2,2′-(2,6-Pyridinediyl)diquinoline

The title molecule, C23H15N3, is a terpyridine derivative resulting from the Friedländer annulation between 2,6-diacetylpyridine and N,N′-bis(2-aminobenzyl)ethylenediamine. The asymmetric unit contains one half-molecule, the complete molecule being generated by a mirror plane (one N atom and one C atom lie on the plane). The molecule, although aromatic, is deformed from planarity as a result of crystal packing forces: molecules are stacked along the short c axis, with a short separation of 3.605 (1) Å between the mean planes. The bent molecular shape is reflected in the dihedral angle of 16.10 (5)° between the essentially planar quinoline groups. In addition to π⋯π interactions, the crystal structure features weak inter-stack C—H⋯N contacts involving atoms of the central pyridine rings which lie in a common crystallographic m plane.

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL-Plus (Sheldrick, 2008); program(s) used to refine structure: SHELXTL-Plus; molecular graphics: SHELXTL-Plus and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL-Plus. Thirty years after cisplatin was approved by the FDA for its use as a chemotherapy drug, studies regarding interactions between platinum-based complexes and basic sites in DNA remain actives. Recently, Bertrand et al. (2009) showed that Pt II cationic complexes bearing 2,2':6',2"-terpyridine or a terpyridine derivative as ligand have the ability to platinate the human telomeric G-quadruplex. Interestingly, both the binding affinity and the platination activity seem to be determined by the extension of the aromatic surface of the terpyridine derivative. One of the ligands used in that work was 2,2'-(2,6pyridinediyl)bis-quinoline, synthesized through the Friedländer condensation (Da Costa et al., 2009;Sridharan et al., 2009) between 2,6-diacetylpyridine and 2-nitrobenzaldehyde. We now report the crystal structure of this aromatic ligand.

Structure Reports Online
The title terpyridine derivative was obtained as a by-product during the preparation of a macrocyclic ligand (see Experimental). More suitable synthesis are however available in the literature (Harris et al., 1969;Klassen et al., 1975;Bertrand et al., 2009). The molecule ( Fig. 1) displays the crystallographic m symmetry, with atoms N1, C1 and H1A placed in the mirror planes normal to [010]. The molecular conformation observed in the solid-state is not suitable for coordination through the three N atoms: the quinoline N atoms are placed in a trans arrangement with respect to the central pyridine N atom, while a cis,cis conformation is required for the molecule to be a terdentate ligand. However, as invariably found in non-hindered terpyridine derivatives, aromatic fragments are free to rotate, for example about the C3-C4 bond in the case of the title molecule. Such a behavior has been reported, for example, for the coordination to Ru II of a closely related terpyridine ligand, namely 2,6-bis(5,6,7,8-tetrahydroquinol-2-yl)pyridine (Sasaki et al., 1998).
Molecules are stacked along the short axis c, at a distance of 3.605 Å (separation between two mean planes passing through two neighboring molecules in the [001] direction, see Fig.2, inset). This short separation, although larger than that observed in graphite (ca. 3.36 Å), results in strong π···π interactions in the stacks, which, in turn, deform the molecules from planarity. The dihedral angle between the central pyridine ring and the quinoline substituent is 8.13 (8)°. The bent shape is also reflected in the dihedral angle between quinoline systems, 16.10 (5)° (Fig. 2, inset). Finally, the crystal structure is completed by weak intermolecular C-H···N contacts (Table 1 and Fig. 2), linking the stacks in the [100] direction.

Experimental
A mixture of 305 mg of 2,6-diacetylpyridine and 823 mg of Ce(NO 3 ) 3 .6H 2 O in methanol (25 ml) was refluxed for 30 min, followed by slow addition of a dissolution of N,N'-bis(2-aminobenzyl)ethylenediamine (530 mg in 25 ml methanol). The mixture was kept under these conditions for 3.5 h, and then cooled to room temperature, giving a red precipitate. After 1.5 month, the resulting solid was filtered, washed with cold methanol, diethyl ether, and air dried. Suitable single crystals were picked off from the solid. m.p. 495-497 K (lit. 500-501 K: Klassen et al., 1975).
supplementary materials sup-2 Refinement All H atoms were placed in idealized positions, with C-H bond lengths fixed to 0.93 Å. Isotropic displacement parameters for H atoms were calculated from displacements of parent C atoms: U iso (H) = 1.2U eq (C). Fig. 1. The structure of the title compound, with displacement ellipsoids at the 30% probability level for non-H atoms. Non-labeled atoms are generated by symmetry code x, 1/2-y, z. . Two least-squares planes are represented (red), which were computed using all atoms in each selected molecule (Macrae et al., 2008). sup-5