catena-Poly[[bis(diaqualithium)]-μ4-3,3′,5,5′-tetranitro-4,4′-bipyrazole-1,1′-diido]: a new moisture-insensitive alkali-metal energetic salt with a well-defined network structure

Abstract

In the structure of the title salt, [Li2(C6N8O8)(H2O)4]n, the 3,3′,5,5′-tetranitro-4,4′-bipyrazole-1,1′-diide dianion [{TNBPz}2−] is situated across the twofold axis. The distorted coordination octahedra around Li+ involve four short bonds with two pyrazolate N atoms and two aqua ligands [Li—N(O) = 1.999 (3)–2.090 (2) Å] and two longer contacts with nitro-O atoms [2.550 (2), 2.636 (2) Å]. When combined with μ4-{TNBPz}2−, this generates a mono-periodic polymeric structure incorporating discrete centrosymmeric [(H2O)2Li–(dinitropyrazolato)2–Li(H2O)2] units. The three-dimensional stack of mutually orthogonal coordination chains is reminiscent of a Lincoln log pattern. It is influenced by conventional hydrogen bonding [O...O = 2.8555 (17)–3.0010 (15) Å] and multiple lone pair–π hole interactions of the nitro groups [N...O = 3.0349 (15) and 3.0887 (15) Å]. The Hirshfeld surface and two-dimensional fingerprint plots also support the significance of non-covalent bonding. Coordinative saturation and a favorable geometry at the Li+ ions, dense packing of the polymeric subconnectivities and particularly extensive interanion interactions may be involved in the stabilization of the structure. The title salt is a rare example of an energetic Li nitroazolate, which nicely crystallizes from aqueous solution and is neither hygroscopic nor efflorescent. The TG/DTA data reveal total dehydration in the range of 330–430 K and stability of the anhydrous material up to 633–653 K.

Keywords

• crystal structure
• lithium
• nitropyrazoles
• energetic materials
• hydrogen bonding