Crystal structure of a host–guest complex of the tris-urea receptor, 3-(4-nitrophenyl)-1,1-bis{2-[3-(4-nitrophenyl)ureido]ethyl}urea, that encapsulates hydrogen-bonded chains of dihydrogen phosphate anions with separate tetra-n-butylammonium counter-ions

The title compound is a host–guest complex of the tris-urea receptor, 3-(4-nitrophenyl)-1,1-bis{2-[3-(4-nitrophenyl)ureido]ethyl}urea, encapsulating a hydrogen-bonded chain of dihydrogen phosphate anions.


Structural commentary
The title compound crystallizes with two independent formula units in the asymmetric unit. The molecular structure of the two tris-urea receptors (R1 and R2) and the dihydrogen phosphate anions (P1 and P2) are illustrated in Fig. 1. In each receptor an intramolecular N-HÁ Á ÁO hydrogen bond is present (N8-H8NÁ Á ÁO7 in R1 and N17-H17NÁ Á ÁO16 in R2), each forming an S(9) ring motif,. Four intramolecular C-HÁ Á ÁO hydrogen bonds are also present in receptor R1 and three in R2 ( Fig. 1 and Table 1). Both receptors display a cis orientation of the urea subunits (N2/N3 and N5/N6 in R1, and N11/N12 and N14/N15 in R2). The urea subunits N5/N6 in R1 and N11/N12 in R2 are orientated towards the dihydrogen phosphate ions (P1 and P2) forming a 2:2 adduct via N-HÁ Á ÁO hydrogen bonds with anion P1 and enclosing R 2 2 (8) ring motifs (Table 1 and Fig. 1). Both NH functions of each urea subunit are trans to the C O group across the respective C-N bond. Anions P1 and P2 interact with each other via two O-HÁ Á ÁO hydrogen bonds (O20-H20OÁ Á ÁO24 and O23-H23OÁ Á ÁO22), enclosing an R 2 2 (8) ring motif ( Fig. 1 and Table 1). The dihedral angle between the urea plane [N-C( O)-N] and the benzene ring to which it is attached vary from 6.66 (14) to 18.96 (14) in R1 and from 6.87 (14) to 13.82 (14) in R2. The dihedral angle between the nitro group and the benzene ring to which it is attached also vary, from 7.1 (3) to 13.4 (4) in R1 and from 8.3 (4) to 16.7 (7) in R2.

Figure 2
A partial view along the b axis of the crystal packing of the title complex.
Synthesis of the title complex (I): Tetra-n-butylammonium dihydrogen phosphate (1.68 mmol) was added to 5 ml of a DMF solution of R (0.168 mmol) and the mixture was stirred for 2 h. After filtration the solution was left to evaporate slowly and yielded colourless prismatic crystals of the title complex within three weeks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to O and N were located in a difference-Fourier map and refined with distance restraints: O-H = 0.93 and N-H = 0.90 Å , with U iso (H) = 1.5U eq (O) and 1.2U eq (N). The C-bound H atoms were positioned geometrically and refined using a riding model: C-H = 0.95-0.99 Å , with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms.

3-(4-Nitrophenyl)-1,1-bis{2-[3-(4-nitrophenyl)ureido]ethyl}urea tetrabutylammonium dihydrogen phosphate
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.