A purple odyssey: synthesis and structure of 3-amino-4-hydroxy-6-oxocyclohexa-2,4-dien-1-iminium chloride monohydrate

The unequal C—C bond lengths in the six-membered ring of the C6H7N2O2 + cation of the title compound can be understood in terms of two separate delocalized systems.

By careful oxidation of the tetrahydrochloride salt of amine 1 with potassium dichromate, we isolated and crystallized the chloride salt of the parent 3-amino-4-hydroxy-6-oxocyclohexa-2,4-dien-1-iminium cation, 8, as a monohydrate [C 6 H 7 N 2 O 2 + ÁCl À ÁH 2 O, (I)] in the form of purple needles. This reaction must proceed via the elusive intermediate 6 which spontaneously hydrolyses. The first hydrolysis product should be intermediate 7. This contains a conjugated iminium salt and a vinylogous amide, which must hydrolyse rapidly, possibly because of the stability of the acidic enol formed. It appears to be a rapid hydrolysis for an amide under mild conditions and so stabilization of a tetrahedral intermediate by the positive iminium salt might occur.

Structural commentary
The asymmetric unit of (I) consists of one essentially planar C 6 H 7 N 2 O 2 + cation (r.m.s. deviation for the non-hydrogen atoms = 0.028 Å ), a chloride counter-ion and a water molecule of crystallization ( Fig. 1). Despite being a nominal 6 aromatic system, the bond lengths of the C1-C6 ring in (I) are far from equal and are split into three groups of two: the shortest are C1-C6 [1.354 (5) The short C3-C4 and C4-C5 bonds correlate with the approximately equal C3-N1 [1.320 (4)] and C5-N2 [1.306 (4) Å ] bond lengths, which imply equal delocalization of the positive charge of the cation over atoms N1 and N2, mediated via the C-N and C-C bonds between them. In terms of the 'oxygen side' of the cation, the C6-O2 bond [1.320 (4) Å ] is short for a C-O single bond whereas C2-O1 [1.227 (4) Å ] is slightly lengthened for a nominal C O double bond. This in combination with the C1-C2 and C1-C6 bond lengths again implies a degree of delocalization over these five atoms. However, the long C2-C3 and C5-C6 bonds imply little, if any, conjugation between the two delocalized components (O2/C6/C1/C2/O1 and N2/C5/C4/C3/N1) of the cation.

Supramolecular features
In the crystal, the components are linked by N-HÁ Á ÁCl, N-HÁ Á ÁO, O-HÁ Á ÁCl and O-HÁ Á ÁO hydrogen bonds (Table 1). If the cation and chloride anion are considered together, then [001] chains arise (Fig. 2) in which adjacent cations are related to each other by c-glide symmetry. Each link in the chain comprises two cations and two anions and R 2 4 (12) loops are apparent.
When the cation and water molecule are considered together, an [001] chain also arises (Fig. 3). The water molecule plays a key role in terms of both accepting hydrogen bonds from O2 and N1 and donating a hydrogen bond to O1 (it also acts as a donor to the chloride ion). The end result is a chain The molecular structure of (I) showing 50% displacement ellipsoids. Hydrogen bonds are shown as double-dashed lines.
When all components are considered together, (100) double sheets result (Fig. 4), with the water-O3-H2wÁ Á ÁCl1 hydrogen bond providing the key link between the sheets. Overall, the chloride ion accepts four hydrogen bonds (three N-HÁ Á ÁCl and one O-HÁ Á ÁCl interactions) in an irregular geometry.

Figure 4
The packing in (I) viewed along [001] showing the formation of (100) double layers.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-bound H atoms were geometrically placed (C-H = 0.95 Å ) and refined as riding atoms. The N-and O-bound H atoms were located in difference maps and their positions were freely refined. The constraint U iso (H) = 1.2U eq (carrier) was applied in all cases. The crystal studied was found to be a twin with the components related by a 180 rotation about [001]. Computer programs: CrysAlis PRO (Rigaku, 2015), SHELXS97 and SHELXL97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 2012) and publCIF (Westrip, 2010).

3-Amino-4-hydroxy-6-oxocyclohexa-2,4-dien-1-iminium chloride monohydrate
Crystal data 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.  (10)