Tetrabutylammonium cation in a homoleptic environment of borohydride ligands: [(n-Bu)4N][BH4] and [(n-Bu)4N][Y(BH4)4]

A novel solvent-free dual-cation organic–inorganic derivative, tetrabutylammonium yttrium borohydride (TBAYB), has been prepared for the first time and structurally characterized together with its organic precursor, tetrabutylammonium borohydride (TBAB). Both compounds crystallize in monoclinic unit cells (TBAYB: P21/c, TBAB: P2/c) and they contain [(n-Bu)4N]+ in a homoleptic environment consisting of BH4− ligands. Presence of large and lightweight Bu4N+ cations results in loose packing and low densities of both solids close to 1 g cm−3. TBAB melts at ca. 130 °C and it decomposes thermally above 160 °C while TBAYB melts at temperature as low as 78 °C, and the melt is stable over an appreciable temperature range of ca. 150 °C. The low melting point of TBAYB is the second lowest among derivatives of yttrium rendering this compound a new ionic liquid above 78 °C.