N-[3-(2,6-Dimethylanilino)-1-methylbut-2-enylidene]-2,6-dimethylanilinium chloride1

The title salt, C21H27N2 +·Cl− resulted from the condensation between 2,6-dimethylaniline and acetylacetone in acidified ethanol. The bulky cation is stabilized in a β-iminoenamine tautomeric form, and presents a W-shaped conformation. The benzene rings are arranged almost parallel, with a dihedral angle of 6.58 (4)° between the mean planes. Both N—H groups in the cation form strong hydrogen bonds with two symmetry-related chloride anions. The resulting supramolecular structure is a one dimensional polymer running along [001], alternating cations and anions. The π–π interaction observed in the molecule, characterized by a centroid–centroid separation of 4.298 (4) Å, is thus extended to the chains, with separations of 5.222 (4) Å between benzene rings of neighbouring cations in the crystal.

The title salt, C 21 H 27 N 2 + ÁCl À resulted from the condensation between 2,6-dimethylaniline and acetylacetone in acidified ethanol. The bulky cation is stabilized in a -iminoenamine tautomeric form, and presents a W-shaped conformation. The benzene rings are arranged almost parallel, with a dihedral angle of 6.58 (4) between the mean planes. Both N-H groups in the cation form strong hydrogen bonds with two symmetry-related chloride anions. The resulting supramolecular structure is a one dimensional polymer running along [001], alternating cations and anions. Theinteraction observed in the molecule, characterized by a centroidcentroid separation of 4.298 (4) Å , is thus extended to the chains, with separations of 5.222 (4) Å between benzene rings of neighbouring cations in the crystal.

Comment
Many synthetic routes are currently available to synthesize symmetric and unsymmetric β-diketimines, including the cocondensation reaction of a ketone and a primary amine (Dorman, 1966;Park, 2007). These ligands proved to be very versatile and diverse, considering the possible variation of both coordination modes and of groups bonded to the N atoms and to the α-and β-C atoms. Indeed, β-diketiminates are the most used ligands in coordination chemistry for the stabilization of low coordinate and low oxidation states of main group or transition elements (Bourget-Merle et al., 2002;Nagendran & Roesky, 2008). β-Diketiminates complexes have been used as catalysts and even as structural models of protein active sites (Holland & Tolman, 2000).
Neutral β-diketimines invariably show a U-shaped conformation, favoured over other possible conformers by the formation of an intramolecular hydrogen bond involving the amine and imine N atoms (Dorman, 1966;Stender et al., 2001;Carey et al., 2003). However, if the β-diketimine is protonated, the backbone molecule adopts a new W-conformation, since the protonated N atom is able to form a strong intra-ionic N-H···Cl bond. Such behaviour has been observed in three salts closely related to the title compound where phenyl (Brownstein et al., 1983), methyl (Kuhn et al., 2000), and mesityl (Lesikar & Richards, 2006) have replaced the 2,6-dimethylphenyl. The supramolecular one-dimensional structure generated by this hydrogen bond is expected to be a strong stabilizing factor, as anions and cations alternate in a chain structure, which has a theoretical Madelung constant of 1.38.
In the title salt, the cation adopts the W-shaped conformation, and thus presents a non-crystallographic twofold axis passing through the central atom C9 (Fig. 1). Both N atoms are protonated, indicating that the β-iminoenamine tautomeric form is stabilized in the solid state. A parallel arrangement is observed for benzene rings, which are separated by 4.298 (4) Å. A similar arrangement was found with the mesityl-including cation, although the benzene separations are larger, probably because of the hindering character of mesityl (centroid-to-centroid distances: 4.348, 4.823, or 4.881 Å, depending of the nature of the counterion; Lesikar & Richards, 2006). Unexpectedly, the phenyl-containing salt has little intramolecular π-π interaction, with non-parallel phenyl rings separated by 5.480 Å (Brownstein et al., 1983). In the title cation, benzene rings are close to be parallel, the dihedral angle between mean planes being 6.58 (4)°.
Regarding the crystal structure, both amine and imine NH functionalities, N7 and N11, are involved in strong N-H···Cl hydrogen bonds with symmetry-related Clions. The network of hydrogen bonds forms a one-dimensional supramolecular structure along the short cell axis c (Fig. 2). Cations and anions alternate in a chain, with all benzene rings oriented in the same direction. This arrangement allows to extend the π-π interactions to the whole polymeric structure, with a separation of 5.222 (4) Å between benzene rings of neighbouring cations. No significant contacts are observed between chains in the crystal.

Experimental
The title salt was prepared by mixing acetylacetone (25.03 g, 0.25 mol) and 2,6-dimethylaniline (60.5 g, 0.5 mol) in 12 N hydrochloric acid (20.8 ml, 0.25 mol HCl). The mixture was heated to 393 K for 4 h., allowing the water to distil. The reaction was then further heated to 413 K overnight. The resulting solid was recrystallized from hot ethanol, yielding 87.6 g of the title salt (94%). NMR data are in agreement with the X-ray structure (see archived CIF).

Refinement
All C-bonded H atoms were placed in calculated positions and refined as riding to their carrier atoms, with bond lengths fixed to 0.93 (aromatic CH) or 0.96 Å (methyl CH 3 ). N-bonded H atoms (H7 and H11) were found in a difference map and refined freely. Isotropic displacement parameters for H atoms were calculated as U iso (H) = 1.5U eq (carrier atom) for methyl groups and U iso (H) = 1.2U eq (carrier atom) otherwise. Fig. 1. The title compound (asymmetric unit) with displacement ellipsoids at the 25% probability level. The dashed bond corresponds to the N-H···Cl hydrogen bond.