Hydrogen-bonding landscape of the carbamoylcyanonitrosomethanide anion in the crystal structure of its ammonium salt

The structure of the title salt, ammonium carbamoylcyanonitrosomethanide, NH4 +·C3H2N3O2 −, features the co-existence of different hydrogen-bonding patterns, which are specific to each of the three functional groups (nitroso, carbamoyl and cyano) of the methanide anion. The relatively simple scheme of these interactions allows the delineation of the supramolecular synthons, which may be applicable to crystal engineering of hydrogen-bonded solids containing polyfunctional methanide anions.


Chemical context
Resonance-stabilized methanide-type anions are excellent ligands in metal-organic chemistry, which reveal a variety of coordination modes toward metal ions (Gerasimchuk, 2019;Turner et al., 2011). The rich molecular functionality of such species, as is exemplified by different nitrile-, nitroso-and carbamoyl-substituted derivatives, also predetermines their special properties as potent acceptors of conventional hydrogen bonds. These kinds of interactions are important for the solvation and solvatochromism of cyanoanions (Gerasimchuk et al., 2010) and intermolecular bonding in the crystal structures of metal complexes , but it could also influence the specific targeting of cyanoanions in biomedical systems (Gerasimchuk et al., 2007) and their behavior as anionic components for ionic liquids (Janikowski et al., 2013). It is worth noting that extensive conjugation and charge delocalization within the molecular frameworks support higher electron densities at all three functional sites (Chesman et al., 2014), which is beneficial for stronger and more directional interactions. Therefore, methanide-type anions are well suited for the crystal engineering of hydrogenbonded solids with cationic H-atom donors (Turner et al., 2009).
The specific hydrogen-bonding preferences associated with each of the different functional groups at the methanide core could result in a variety of predictable patterns, as well as providing a degree of selectivity for the interactions with hydrogen-bond donors. In this view, structurally similar methanides possess a distinct potential for crystal design. For example, either nitroso or carbamoyl groups equally well complement the cyano groups in methanide systems, but the chemical outputs of such functionalization, represented by closely related [ONC(CN) 2 ] À and [C(CN) 2 (CONH 2 )] À anions, are rather different with regard to their hydrogen-bonding behavior. The nitroso groups favor direct interactions with hydrogen-bond-donor cations and the assembly of cation/ anion pairs (Arulsamy et al., 1999), while the crystal chemistry of carbamoyldicyanomethanide is dominated by mutual amide/amide and amide/cyano interactions with the generation of less-common anion-anion networks . The particular combination of nitrile, nitroso and carbamoyl groups in carbamoylcyanonitrosomethanide [ONC(CN)(CONH 2 )] À , which is a well known product of the nucleophylic addition of water to [ONC(CN) 2 ] À (Arulsamy & Bohle, 2000), presumably allows one to unite the individual structural trends for the two kinds of anions. One can anticipate the assembly of such hybrid hydrogen-bonded structures in a predictable fashion, while taking into account the hierarchy of homo-and heterosynthons formed by each of the functional groups and appropriate hydrogen-bond donors.
In the present contribution, we report the construction of a three-dimensional hydrogen-bonded framework in ammonium carbamoylcyanonitrosomethanide NH 4 (nccm), which features the co-existence and interplay of the abovementioned anion-cation and mutual anion-anion interactions.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. This salt is isomorphous with the previously examined Cs analog (Domashevskaya et al., 1989), which is slightly unusual when considering the very different nature and ionic radii of the cations.
The main geometries of the (nccm) À (or C 3 H 2 N 3 O 2 À ) anion reveal a highly conjugated structure. The nitrosocyanomethanide O1/N1/C1/C2/N2 fragment itself is planar within 0.004 Å , being almost coplanar also with the C3/N3/O2 amide fragment [dihedral angle = 3.93 (14) ]. The nitroso group adopts a trans-anti configuration with respect to the carbamoyl C O group, which is the most favorable either for neutral or anionic ONC(CN)-COR species (Ponomareva et al., 1997;Ponomarova & Domasevitch, 2012). When compared with the parameters for neutral H(nccm) (Arulsamy & Bohle, 2000), the deprotonation results in a perceptible lengthening of the double bonds. For example, the carbonyl O2-C3 bond in the title compound is 1.252 (2) Å versus 1.228 (3) Å for H(nccm), but the same elongation is relevant also to the N1-C1 bond [1.303 (2) Å ], which is significantly longer