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.

Structural Behavior of Li2B10H10

“…Polyhedral boron–hydrogen compounds containing the relatively stable decahydro- closo -decaborate (B 10 H 10 2– ) and dodecahydro- closo -dodecaborate (B 12 H 12 2– ) anions have been studied more intensely in recent years because of their possible formation and persistence as side-products during the dehydrogenation of tetrahydroborate-based hydrogen-storage materials, − as well as for their own potential as host materials for ammonia and indirect hydrogen storage. − Of particular interest are the lighter alkali- and alkaline-earth-metal congeners containing Li, Na, Mg, and Ca because the incorporation of these elements yields storage materials with larger hydrogen mass fractions. More recently, polyhedral boron–hydrogen compounds of Na and potentially Li have also shown great promise as superionic electrolytes for next-generation, solid-state rechargeable batteries. − Na 2 B 10 H 10 is noteworthy for displaying an exceptional conductivity of about 0.01 S cm –1 at 383 K .…”

Syntheses, Crystal Structures, and Properties of the Isotypic Pair [Cr(H2O)6]2[B12H12]3·15H2O and [In(H2O)6]2[B12H12]3·15H2O

“…Together with these strong classical hydrogen bonds, there are two types of non‐classical B–H δ – ··· δ + H–O hydrogen bonds17 existing in both title compounds. These unconventional hydrogen bonds can also be found in other structures, containing donor molecules with N or O and [B 12 H 12 ] 2– dianions 18,19. The first type is the one between the negatively polarized hydrogen atoms of the [B 12 H 12 ] 2– cages and the twelve surrounding zeolitic water molecules with d (H δ – ··· δ + H) = 176–295 pm ( r w (H) = 120 pm for the van‐der‐Waals radius of hydrogen),20 forming distorted cuboctahedra.…”

Analyzing Ammonia Bridges – and more about Bonding in Boron-rich Solids

“…Two of our synthetic procedures are novel for hydroborates and gave access to new compounds and their crystal structures: i) the reaction in liquid ammonia at very low temperatures, [5,9,10] and ii) the gel crystallization (Fig. 3).…”

“…Nowadays, hydroborates are being discussed in the context of energy-related materials being potentially useful for hydrogen storage. [5] We have synthesized, crystallized, and structurally analyzed many closo-hydroborates because they are fascinating as model compounds for comparison of molecular solids to solids with extended frameworks. [6,7] Boron-rich borides like BaB 6 for example show structural (B 6 octahedra) and bonding (electron deficiency) features that resemble the corresponding molecular salt with a B 6 H 6 2-anion (Fig.…”