesis products are characterized employing synchrotron radiation-powder X-ray diffraction, infrared spectroscopy and thermogravimetric analysis in combination with mass spectroscopy. Thermal analysis reveals the decomposition of Mn(BH4)(2) to occur at 160 degrees C, accompanied by a mass loss of 14.8 wt%. A small quantity of the desorbed gaseous species is identified as diborane (rho(m)(Mn(BH4)(2)) = 9.5 wt% H-2), while the remaining majority is found to be hydrogen.