4-Bromo-N,N′-bis(4-methoxyphenyl)benzamidine

The title compound, C21H19BrN2O2, is an amidine containing electron-donating methoxy groups and a bulky Br atom on the benzene rings. The solid-state structure reveals a non-centrosymmetric molecule, with an E configuration around the C=N double bond. The C—N bonds show distinct amine [1.3689 (19) Å] and imine [1.285 (2) Å] characteristics. In the crystal, symmetry-related molecules are linked via a very weak N—H⋯N interaction, and C—H⋯O and C—H⋯π interactions.

The title compound, C 21 H 19 BrN 2 O 2 , is an amidine containing electron-donating methoxy groups and a bulky Br atom on the benzene rings. The solid-state structure reveals a noncentrosymmetric molecule, with an E configuration around the C N double bond. The C-N bonds show distinct amine [1.3689 (19) Å ] and imine [1.285 (2) Å ] characteristics. In the crystal, symmetry-related molecules are linked via a very weak N-HÁ Á ÁN interaction, and C-HÁ Á ÁO and C-HÁ Á Á interactions.

Experimental
It is already known that the difference between C-N and C═N depends on the degree of delocalisation in the N-C═N skeleton. In the title compound the difference is 0.0839 Å, whereas it is 0.058 Å in N,N'-diphenylbenzamidine (Alcock et al., 1988), 0.046 Å in acetamidine (Norrestam and Mertz, 1983), and 0.06 Å in N,N'-di(p-tolyl)benzamidine (Alcock et al., 1994). This correlation clearly proves that the degree of delocalisation depends on the substituents on the phenyl rings. In the title compound the bromine atom and the methoxy groups strongly influence the N-C═N congugation (0.0839 Å), in comparison with the unsubstituted compound N,N'-diphenylbenzamidine (0.058 Å).
From the torsion angles, C8-N1-C1-N2 = 13.9 (3)° and C8-N1-C1-C2 = 166.9 (14)°, it is revealed that the H-atom, H1, along with atom N1 and the phenyl substituent (ring A) at N2 are in an E (trans) configuration with respect to the C1═N2 bond. The solid state structure of the title compound indicates that the imine lone pair and the N1-H1 bond are on opposite sides of the molecule. This orientation hinders self association to give cyclic dimer formation, as observed in N, N'-di(p-chlorophenyl)formamidine (Cotton et al., 1997). The widening of the N1-C1-N2 bond angle [121.89 (14)°] and the slight deviation from the ideal sp 2 bond angle (120°), also observed in N,N'-diphenylbenzamidine (120.4°) and N,N'-di(p-tolyl)benzamidine (120.8), is assumed to be due to intermolecular interactions.
In the crystal symmetry related molecules are linked by a very weak N-H···N, interaction and by C-H···O and C-H···π interactions (see Table 1 for details).

Experimental
The title compound was prepared according to following procedure. A 250 ml round-bottomed flask was charged with p-bromobenzoic acid (10.0 g, 49.0 mmol) and SOCl 2 (45 ml). The resulting slurry was refluxed under a N 2 atmosphere for 2 h to give a clear solution. The solution was cooled to rt and the unreacted SOCl 2 was removed by distillation. The residue was then dried for 2 h under vacuum to give colourless crystalline p-bromobenzoyl chloride (11.0 g, 100%). Dry supplementary materials sup-2 DCM (60 ml) and dry Et 3 N (20 ml, 143 mmol) were added to the residue at 283 K under a N 2 atmosphere to give a brown precipitate. A solution of p-anisidine (7.0 g, 56 mmol) in another aliquot of dry DCM (40 ml) was then added to the reaction mixture over 30 min by syringe at 283 K to give a pale brown precipitate. The flask was then fitted with a reflux condenser and the slurry was heated to reflux for 10 h and a yellowish brown precipitate formed. After 10 h the mixture was cooled to rt and then evaporated to dryness to give a yellowish brown residue. Distilled water (200 ml) was then added and the resulting slurry was filtered and the solid rinsed with methanol (3 × 100 ml) to give colourless spongy crystalline 4-bromo-4'-methoxybenzanilide (15 g, 98%). (2) To a stirred solution of 4-bromo-4'-methoxybenzanilide (4.0 g, 13 mmol) in dry DCM (40 ml), in a 250 ml round-bottomed flask, was added a solution of PCl 5 (5.0 g, 24 mmol) in dry DCM (30 ml) by a syringe at 283 K under a N 2 atmosphere. The resulting slurry was then allowed to come to rt and was stirred for 2 h to give a clear bright yellow solution. After 2 h a solution of p-anisidine (4.8 g, 39 mmol) in another aliquot of dry DCM (30 ml) was added with stirring under a N 2 atmosphere, maintaining the temperature at 283 K, and then the solution was allowed to reach rt. This solution was then stirred at rt for 1 h, giving pale yellow precipitate, and then was evaporated to dryness yielding a pale yellow residue, which was poured into a beaker containing basic aqueous KOH solution (200 ml, pH>12) to give an off-yellow residue. This residue was then filtered off and rinsed with water (5 × 100 ml) and dried under vacuum. Slow evaporation of an EtOAc solution of this pale-yellow solid gave pale-yellow crystals of the title compound

Refinement
The H-atoms were included in calculated positions and treated as riding atoms: amine N-H 0.88 Å, aromatic C-H 0.95 Å, methyl C-H 0.98 Å, with U iso (H) = k × U eq (parent N-or C-atom), where k = 1.2 for the amine and aromatic H-atoms and 1.5 for the methyl H-atoms. Fig. 1. The molecular structure of the title compound, showing the numbering scheme and displacement ellipsoids drawn at the 50% probability level.