Isostructural rubidium and caesium 4-(3,5-di-nitro-pyrazol-4-yl)-3,5-di-nitro-pyrazolates: crystal engineering with polynitro energetic species.

In the structures of the title salts, poly[[μ-4-(3,5-di-nitro-pyrazol-4-yl)-3,5-di-nitro-pyrazol-1-ido]rubidium], [Rb(CHNO)] , (), and its isostructural caesium analogue [Cs(CHNO) , (), two independent cations 1 and 2 ( = Rb, Cs) are situated on a crystallographic twofold axis and on a center of inversion, respectively. Mutual inter-molecular hydrogen bonding between the conjugate 3,5-dinito-pyrazole NH-donor and 3,5-di-nitro-pyrazole N-acceptor sites of the anions [N⋯N = 2.785 (2) Å for () and 2.832 (3) Å for ()] governs the self-assembly of the translation-related anions in a predictable fashion. Such one-component modular construction of the organic subtopology supports the utility of the crystal-engineering approach towards designing the structures of polynitro energetic materials. The anionic chains are further linked by multiple ion-dipole inter-actions involving the 12-coordinate cations bonded to two pyrazole N-atoms [Rb-N = 3.1285 (16), 3.2261 (16) Å; Cs-N = 3.369 (2), 3.401 (2) Å] and all of the eight nitro O-atoms [Rb-O = 2.8543 (15)-3.6985 (16) Å; Cs-O = 3.071 (2)-3.811 (2) Å]. The resulting ionic networks follow the CsCl topological archetype, with either metal or organic ions residing in an environment of eight counter-ions. Weak lone pair-π-hole inter-actions [pyrazole-N atoms to NO groups; N⋯N = 2.990 (3)-3.198 (3) Å] are also relevant to the packing. The Hirshfeld surfaces and percentage two-dimensional fingerprint plots for () and () are described.