Bis[μ-1-hexyl-3-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-15-yl)urea]bis(azidosodium) chloroform disolvate

In the title compound, [Na2(N3)2(C21H34N2O6)2]·2CHCl3, the sodium cation is heptacoordinated by five O atoms of the crown ether unit of the 1-hexyl-3-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-15-yl)urea (L) ligand, the O atom of the urea group of the second, symmetry-related L ligand, and one N atom of the azide anion. The experimentally determined distance 2.472 (2) Å between the terminal azide N atom and the sodium cation is substantially longer than that predicted from density functional theory (DFT) calculations (2.263 Å). The crown ethers complexing the sodium cation are related by an inversion centre and form dimers. The urea groups of the two L ligands are connected in a head-to-tail fashion by classical N—H⋯N hydrogen-bonding interactions and form a ribbon-like structure parallel to the b axis. Parallel ribbons are weakly linked through C—H⋯N, C—H⋯O and C—H⋯π interactions.

In the title compound, [Na 2 (N 3 ) 2 (C 21 H 34 N 2 O 6 ) 2 ]Á2CHCl 3 , the sodium cation is heptacoordinated by five O atoms of the crown ether unit of the 1-hexyl-3- (2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-15-yl)urea (L) ligand, the O atom of the urea group of the second, symmetry-related L ligand, and one N atom of the azide anion. The experimentally determined distance 2.472 (2) Å between the terminal azide N atom and the sodium cation is substantially longer than that predicted from density functional theory (DFT) calculations (2.263 Å ). The crown ethers complexing the sodium cation are related by an inversion centre and form dimers. The urea groups of the two L ligands are connected in a head-to-tail fashion by classical N-HÁ Á ÁN hydrogen-bonding interactions and form a ribbon-like structure parallel to the b axis. Parallel ribbons are weakly linked through C-HÁ Á ÁN, C-HÁ Á ÁO and C-HÁ Á Á interactions.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: RK2333).

Comment
In this contribution we present the structure of a 15 C5 crown ether (4-hexylurea-benzo-15-crown-5) co-crystallized with sodium azide and chloroform as solvate. Structures with 4-hexylurea-benzo-15-crown-5 have been recently described (Brown et al., 2006;2008). These authors focused in particular on the M-N terminal metal-azide bond length and charge densities on the metal (M) and terminal nitrogen centre (N terminal ) by X-ray crystallography and DFT calculations. They failed however to get the intended crystal structure of the coordinated sodium azide with the crown ether, obtaining only a crown ether coordination with water. In this paper, we describe the crown ether derivative, bearing a benzene ring along with a urea and hexyl alkyl chain, coordinated with sodium azide. Examples of such derivatives have been reported, but with different salts such as potassium nitrate (Cazacu et al., 2009) or without any salts (Cazacu et al., 2006).
In the asymmetric unit of the title compound, one 15 C5 crown ether molecules, one sodium cation, one azide molecule and one solvent chloroform molecule are present (Fig. 1). The crown ethers appear as a head-to-tail dimer, the crown ether facing the urea portion of the second molecule. Sodium is hepta-coordinated by the five oxygen atoms of the crown ether, one of the N atoms of the azide group and with the oxygen of the urea group from the second molecule. The experimental Na-N distance (2.472 (2)Å) is significantly longer than the DFT calculated Na-N distance (2.263 Å; Brown et al., 2006). A possible explanation is that the azide group in the present structure is also linked through H-bonds to NH from the urea function of the second molecule, one branch being significantly stronger than the other (N15H151···N37 ii = 2.03 (2)Å versus N8H81···N37 i = 2.39 (3)Å). This will pull the azide group slightly outside the interaction sphere of the sodium cation. In this way coordination and H-bonds are forming a ribbon like structure along the b-axis (Fig. 2). The individual ribbons are weakly linked through CH···π interactions (centroid Cg to H281 distance 2.79Å and C28-H281···Cg angle 131°, where Cg is the centroid formed by the aromatic ring C16-C19/C33/C34). A weak CH···O nonclassical interaction is present between C31 and O29 -C31H311···O29 iii = 2.56Å. Symmetry codes:

Experimental
To a solution of 4′-aminobenzo-15-crown-5 (300 mg, 1.06 mmol, 1eq.) in acetonitrile was added hexyl isocyanate (175 mg, 1.38 mmol, 1.3 eq.). After 12 h under reflux, the solution was evaporated. The residue was dissolved in chloroform and precipitated by addition of hexane. The resulting precipitate was isolated by filtration and washed with hexane to give the crude title compound (302 mg, 70% yield). The compound was dissolved in chloroform, a small amount of NaN 3 was added and the mixture was sonicated for 30 minutes. The compound crystallized after a few hours.

Refinement
The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically.
The H atoms attached to carbon atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry -C-H in the range 0.93Å-0.98Å) and U iso (H) = 1.2(1.5)U eq (C), after which the positions were refined with riding constraints. The positions of the H atoms attached to nitrogen atoms were freely refined, but their isotropic atomic displacement parameter were constrained as for the other H atoms.   Packing of the title complex with hydrogen bonds between urea groups and azide anions indicated by dotted lines.