Crystal structure of the high-energy-density material guanylurea dipicrylamide

1-Carbamoylguanidinium bis(2,4,6-trinitrophenyl)amide (= guanylurea dipicrylamide) builds up an array of mutually linked guanylurea cations and dipicrylamide anions. The crystal packing is dominated by an extensive network of N—H⋯O hydrogen bonds, resulting in a high density of 1.795 Mg m−3 which makes the title compound a potential secondary explosive.


Chemical context
High-energy-density materials (HEDMs) form an important class of explosive compounds. Several significant advantages such as high heats of combustion, high propulsive power, high specific impulse, as well as smokeless combustion make them highly useful as propellants, explosives, and pyrotechnics (Oestmark et al., 2007;Rice et al., 2007;Badgujar et al., 2008;Gö bel & Klapö tke, 2009;Nair et al., 2010;Klapö tke, 2011). An important class of such high-energy-density materials are polynitro aromatics such as trinitrotoluene (TNT), picric acid, trinitroresorcinol (= styphnic acid), and 2,2 0 ,4,4 0 ,6,6 0 -hexanitrodiphenylamine (= dipcrylamine). Dipicrylamine combines several very interesting structural features: It contains six nitro groups, which are flexible and can interact and adjust in the crystal lattice. Moreover, dipicrylamine has a secondary amine group which can be deprotonated with alkali and alkaline earth-metal hydroxides to form water-soluble dipicrylamide salts. In the resulting dipicrylamide anion (= DPA À ), partial delocalization of the negative charge mediated by the aromatic rings is possible, which may facilitate coordination of the oxygen atoms of the nitro groups with suitable metal ions (Eringathodi et al., 2005;Agnihotri et al., 2006). Moreover, the DPA À anion has various sites which are capable of forming different types of hydrogen bonds in the solid state.
The ammonium salt of dipicrylamine, also known as Aurantia or Imperial Yellow, was discovered in 1874 by Gnehm and used as a yellow colorant for leather, wool, and silk until the early 20 th century (Gnehm, 1874(Gnehm, , 1876. However, these practical uses have been terminated due to the highly toxic and explosive nature of dipicrylamine (Kjelland, 1971). Dipicrylamine can also be used for the extraction of K + ISSN 1600-5368 ions from sea bittern, which contains a mixture of K + , Na + , and Mg 2+ salts (Winkel & Maas, 1936). A related study carried out with a mixture of K + , Rb + , and Cs + ions revealed that the Cs + ion shows maximum selectivity towards DPA À (Bray et al., 1962). In fact, it has been reported that DPA À can be used for the recovery of Cs + from radioactive wastes (Kyrš et al., 1960). Only in recent years has the structural chemistry of alkali metal and alkaline earth metal as well as ammonium and azolium dipicrylamides been investigated in detail. All these compounds were found to display interesting hydrogenbonded supramolecular structures in the solid state (Eringathodi et al., 2005;Agnihotri et al., 2006;Huang et al., 2011).

Spectroscopic features
In the course of our ongoing studies on the crystal structures of energetic compounds (Deblitz et al. 2012a,b;Stock et al., 2014), we investigated the preparation and structural characterization of the title compound, guanylurea dipicrylamide, [H 2 NC(=O)NHC(NH 2 ) 2 ] 2 [N{C 6 H 2 (NO 2 ) 3 -2,4,6} 2 ]. The guanylurea cation has frequently been reported to be a useful component in energetic nitrogen-rich salts, e.g. guanylurea dinitramide (Langlet, 1998) or guanylurea tetrazolate salts (Wang et al., 2009). An aqueous solution of sodium dipicrylamide was prepared in situ by deprotonating dipicrylamine with NaOH. Treatment of this dark-red solution with solid 1carbamoylguanidinium sulfate, [H 2 NC(=O)NHC(NH 2 ) 2 ] 2 SO 4 (also known as guanylurea sulfate) (Lotsch & Schnick, 2005), afforded dark-red block-like crystals of the title compound after undisturbed standing of the reaction mixture for 10 d. The product was characterized by spectroscopic methods and elemental analysis. The 1 H NMR spectrum displayed a sharp singlet at = 8.78 p.p.m. for the aromatic protons of the DPA À anion, which is in excellent agreement with the literature values (Eringathodi et al., 2005;Agnihotri et al., 2006;Huang et al., 2011). However, the NH and NH 2 protons only gave rise to two very broad resonances spread over a range of ca 4 p.p.m. [(C(O)NH 2 ] = ca 6.3-7.1 p.p.m., [NHC(NH 2 ) 2 ] = ca 3.3-5.8 p.p.m.). In contrast, interpretation of the 13 C NMR spectrum was straightforward. In perfect agreement with the 13 C NMR data of previously reported ammonium and azolium DPA salts (Huang et al., 2011), the spectrum of the title compound displayed signals of the aromatic ring carbons at = 143.4, 139.5, 132.6, and 125.1 p.p.m. The two carbon resonances of the guanylurea cation were well separated at = 157.4 p.p.m. (C O) and = 155.5 p.p.m. [NHC(NH 2 ) 2 ). IR bands in the range of 3200-3400 cm À1 were characteristic for the N-H valence vibrations in the guanylurea cation. A strong carbonyl band was observed at 1632 cm À1 , whereas the band at 1532 cm À1 is characteristic for the nitro groups.