than in the latter case [1.275 (3) Å ]. This is accompanied by a shortening of the N1-O1 bonds, which are particularly sensitive to the protolytic effects. These effects can be precisely traced by gradual shortening of the nitroso bonds for the series H(nccm) [1.356 (2) Izgorodina et al., 2010], in line with the strength of the N-OÁ Á ÁH bonding. Thus, with relatively strong multiple hydrogen bonds sustained by the nitroso O atoms, the N-O bond order in the title compound is still greater than for the symmetrical hydrogen dioximate anion H(nccm) 2 À [which is structurally similar to more common hydrogen carboxylates (Speakman, 1972)], but is lower than in NMe 4 (nccm) (one N-HÁ Á ÁO bond) and also Cs(nccm) [1.297 (8) Å ; Domashevskaya et al., 1989] showing only distal ion-dipole interactions of the nitroso group. Such an evolution is clearly reflected in the positions of the (NO) bands in the IR spectra (cm À1 ): they are 1098 for H(nccm); 1140 for H(nccm) 2 À ; 1212 for the title compound; 1253 for NMe 4 (nccm) and 1290 for Cs(nccm), demonstrating the systematic blue shift as the N-O bond order increases.
An important result from the multiple NH 4 + Á Á ÁON interactions is the assembly of infinite chains running along the caxis direction in the crystal, with the [(NH 4 ) 2 (O) 2 ] rhombs sharing their opposite edges (Fig. 2). Two such N-HÁ Á ÁO bonds are relatively strong [NÁ Á ÁO = 2.688 (3) and 2.848 (2) Å , Table 1], whereas N4Á Á ÁO1 ii [3.000 (3) Å , symmetry code (ii) Àx, Ày, x + 1 2 ] exists as a branch of a weaker bifurcated N4-H5Á Á Á(O1,O2) interaction with the nitroso and carbamoyl acceptors. The present motif is noticeably different from the bonding of NH 4 + cations and nitrosodicyamomethanide, with the ionic pairs assembled via both the O and N atoms of the nitroso groups and only two N-HÁ Á ÁO interactions retained at NÁ Á ÁO distances of 2.822 (2), 2.881 (2) Å , which are comparable to the two strongest bonds in the title salt (Arulsamy et al., 1999). Such a discrimination of the nitroso N atom in (nccm) À may be attributed to its lower accessibility, which is in line with the higher steric demands of the carbamoyl group. At the same time, one of the carbamoyl H atoms (which is trans-positioned to the C O bond) is also less accessible and it selectively maintains weaker N-HÁ Á ÁN bonding to the nitrile acceptor [N3Á Á ÁN2 vi = 3.004 (3) Å ; symmetry code (vi) x À 1 2 , Ày + 1 2 , z + 1], very similar to the structure of parent H(nccm) (Arulsamy & Bohle, 2000).
The columnar packing of (nccm) À anions yields slipped stacks down the c-axis direction, with an interplanar distance of 3.32 Å (Figs. 2 and 3). This feature is similar to the structures of cyanomethanide species examined by Chesman et al. (2014), which typically support stacks at 3.15-3.30 Å . However, the overlaps of the (nccm) À skeletons are minor [as indicated by a large slippage angle of 54.9 (2) ] and actually only the nitrile fragment is involved in the stacking with the methanide fragment. The shortest contact between translation-related anions is N2Á Á ÁC1 viii = 3.357 (2) Å [symmetry code: (viii) x, y, z -1]. This stacking is less significant for (nccm) À salts due to the prevalent role of hydrogen bonding, which is a primary anion-anion interaction for carbamoylsubstituted methanides (Chesman et al., 2014).
The two-dimensional fingerprint plots (Fig. 7) are consistent with the prevalence of hydrogen bonding in the structure. For the individual NH 4 + cations, as much as 57.3% of their surface are HÁ Á ÁO contacts. The HÁ Á ÁN contacts account for only 20.1% (HÁ Á ÁH and HÁ Á ÁC are 20.1% and 2.5%, respectively), which suggests a rather high selectivity in the bonding of NH 4 + cations to the O-acceptor sites. The plots for the anion are even more informative. The short separations are overwhelmingly hydrogen-bond contacts, accounting for 64.1% of the surface. The OÁ Á ÁH/HÁ Á ÁO fraction of 34.5% appears on the plot as a pair of sharp spikes pointing to the lower left, with the upper spike representing entirely HÁ Á ÁO of the amide/ amide synthon (the shortest contact is 2.0 Å ), while the more intense and longer lower spike is due to a reciprocal OÁ Á ÁH bond superimposed with points from stronger and more numerous OÁ Á ÁH (NH 4 + ) contacts (the shortest is 1.7 Å ). In the case of NÁ Á ÁH/HÁ Á ÁN type (29.6%), two spikes are shorter Structure of the title compound, viewed in a projection onto the ab plane, showing the co-existence and interplay of the three main supramolecular motifs in the form of ammonium/nitroso chains, amide/amide chains (both of which are situated across 2 1 axes and are orthogonal to the drawing plane) and amide-nitrile mutual bonding [symmetry codes: (iv) Àx + 1 2 , y À 1 2 , z À 1 2 ; (v) Àx, Ày + 1, z + 1 2 ; (ix) x + 1 2 , Ày + 1 2 , z].

