Confinement Inside a Crystalline Sponge Induces Pyrrole to Form N-H…π Bonded Tetramers.

Based on the DFT-level calculated molecular volume (V mol ) of pyrrole and its liquid density, pyrrole manifests the highest liquid density coefficient LD c (defined as [V mol • density • 0.6023]/FW) value of 0.7. Normal liquids have LD c < 0.63. This very high LD c is due to the strong N-H … π interactions in solution and hence pyrrole can be considered to be a pseudo-crystalline liquid. When trapped inside the confined space of the crystalline sponge a reorientation of the N-H … π interaction is observed leading to specific cyclic N-H … π tetramers and N-H … π dimers, verified by single crystal X-ray crystallographic and computational methods. These tetramers are of the same size as four pyrrole molecules in the solid-state of pyrrole, yet the cyclic N-H … π intermolecular interactions are circularly oriented instead of the linear zig-zag structure found in the X-ray structure of a solid pyrrole. The confinement thus acts as an external driving force for the tetramer formation.