The appearance of the metal organic frameworks (MOFs) has opened a new branch of research, the new structures are suitable for as sensing, drug delivery, heterogeneous catalysis, separation and storage etc. Following the discovery of MOFs, new covalent organic frameworks (COFs) were synthesised as metal-free analogous. Recently highly porous hydrogen-bond assisted organic frameworks (HOF) were reported. Most of the published organic frameworks are electronically neutral, but some of them are ionic metal–organic frameworks (iMOFs) or ionic covalent organic frameworks (iCOFs). I Intermolecular forces and molecular inflexibility are the conditions of the formation of non-covalently bonded organic frameworks.
Three different polymorphic- and solvatomorphic iHOF frameworks of the investigated molecule (1: 5,5,11,11-tetrabutyl-1,3,7,9-tetraphenyl-4,5,6,10,11,12-hexahydro-5,11- diazadibenzo[ef,kl]heptalene-5,11-diium-bromide) were prepared by using different crystallisation techniques and were structurally characterised using single crystal X-ray diffraction technique . Crystals of 1a were obtained by recrystallization from the saturated THF solution in the presence of the solid powder of compound 1 in a closed ampule. 1a crystallizes in space group Fddd and almost the half of the unit cell (42%) was found to be filled with disordered THF solvent molecules. Br- ions are placed along the channels close to the quaterner nitrogen groups. Crystals of 1b were grown by slow evaporation of the THF solvent, the dimorphic structure has the Pnna space group. In this case the void volume is ~39% per unit cell. The voids are filled with disordered solvent molecule similarly to 1a. The crystals of 1c were received by fast evaporation of the solvent mixture of THF and pyridine to almost dryness. The molecules crystallise in the P21/n space group and contain voids of ~18% where the pyridine molecules take part in the framework construction as neutral linkers. 1a and 1b are polymorphs of 1, while 1b and 1c are solvatomorphs.
In all reported three crystalline frameworks the structures are stabilized by C-H…Br- and Br-…π interactions, while pyridine is part of C-H…π interactions. The solvent THF molecules are disordered throughout the channels. The residual electron density in the channels could be taken into account by the refinement of the data with squeeze program (part of the Platon program). The necessity of molecular inflexibility in the formation of these highly porous crystals is also discussed.