Thermal Desorption, Vibrational Spectroscopic, and DFT Computational Studies of the Complex Manganese Borohydrides Mn(BH₄)₂and [Mn(BH₄)₄]²⁻

The mechanochemical reaction of LiBH4 with MnCl2 produces the neutral complex Mn(BH4)(2). Thermal desorption studies show that the mechanochemical reaction of NaBH4 with MnCl2 produces a different species, apparently Na2Mn(BH4)(4), that undergoes dehydrogenation of a much lower weight percent H at a similar to 20 degrees C higher temperature than the neutral Mn(BH4)(2). Vibrational spectroscopy also reveals that a complex manganese borohydride(s) in addition to Mn(BH4)(2) are formed from the mechanochemical reactions. Analysis of the vibrational spectra in conjunction with DFT calculations on a model Mn(BH4)(4)(2-) complex suggest bidentate binding of the [BH4](-) ligands to the Mn center in the anionic complex. The calculated highest frequencies of the B-H stretching modes (corresponding to the "free" B-H bonds) agree well with the experimental frequencies and support the presence of this structural feature.