Crystal structures of the three closely related compounds: bis[(1H-tetrazol-5-yl)methyl]nitramide, triaminoguanidinium 5-({[(1H-tetrazol-5-yl)methyl](nitro)amino}methyl)tetrazol-1-ide, and diammonium bis[(tetrazol-1-id-5-yl)methyl]nitramide monohydrate

The crystal packing and intermolecular hydrogen-bonding schemes vary greatly between the three compounds. In all three structures, the nitramide is mainly sp 2-hybridized and the bond lengths indicate delocalization of charges on the tetrazole rings.


Chemical context
Materials which release large amount of energy during chemical transformations are characterized as energetic materials. Interest is high in improving energetics to reduce environmental impact and to improve safety and performance (Talawar et al., 2009). These materials can pose a hazard if they have high sensitivities to friction, heat, electrostatic discharge or impact. Compounds containing both tetrazole and nitro groups have frequently been used in the development of energetic materials (Klapö tke et al., 2009;Wei et al., 2015). Tetrazoles have been of special interest because of their high nitrogen content, which lead to high heats of formation and to more environmentally benign decomposition products like N 2 (Jaidann et al., 2010). Nitro groups have been commonly utilized to achieve an optimum oxygen balance (Wu et al., 2014). Herein is a discussion of the X-ray crystal structures of three nitro-containing tetrazole complexes. Structure (I), bis[(1H-tetrazol-5-yl)methyl]nitramide, is the neutral form, ISSN 2056-9890 structure (II), triaminoguanidinium 5- ({[(1H-tetrazol-5yl)methyl](nitro)amino}methyl)tetrazol-1-ide, has one deprotonated tetrazole ring with a triaminoguandidinium counterion, and structure (III), diammonium bis [(tetrazol-1-id-5yl)methyl]nitramide monohydrate, has both tetrazole rings deprotonated with ammonium counter-ions.

Structural commentary
In the molecule of complex (I), two intramolecular hydrogen bonds, N4-H4Á Á ÁO15 and N10-H10Á Á ÁO16, both between tetrazole donors and nitro acceptors are present (Fig. 1). This molecule adopts a chair-like conformation where the tetrazole rings are trans to one another. Molecule (III) adopts a similar conformation, despite not having any similar intramolecular hydrogen bonds (Fig. 2). Surprisingly, while structures (I) and (III) are both in a chair conformation, with respect to the tetrazole rings, structure (II) is bent into a boat where the tetrazole rings are cis to one another (Fig. 3).
This unusual conformation is likely due to the intramolecularstacking interactions observed between the tetrazole rings [centroid-centroid distance = 3.4978 (10) Å ]. Both tetrazole rings are nearly planar with an r.m.s. deviation of 0.0007 for the protonated ring and 0.00004 Å for the deprotonated ring.    For all three compounds, the C-N (ranging from 1.321 to 1.338 Å ) and N-N (ranging from 1.301 to 1.362 Å ) bond lengths for the tetrazole rings were found to match more closely with bonds of multiple character than of discrete single and double bonds, signifying a delocalized aromatic system (Allen et al., 1987).
In structure (II), the N18-C17, N20-C17, and N22-C17 bond lengths for the triaminoguandidinium cation were all found to be relatively equal (maximum difference 0.006 Å ), indicating delocalization of the charge over all three branches.
The pyramidality of the amine functionality for the central tertiary amine was examined for all three structures by examining n , the angle between the N amine -N nitro vector and the C methylene1 /N amine /C methylene2 plane, described by Allen et al. (1995). Structure (I) has a n of 13.1 (5) for vector N2-N1 and plane C11/C5/N1, structure (II) has a n of 26.11 (18) for vector N14-N7 and plane C6/N7/C8, and structure (III) has a n of 6.21 (11) for vector N7A-N7 and plane C6/N7/C8. This indicated the hybridization of the central nitrogen atom is mainly sp 2 hybridized for all three structures (sp 2 n ' 0 , sp 3 n ' 60 ).

Figure 5
Packing diagram for structure (II) viewed along the a axis. Dashed lined indicate intermolecular hydrogen bonds. the interactions between the main residue and the triaminoguandidinium counter-ion. The additional hydrogen bonds link the molecules into a three-dimensional network. The compound packs into columns of alternating anions and cations along the c axis (Fig. 5).
Structure (III) contains several intermolecular hydrogen bonds, which also form a three-dimensional network (Table 3). There are seven N-HÁ Á ÁN bonds between ammonium donors and tetrazole acceptors, two O-HÁ Á ÁN bonds between water donors and tetrazole acceptors, two N-HÁ Á ÁO bonds between ammonium donors and water acceptors, and one N-HÁ Á ÁO bond between an ammonium donor and a nitro acceptor. The ions and molecules pack into columns along the b axis (Fig. 6).
The neutral complex, compound (I), has a density of 1.825 g cm À3 (173 K). This is similar to the density, determined by X-ray crystallography, of the well known energetics RDX (-hexahydro-1,3,5-trinitro-1,3,5-triazine) and HMX (1,3,5,7tetra-nitro-1,3,5,7-tetraazacyclooctane) at 1.794 g cm À3 (298 K) and 1.948 g cm À3 (120 K) respectively (Zhurov et al., 2011). The ionic compounds have much lower densities. The density of compound (II) is 1.611 g cm À3 (293 K), and the density of compound (III) is 1.579 g cm À3 (296 K).  , 2008). A search for triaminoguandidinium containing compounds found 47 hits. The compounds from the CSD had similar bond lengths and angles to the triaminoguandidinium cation in complex (II). The average difference in C-N bond lengths for the triaminoguandidinium complexes in the CSD was found to be 0.015 Å , indicating a high level of charge delocalization, similar to that seen in complex (II).

Figure 6
Packing diagram for structure (III) viewed along the b axis. Dashed lined indicate intermolecular hydrogen bonds.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The methylene H atoms were positioned geometrically and refined using a riding model, with C-H = 0.99 Å and U iso (H) = 1.2U eq (C). All other H atoms were located in a difference Fourier map using. Compound (II) was found to be a non-merohedral twin and was solved and refined in the major component. The N10-H10 bond length in structure (I) was restrained.   Computer programs: APEX2, SAINT and XPREP (Bruker, 2008), SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) within WinGX (Farrugia, 2012) and publCIF (Westrip, 2010

Computing details
For all compounds, data collection: APEX2 (Bruker, 2008); cell refinement: APEX2 (Bruker, 2008); data reduction: SAINT (Bruker, 2008) and XPREP (Bruker, 2008); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015) within WinGX (Farrugia, 2012); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010). 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.

(II) Triaminoguanidinium 5-({[(1H-tetrazol-5-yl)methyl](nitro)amino}methyl)tetrazol-1-ide
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.

Special details
Experimental. Output from intergration and final cell refinement: A B C Alpha Beta Gamma Vol 7.59208 7.60543 11.22509 85.5941 85.5165 64.9686 584.79 0.00008 0.00008 0.00012 0.0004 0.0004 0.0004 0.01 Corrected for goodness of fit: 0.00040 0.00041 0.00058 0.0020 0.0022 0.0019 0.07 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.