Indole-3-thiouronium nitrate

In the title compound, C9H10N3S+·NO3 −, the indole ring system and the thiouronium group are nearly perpendicular, with a dihedral angle of 88.62 (6)°. Hydrogen bonding generates two-dimensional networks which are linked to each other via π stacking interactions of the indole groups [average inter-planar ring–ring distance of 3.449 (2) Å].

In the title compound, C 9 H 10 N 3 S + ÁNO 3 À , the indole ring system and the thiouronium group are nearly perpendicular, with a dihedral angle of 88.62 (6) . Hydrogen bonding generates two-dimensional networks which are linked to each other via stacking interactions of the indole groups [average inter-planar ring-ring distance of 3.449 (2) Å ].
Indole-3-thiouronium nitrate M. Lutz, A. L. Spek, E. P. L. van der Geer, G. van Koten and R. J. M. Klein Gebbink Comment Thiourea derivatives have found widespread application in molecular recognition and supramolecular chemistry, largely due to their hydrogen-bonding complementarity with carboxylate groups (Takemoto, 2005;Fitzmaurice et al., 2002;Schmidtchen & Berger, 1997). Of all thiourea derivatives, positively-charged thiouronium salts may be among the strongest anion receptors due to their increased acidity and the electrostatic stabilization of the anion-receptor complex (Esteban Gómez et al., 2005). Recently, we have demonstrated that N-substituted indole-3-thiouronium salts are readily available from indole by nucleophilic substitution at the nitrogen atom followed by electrophilic aromatic substitution with thiourea (van der Geer et al., 2007). In order to gain more insight into the hydrogen-bonding properties of indole-3-thiouronium salts, we have obtained the crystal structure of the title compound indole-3-thiouronium nitrate (I).
Bond distances and angles are as expected. The thiouronium group itself is planar, with the C-N bond lengths of 1.3076 (19) and 1.3162 (19) Å indicating a significant degree of double-bond character. Reflecting the resulting hindered rotation about the C-N bonds, solution-phase 1 H NMR shows separate signals for the thiouronium hydrogen atoms cis and trans to sulfur at room temperature. The least-squares plane of the thiouronium moiety forms an interplanar angle of 88.62 (6)° with respect to the least-squares plane of the indole group ( Fig. 1).
A thermal motion analysis using the program THMA11 (Schomaker & Trueblood, 1998) results in a low weighted R value (R = SQRT[(Σ (wΔU) 2 ) / (Σ (wU obs ) 2 )]) of 0.084 indicating that the molecule behaves as a rigid body in the solid state.
All N-H groups act as hydrogen bond donors with the oxygen atoms of the nitrate anion as acceptors. O1 and O2 accept two hydrogen bonds, respectively, while O3 accepts only one. By this hydrogen bonding scheme a two-dimensional network in the a,b-plane is formed (Fig. 2).
Via π stacking interactions the indole ring systems form parallel, centrosymmetric dimers with an average ring···ring distance of 3.449 (2) Å (Fig. 3). These π stacking interactions occur between the two-dimensional hydrogen bonded layers.
supplementary materials sup-2 Refinement 364 frames were collected as φ scans and 397 frames as ω scans with a rotation angle of 1° and an exposure time of 60 s, respectively. Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. C-H hydrogen atoms are omitted for clarity. Symmetry operations i: 1 − x, 1/2 + y, 0.5 − z; ii: 1/2 + x, y, 0.5 − z; iii: 2 − x, 1/2 + y, 0.5 − z. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq S1 0.69778 (