Isostructural rubidium and caesium 4-(3,5-dinitropyrazol-4-yl)-3,5-dinitropyrazolates: crystal engineering with polynitro energetic species

In the structures of the title salts, two independent cations (Rb, Cs) are situated on a crystallographic twofold axis and on a center of inversion, respectively. Mutual intermolecular hydrogen bonding between the conjugate 3,5-dinitopyrazole NH-donor and 3,5-dinitropyrazolate N-acceptor sites of the anions governs the self-assembly of the translation-related anions in a predictable fashion. The anionic chains are further linked by multiple ion–dipole interactions involving the 12-coordinate cations bonded to two pyrazole N-atoms and all of the eight nitro O-atoms. 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 interactions are also relevant to the packing.

In the structures of the title salts, poly [[ 4 -4-(3,5-dinitropyrazol-4-yl)-3,5dinitropyrazol-1-ido]rubidium], [Rb(C 6 HN 8 O 8 )] n , (1), and its isostructural caesium analogue [Cs(C 6 HN 8 O 8 ) n , (2), two independent cations M1 and M2 (M = Rb, Cs) are situated on a crystallographic twofold axis and on a center of inversion, respectively. Mutual intermolecular hydrogen bonding between the conjugate 3,5-dinitopyrazole NH-donor and 3,5-dinitropyrazole N-acceptor sites of the anions [NÁ Á ÁN = 2.785 (2) Å for (1) and 2.832 (3) Å for (2)] 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 interactions involving the 12-coordinate cations bonded to two pyrazole N-atoms [Rb-N = 3.1285 (16) Many issues of crystal engineering, in regard to control over bonding patterns, supramolecular topologies, molecular packing, and crystal morphologies are highly relevant to the area of energetic materials. In particular, non-covalent contacts involving common explosophore nitro groups (Bauzá et al., 2017) establish pathways to transmit intermolecular interactions and they are often responsible for higher densities of the solids (Zhang et al., 2000). The layered architectures of the energetic solids provide better buffering against external mechanical stimuli, which is essential for developing insensitive materials (Zhang et al., 2008). At the same time, incorporation of specific coordination geometries for the assembly of metal-organic solids offers potential for the synthesis of new perchlorate-free flame colorants and pyrotechnics (Glü ck et al., 2017). However, successful applications of the crystal-engineering methodology toward designing the structures of polynitro compounds are relatively rare, so far . This is predetermined by a lack of reliable supramolecular synthons comprising the nitro groups, which are only weak acceptors of conventional hydrogen ISSN 2056-9890 bonds (Robinson et al., 2000) and are only weak donors with respect to the metal ions. A more severe limitation is associated with the need for direct bonding between the nitro-rich functionalities only, since the incorporation of any low-energetic component or solvent molecules is an inevitable penalty to the performance. Such dilution of the energetic moieties in the crystals is relevant, for example, to a series of hydrogenbonded solids prepared by Aakerö y et al. (2015) with acidic ethylenedinitramine and common bitopic pyridine-N bases.
Recently, we have reported a new strategy for the construction of energetic salts, which offers higher degree of control over the structure. Double functionality of the wellperforming material 3,3 0 ,5,5 0 -tetranitro-4,4 0 -bipyrazole [H 2 (TNBP)] (Domasevitch et al., 2019) grants synthetic access either to singly or doubly anionic species [{H(TNBP)} À and {TNBP} 2À , respectively]. The former combine conjugate dinitropyrazole donor and dinitropyrazolate acceptor sites for sustaining particularly strong N-HÁ Á ÁN bonding. In fact, such bonding of two explosophores dominated the self-assembly in a very predictable fashion and it was traced in all of the previously examined salts with a range of nitrogen-rich cations (Gospodinov et al., 2020). That the resulting networks are ionic may find further applications to the synthesis of inorganic nitro-rich salts, based upon Li + , Rb + , Cs + , Sr 2+ , Ba 2+ and other s-and p-block cations, which are a new generation of 'green' pyrotechnic formulations (Steinhauser & Klapö tke, 2008).

Structural commentary
The title compounds are isostructural, crystallizing in space group C2/c. The molecular structure of the rubidium salt (1) is shown in Fig. 1, with the unique part comprising one organic anion {H(TNBP)} À (or C 6 HN 8 O 8 À ) and two cations situated on a crystallographic twofold axis [Rb1] or on a center of inversion [Rb2]. The easy formation of such salts is conditioned by the appreciable acidity of polynitropyrazoles, c.f. pK a = 3.14 for 3,5-dinitropyrazole versus 14.63 for the parent pyrazole (Janssen et al., 1973), while for the crystallization of singly charged hydrogen bipyrazolate derivatives, the weakly polarizing, large Rb + and Cs + cations are important.
interactions. This may be best related to the bonding in the ionic salts with polynitro anions lacking conventional donor sites. For example, in caesium picrate, the cations display a comparable 12-fold coordination and a wide spread of Cs-O separations of 3.028 (3)-3.847 (2) Å (Schouten et al., 1990). The coordination polyhedra of the two unique cations are very similar and represent essentially distorted icosahedra (Fig. 2). These are completed with a twofold axis [for M1] or inversion [for M2] related pairs of chelating nitropyrazole-N,O groups, pseudo-chelating NO 2 groups and two singly coordinated NO 2 groups. Both kinds of cations reside in a closest environment of eight {H(TNBP)} À anions, which maintain supramolecular boxes with a small internal cavity for the cation (Fig. 3). It is notable that all of the eight O atoms present and the two pyrazole N atoms coordinate to the metal ions.

Supramolecular features
The ionic structures of the title compounds may be regarded as three-dimensional networks, which are related to the structure of CsCl. The metal ions themselves constitute a distorted primitive cubic framework with the cells representing elongated prisms [the MÁ Á ÁM edges are 5.2560 (3), 6.5962 (3), 8.8395 (8) and 5.4775 (4), 6.3932 (5), 9.1482 (12) Å for (1) and (2), respectively]. Every such cell is populated with the organic anion and, conversely, every cation resides inside the distorted prismatic box of eight anions (Figs. 3 and 4).
Beyond Coulombic attraction, the principal supramolecular interaction is strong and directional N-HÁ Á ÁN hydrogen bonding between the pyrazole and pyrazolate halves of The Rb2 ion resides inside a supramolecular prism (represented here as a gray box) adopted by eight anions, which complete the coordination environment. The vertices of the prism are built through the mid-points of the central C-C bonds of the molecules. The environments of Rb1 and the respective Cs ions in the structure of (2) are similar. [Symmetry codes: (i) x, y + 1, z; (ii) Àx + 1 2 , y + 1 2 , Àz + 1 2 ; (iv) x À 1 2 , y + 1 2 , z; (ix) Àx, Ày + 1, Àz.]
NÁ Á Áplane is a distance of an N-donor to the mean plane of a nitro group and ' is an angle of the NÁ Á ÁN axis to the plane of the nitro group.

Hirshfeld analysis
The supramolecular interactions in the title structures were also assessed by Hirshfeld surface analysis (Spackman & Byrom, 1997;McKinnon et al., 2004;Hirshfeld, 1977;Spackman & McKinnon, 2002) performed with Crystal-Explorer17 (Turner et al., 2017). The contributions of different kinds of interatomic contacts to the Hirshfeld surfaces of the individual anions are listed in Table 5 and the fingerprint plots for (1) are shown in Fig. 7. The most significant contributors are OÁ Á ÁO contacts (37.4%), while the fraction of O,NÁ Á ÁRb (15.4%) is relatively modest due to the larger lengths of the ion-dipole interactions. The shortest OÁ Á ÁO separation on the plot of $2.8 Å corresponds to the contact O1Á Á ÁO8 xiii = 2.741 (2) Å [2.732 (3) Å in (2); symmetry code (xiii) Àx + 1 2 , Ày À 1 2 , Àz]. We note that slight contraction of the OÁ Á ÁM fraction in the case of M = Rb [13.6% for (1) and 13.0% for (2)] coincides with a larger contribution of less favorable OÁ Á ÁO contacts [37.4% for (1) and 35.5% for (2)]. This may be an additional factor destabilizing the structure: the crystals of (1) eventually decompose under the mother solution, unlike the stable Cs analogue. The lone pair--hole pyrazole-NO 2 interactions generate 5.3% (1) and 6.3% (2) Table 4 Geometry of stacking interactions involving nitro and pyrazole groups (Å , ) in (1) and (2). AtomÁ Á ÁCg is the shortest distance from the nitro group atom to the centroid of the ring; AtomÁ Á Áplane is the deviation of the given atom from the mean plane of the ring and ' is the angle of the atomÁ Á ÁCg axis to the plane of the ring.

Figure 7
Two-dimensional fingerprint plots for the individual anions in (1)   . The contributions of the OÁ Á ÁH/ HÁ Á ÁO and NÁ Á ÁH/HÁ Á ÁN contacts are comparable and perceptible [5.4 and 6.9% for (1) and 5.2 and 6.6% for (2)], but only the latter correspond to hydrogen bonding, as is reflected in the plots. These bonds are responsible for a pair of very sharp features pointing to the lower left, with a shortest contact of 1.9 Å , whereas OÁ Á ÁH/HÁ Á ÁO contacts are identified only with a diffuse collection of points between the above features and with a shortest contact of 2.8 Å .
To prepare the Rb salt (1), 0.332 g (1.0 mmol) of H 2 (TNBP)ÁH 2 O was added to a solution of 0.116 g (0.5 mmol) of Rb 2 CO 3 in 8 ml of water and the mixture was heated at 353-363 K until total dissolution was observed. The solution was cooled to room temperature and left for a few hours for crystallization. Pale-yellow crystals of Rb{H(TNBP)} were isolated in a yield of 0.325 g (82%) and dried in air. The compound is unstable when stored under the reaction solution as the initially formed crystals dissolve in a period of 10-15 d and colorless H 2 (TNBP)ÁH 2 O deposits. In a similar way, the reaction of 0.332 g (1.0 mmol) of H 2 (TNBP)ÁH 2 O and 0.163 g (0.5 mmol) of Cs 2 CO 3 in 8 ml of water gives 0.415 g (93%) of pale-yellow Cs{H(TNBP)} (2). Unlike (1), this material is stable under the mother solution. Similar reactions with Na 2 CO 3 and K 2 CO 3 did not afford any hydrogen bipyrazolates and led to soluble M 2 {TNBP} (M = Na, K) and precipitation of the excess amount of H 2 (TNBP)ÁH 2 O. Analysis (%) calculated for (1)
1.5U eq (N). For (2), the H atom is equally disordered over two positions corresponding to the N1 and N4 carrier atoms.

Poly[[µ 4 -4-(3,5-dinitropyrazol-4-yl)-3,5-dinitropyrazol-1-ido]rubidium] (1)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.