Crystal structure of N′-[2-(benzo[d]thiazol-2-yl)acetyl]benzohydrazide, an achiral compound crystallizing in space group P1 with Z = 1

The title compound crystallizes in space group P1 despite being achiral. Two classical hydrogen bonds link the molecules to form a layer structure.


Chemical context
Heterocycles represent a link between organic synthesis and pharmaceutical chemistry, thereby encouraging researchers to discover new hetereocyclic drug candidates. One of the most prominent heterocycles is benzothiazole, a privileged scaffold in the field of synthetic and medicinal chemistry (Elgemeie et al., 2000a,b). Its derivatives and metal complexes possess a wide range of pharmacological properties and a high degree of structural diversity that have proved vital for the investigation for novel therapeutics (Elgemeie et al., 2020;Gill et al., 2015). The carbon atom C2 (standard numbering; the carbon atom between nitrogen and sulfur) is the most attractive site both from a synthetic and medicinal point of view (Azzam et al., 2020a,b). As structure-activity relationships have shown, changes in the substituent at C2 can induce marked changes in the biological activity (Azzam et al., 2017a,b). Numerous biologically potent molecules containing 2-substituted benzothiazole scaffolds have extensive biological applications (Keri et al., 2015), such as anti-microbial (Kö nig et al., 2011), anti-malarial (Bowyer et al., 2007) and anti-inflammatory (Wang et al., 2009). Among the 2-substituted benzothiazoles, 2-aryl benzothiazoles are versatile scaffolds that have major biological and industrial applications (Kamal et al., 2011). Part of our research has therefore concentrated on the synthetic pathways of 2-arylbenzothiazoles (Azzam et al., 2019;Elgemeie & Elghandour, 1990). Recently, we contributed to current progress in the manufacturing and biological estimation of 2-aryl, 2-pyridyl and 2-pyrimidylbenzothiazoles and other antimetabolites as potent chemotherapeutic agents (Azzam et al., 2020c;Metwally et al., 2021). Here we deal with synthetic approaches to the new compound N 0 -(2-(benzo-[d]thiazol-2-yl)acetyl)benzohydrazide (3). Compound 3 was prepared by the reaction of 2-(benzo[d]thiazol-2-yl)acetohydrazide (2) with benzoyl chloride in the presence of pyridine at room temperature. The structure of 3 was initially determined on the basis of spectroscopic data and elemental analysis. In order to establish the structure of the product unambiguously, its crystal structure was determined and is presented here.

Structural commentary
The structure determination confirms the formation of compound 3 (Fig. 1). Bond lengths and angles may be regarded as normal (Allen et al., 1987); a selection is presented in Table 1. The geometry at the hydrazinic nitrogen atom N1 is essentially planar, but N2 is slightly pyramidalized [angle sum 355 (2) ; the nitrogen atom lies 0.15 (1) Å out of the plane of its substituents]. The general shape of the molecule is defined by the torsion angles along the atom chain S1-C2-C8-C9-N1-N2-C10-C11-C12, which are also given in Table 1; in particular, the torsion angle about the hydrazine N1-N2 bond is 66.44 (15) [cf. H01-N1-N2-H02 101 (3) ]. Each hydrazinic hydrogen atom lies antiperiplanar to a carbonyl oxygen atom across the respective N-C bond. The interplanar angle between the benzothiazol group and the phenyl ring is 75.65 (3) .

Figure 2
Packing diagram of compound 3 viewed perpendicular to the ab plane. Dashed lines represent classical hydrogen bonds. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. Selected atoms of the asymmetric unit are labelled.

Database survey
A database search (CSD Version 5.41) for other structures containing the same benzothiazol-acetylhydrazide moiety gave only one hit, refcode JEBQOZ, with a p-tosylate group replacing the benzoyl group of 3; this was our previous publication (Azzam et al., 2017b). There are major conformational differences between the two structures, e.g. the C-C-C( O)-N torsion angle of JEBQOZ is À109.79 (19) in contrast to À152.41 (11) in the title structure. The average database bond lengths C2-S and C2-N for the benzo[d]thiazole ring system were calculated; for 444 hits (600 different molecules) the values were 1.750 (16) and 1.300 (29) Å , respectively, virtually unchanged from the values we obtained previously (Azzam et al., 2017b); however, we regret having mistyped the latter value as 1.200.
Anecdotal evidence, combined with previous experience, would suggest that it is unusual for an achiral compound to crystallize in space group P1, which may be considered as a moderately rare space group; of the over 1.1 million structures in the Cambridge database, only 9843 are in P1 (8832 with coordinates available, 6730 of these without disorder).
We therefore wished to see how many of the P1 structures in the CSD, particularly those with Z = 1, were achiral. Unfortunately, there is at present no means of identifying, labelling and searching for chirality or chiral ('asymmetric') atoms using the standard ConQuest search routines, and it is clearly unfeasible to check all the P1 structures by hand. We therefore began by simply considering the small and possibly non-representative subset of 20 P1 structures (13 with Z > 1) that were determined by PGJ. Of these, 14 were pure enantiomers; for 12 of these, the absolute configuration was determined. Of the remaining six, five were not organic compounds [two metal complexes with Z = 1 (Jones et al., 1996;Filimon et al., 2014), two organotellurium compounds (Jones et al., 2015, Z = 1;du Mont et al., 2010, Z = 4), and one phosphane sulfide (Taouss & Jones, 2013, Z = 2)], and the remaining structure (Focken et al., 2001, Z = 4) displayed planar chirality, but contained no 'asymmetric' atom. On this limited basis, we would therefore postulate that is very rare for achiral organic compounds to crystallize in P1, especially with Z = 1. An extension of this survey to all P1 structures in the CCDC is being planned.

Synthesis and crystallization
A mixture of 2-(benzo[d]thiazol-2-yl)acetohydrazide 2 (0.08 mol) and pyridine (10 mL) was stirred for 15 min at room temperature. Benzoyl chloride (0.16 mol) was then added gradually to the reaction mixture, which was stirred for 15 min at 273 K. The reaction mixture was left to stand at room temperature for another 3 h, then poured onto ice water and neutralized with HCl. The precipitate thus formed was filtered off and dried to produce a white solid product 3.  122.5, 122.8, 125.5, 126.5, 127.9, 128.9, 132.4, 132.8, 136.9, 152.7, 165

Refinement
Crystal data, data collection and structure refinement details are summarized in

N′-[2-(Benzo[d]thiazol-2-yl)acetyl]benzohydrazide
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. Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane) 3.5462 (0.0009) x + 3.6978 (0.0019) y -0.2128 (0.0045) z = 6.1650 (0.0025) * -0.0069 (0.0007) S1 * 0.0023 ( x y z U iso */U eq S1 0.88408 (5) (17)