2,2′-(Propane-2,2-diyl)bis(1H-pyrrole)

The title compound, C11H14N2, crystallized with two independent molecules (A and B) in the asymmetric unit. The two molecules differ only slightly, with the pyrrole rings being inclined to one another at a dihedral angle of 87.67 (8)° in molecule A and 88.09 (7)° in molecule B. In the crystal, there are no classical hydrogen bonds, but the two pyrrole NH groups of one molecule are involved in N—H⋯π interactions with the pyrrole rings of the other molecule. In this manner, a compact box-like arrangement of the two independent molecules is formed.


Comment
The title compound was prepared as a building block for the formation of substituted calix[4]pyrroles. The latter have been shown to form extremely interesting host-guest complexes with various anions (Gale et al., 1998;Sessler & Davis, 2001;Sessler et al., 2003).
The structure of the title compound is shown in Fig. 1, and the geometrical parameters are given in the Supplementary Information and the archived CIF. The compound crystallized in the centrosymmetric triclinic space group P1 with two independent molecules (A and B) in the asymmetric unit. The bond lengths and angles are similar to those observed in the diethyl analogue (Sobral et al., 2003), which also crystallized with two independent molecules, but in the non-centrosymetric monoclinic space group C2.
In the title compound the quateranry centers, C9 in A and C29 in B, impose a twist to the molecules with the pyrrole ring mean-planes being almost perpendicular to one another; 87.67 (8) ° in molecule A and 88.09 (7)° in molecule B. This is similar to the situation in the diethyl analogue where the two dihedral angles are 86.5 (2) and 86.7 (2) °.
N-H···π interactions are extremely important in biological systems and this aspect as been reviewed by (Meyer et al., 2003). The spectroscopic aspects of the N-H···π interactions of the pyrrole dimer have also been studied recently by (Dauster et al., 2008). In the crystal of the title compound the two independent molecules are linked by N-H···π interactions involving the pyrrole NH H-atoms of molecule A with the pyrrole rings of molecule B, and visa-versa (Table 1). This leads to the formation of a compact box-like arrangement of the two molecules, as shown in Fig. 2. Again this arrangement is similar to that observed in the crystal of the diethyl analogue.
The C-bound H-atoms were included in calculated positions and treated as riding atoms: C-H = 0.95 and 0.99 Å for CH and CH 3 H-atoms, respectively, with U iso (H) = k × U eq (C), where k = 1.2 for CH H-atoms, and 1.5 for CH 3 H-atoms. Fig. 1. A view of the molecular structure of the two independent molecules (A and B) of the title compound, with the displacement ellipsoids drawn at the 50% probability level.  Table 1 for details; C-bound H-atoms have been omitted for clarity).