Crystal structure of tetramethylammonium 1,1,7,7-tetracyanohepta-2,4,6-trienide

In the crystal, C—H⋯N(nitrile) short contacts and stacking interactions combine to link the anions into layers parallel to the (01) plane and separated by columns of tetramethylammonium cations.


Chemical context
Polymethines, being fully conjugated hydrocarbons, represent the simplest 'molecular wires' with potential uses in organic electronic applications thanks to their easily tuned band gaps, and their wide range of absorption covering the visible spectrum (Etemad & Heeger, 1982;Meisner et al., 2012;Jayamurugan et al., 2014). Crystallographic data for polymethines are rather scare because of their instability and low solubility (Chetkina & Bel'skii, 2002;Meisner et al., 2012;Tsuji & Hoffmann, 2016). A successful strategy to increase the chemical stability with respect to oxidative decomposition has been reported (Meisner et al., 2012) that includes the decoration of polyenes with cyano groups and which resulted in the synthesis of a library of odd-numbered members from three to thirteen linear conjugated olefins and the determination of their crystal structures.

Structural commentary
The title compound (Fig. 1) crystallizes with one cation and one anion per asymmetric unit, both entities residing in general positions. The trimethylammonium cation has a common tetrahedral geometry (2460 hits for this cation in CSD version 5.40, last update November 2018; Groom et al., 2016), with three of the four methyl groups being disordered (see Refinement). In the linear anion, the bond lengths vary in the narrow range 1.382 (2)-1.394 (2) Å , thus indicating a significant degree of conjugation along the hydrocarbon chain. Such a structural and electronic configuration in which the difference in bond lengths along the conjugated backbone approaches zero is known as a cyanine-like structure (Marder et al., 1994). The anion is slightly distorted from a planar arrangement as shown by the r.m.s. deviation of 0.098 Å for non-hydrogen atoms from the least-square plane calculated through the entire carbanion. The dihedral angles between the perfectly planar terminal dicyano-groups, C(CN) 2 and the linear C4-C8 central fragment in the anion are 6.1 (2) and 7.1 (1) . The bond lengths and angles and the overall conformation of the anion are close to those reported for the same anion in N-(7-(dimethylamino)hepta-2,4,6-trienylidene)-N,Ndimethylammonium 1,1,7,7-tetracyanohepta-2,4,6-trienide (NEQHOH; Reck & Dahne, 2006), and for its dicyano derivative, 1,1,2,6,7,7-hexacyanoheptatrienide in the ammonium salt (Edmonds et al., 1970).

Supramolecular features
In the crystal, anions related by the twofold screw axis are linked by C4-H4Á Á ÁN3 i short contacts (Table 1), forming zigzag chains along the [101] direction in which adjacent molecules have a nearly orthogonal arrangement, as indicated by the dihedral angle between their skeletons of 87.62 . The antiparallel chains stack along the [110] direction with alternating separations between neighboring anions in the stacks of 3.291 and 3.504 Å . The C-HÁ Á ÁN short contacts (Table 1) and stacking interactions of 3.291 and 3.504 Å combine to form layers of anions parallel to the (101) plane and separated by columns of tetramethylammonium cations (Fig. 2). A similar arrangement with separation of the anionic and cationic regions was noted in the crystal structure of tetramethylammonium 1,1,2,4,5,5-hexacyanopentadienide (HXCPEN; Sass & Nichols, 1974 Symmetry codes: (i) x þ 1 2 ; Ày þ 3 2 ; z þ 1 2 ; (ii) x þ 1 2 ; Ày þ 1 2 ; z þ 1 2 .

Figure 2
The crystal packing in the title compound showing (a) supramolecular anionic chains with C-HÁ Á ÁN interactions packed in a layer parallel to the (101) plane and (b) the packing. The minor disorder component is omitted for clarity.

Figure 1
The formula unit of the title compound with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Only the major components of the disordered methyl groups in the cation are shown.

Synthesis and crystallization
The synthesis is shown in Fig. 5.
Crystallization. Crystals of the title compound were grown over a period of 2-4 weeks by the vapour-diffusion method using dichloromethane as the solvent and hexane as the nonsolvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were fixed geometrically (C-H = 0.95-0.98 Å ) and refined using a riding model, with U iso (H) set to 1.2U eq (C) for aromatic and 1.5U eq (C-methyl). To obtain an idealized geometry of the cation, 1,2 and 1,3 restraints for C-N mean bond distances and C-N-C bond angles were used. In the tetramethyl ammonium cation, three methyl groups are each disordered over two positions about the N5-C12 axis and were refined with partial occupancies of 0.66 (1)  Chemical structure of AHCNPI.

Figure 4
Chemical structure of NEQHOH.

Figure 5
Synthesis of the title compound. of all disordered atoms were refined in an isotropic approximation.

Funding information
Funding for this research was provided by: National Science Foundation (grant No. DMR-1523611).  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.