4-Chloro-7-hydroxy-6-methyl-1,7-naphthyridin-8(7H)-one

The title compound, C9H7ClN2O2, was prepared by reaction of methyl 4-chloro-3-(prop-1-ynyl)picolinate with hydroxylamine in MeOH/KOH solution. The two essentially planar molecules which make up the asymmetric unit have almost identical geometries and and are linked into dimeric aggregates via pairs of O—H⋯O hydrogen bonds. These aggregates have almost perfect inversion symmetry; however, quite unusually, the inversion center of the dimer does not coincide with the crystallographic inversion center.

The title compound, C 9 H 7 ClN 2 O 2 , was prepared by reaction of methyl 4-chloro-3-(prop-1-ynyl)picolinate with hydroxylamine in MeOH/KOH solution. The two essentially planar molecules which make up the asymmetric unit have almost identical geometries and and are linked into dimeric aggregates via pairs of O-HÁ Á ÁO hydrogen bonds. These aggregates have almost perfect inversion symmetry; however, quite unusually, the inversion center of the dimer does not coincide with the crystallographic inversion center.

Comment
The title compound was obtained using the reaction of of methyl 4-chloro-3-(prop-1-ynyl)picolinate with hydroxylamine in MeOH/KOH solution (Knight et al., 2002). The structural formula of the product was confirmed by the present study (Fig. 1).
There are two independent molecules in the structure, which show almost identical geometry. The molecules are essentially planar (with the exception of methyl H atoms) and their parameters are quite similar to those found in related structures with analogous carbon-nitrogen bicyclic framework (Ikeura et al., 1998;Natsugari et al., 1995). To the best of our knowledge, however, this is the first structurally characterized system of this kind with the O-substitution at the N atom next to C=O group.
The molecules in the asymmetric unit of the title compound are linked into dimeric aggregates via H-bonds (Table 1).
These aggregates have almost ideal inversion symmetry, however, quite unusually, the inversion center of the dimer does not coincide with the crystallographic inversion center.

Experimental
Warm solutions (50°C) of hydroxylamine hydrochloride (199.0 mg, 2.86 mmol, 6 eq) in methanol (2.0 M, 1.43 ml) and potassium hydroxide (241.0 mg, 4.29 mmol, 9 eq) in methanol (4.0 M, 1.07 ml) were mixed; the resulting solution was cooled to below 40°C and potassium chloride precipitated out. The precipitate was filtered and the filtrate was added to a vial containing methyl 4-chloro-3-(prop-1-ynyl)picolinate (100.0 mg, 0.4770 mmol); the flask containing the filtrate was rinsed with an additional 1 ml of MeOH and added to the reaction vial. The resulting mixture was then heated to reflux.

Refinement
All H atoms were placed in geometrically calculated positions (C-H 0.98 Å and 0.95 Å for methyl and aromatic CH-groups; O-H 0.84 Å) and included in the refinement in riding motion approximation. The U iso (H) were set to 1.2U eq of the carrying atom (1.5U eq for methyl and hydroxyl H atoms).
supplementary materials sup-2 Two independent molecules in the structure of the title compound are related by almost ideal non-crystallographic inversion center, which prompted us to perform additional checks on the presence of higher genuine symmetry by careful inspection of atomic coordinates as well as by using ADDSYM option in PLATON (Spek, 2009

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.
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 > σ(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.