3-Hydroxy-2-methoxybenzamide

The crystal structure of the title compound, C8H9NO3, features centrosymmetric dimers with two amide groups interconnected by a pair of almost linear N—H⋯O hydrogen bonds. Through intermolecular O—H⋯O interactions between phenolic hydroxy groups and carbonyl O atoms, these dimers are assembled into undulating hydrogen-bonded layers parallel to the [101] plane. Additionally, the anti-H(—N) atom of the primary amide group forms an intramolecular hydrogen bond to the O atom of the methoxy group. The amide group froms a dihedral angle of 12.6 (1)° with the phenyl ring.

The crystal structure of the title compound, C 8 H 9 NO 3 , features centrosymmetric dimers with two amide groups interconnected by a pair of almost linear N-HÁ Á ÁO hydrogen bonds. Through intermolecular O-HÁ Á ÁO interactions between phenolic hydroxy groups and carbonyl O atoms, these dimers are assembled into undulating hydrogen-bonded layers parallel to the [101] plane. Additionally, the anti-H(-N) atom of the primary amide group forms an intramolecular hydrogen bond to the O atom of the methoxy group. The amide group froms a dihedral angle of 12.6 (1) with the phenyl ring.

3-Hydroxy-2-methoxybenzamide Sabine Wilbrand, Christian Neis and Kaspar Hegetschweiler Comment
The formation of hydrogen-bonded head-to-head dimers is well established for primary amides. A comprehensive search (McMahon et al., 2005) in the Cambridge Structural Database (Allen, 2002) revealed that 84% of the primary amides form such dimers. In addition, 14% form a catemeric (infinite chain) structure. It has recently been demonstrated that the dimers may, however, readily be disrupted in the presence of additional hydrogen-accepting functional groups (Eccles et al., 2011). Although the title compound possesses two such additional groups, the head-to-head dimer formation in the crystal structure is retained.
In the title compound, the two plains defined by the phenyl ring (C1 -C6) and the amide group (C1, C7, O3, N1) are slightly tilted against each other with an angle of 12.6 (1)°. Since the amide, methoxy and hydroxy group are arranged in a consecutive 1,2,3-arrangement, the ether oxygen atom of the methoxy group would be capable to accept either the O-H or N-H proton from the two adjacent moieties, forming a five-or six-membered ring structure, respectively. It is generally accepted that the latter is favoured, and this has indeed been observed. Owing to the steric demands of the 1,2,3-arrangement, the methoxy methyl group is significantly displaced from the aromatic plain, avoiding congestion: The distance of the methyl carbon atom to the aromatic mean plane is 0.819 (2) Å. This is a common feature for 3-alkoxy-or 3-hydroxy-2-methoxybenzamides. A search in the Cambridge Structural Database revealed a total of 46 entries for this structure type, and all of them show a similar displacement. An unexpected feature in the title compound is, however, the orientation of the three hydrogen atoms of the methyl group with an H8C-C8-O1-C2 torsional angle of 24.2 °. This value approaches an eclipsed rather than a staggered conformation. In the final refinement, these hydrogen atoms were treated as rigid group which was, however, allowed to rotate freely. It is clear that the effect under discussion is related to a rather small amount of electron density. However, a corresponding refinement, where the three hydrogen atoms were forced to adopt a staggered orientation, resulted in an increase of wR2(all) from 8.28 to 13.75%! Moreover, full refinement of the positional parameters revealed again the previously obtained, non-staggered arrangement (H8C-C8-O1-C2 torsional angle = 25.4 °, wR2(all) = 8.12%). Notably, in the above-mentioned 46 structures found in our CSD search, only three of them (LUDXEN, IPUQEP, SIGKIC) exhibit a similar deviation from a staggered orientation.
Obviously, attractive and repulsive interactions account for this particular structure, and it is tempting to interpret the H8A···O2 distance of 2.49 Å in terms of some intramolecular C-H···O hydrogen bonding. However, the small C8-H8A···O2 angle of 98 ° indicates that such an interaction would be -if at all -rather weak (Desiraju, 1996).

Experimental
2,3-Bis(benzyloxy)benzoic acid was obtained from 2,3-dihydroxybenzoic acid (K 2 CO 3 , benzyl bromide). It was converted into the corresponding amide via the acid chloride using thionyl chloride and subsequently an aqueous ammonia solution. The benzyl groups were then removed (ammonium formiate, Pd/C), and the resulting 2,3-dihydroxybenzamide was methylated in DMF using iodomethane and potassium bicarbonate. Some by-products were removed by

Refinement
The 3-hydroxy-2-methoxybenzamide molecule could be refined without problems, and its hydrogen atoms could all be located. The H(-C) positions were calculated (riding model). The methyl group was allowed to rotate freely. All positional parameters of the O-and N-bonded H-atoms were refined using variable isotropic displacement parameters.

Computing details
Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2010); data reduction: SAINT (Bruker, 2010); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).  A section of the hydrogen-bonding network. 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 R-factors(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.