4-Methoxyquinolinium-2-carboxylate dihydrate

The title hydrated quinoline derivative, C11H9NO3·2H2O, crystallizes as a zwitterion in which the quinoline N atom is protonated. The quinoline ring is essentially planar, with a maximum deviation of 0.017 (2) Å. An intramolecular N—H⋯O hydrogen bond between the protonated N atom and the O atom of the carboxylate group in the zwitterion forms an S(5) ring motif. In the crystal, the zwitterions are connected into inversion dimers via pairs of N—H⋯O and C—H⋯O hydrogen bonds with R 2 2(4) and R 1 2(6) motifs. The water molecules are connected via O—H⋯O hydrogen bonds, forming supramolecular chains along the c axis. Furthermore, the chains and the dimers are connected via O—H⋯O hydrogen bonds, forming ladder-like supramolecular ribbons along the c axis.

The title hydrated quinoline derivative, C 11 H 9 NO 3 Á2H 2 O, crystallizes as a zwitterion in which the quinoline N atom is protonated. The quinoline ring is essentially planar, with a maximum deviation of 0.017 (2) Å . An intramolecular N-HÁ Á ÁO hydrogen bond between the protonated N atom and the O atom of the carboxylate group in the zwitterion forms an S(5) ring motif. In the crystal, the zwitterions are connected into inversion dimers via pairs of N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds with R 2 2 (4) and R 1 2 (6) motifs. The water molecules are connected via O-HÁ Á ÁO hydrogen bonds, forming supramolecular chains along the c axis. Furthermore, the chains and the dimers are connected via O-HÁ Á ÁO hydrogen bonds, forming ladder-like supramolecular ribbons along the c axis.

Comment
Quinolines and their derivatives are very important compounds because of their wide occurrence in natural products (Morimoto et al., 1991;Michael, 1997) and biologically active compounds (Markees et al., 1970;Campbell et al., 1988).
The title molecule, (Fig. 1), crystallizes as a zwitterion in which the quinoline N atom is protonated. The asymmetric unit consists of one 4-methoxyquinolinium-2-carboxylate molecule and two water molecules. The quinoline ring (N1/C1-C9) is essentially planar, with a maximum deviation of 0.017 (2) Å for atom C4.
In the crystal structure (Fig. 2), the 4-methoxyquinolinium-2-carboxylate molecules are connected via N-H···O and C-H···O hydrogen bonds to form R 2 2 (4) and R 1 2 (6) (Bernstein et al., 1995) motifs. There is an intramolecular N-H···O hydrogen bond observed between the protonated nitrogen atom of the cationic part of the quinolinium and the oxygen atom of anionic part of the carboxylate group in the zwitterion forming an S(5) ring motif. The water molecules are connected via O-H···O hydrogen bonds to form one-dimensional supramolecular chains along the c-axis. Furthermore, the chains formed by water molecules and the 4-methoxyquinolinium-2-carboxylate molecules are connected via O-H···O (Table 1) hydrogen bonds to form ladder-like supramolecular ribbons along the c-axis.

Experimental
A methanol solution (20 ml) of 4-methoxyquinoline-2-carboxylic acid (50. 8 mg, Aldrich) was warmed over a heating magnetic stirrer for 5 minutes. The resulting solution was allowed to cool slowly at room temperature. Crystals of the title compound appeared from the mother liquor after a few days.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(F 2 ) is used only for calculating Rfactors(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.