2,4-Dimethyl-1,3-thia­zole-5-carb­oxy­lic acid: an X-ray structural study at 100 K and Hirshfeld surface analysis

Non-covalent molecular interactions, in particular hydrogen bonds, play an important role in the field of crystal engineering (Desiraju, 1996). It is well known that electronegative atoms such as N and O can form strong hydrogen bonds, D—H···A (where D is a donor and A an acceptor), with an estimated interaction energy in the range of 15–60 kJ mol^-1 (Aburaya et al., 2008). These hydrogen bonds often lead to the self-assembly of molecules, whereby supramolecular structures are built from a relatively small building block or synthon with simple aromatic functionalities. In recent decades, moderately strong hydrogen bonds (energy range 4–15 kJ mol^-1) and weaker interactions (usually ≤ 4 kJ mol^-1), such as C—H···O, C/N—H···π and S···S interactions, have also received much attention due to their ability to form supramolecular motifs in molecular solids (Desiraju, 2011; Arunan et al., 2011). These interactions are individually weaker and geometrically less well defined, but their combined effect can be just as important as that of strong interactions (Desiraju & Steiner, 1999). In order to improve the reliability of studies of weak intermolecular interactions, accurate positions for the H atoms in the molecule are necessary, and consequently the X-ray structure analysis should be carried out using a high-quality data set collected at a sufficiently low temperature. In this context, the small molecule methylthiazolecarboxylic acid [Should this not refer to 2,4-dimethylthiazole-5-carboxylic acid?], possessing O—H and C—H donors and a carboxyl C═O acceptor, constitutes an ideal structural component with a rigid core. A search of the Cambridge Structural Database (CSD, Version 5.33, November 2011 release; Allen, 2002) for simple aromatic thiazolecarboxylic acids returned only 5 hits (excluding duplicate structure determinations), with refcodes BETKUH (Curtis et al., 2004; CCDC No. 237120), BUFMEV (Quan et al., 2009; CCDC No. 751282), IYULUJ (Fong et al., 2004; CCDC No. 219844), JEVCOD (Chen et al., 2007; CCDC No. 638357) and ONOHEF (Rossin et al., 2011; CCDC No. 781928) (see scheme). In order to analyse the role of intermolecular O—H···N and C—H···O hydrogen bonds in building any possible supramolecular assembly, the structural characterization of 2,4-dimethylthiazole-5-carboxylic acid, (I), has now been undertaken at 100 K. An investigation of the close intermolecular interactions in (I) and in some related thiazolecarboxylic acid derivatives via Hirshfeld surface analysis is also presented.

The molecule of (I) (Fig. 1), containing a thiazole ring (S1/C1/C2/N1/C3), is essentially planar, with r.m.s. fits of the atomic positions (excluding atom C6) of 0.011 Å; the exocyclic methyl atom C6 deviates from the least-squares plane through atoms S1/C1–C5/N1/O1/O2 by 0.087 (2) Å. The C1—S1 [1.7231 (10) Å] and C3—S1 [1.7148 (11) Å] bond lengths indicate single-bond character (Allen et al., 1987). The C3—N1 [1.3180 (13) Å] and C2—N1 [1.3874 (13) Å] (Table 1) distances agree well with the mean values for C═N [1.31 Å] and C—N [1.38 Å] bond lengths based on 193 –Csp²═N—Csp²– fragments obtained using Mogul (Bruno et al., 2004) for a search of the CSD for related organic compounds. The lengthening of the C1—C2 bond [1.3748 (13) Å] in (I) compared with the average Csp²(cyclic)═Csp²(cyclic) distance of 1.35 Å calculated using 52770 organic structures in the CSD is a consequence of extended π-conjugation between the thiazole ring and the carboxylic acid fragment.

