2-(Pyridin-2-ylamino)pyridinium thiocyanate acetonitrile monosolvate

The title compound, C10H10N3 +·NCS−·CH3CN, is the acetonitrile solvate of the thiocyanate salt of protonated dipyridin-2-ylamine. Protonation occurs at one of the pyridine N atoms. The molecular geometry around the central N atom is essentially planar (sum of angles = 359.89°). In the crystal, N—H⋯N hydrogen bonds, as well as C—H⋯S contacts link the different residues into chains along the c-axis direction. Interaction between aromatic systems gives rise to π-stacking, the shortest distance between two π-systems being 3.6902 (6) Å. Both the protonated and the non-protonated pyridyl groups are involved in the latter interaction.


Comment
Chelate ligands have found widespread use in coordination chemistry due to the enhanced thermodynamic stability of resulting coordination compounds as compared to that of the complexes with exclusively monodentate ligands (Gade, 1998).
Combining different sets of donor atoms in one chelate ligand molecule, allows to construct a probe for testing and accommodating metal centers of different Lewis acidities. In our efforts to synthesize a chelate ligand featuring a set of oxygen, sulfur and nitrogen as possible donor atoms, a crystalline reaction product was obtained whose crystal structure analysis revealed the unintentional synthesis of a salt of the starting material, dipyridin-2-ylamine. The crystal structure of free dipyridin-2-ylamine has been reported earlier (e.g. Johnson & Jacobson, 1973;Pyrka & Pinkerton, 1992;Schödel et al., 1996).
The studied compound was proved to be the thiocyanate salt of protonated dipyridin-2-ylamine ( Fig. 1). Protonation occurs at one of the pyridine nitrogen atoms. The central nitrogen atom has a nearly trigonal-planar molecular geometry with H-N-C angles of 115.3 (9) °, 117.0 (9) ° and C-N-C angle of 127.59 (8) °. The aromatic systems are nearly coplanar, the least-squares planes defined by their respective atoms form very small dihedral angle of 1.99 (4) °. These observations are in good agreement with the geometrical parameters reported for similar compounds such as the chloride (Bock et al., 1998), the bromide (Junk et al., 2006) or the nitrate (Du & Zhao, 2004). In contrast to unprotonated dipyridine-2-ylamine, the title compound features the aromatic-ring-containing entity in a conformation with the pyridine nitrogen atoms facing each other. In addition, the pyridine moieties in neutral dipyridine-2-ylamine usually form dihedral angles well above 20 °( Johnson & Jacobson, 1973); this difference is most probably due to the formation of an intramolecular hydrogen bond in the molecule of the title compound ( Fig. 2; vide infra).
In the crystal structure, both nitrogen-bound hydrogen atoms take part in hydrogen bonds. While the nitrogen atom of the acetonitrile molecule serves as acceptor for the hydrogen bond originating from the protonated pyridyl moiety, the nitrogen atom of the thiocyanate anion serves as acceptor for the hydrogen bond involving the central NH group. Apart from these hydrogen bonds, the C-H···S contact, which is about 0.10 Å shorter than the sum of van-der-Waals radii of the corresponding atoms, exists in the crystal. These contacts originate from the H atom in ortho-position to the nitrogen atom in the protonated pyridyl moiety. As a result, all chemical residues of the crystal structure end up being linked into the infinite chains running along the crystallographic c axis (Fig. 2). In terms of graph-set analysis, (Etter et al., 1990;Bernstein et al., 1995), the descriptor for the hydrogen bonding system on the unitary level is DD while the C-H···S contacts necessitate a D descriptor on the same level. Interaction between aromatic systems gives rise to π-stacking. The shortest intercentroid distance between two π-systems was measured at 3.6902 (6) Å and involves the protonated as well as the non-protonated pyridyl moiety.
These connect the molecules into stacks along the a axis. The packing of the cystal of the title compound is shown in Fig. 3.

Experimental
The compound was prepared upon reacting of 4-bromobenzyl chloride (2.5 mmol) with potassium thiocyanate (2.5 mmol) and dipyridin-2-ylamine (2.5 mmol) in refluxing acetonitrile (15 mL) under nitrogen for two hours. Crystals suitable for the X-ray diffraction study were obtained upon free evaporation of the reaction mixture.  Fig. 1. The molecular structure of the title compound; displacement ellipsoids are drawn at 50% probability level. Fig. 2. Intra-and intermolecular contacts, viewed along the a-axis. Symmetry operators: (i) x, y, z + 1; (ii) x, y, z -1. Hydrogen bonds are indicated with green dashed lines, the C-H···S contacts are drawn as red dashed lines. Fig. 3. Molecular packing of the title compound, viewed along the a-axis (anisotropic displacement ellipsoids drawn at 50% probability level).  (12)