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 a 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, as 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 being in the linear zigzag structure found in the X-ray structure of a solid pyrrole. The confinement thus acts as an external driving force for tetramer formation.