2-(3,5-Dioxo-4-azatricyclo[5.2.1.02,6]dec-8-en-4-yl)acetic acid

The asymmetric unit of the title compound, C11H11NO4, contains two molecules, A and B, with different conformations: in molecule A, the norborne and carboxylic acid groups lie to the same side of the heterocycle, whereas in a molecule B, they lie on opposite sides. In the crystal, the A molecules form R 2 2(8) carboxylic acid inversion dimers, linked by pairs of O—H⋯O hydrogen bonds. The B molecules link to one of the ketone O atoms of the A molecule by an O—H⋯O interaction, resulting in tetramers (two A and two B molecules). The tetramers are linked by weak C—H⋯O interactions, generating a three-dimensional network.

The asymmetric unit of the title compound, C 11 H 11 NO 4 , contains two molecules, A and B, with different conformations: in molecule A, the norborne and carboxylic acid groups lie to the same side of the heterocycle, whereas in a molecule B, they lie on opposite sides. In the crystal, the A molecules form R 2 2 (8) carboxylic acid inversion dimers, linked by pairs of O-HÁ Á ÁO hydrogen bonds. The B molecules link to one of the ketone O atoms of the A molecule by an O-HÁ Á ÁO interaction, resulting in tetramers (two A and two B molecules). The tetramers are linked by weak C-HÁ Á ÁO interactions, generating a three-dimensional network.

Comment
As shown in Fig. 1, the asymmetric unit of the title compound contains two independently molecules 1 (with N1) and 2 (with N2). The norbornene units of the molecules 1 and 2 are bound endo with respect to acetic acid. The sum of the three C-N-C angles at the imide N atom is 359.61 (13) lengths. In molecule 2, the corresponding bond lengths are N2-C13 of 1.442 (2) Å, N2-C17 of 1.377 (2) Å and N2-C14 of 1.384 (2) Å, respectively. As expected, this indicates a delocalized π-electron system along the imide parts of the molecules, as in a similar structure (Bartkowska et al., 1997).
In the crystal, pairs of molecules generate a dimer of the R 2 2 (8) motif by O-H···O hydrogen bonds; these two molecules are linked to the other two molecules by O-H···O hydrogen bonds (Table 1, Fig. 2). In addition, C-H···O hydrogen bonds contribute to the overall crystal packing.

Refinement
All H atoms were geometrically placed [(O-H = 0.82 Å (hydroxyl), C-H = 0.93 Å (aromatic), C-H = 0.97 Å (methylene) and C-H = 0.98 Å (methine)] and refined as riding with U iso (H) = 1.5U eq (O) for the hydroxyl group and 1.2U eq (C) for the others.  View of the two molecules of the title compound in the asymmetric unit. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level.

Figure 2
View of the dimer and C-H···O hydrogen bonds of the title compound along the a axis.