Confinement Inside a Crystalline Sponge Induces Pyrrole to Form N−H⋅⋅⋅π Bonded Tetramers

Abstract

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.