Structural characterization of two tetrachloridozincate salts of 4-carboxy-1H-imidazol-3-ium: a salt hydrate and a co-crystal salt hydrate

Two tetrachloridozincate salts of 4-carboxy-1H-imidazol-3-ium were structurally characterized. The first crystallizes with a water molecule of hydration and the second with a water of hydration and two equivalents of the zwitterion 4-carboxy-1H-imidazole per salt formula unit.

As a result of the myriad binding modes available to imidazole ligands that bear carboxylic acid substituents, they have found use in the preparation of several metal organic frameworks, MOFs (Starosta & Leciejewicz, 2006;Yin et al., 2009Yin et al., , 2012Sun & Yang, 2007;Sun et al., 2006). The synthesis and characterization of novel MOFs is an area of active research because of their potential use in such diverse areas as gas storage, catalysis, chemical sensors and molecular separation (Dey et al., 2014;Kreno et al., 2012;Farha & Hupp, 2010). Neutral carboxyimidazoles exist in their zwitterionic form and none of the reported compounds have the carboxyimidazole ligand in the fully protonated form. However, there are examples of MOFs with anionic repeating units and imidazolium cations (Shao & Yu, 2014;Wang et al., 2013). The molecular structure of (I), showing the atom-labeling scheme. Nonhydrogen anisotropic displacement parameters are drawn at the 50% probability level.

Figure 2
The molecular structure of (II), showing the atom-labeling scheme. Nonhydrogen anisotropic displacement parameters are drawn at the 50% probability level.
examples of tetrachloridozincate salts (Govindan et al., 2014a,b;Leesakul et al., 2012;Goh et al., 2012;Kefi et al., 2011). The same example structures exhibit Cl-Zn-Cl angles in the range 102.256 (10) to 112.72 (3) . The average angles found in (I) and (II) are 109 (2) and 109 (3) , respectively, and the individual values exhibit comparable ranges (Tables 1 and 2). There are no noteworthy differences in the C-C and C-N bond lengths between the ImHCO 2 H + cations and ImHCO 2 zwitterions found in (I) and (II). The N-C 0 bond length, where C 0 is the carbon atom bonded to both nitrogen atoms in a given ring (formally, the 2 position in the ring), is consistently shorter than the N-C 00 bond length, where C 00 represents the carbon in the formal 4 or 5 position in the ring, for all of the ImHCO 2 H + and ImHCO 2 residues. This observation is consistent with other reported imidazoles and imidazolium salts (e.g., Mohamed et al., 2014;Trifa et al., 2013;Ché rif et al., 2013;Yu, 2012;Zhu, 2012).
The carboxy and carboxylate groups are tilted slightly from the imidazole plane in all cases. The N-C-C-O torsion angles are reported in Tables 1 and 2. In both (I) and (II), the carboxy and carboxylate groups are unsymmetrical. For the carboxy groups, the C-OH bond is longer than the C O bond. These observations are consistent with the geometric parameters found in similar imidazolecarboxylic acids (Cao et al., 2012;Du et al., 2011;Guo, 2009). The observed O-C-O bond angles of the fully protonated form in (I) and (II) and the zwitterionic form in (II) are the same within the standard uncertainties of the refinement.

Figure 4
Packing diagram of (I), showing the hydrogen-bonding scheme. Only H atoms involved in the interactions are shown. Table 4 Hydrogen-bond geometry (Å , ) for (II). the chains (Fig. 4). N-HÁ Á ÁO(water), N-HÁ Á ÁCl, and O-HÁ Á ÁO hydrogen bonds incorporate the ImHCO 2 H + cations into the three-dimensional extended structure. Using graphset analysis to describe the hydrogen bonding (Etter et al., 1990), an R 4 4 (20) ring is observed with four oxygen acceptors, two oxygen donors and two nitrogen donors. One oxygen donor, two oxygen acceptor rings, R 2 1 (4), involving a carboxy group are also present.
Figs. 5 and 6 show two views of the crystal packing observed in (II). Hydrogen-bonding parameters are found in Table 4. As seen in Fig. 5, there are several hydrogen-bonding ring motifs that are common to (I) and (II). An R 4 4 (20) ring is observed with four oxygen acceptors, two oxygen donors and two nitrogen donors, and there is a one oxygen donor, two oxygen acceptor ring, R 2 1 (4), involving a carboxy group. R 2 2 (7) rings involving two nitrogen donors and two oxygen acceptors are also observed. There are two rings containing chlorine acceptor atoms: an R 4 4 (15) system with one oxygen donor, three nitrogen donors, one oxygen acceptor and three chlorine acceptors; and an R 2 1 (4) ring with a single oxygen donor and two chlorine acceptors. Similarly to (I), chains of hydrogenbonded tetrachloridozincate anions and water molecules of hydration are found parallel to [220].
In (I), a weakinteraction between ImHCO 2 H + cations related by a crystallographically imposed center of symmetry is observed with a centroid-to-centroid distance of 3.5781 (15) Å and an interplanar distance of 3.4406 (9) Å , corresponding to 0.983 Å slippage (Spek, 2009). Two independent weakinteractions between ImHCO 2 H + cations and ImHCO 2 zwitterions are observed in (II). The principal one involves the rings containing N1 and N7 with a centroidto-centroid distance of 3.5871 (3) Å , an interplanar distance of 3.3591 (18) Å and a dihedral angle of 2.6 (2) between rings (Spek, 2009). The weakerinteraction involves the rings containing N3 and N5 and has a centroid-to-centroid distance of 3.740 (3) Å , an interplanar distance of 3.3140 (17) Å and a dihedral angle of 1.2 (2) Å between planes. Fig. 7 shows representations of the observed stacking in which members of interacting pairs of molecules are projected into the same plane. Ainteraction is also observed in the solid-state structure of the ImHCO 2 zwitterion, labeled (III) (Cao et al., 2012). In (III), the centroid-centroid distance is longer [3.674 (4) Å ] than that observed between the fully protonated form in (I) and the principal interaction between the zwitterion and the protonated form in (II). In all of the pairs except in (I), the members of the pairs are arranged in a head-to-head configuration.

Figure 5
A view of the hydrogen bonding in (II), showing the ring motifs. Only H atoms involved in the interactions are shown. Only the major contributor to the disorder model of the water molecule is shown.

Figure 6
A second view of the hydrogen bonding in (II). Only H atoms involved in the hydrogen bonds are shown. Only the major contributor to the disorder model of the water molecule is shown.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. For (I), data completeness was 97.9% and for (II) it was 95.2%. For both (I) and (II), all hydrogen atoms were located in difference Fourier maps. The hydrogen atoms bonded to carbon were refined using a riding model with a C-H distance of 0.95 Å and hydrogen-atom isotropic displacement parameters were set using the approximation U iso (H) = 1.2U eq (C). The O-H and N-H distances were restrained to 0.84 and 0.88 Å , respectively. The isotropic displacement parameters of the hydrogen atoms bonded to nitrogen were set using the approximation U iso (H) = 1.2U eq (N). In (I), isotropic displacement parameters of the hydrogen atoms bonded to oxygen were refined freely, but for (II) they were set using the approximation U iso (H) = 1.5U eq (O). For (II), the water molecule is disordered over two positions. In addition to the aforementioned distance restraint,  For both compounds, data collection: APEX2 (Bruker, 2013); cell refinement: APEX2 (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2006); 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.