A centrosymmetric monoclinic polymorph of N 1,N 4-bis(pyridin-3-ylmethylidene)benzene-1,4-diamine

The complete molecule of the title compound, C18H14N4, is generated by the application of a centre of inversion. The dihedral angle between the central benzene ring and the pyridine ring is 31.88 (7)°. In the crystal, molecules are stacked in columns along the c axis and several intermolecular π–π interactions are present between the six-membered rings, the shortest centroid–centroid distance being 3.937 (2) Å. The structure reported herein represents a centrosymmetric polymorph of the previously reported non-centrosymmetric (P21) form [Kim et al. (2005 ▶). Bull. Korean Chem. Soc. 26, 892–898].

The complete molecule of the title compound, C 18 H 14 N 4 , is generated by the application of a centre of inversion. The dihedral angle between the central benzene ring and the pyridine ring is 31.88 (7) . In the crystal, molecules are stacked in columns along the c axis and several intermolecularinteractions are present between the six-membered rings, the shortest centroid-centroid distance being 3.937 (2) Å . The structure reported herein represents a centrosymmetric polymorph of the previously reported non-centrosymmetric (P2 1 ) form [Kim et al. (2005). Bull. Korean Chem. Soc. 26,[892][893][894][895][896][897][898].

Comment
The title compound is a polydentate Schiff-base ( Fig. 1), which can act as a monodentate or bis(monodentate) ligand, that is, one N atom or two N atoms of the pyridyl groups can coordinate to a metal ion or metal ions. The crystal structure of the this compound was previously reported in the non-centrosymmetric space group P2 1 (Kim et al., 2005). The centrosymmetric structure presented here is essentially the same as the published structure and represents a new monoclinic polymorph. In the present study the compound is isomorphous with the analogous compound N 1 ,N 4 -bis(pyridin-2-ylmethylene)benzene-1,4-diamine (Chanda et al., 2002;Ball et al., 2004).
The asymmetric unit of the title molecule contains one half of the formula unit (Fig. 1); a centre of inversion is located in the midpoint of the compound, and therefore the two pyridyl rings are exactly parallel. The dihedral angle between the central benzene ring and the pyridine ring is 31.88 (7) °. The N2-C6/7 bond lengths and the C6-N2-C7 bond angle indicate that . In the crystal structure, the molecules are stacked in columns along the c axis and several intermolecular π-π interactions are present between the six-membered rings, with a shortest centroid-centroid distance being 3.937 (2) Å (Fig. 2).
Experimental 1,4-Phenylenediamine (1.0812 g, 9.998 mmol) and 3-pyridinecarboxaldehyde (2.1447 g, 20.023 mmol) in CH 3 CN (30 ml) were stirred for 3 h at room temperature. The precipitate was then separated by filtration, washed with CH 3 CN and ether, and dried under vacuum, to give a yellow powder (1.8899 g). Crystals suitable for X-ray analysis were obtained by slow evaporation from its ethylacetate solution.

Refinement
H atoms were positioned geometrically and allowed to ride on their respective parent atoms [C-H = 0.95 Å and U iso (H) = 1.2U eq (C)]. Fig. 1. The structure of the title compound, with displacement ellipsoids drawn at the 50% probability level; H atoms are shown as small circles of arbitrary radius. Unlabelled atoms are related by the symmetry transformation: -x, -y, 1 -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 > σ(F 2 ) is used only for calculating Rfactors(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.

Figures
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )