3-[(Z)-(4-Diethylamino-6-oxocyclohexa-2,4-dien-1-ylidene)methylamino]benzoic acid

The title compound, C18H20N2O3, crystallizes as the keto tautomer, unlike the vast majority of similar structures that have been reported that contain the hydroxy tautomer. There are two strong hydrogen bonds in the crystal structure, both accepted by the same carbonyl group: one intramolecular N—H⋯O and one intermolecular O—H⋯O. As a result, the carbonyl C=O distance is long, at 1.310 (2) Å, which may suggest the molecule has a significant zwitterionic character. The dihedral angle between the benzene ring planes is 15.05 (7)°. As a result of the intramolecular hydrogen bond, the bridging C—C=N—C group is almost coplanar with the benzene ring that has the diethylamino substituent [dihedral angle 2.35 (15)°].

The title compound crystallizes as the keto-tautomer ( Fig. 1), unlike the majority of the similar compounds. In the CSD (Allen, 2002) the hydroxy tautomers were found in 218 compounds while the keto ones only in 17 compounds (Version Nov. 2006, updates up to August 2007only organic compounds). In this case the presence of the certain tautomer is proven by the succesful location and refinement of the hydrogen atom bonded to N7 nitrogen atom (Fig. 2a). Short and relatively linear intramolecular N-H···O hydrogen bond forms an almost planar (maximum deviation 0.017 (1) Å) sixmembered ring. The same O10 oxygen atom which accepts the intramolecular hydrogen bond is involved in a very short (O···O distance is 2.467 (1) Å) intermolecular O-H···O hydrogen bond. As a result the C10-O10 bond of 1.310 (2)Å is significantly longer than a typical C?O double bond. CSD search results show that such elongation is typical for similar compounds, the mean C-O distance being 1.348 (17)Å for hydroxy tautomers, but it may also be as large as 1.299 (17)Å also for the keto-tautomers. Together with the observation of bond lengths around N7 atom this implies some degree of zwitterionic character of the molecule, with partial positive charge at N7-H7 group and negative at O10 atom.
These perturbations disturb also the benzene ring. The ring A (C1 -C6) is closer to planarity than the ring B (C9 -C14).
Maximum deviations from the least-squares planes are 0.0103 (11)Å for ring A and 0.0219 (12)Å for ring B. Also the bond lengths and angles are much more uniform within the ring A than in ring B.
The conformation of the molecule is described by the dihedral angles between benzene ring planes of 15.05 (7)°. As a result of the intramolecular hydrogen bond, the bridging C-C?N-C group is almost coplanar with the ring B (dihedral angle 2.35 (15)°). The COO group is significantly, by 21.73 (10)°, twisted with respect to its parent ring's plane. On the other end of the molecule, the C-N-C fragment is twisted by 17.0 (2)°, while the terminal C-C bonds are almost perpendicular to the CNC plane.
In the crystal structure the molecules are connected by strong intermolecular O-H···O hydrogen bonds into tapes along the y-direction. The O-H bond is significantly elongated, to 1.07 (3)Å due to the formation of the hydrogen bond (cf. Fig. 2 b). The tapes are connected by relatively strong inter-tape C-H···O hydrogen bonds into the layers (Fig. 3). Some additional C-H···O interactions (Table 1)

S3. Refinement
The hydrogen atoms were located in the difference Fourier maps and refined as 'riding model′. Isotropic displacement parameters for hydrogen atoms were set at 1.2 (1.3 for methyl group) times the U eq values of appropriate carrier atoms.