3,3′-Di-tert-butyl-1,1′-[1,3-phenylenebis(methylene)]diurea

The title compound, C18H30N4O2, contains two tert-butyl urea groups, each connected to a benzene ring though a methylene group. One of the groups occupies a position almost normal to the aromatic plane with a C—N—C—C torsion angle of −94.4 (4)°, while the other is considerably twisted from the ring with a C—N—C—C torsion angle of −136.1 (4)°. In the crystal, pairs of molecules are connected to each other, forming centrosymmetric dimers in which two NH groups of one molecule act as hydrogen-bond donors to one carbonyl O atom of the other molecule. The dimers are linked into sheets parallel to (100) by N—H⋯O hydrogen bonds involving the remaining N—H and C=O groups.


Comment
Because of the ability of urea functional groups to form hydrogen bonds with an anion, urea-based compounds are known as effective hosts for a variety of anions (Brooks et al., 2008;Carr et al., 1998;Chauhan et al., 2008;Hiscock et al., 2009;Lorenzo et al., 2009;Tejeda et al., 2000) as well as neutral species (Kyne et al., 2001) and are often used for colorimetric detection for a specific anion in solution (Ghosh et al., 2007;Pérez-Casas et al., 2008). For example, simple acyclic ligands with mono-functional urea or thiourea groups are known to form 1:1 complexes with carboxylates, halides and phosphate in DMSO (Gomez et al., 2005). Encapsulation of sulfate anion was also structurally identified within the cavity formed by two tren-based urea ligands (Jose et al., 2007). In an effort to design multifunctional anion receptors (Hossain, 2008), we synthesized a urea-based compound containing two urea units, that can be useful in anion binding.
Single crystal X-ray analysis reveals that the bis-urea cleft crystallized in an orthorhombic space group without the involvement of solvent molecules. As illustrated in Fig. 1, the carbonyl groups of the two urea fragments are oriented in opposite directions. The methylene groups connected to the aromatic units are almost co-planar with the NH groups, as indicated by C13-N3-C14-N4 and C7-N1-C8-N2 torsion angles of -164.8 (3) and 177.6 (3)°, respectively. The NH groups are oriented almost perpendicular to the aromatic plane.
There are no intramolecular hydrogen bonding in the molecule, however, each C═O group forms two hydrogen bonds with two adjacent NH fragments resulting in the formation of centrosymmetric dimers with N···O distances of 2.889 (4) Å and 2.941 (4) Å ( Fig. 2 and Table 1). Each dimer is then connected with four adjacent dimers forming a sheet parallel to the (100) (Fig. 3). Similar intermolecular bonding was previously reported for a mono-functional urea-based compound (Lo & Ng, 2008).

Experimental
To a solution of m-xylylenediamine (0.10 g, 1.0 mmol) in CH 3 CN (20 ml) was added two equivalents of tert-butyl isocyanate (0.20 g, 2.0 mmol) and the mixture was stirred overnight at room temperature. The white precipitate formed was separated A small portion of the sample was re-dissolved in CHCl 3 , and crystals suitable for X-ray analysis were grown by slow evaporation of the solvent at room temperature. supplementary materials sup-2 U iso (H) values were assigned as 1.2 times U eq of the attached atom (1.5 for methyl). A torsional parameter was refined for each methyl group. The highest residual peak is located 1.50 Å from O2. Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

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.