1,4,10,13-Tetraoxa-7,16-diazoniacyclooctadecane bis(1H-pyrrole-2-carboxylate)

In the title salt, C12H28N2O4 2+·2C5H4NO2 −, the 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane dication possesses inversion symmetry. In the crystal, the pyrrole-carboxylate anions are linked via pairs of N—H⋯O hydrogen bonds, forming inversion dimers. These dimers are linked by the dications, via N—H⋯O hydrogen bonds, forming chains propagating along [110].


Fanglei Zeng and Zhenming Yin
Comment Hydrogen-bond-mediated self-assembly represents an area of considerable current interest (Burrows, 2004). It has recently been found that pyrrole-based entities are also capable of undergoing self-assembly through hydrogen bonds, especially in the solid state. In our previous works, we have reported the hydrogen-bonded assemblies of 4-pyridylmethyl 1H-pyrrole-2-carboxylate (Wang & Yin, 2007) and some other pyrrole-based compounds (Yin & Li, 2006;Cui et al. 2009;Li et al. 2012) in the solid state. Here we report the self-assembly of the title compound, (I), via conventional N-H···O hydrogen bonds.
The molecular structure of (I) is shown in Fig.1. In the solid state, the compound adopts central symmetrical conformation. Each pyrrole-2-carboxylate group is planar and interact with protonated amino group through two charge assisted N-H···O hydrogen bonds.
In the crystal structure, the molecules of (I) are held together by a pair of N-H···O hydrogen bonds between the pyrrole and carbonyl groups (Fig.2). Consequently, the molecules of (I) form a one-dimensional infinite chain structure.

Refinement
The N-bound H atoms were located in a difference map and refined freely. Other H atoms were positioned geometrically (C-H = 0.93 or 0.97 A°) and refined using a riding model, with U iso (H) =1.2U eq (C).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 > σ(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.