2-(2-Pyridylamino)pyridinium tetrachloridozincate(II)

The structure of the title compound, (C10H10N3)2[ZnCl4], is composed of C10H9N3H+ (DPAH+) cations and [ZnCl4]2− anions. The two pyridyl rings of DPAH+ are approximately coplanar, with a dihedral angle of 7.2 (2)° between their corresponding least-squares planes. The proton is disordered in a one-to-one ratio over the two chemically equivalent pyridyl N atoms. An intramolecular hydrogen bond is formed between the pyridinium H atom and the pyridyl N atom of the other pyridyl ring. The Zn atom lies on a twofold rotation axis. There are also some weak N—H⋯Cl hydrogen bonds. These interactions lead to the formation of an alternating zigzag chain in the solid state. The results clearly show that reducing agents normally used in hydrothermal syntheses, such as metallic zinc employed here, are also active in terms of coordination chemistry.

The structure of the title compound, (C 10 H 10 N 3 ) 2 [ZnCl 4 ], is composed of C 10 H 9 N 3 H + (DPAH + ) cations and [ZnCl 4 ] 2À anions. The two pyridyl rings of DPAH + are approximately coplanar, with a dihedral angle of 7.2 (2) between their corresponding least-squares planes. The proton is disordered in a one-to-one ratio over the two chemically equivalent pyridyl N atoms. An intramolecular hydrogen bond is formed between the pyridinium H atom and the pyridyl N atom of the other pyridyl ring. The Zn atom lies on a twofold rotation axis. There are also some weak N-HÁ Á ÁCl hydrogen bonds. These interactions lead to the formation of an alternating zigzag chain in the solid state. The results clearly show that reducing agents normally used in hydrothermal syntheses, such as metallic zinc employed here, are also active in terms of coordination chemistry.

Comment
Core to the crystal engineering of supramolecular compounds are the different strategies that are used to control the molecular aggregation processes. For example, the selection of metal ions that promote the formation of extended arrays, and organic molecules capable of forming hydrogen bonding or close packed structures are fundamental to control the molecular aggregation (Guillon et al., 2000, Rice et al., 2002. Therefore, the strategies adopted in the preparation of crystal structures involve two main concepts. One of them is related with the use of the shape-controlled close packing of molecules, and the second one with the use of the specific interactions to control aggregation of molecular species (Guillon et al., 2000).
2,2-Dipyridylamine (DPA) is used as an organic linker presenting different coordination modes ranging from monodentate to chelating bidentate or bridging tridentate. Furthermore, this organic ligand contains the amino group and aromatic rings that may induce intermolecular hydrogen bonding and π-π stacking interactions that may lead to different structures. (Camus, et al. 2000, Bose et al., 2004, Rahaman et al.2005, Youmgme et al., 2005, Marinescu et al., 2005.
In this work we inform on the synthesis of a novel crystal structure with a tetrachlorozincate anion obtained by hydrothermal synthesis. The reaction conditions described in the experimental section lead to the oxidation of the metallic zinc used as the reducing agent and, to the formation of a crystalline tetrahalide species, which is present together with the cationic counterion, DPAH + .
As expected, the tetrachlorozincate anion, with the zinc atom lying on the twofold axis, displays an almost perfect tet- distortion from square planar geometry in this latter molecule has been addressed to the hydrogen bonding interactions (Willett, 1995).
As can be seen in Figure 1, there are two pairs of strictly parallel cations, which are separated by approximately 3.3 Å, a value that is in the range for π-π interactions (Marinescu et al., 2005), but somewhat shorter than the 3.534 (5) Å reported for (DPAH + )NO 3 - (Du & Zhao, 2004). Each pair of DPAH + cations is in connected with a neighboring pair in a "head to tail" fashion (see Figure 1), thus leading to a packing arrangement with a zigzag chain with alternating organic cations and tetrachlorozincate anions as seen in Figure 2. These chains interact in the solid by means of π-π contacts.

Experimental
The compound was obtained by hydrothermal synthesis employing the following reagents: CrCl 3 .6H 2 O, DPA, V 2 O 5 , Zn, H 3 PO 4 (85%) and H 2 O (all Aldrich, used without further purification) in the following quantities: 0.1523 g, 0.28 g, 0.1438 g, 0.1054 g, 0.58 ml and 5 ml respectively. This mixture was sealed in a 23 ml PTFE-lined stainless steel autoclave, and heated to 393 K (120°C) for 72 h. The yield of the studied compound was approximately 10%, and X-ray quality crystals were directly selected from the bulk mixture of products.

Refinement
The hydrogen atoms positions were calculated after each cycle of refinement with SHELXL (Bruker,1999) using a riding model with C-H distance equal to 0.96 Å. U iso (H) values were set equal to 1.2U eq of the parent carbon atom. At the final stages of refinement, the hydrogen bonded to the nitrogen atoms becomes evident in the Fourier Difference Map, with some disorder. This was modeled considering two half-occupied hydrogen sites, one on each pyridyl nitrogen atom. These N-H hydrogen atoms were then refined using a riding model with C-N-H angles idealized for amide H atoms, but the the N-H distances were allowed to refine freely.