Crystal structure of 2-[(naphthalen-2-yl)methyl]isothiouronium bromide

The crystal structure of the title salt is held by N—H⋯Br charge-assisted hydrogen bonds and dipole–dipole interactions. In comparison with reference structures, elongation of C—S bonds is observed.

Herein we report the crystal structure of 2-[(naphthalen-2-yl)methyl]isothiouronium bromide, C 12 H 13 N 2 S + ÁBr À , which crystallizes in the monoclinic P2 1 /c centrosymmetric space group. The asymmetric unit contains one 2-[(naphthalen-2-yl)methyl]isothiouronium cation and one bromide anion. The methylene carbon lies in plane of the naphthalene core. In comparison with reference structures, elongation of C-S bonds as well as tilting of the isothiouronium group is observed. Given the ionic nature of the compound, the structure is held by charge-assisted N-HÁ Á ÁBr hydrogen bonds, with a noteworthy contribution of dipole-dipole interactions, which form bilayers in the structure. The bilayers are held by the weak London forces.

Structure description
Isothiouronium salts have been investigated because of their ability to bind anions by charge-assisted hydrogen bonds (Yeo & Hong, 1998;Seong et al., 2004). Despite their potential in crystal engineering, only 23 crystal structures of 2-(arylmethyl)isothiouronium salts are present in the CSD (Groom et al., 2016). In our studies of isothiouronia, we have managed to synthesize and crystallize naphthalene-2-ylmethyl-bearing isothiouronium bromide, and we report here its structure and a comparison with similar crystal structures.
The title compound crystallizes in the monoclinic P2 1 /c centrosymmetric space group with one 2-[(naphthalen-2-yl)methyl]isothiouronium cation and one bromide anion in the asymmetric unit, see Fig. 1. The naphthalene core is almost perfectly planar, with a maximum deviation of 0.026 (3) Å for atom C6. The C11 atom can be considered to be in the plane of naphthalene core, with a deviation of 0.039 (3) Å from the mean plane. The single bonds around carbon C11 and S1 allow for the free rotation of the isothiouronium group.
The C11-S1 bond is almost perpendicular to the naphthalene core, with C1-C2-C11-S1 and C3-C2-C11-S1 torsion angle values of 91.8 (3) and À88.0 (3) , respectively. Among the reference group, such a conformation is unusual, with average values being 61.45 and 120.53 . The isothiouronium group is significantly tilted, with C11-S1-C12-N1 and C11-S1-C12-N2 torsion angles of À68.5 (3) and 110.7 (3) , respectively. Such a tilt is not observed among the reference group, where the average values are 20.46 and 160.94 . The tilting of the group can be explained by the steric demands of the 2-naphthylmethyl unit on the packing.

Figure 2
The charge-assisted hydrogen bonds in the title compound. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. Symmetry codes:

Figure 3
The bilayers formed in the title compound by charge-assisted hydrogen bonds, depicted in red, and dipole-dipole interactions, depicted in green. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity.

Figure 1
The title compound showing the numbering scheme with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius.

Synthesis and crystallization
Thiourea (0.23 g, 3 mmol) was dissolved in 25 ml of anhydrous acetonitrile. The solution was then treated with 2-(bromo-methyl)naphthalene (0.55 g, 2.5 mmol). The reaction mixture was stirred for 4 h at room temperature. The resulting white precipitate was filtered and washed with diethyl ether and left to dry at room temperature, resulting in yield 0.72 g (97%) of 2-[(naphthalen-2-yl)methyl]isothiouronium bromide.