4,4′-Dimethyl-1,1′-(p-phenylenedimethylene)dipyridinium bis[7,7,8,8-tetracyanoquinodimethanide(1−)]

In the title salt, C20H22N2 2+·2C12H4N4 −, the cations and anions stack along the b axis into segregated columns. In the cation, which has a crystallographically imposed centre of symmetry, the dihedral angle between the benzene and pyridine rings is 89.14 (4)°. Centrosymmetrically related anions form dimers by π–π stacking interactions, with centroid–centroid separations of 3.874 (4) Å. The crystal packing is stabilized by intercolumnar C—H⋯N hydrogen bonds.

In the title salt, C 20 H 22 N 2 2+ Á2C 12 H 4 N 4 À , the cations and anions stack along the b axis into segregated columns. In the cation, which has a crystallographically imposed centre of symmetry, the dihedral angle between the benzene and pyridine rings is 89.14 (4) . Centrosymmetrically related anions form dimers by stacking interactions, with centroid-centroid separations of 3.874 (4) Å . The crystal packing is stabilized by intercolumnar C-HÁ Á ÁN hydrogen bonds.

Comment
The search for new compounds with promising electronic, and magnetic properties has prompted chemists to combine different spin carriers within the same molecular or supramolecular entity (Madalan et al., 2002). One of the most extensively used radicals in these studies has been the planar organic molecule 7,7,8,8-tetracyanoquinodimethane, [C 8 H 4 (CN) 4 ], TCNQ, since it shows a low reduction potential which makes it a suitable acceptor in charge-transfer processes. Another significant feature of this acceptor is its tendency to overlap its π-delocalized system with those of neighbouring molecules to form stacks with different degrees of electron delocalization (Alonso et al., 2005). Previous work has shown that molecular stacks of charge-transfer salts exhibit low-dimensional properties in some cases, which have intriguing anisotropic magnetic, electronic and structural characteristics (Ren, Meng et al., 2002;Ren et al., 2003;Liu et al., 2005). Furthermore, the size and shape of the counter-cations play an important role in determining the ground-state properties of the resulting materials (Ren, Chen et al., 2002;Liu et al., 2008). As a result, charge-transfer salts consisting of the TCNQ anion and benzylpyridinium cations could offer the possibility of systematically studying the fundamental relationship between the stack structure and the size and steric properties of substituent groups. In this communication, the crystal structure of the title complex is reported.
The asymmetric unit of the title compound contains a half of a (C 20 H 22 N 2 ) 2+ cation and one [C 8 H 4 (CN) 4 ]anion ( Fig.   1). Anions and cations stack into completely segregated columns along the b axis, as illustrated in Fig. 2. Within an anion column, [(TCNQ) 2 ] 2dimers are formed by π···π stacking interactions with a centroid-to-centroid distance of 3.874 (4) Å, and adjacent units are displaced relative to each other along the direction of the shorter molecular axis of TCNQ with centroid-to-centroid separations of 6.556 (4) Å (Fig. 3). The (C 20 H 22 N 2 ) 2+ cation affords a trans conformation, with a dihedral angle between the benzene ring and the pyridine rings of 89.14 (4)°. The crystal packing is stabilized by C-H···N intercolumar linkages (Table 1).

Refinement
H atoms were positioned geometrically, with C-H = 0.93, 0.97 and 0.96 Å for aromatic, methylene and methyl H atoms, respectively, and constrained to ride on their parent atoms, with Uiso(H) = xUeq(C), where x = 1.5 for methyl H and x = 1.2 for all other H atoms. Fig. 1. The asymmetric unit of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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.