Structural commentary
Single crystals obtained directly from the reaction mixture were found to be suitable for X-ray diffraction. The title compound crystallizes in the triclinic space group P1. The crystal structure consists of mutually linked 1-carbamoylguanidinium cations and dipicrylamide anions (Fig. 1). The angle C1-N1-C7 at the amide nitrogen atom of the DPA À anion is 131.66 (10) , with C-N bond lengths of 1.3021 (15) (C1-N1) and 1.3403 (15) Å (C7-N1). These values are somewhat shorter than the C-N bond lengths in free dipicrylamine (1.373, 1.375 Å ; Huang et al., 2011) but comparable to those reported for related azolium dipicrylamides which have central C-N bond lengths in the range of 1.281-1.338 Å (Huang et al., 2011). These values indicate delocalization of the nitrogen lone pair on N1 in dipicrylamide salts, thereby stabilizing the anion by strengthening the C1-N1 and C7-N1 bonds. As a structural consequence, not only is the central C1-N1-C7 angle widened, but there is also a significant elongation of the four C-C bonds adjacent to C1 and C7 (average 1.433 Å ) as compared to the other aromatic C-C bonds (average 1.378 Å ). The C-N bond lengths in the nearly planar (r.m.s. deviation = 0.0371 Å ) 1-carbamoylguanidinium cation also indicate significant electron delocalization. The   geometry around the carbon atom in the amidinium fragment NHC(NH 2 ) 2 is nearly trigonal-planar with N-C-N angles between 117.52 (11) and 121.48 (11) and C-N distances in the very narrow range of 1.3064 (15)-1.3158 (15) Å . Overall, the structural parameters of the cation in the title compound do not differ significantly from those in 1-carbamoylguanidinium sulfate, [H 2 NC(=O)NHC(NH 2 ) 2 ] 2 SO 4 (Lotsch & Schnick, 2005).

Supramolecular features
Both the cation and the anion comprise numerous sites capable of forming different types of hydrogen bonds. Thus it is not surprising that the crystal packing ( Fig. 2) is controlled by an extensive hydrogen-bonding network (Table. 1). Six distinct N-HÁ Á ÁO hydrogen bonds are found in the crystal packing of the title compound. First of all, pairs of cations are formed through dimerization via two N-HÁ Á ÁO hydrogen bonds between the ureic fragments, which is also very typical for carboxylic amides. Furthermore, the NH 2 groups in the amidinium fragments NHC(NH 2 ) 2 engage in four N-HÁ Á ÁO hydrogen bonds to three different nitro groups of the DPA À anion. The calculated density of 1.795 Mg m À3 is not only higher than the densities reported for other DPA-based salts (1.69-1.78 Mg m À3 ), but also much higher than the density of TNT (1.65 Mg m À3 ) (Huang et al., 2011). The high density of the title compound can be traced back in large part to the hydrogen bonding in the crystal structure. The energetic properties (e.g. impact and friction sensitivity) of guanylurea dipicrylamide have not been tested, but recent findings have shown that the impact sensitivities of various ammonium and azolium dipicrylamides are in the range of that of the secondary explosive TNT (Huang et al., 2011).

Synthesis and crystallization
Cautionary note: Dipicrylamine and dipicrylamide salts are potentially explosive and should be handled only in small amounts using proper safety equipment (Klapö tke, 2011).
Preparation of guanylurea dipicrylamide: To a suspension of dipicrylamine (1.0 g, 2.3 mmol) in 10 ml water were added two pellets of NaOH to give a dark red solution of sodium dipicrylamide; 0.23 g (1.5 mmol) of guanylurea sulfate were added as solid, and the mixture was allowed to stand undisturbed at room temperature. After 10 d, 0.87 g (70%) dark-red crystals of the title compound had formed, which were isolated by filtration and dried in air.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Positions and isotropic thermal parameters of hydrogen atoms were freely refined. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 2; Àz; (ii) Àx; Ày þ 1; Àz þ 1; (iii) Àx þ 2; Ày þ 1; Àz; (iv) x þ 1; y; z.  SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).  Dwiggins Jr (Acta Cryst.(1975) A31,146-148) is used with some modification. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.