New Energetic Dodecahydro-closo-Dodecaborates (N₂H₅)₄X₂[B₁₂H₁₂] (X = Cl and Br) with a Novel Hexagonal A₄B₂C Tunnel Structure

Two new tetrahydrazinium dihalide dodecahydro- closo -dodecaborates (N 2 H 5 ) 4 X 2 [B 12 H 12 ] ( X – = Cl – and Br – ) were successfully synthesized via the direct reaction of dihydrazinium dodecahydro- closo -dodecaborate (N 2 H 4 ) 2 [B 12 H 12 ] with hydrazinium halides (N 2 H 5 )Cl and (N 2 H 5 )Br in aqueous media. The resulting isotypic compounds crystallize monoclinically in the space group P 2 1 / c with similar unit-cell parameters ( a = 681.98(4) pm, b = 1025.38(6) pm, c = 1325.38(8) pm, and β = 98.393(3)° for the chloride and a = 686.05(4) pm, b = 1032.51(6) pm, c = 1316.04(8), and β = 97.449(3)° for the bromide). Their crystal structure was elucidated using single-crystal X-ray diffraction techniques, unveiling a new unique arrangement for the composition A 4 B 2 C ( A = hydrazinium, B = halide, and C = dodecahydro- closo -dodecaborate). This distinctive structural configuration sets it apart from compounds exhibiting other A 4 B 2 C arrangements, such as the spinel or double-perovskite structure type. In this new structure, the halide anions are coordinated by five hydrazinium cations, creating triangular bipyramidal polyhedra. These are interconnected via edge- and corner-sharing, resulting in the formation of hexagonal tunnels along the [100] direction. Within each of these tunnels, the [B 12 H 12 ] 2– clusters reside centrally located. Differential scanning calorimetry analyses demonstrate that these compounds contain a substantial amount of energy that is released during their thermal decomposition.