Crystal structure and Hirshfeld surface analysis of dibutyl 5,5′-(pentane-3,3-diyl)bis(1H-pyrrole-5-carboxylate)

The molecular structure of the title compound, C23H34N2O4, has C2 symmetry. In the crystal, interlocked dimers are formed through quadruple N—H⋯O hydrogen bonds between pyrrole N—H groups and carbonyl O atoms.


Chemical context
Hydrogen-bonding interactions play an important role in the design of functional assemblies that exhibit a variety of properties and functions (Prins et al., 2001;Steiner, 2002). Pyrrole-2-carboxylate possesses one hydrogen-bond donor (N-H pyrrole ) and one acceptor (C O), which favour the formation of centrosymmetric dimers with pairs of N-HÁ Á ÁO hydrogen bonds (Figueira et al., 2015). The dimer motif is structurally similar to classic Watson-Crick nucleotide basepairs. Calculations have revealed the dimer motif to be a robust supramolecular synthon in crystal engineering (Dubis et al., 2002). In previous work, we have shown a way to use the 2-carbonyl pyrrole dimer as a supramolecular connector to construct hexagonal and grid architectures (Yin et al., 2006). Here, we report the self-assembly of the title compound, via quadruple N-HÁ Á ÁN hydrogen bonds.

Structural commentary
The structure of the title compound is shown in Fig. 1. The asymmetric unit contains one half-molecule as it possesses C2 symmetry. In the molecule, the two pyrrole-2-carboxylate groups are both in a syn conformation, with the carbonyl group arranged syn to its adjacent pyrrole NH group. The O1-C8-C7-N1 torsion angle is À8.2 (5) . The dihedral angle between the pyrrole rings is 72.8 (2) .

Supramolecular features
Pairs of molecules of the title compound form interlocked dimers through four N1-H1Á Á ÁO1 hydrogen bonds between the pyrrole carbonyl oxygen atoms and pyrrole NH protons (Table 1, Fig. 2). This type of dimer has also been observed in our previous work (Yin et al., 2007). The dimers are connected into a three-dimensional supramolecular structure through C-HÁ Á Á contacts (Table 1).

Hirshfeld surface
A Hirshfeld surface analysis with CrystalExplorer (Turner et al., 2017) was performed to give insights into the important intermolecular interactions. These are normalized by van der Waals radii through a red-white-blue color scheme, where the red spots denote close contacts of molecules. The threedimensional d norm surface of the title compound is shown in Fig. 3. The red points represent closer contacts and negative d norm values on the surface corresponding to the N-HÁ Á ÁO and C-HÁ Á Á interactions mentioned above. The twodimensional fingerprint plots in Fig. 4 shown the intermolecular contacts and their percentage distributions on the Hirshfeld surface. HÁ Á ÁH interactions (74.8%) are present as a major contributor while HÁ Á ÁO/OÁ Á ÁH (14.5%), HÁ Á ÁC/CÁ Á ÁH (5.4%), CÁ Á ÁC (2.7%) and HÁ Á ÁN/NÁ Á ÁH (0.9%) contacts also give significant contributions to the Hirshfeld surface.

Figure 3
The Hirshfeld surface of the title compound mapped over d norm in the range À0.486 to 1.895 a.u. The intermolecular contacts can be seen in the red regions.

Figure 1
ORTEP diagram for the title compound, with displacement ellipsoids drawn at the 30% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. N-H hydrogen atoms were located from a difference-Fourier map and freely refined.
Other H atoms were placed in difference calculated positions (C-H = 0.96 or 0.97 Å ) and included in the final cycles of refinement using a riding model, with U iso (H) = 1.2U eq (C).

Funding information
Funding for this research was provided by: National Natural Science Foundation of China (award No. 21172174).    program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).  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.