3,3′-({4-[(4,5-Dicyano-1H-imidazol-2-yl)diazenyl]phenyl}imino)dipropionic acid

The title compound, C17H15N7O4, is a push–pull non-linear optical chromophore containing a dialkylamino donor group and the dicyanoimidazolyl acceptor separated by a π-conjugated path. The benzene and imidazole rings are not coplanar, making a dihedral angle of 10.0 (2)°. In the crystal, molecules are linked by an extended set of hydrogen bonds and several motifs are recognized. Pairs of molecules are held together by hydrogen bonding between carboxy O—H donor groups and diazenyl N-atom acceptors, forming R 2 2(24) ring patterns across inversion centres. Four-molecule R 4 4(28) ring motifs are formed, again across inversion centres, through hydrogen bonding involving carboxy O—H donor groups and diazenyl and imidazole N-atom acceptors. Four-molecule R 4 4(42) patterns are formed among molecules related by translation and involve carboxy O—H and imidazole N—H donor groups with carbonyl O-atom and imidazole N-atom acceptors.

The title compound, C 17 H 15 N 7 O 4 , is a push-pull non-linear optical chromophore containing a dialkylamino donor group and the dicyanoimidazolyl acceptor separated by aconjugated path. The benzene and imidazole rings are not coplanar, making a dihedral angle of 10.0 (2) . In the crystal, molecules are linked by an extended set of hydrogen bonds and several motifs are recognized. Pairs of molecules are held together by hydrogen bonding between carboxy O-H donor groups and diazenyl N-atom acceptors, forming R 2 2 (24) ring patterns across inversion centres. Four-molecule R 4 4 (28) ring motifs are formed, again across inversion centres, through hydrogen bonding involving carboxy O-H donor groups and diazenyl and imidazole N-atom acceptors. Four-molecule R 4 4 (42) patterns are formed among molecules related by translation and involve carboxy O-H and imidazole N-H donor groups with carbonyl O-atom and imidazole N-atom acceptors.
The rationalization of the local packing modes of chromophore units (Thallapally et al., 2002;Centore & Piccialli, 2012;Centore, Piccialli & Tuzi, 2013) is another crucial point, because many properties required for optimum device performances (e. g. electron mobility) critically depend on the packing not less than on strictly molecular properties. In our research group we are interested in the synthesis of new heterocyclic compounds, including metal containing heterocyclic compounds (Takjoo et al., 2011;Takjoo & Centore, 2013), for applications as advanced materials and bioactive compounds , and in the analysis of crystal structures controlled by the formation of H bonds (Centore, Jazbinsek et al., 2012;Centore, Fusco, Jazbinsek et al., 2013). Following these issues, we report, in the present paper, the structural investigation of the title compound, shown in the Scheme. The title compound is a typical push-pull azo-dye, containing the dialkylamino as donor group and two cyano acceptor groups. Moreover, the cyano groups are attached to an electron poor imidazole ring. The chromophore unit has been used in the synthesis of polymers showing quadratic NLO behaviour (Carella, Centore, Sirigu et al., 2004).
The molecular structure is shown in Fig. 1. The geometry around the donor N1 atom is substantially planar indicating sp 2 hybridization (the sum of valence angles at N1 is 360°) and the pattern of bond lenghts within the adjacent phenyl ring shows a certain degree of quinoidal character. All these structural features are in accordance with the expected π conjugation and push-pull character of the chromophore group.
The two aromatic rings are not coplanar, the dihedral angle between the mean planes being 10.0 (2)°; the π-conjugated part of the molecule has a slighlty curved shape, as the result of small torsions around the bonds C10-N2, N2-N3 and N3-C13.
The molecules of the title compound have several H bonding donor and acceptor groups, and the crystal packing is dominated by the formation of H bonds, Table 1. Several H bonding motifs are recognized in the crystal packing (Allen et al., 1999;Steiner, 2002) and some of them are shown in Fig. 2

Refinement
The H atoms of the carboxy groups and of the imidazole ring were located in difmaps and their coordinates were refined.
All other H atoms were generated stereochemically and were refined by the riding model. For all H atoms U iso =1.2×U eq of the carrier atom was assumed.

Computing details
Data collection: MACH3/PC Software (Nonius, 1996); cell refinement: CELLFITW (Centore, 2004); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012).  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.30 e Å −3 Δρ min = −0.33 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 R-factors(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. Several crystal specimens were tested but their quality was, in general, rather poor, as witnessed by the relatively high fraction of low intensity reflections. The poorly diffracting nature of the crystals is the reason for the relatively high R factors.