Figure 5
The hydrogen-bonding capacity of the (nccm) À anion. (a) Two kinds of mutual interactions marked in black and red and bonding with NH 4 + cations marked in blue; (b-d) three types of supramolecular synthons identified for the the title compound taking into account a set of strongest interactions: ammonium/nitroso chain (b), amide/amide chains (c) and mutual amide/nitrile bonding (d).

Synthesis and crystallization
The 2-cyano-2-isonitrosoacetamide H(nccm) was prepared by nitrosation of cyanoacetamide (Gerasimchuk et al., 2010). It is a relatively weak acid (pK = 5.03; Klaus et al., 2015) and therefore the compound NH 4 (nccm) is unstable, readily losing ammonia in air within a period of several days. When slowly evaporated, its aqueous or methanolic solutions lose ammonia first and then H(nccm) crystallizes.
For the preparation of the title compound, 0.339 g of H(nccm) (3 mmol) was dissolved in 10 ml of methanol at 303-313 K and 0.6 ml of 25% aqueous ammonia (8 mmol) were added to form a clear pale-yellow solution. It was placed, in an open vial, inside the larger stoppered flask containing mixture of 50 ml of 2-propanol and 1 ml of 25% aqueous ammonia. Slow interdiffusion of the solvents through the gaseous phase resulted in the precipitation of large pale-yellow NH 4 (nccm) crystals over a period of 30 d. The yield was 0.250 g (64%). Analysis (%) calculated for C 3 H 6 N 4 O 2 : C 27. 69, H 4.65, N 43.07; found: C 28.01, H 4. 85, N 42.68

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were located and then refined isotropically. Soft similarity restraints were applied to four N-H bond lengths and six H-N-H bond angles of the ammonium cations.    All H-atom parameters refined Á max , Á min (e Å À3 ) 0.19, À0.14 Computer programs: SMART-NT (Bruker, 1998), SAINT-NT (Bruker, 1999), SHELXS97 (Sheldrick, 2008), SHELXL2014/7 (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and WinGX (Farrugia, 2012 Data collection: SMART-NT (Bruker, 1998); cell refinement: SAINT-NT (Bruker, 1999); data reduction: SAINT-NT (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012). 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.