Closo-Hydroborates from Liquid Ammonia: Synthesis and Crystal Structures of [Li(NH3)4]2[B12H12]·2NH3, Rb2[B12H12]·8NH3, Cs2[B12H12]·6NH3 and Rb2[B10H10]·5NH3

Abstract

Ammonia complexes of hydroborates may be potentially promising materials for ammonia storage or indirect hydrogen storage. The title compounds contain 20.04-52.23 wt % ammonia and 5.94-13.01 wt % hydrogen. They were synthesized in liquid ammonia, using (NBu4)2[B12H12] and Li2[B10H10] as starting materials. [Li(NH3)4]2[B12H12]·2NH3 (1) crystallizes in the monoclinic crystal system (space group P2(1)/c, a = 9.183(2) Å, b = 8.133(1) Å, c = 16.375 Å, β = 110.54(1)°, V = 1143.97(40) Å(3), Z = 2). The compound is a direct precursor of the hydrogen storage compound Li2[B12H12]·7NH3. Hydroborates of the heavier alkali metals were found to crystallize in the orthorhombic crystal system: Rb2[B12H12]·8NH3 (2) (space group Pnnm, a = 14.4166(6) Å, b = 7.8221(3) Å, c = 9.5792(4) Å, V = 1080.23(8) Å(3), Z = 2), Cs2[B12H12]·6NH3 (3) (space group Pbca, a = 7.7569(8) Å, b = 14.087(2) Å, c = 18.075(2) Å, V = 1974.99(30) Å(3), Z = 4), and Rb2[B10H10]·5NH3 (4) (space group Pnma, a = 13.9510(7) Å, b = 8.675(2) Å, c = 13.966(2) Å, V = 1690.2(3) Å(3), Z = 4), as determined by single crystal X-ray diffraction. The structures are discussed briefly.