6,7-Dihydro-5H-pyrrolo[1,2-a]imidazole

In the title compound, the pyrrolidine ring fused to the imidazole ring has an envelope conformation.


Structure description
Ionic liquids have emerged as a promising area in material science because of their tunable properties, allowing them to be used in a wide range of application such as: carbon dioxide capture, fuel cells, nanoparticle stabilization, and many more (Song et al., 2019;Huang et al., 2017;Wang et al., 2017). In this context, our group has been working on the synthesis of new cation moieties for ionic liquid designs. From the different chemical entities employed in ionic liquids, imidazolium derivatives are widely used in the field due to their versatility and relatively high stability. Imidazolium ionic liquid research is dominated by fluorine containing anions (Xue et al., 2006). Thus, we intended to explore imidazole derivatives (II) at position 2 of the principal structural component, imidazole (I), in order to decrease its reactivity (Fig. 1).
To understand how cations and anions interact in ionic liquids, characterization of the starting materials is important. Towards that end, the crystal structure of 6,7-dihydro-5Hpyrrolo[1,2-a]imidazole (II) is presented in order to characterize and establish a structure stability relationship of cyclic imidazole derivative families for imidazolium ionic liquid research applications.

data reports
The molecular structure of imidazole derivative (II) is displayed in Fig. 2. We initially envisioned that the electronic and steric effects would be similar to a pyrrole fused to the imidazole moiety and, thus, provide comparison to pyrrolidine below. In order to put into perspective the results found in the molecular and crystal structure of (II), we compare the fused imidazole moiety of (II) with the imidazole crystal structure. There is no significant difference in bond length of the imidazole moiety of (II) compared to the imidazole crystal structure (McMullan et al., 1979). However, the C2-N2 (N3-C4 in imidazole; McMullan et al., 1979) bond length of (II) is larger than the same bond found in the imidazole crystal structure [1.390 (2) vs1.375 (1) Å ]. This bond-length difference might be due to the new substituted imidazole ring system, which can help justify the chemical shifts observed in the 1 H-NMR spectrum between (II) and (I) of those hydrogen atoms in C1 and C2 of (II), based on the inductive effects of the substituents. On the other hand, we also compare the pyrrolidine fused ring to the pyrrolidine crystal structure (Dziubek & Katrusiak, 2011). We found that the major bond length difference occurs between bonds N1-C3 and C3-C4 [1.353 (2) vs 1.457 (2) Å and 1.492 (2) vs 1.528 (2) Å ], respectively; only the shortest bond length of pyrrolidine was used due to its symmetry). The C5-C6 bond length was found to be 1.543 (2) Å , which is slightly larger than the one found for pyrrolidine [1.528 (2) Å ]. These differences in bond length could be attributed to the new sp 2 carbon atom (C3) of (II), which also might be responsible for the differences in bond angles in (II). For example, angles N1-C3-C4 and C6-N1-C3 within the fused ring system are much larger compared to those of the pyrrolidine structure [111.1 (1) vs 107.2 (1) and 113.99 (9) vs 103.37 (1) Å , respectively], but angle N1-C6-C5 [102.10 (9) vs 107.05 (1) Å ] becomes smaller. These angle differences, despite being small, can help relieve the ring's stress. Finally, it is important to point out that N1 in pyrrolidine is out of plane (envelope conformation) in order to reduce lone-pair interactions, but in (II), this envelope conformation is adopted by C5 as the flap atom where C5 is 0.317 (2) Å out of the plane of the remaining four atoms. Also, the imidazole ring and the planar part of the pyrrolidine ring make a dihedral angle of 3.85 (9) . By N1 becoming part of the plane, its lone pairs could add new repulsion interactions, suggesting why the ring has to adapt to this conformation to avoid repulsions.
A look into possible intermolecular hydrogen-bonding interactions of (II), we found a value of 3.37 (3) Å between C1Á Á ÁN2, indicative of a weak hydrogen bond (Fig. 3, Table 1) that leads to the formation of supramolecular chains extending parallel to [101]. Nevertheless, we believe that the major intermolecular force contribution to the stabilization of the crystal structure is by aliphatic C-HÁ Á Á interactions. We observed how C6 (non-aromatic ring atom) interacts with C2 i , N2 i , and C3 i [distances are 3.672 (3), 3.692 (4), and 3.620 (3), respectively; symmetry code: (i) Àx + 1, Ày + 2, Àz). Analyzing the possibility of anyinteractions we determine that due to the reciprocal stacking of the molecules and the offset distance of the aromatic centroids (4.487 Å ), we suggest that the possibility of ainteraction between the molecules is not found. We conclude that C-HÁ Á Á interactions, although weak compared to a conventional hydrogen bond, could serve together with other intermolecular forces to impose directionality and order through the crystal lattice. Previously, C-HÁ Á Á interactions have been proven to show The molecular structure of (II) showing the atom-labeling scheme. Displacement ellipsoids are scaled to the 50% probability level.

Figure 3
Packing diagram for (II) projected along the b axis (A), and the intermolecular arrangement and distance of (II) found in the crystal structure (B) (a shows the centroid-to-centroid distance, b the centroidto-atom distance). Hydrogen atoms were omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).  Figure 1 Schematic representation of imidazole (I) with atom numbering and of the title derivative (II). stability in crystal structures. In fact, it has been suggested that aliphatic-aromatic interactions could play a greater stabilization role than aromatic-aromatic interactions (Ninković et al., 2016;Carmona-Negró n et al., 2016). Fig. 3 shows the packing diagram of (II).
Crystals of the title compound grew as very large, colorless prisms by slow sublimation at 313 K and 1.5 mbar. The crystal under investigation was cut from a larger crystal.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
The data were collected using instrumentation purchased with funds provided by the National Science Foundation grant No. 0741973.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.22 e Å −3 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.