Pimelic acid–urea (1/2)

The asymmetric unit, 2CH4N2O·C7H12O4, of the title cocrystal contains one urea molecule and a half-molecule of pimelic acid; the latter, together with a second urea molecule, are completed by symmetry, with the central atom of the whole pimelic acid moiety placed on a twofold crystallographic axis. The crystal packing is stabilized by O—H⋯O and N—H⋯O hydrogen-bond, generating a chain along [10]. Additionally, the chains are assembled into a three-dimensional framework via weak N—H⋯O interchain interactions.

The asymmetric unit, 2CH 4 N 2 OÁC 7 H 12 O 4 , of the title cocrystal contains one urea molecule and a half-molecule of pimelic acid; the latter, together with a second urea molecule, are completed by symmetry, with the central atom of the whole pimelic acid moiety placed on a twofold crystallographic axis. The crystal packing is stabilized by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen-bond, generating a chain along [101]. Additionally, the chains are assembled into a three-dimensional framework via weak N-HÁ Á ÁO interchain interactions.

Comment
This crystal structure study is part of a broader program of urea-dicarboxylic acid co-crystal engineering with predesigned crystal building blocks (Videnova-Adrabińska, 1996a,b;Chang & Lin, 2011). In these solids, the urea molecules form an extensively hydrogen-bonded host structure (Harris & Thomas,1990), containing linear, parallel tunnels with guest molecules packed densely along these tunnels (Yeo et al., 1997). The phase diagram of a related urea-dicarboxylic acid co-crystal has also been reported (Chadwick et al. 2009). In this contribution, we present the crystal structure of the 2:1 urea/pimelic acid co-crystal.
The asymmetric unit of the title co-crystal, CH 4 N 2 O. 0.5(C 7 H 12 O 4 ), contains one urea molecule and a half-molecule of pimelic acid, with the complete pimelic acid molecule and the additional urea unit generated via crystallographic rotation symmetry, with the central carbon atom of the whole pimelic acid molecule positioned on a twofold axis (Fig. 1).

Experimental
Pimelic acid acid (0.0815 g, 0.5 mmol) and urea (0.0316 g, 0.05 mmol) were dissolved in 15 ml of water (pH = 3.23) under stirring. After slow evaporation of the solution for one week at 50°C, colorless block sized crystals were obtained.

Refinement
H atoms bonded to C and N atoms were placed in their geometrically calculated position and refined using a riding model,

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (