Dimethyl 5,5′-methylenebis(2-hydroxybenzoate)

In the title compound, C17H16O6, the two methyl salicylate moieties are related by crystallographic twofold rotational symmetry with the two benzene rings close to being perpendicular [inter-ring dihedral angle = 86.6 (8)°]. Intramolecular phenolic O—H⋯O hydrogen bonds with carboxyl O-atom acceptors are present, with these groups also involved in centrosymmetric cyclic intermolecular O—H⋯O hydrogen-bonding associations [graph set R 2 2(4)], giving infinite chains extending across (101).

In the title compound, C 17 H 16 O 6 , the two methyl salicylate moieties are related by crystallographic twofold rotational symmetry with the two benzene rings close to being perpendicular [inter-ring dihedral angle = 86.6 (8) ]. Intramolecular phenolic O-HÁ Á ÁO hydrogen bonds with carboxyl O-atom acceptors are present, with these groups also involved in centrosymmetric cyclic intermolecular O-HÁ Á ÁO hydrogen-bonding associations [graph set R 2 2 (4)], giving infinite chains extending across (101).

Experimental
The title compound was prepared in two steps starting with salicylic acid. 5,5′-Methylenebis(salicylic acid) was prepared according to a known procedure (Cushman et al., 1990), and was then esterified with methanol and a catalytic amount of sulfuric acid (Méric et al., 1993). Slow evaporation of a saturated solution in dichloromethane gave single crystals suitable for X-ray diffraction.

Refinement
The phenolic H-atom (H1) was located in a difference Fourier map and both positional and isotropic displacement parameters were refined. All other H-atoms were placed in geometrically idealized positions and refined using a riding model with C-H = 0.95 Å (aromatic), 0.98 Å (methylene) or 0.97 Å (methyl) and U iso (H) = 1.2U eq (C) (aromatic or methylene) or U iso (H) = 1.5U eq (C) (methyl).

Figure 1
The molecular structure of the title compound showing atom numbering and displacement ellipsoids drawn at the 30% probability level. The intramolecular hydrogen bonds are shown as dashed lines. Symmetry code: (i) -x + 1, y, -z + 1/2.

Figure 2
The one-dimensional hydrogen-bonded chains in the title compound, with hydrogen bonds shown as dashed lines.
Displacement ellipsoids are drawn at the 30% probability level.

Special details
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 cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s 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 > 2σ(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.