Confinement inside a Crystalline Sponge Induces Pyrrole To Form N-H⋠⋠⋠π Bonded Tetramers.

Based on the DFT-level-calculated molecular volume (Vmol ) of pyrrole and its liquid density, pyrrole manifests the highest liquid density coefficient LDc (defined as [Vmol ×density ×0.6023]/FW) value of 0.7. Normal liquids have LDc <0.63. This very high LDc 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.

Halogen Bonding in a Crystalline Sponge.

Host-guest interactions are the key to the supramolecular chemistry and the further application of the receptors to study the structural details of the small guest molecules. Crystalline sponges as a kind of supramolecular receptor need to be investigated in terms of the binding ability with the guests. We found in this work that one guest with σ-hole donors and another with electron-donating species were separately entrapped in two distinct channels of the host framework via the crystalline sponge method. Halogen bonding and weak hydrogen bonding were detected between the host and the two guests, respectively. The ability of the crystalline sponge to absorb and sort guests of different types was unambiguously confirmed by X-ray crystallography.

Polymorphs of 2,4,6-tris(4-pyridyl)-1,3,5-triazine and their mechanical properties.

The second polymorph of the compound 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) is reported, TPT-II, which crystallizes in space group I2/a. Its higher density and more efficient space filling indicate the lower entropy of TPT-II, while its slightly lower melting point indicates its weaker intermolecular interactions. The conditions of the crystallization experiments for TPT-I and TPT-II are the dominant factors that determine the final crystalline products. The crystals of TPT-II are long needles. They exhibit bending behaviour along the crystallographic b direction when a mechanical force is imposed perpendicular to it, and regain their original shape after the external stress is removed. The elasticity of the single crystals is interpreted in terms of intermolecular interactions and an energy framework analysis.