In the crystal structure of (I), a combination of O—H···N and C—H···O hydrogen bonds and an S···S interaction (Table 2) generates a supramolecular framework. Considering the criteria X···A < 3.0 Å, H···A < 2.2 Å and 150 < X—H···A < 180°, the observed O—H···N interaction in (I) can be classified as a strong hydrogen bond (Desiraju & Steiner, 1999), whereas the corresponding C—H···O interactions can be termed weak hydrogen bonds (Desiraju & Steiner, 1999) on the basis of the criteria 3.0 < X···A < 3.5 Å, 2.2 < H···A < 2.8 Å and 120 < X—H···A < 180°. The O1—H1···N1(x + 1, -y + 3/2, z - 1/2) and C6—H6A···O2(x - 1, -y + 3/2, z + 1/2) hydrogen bonds connect the molecules of (I) into an R₂²(8) graph-set dimer (Bernstein et al., 1995). Adjacent R₂²(8) rings (marked A in Fig. 2) are linked to form a one-dimensional C₁¹(6)R₂²(8) polymeric chain running along the [201] direction. Further linking of parallel chains via intermolecular C5—H5A···O2(x - 1, y, z) and C6—H6B···O2(x, -y + 3/2, z + 1/2) hydrogen bonds generates two R₃²(10) and R₃³(15) rings, labelled B and C, respectively, in Fig. 3. The resulting molecular assembly can be visualized as a two-dimensional framework built from edge-fused ABCABC– synthons (Fig. 4). Additional reinforcement within the framework is provided by a non-bonded S···S interaction with an interatomic separation of 3.4685 (5) Å. Similar S···S interactions have been reported for related sulfur-containing compounds (Ksiazek et al., 2009).

The Hirshfeld surface (Spackman & Jayatilaka, 2009) and associated two-dimensional fingerprint plot (Spackman & McKinnon, 2002) for (I) were calculated using CrystalExplorer (Wolff et al., 2007) and are illustrated in Fig. 4. The dominant interactions between the carboxylic acid OH group and the thiazole ring N atom can be seen in the Hirshfeld surface as bright-red areas marked a and b in Fig. 4(a). The pale-red spots labelled c, d, e and f are due to C—H···O interactions (Fig. 4a). Other visible spots in Fig. 4(a) correspond to H···S and S···S contacts. In the corresponding two-dimensional fingerprint plot (Fig. 4b), two pairs of sharp spikes labelled a and b, and c and d, are characteristic of a cyclic hydrogen-bonded R₂²(8) synthon. The difference between the lengths of the outer pair of spikes, a and b, in the region 1.7 < d_e + d_i < 2.7 Å, and the inner pair of spikes, c and d, in the region 2.2 < d_e + d_i < 2.7 Å, is a consequence of different types of hydrogen bonds (O—H···N and C—H···O) involved in building the R₂²(8) synthon. The upper spikes a and c (Fig. 4b) represent the donor spikes (carboxylic acid and methyl H atoms interacting with the N atom of the thiazole ring and the O atom of the COOH group), and the lower spikes b and d the acceptor spikes (thiazole N and carboxylic acid O atoms interacting with the H atoms of the COOH and CH₃ fragments). The wings (Fig. 4b, marked with black circles) in the (d_i, d_e) regions of (1.9 Å, 1.2 Å) and (1.2 Å, 1.9 Å) are attributable to the S···H interactions. The density of points in the (d_i, d_e) regions of (1.7 Å, 1.7 Å), marked g in the fingerprint plot (Fig. 4b), indicates the presence of S···S contacts.

The relative contributions of different interactions to the Hirshfeld surface were calculated for (I) and for a few thiazolecarboxylic acid derivatives (Fig. 5) retrieved from the CSD. Due to differences in the number and type of substituents on the thiazolecarboxylic acid core in the structures used in Fig. 5, they are not directly comparable across the compounds, but it offers some insight into the effect of different substituents on the thiazolecarboxylic acid backbone. The H···H, N···H and O···H contacts contribute about 50–70% of the Hirshfeld surface area in (I) (65.1%), BETKUH (62.7%), IYULUZ (66.5%), JEVCOD (52.3%) and ONOHEF (52.7%); the remaining contributions were distributed among the S···H, C···H, C···C, C···N and C···O interactions. With an additional CH₃ substitution at the thiazole ring 4-position in (I) compared with IYULUJ, the contribution of the S···H contacts to the Hirshfeld surface increases significantly, from 1.4% in IYULUJ to 13.6% in (I), with a corresponding decrease in the N···H interactions, from 13.1% in IYULUJ to 7.3% in (I). Although (I) and BUFMEV differ only in terms of substitution at the thiazole 4-position [CH₃ in (I) and CF₃ in BUFMEV], the contributions of various contacts to the Hirshfeld surfaces of the two compounds are widely different. In BUFMEV, intermolecular contacts involving F atoms can account for 52.4% of the Hirshfeld surface area, while the corresponding contribution of H···H, N···H and O···H contacts combinedr is only 26.5%. The variation in the contributions of different interactions to the Hirshfeld surfaces of BUFMEV and other compounds used in Fig. 5 is a consequence of differences in the stereoelectronic properties of C—H and C—F bonds, due to the highly polar nature of the latter (O'Hagan, 2008).