The selectivity behaviour of 9,9′-bifluorenyl-9,9′-diol as a host compound for highly efficient separations of mixed pyridines

Abstract

This investigation explored the selectivity behaviour of 9,9′-bifluorenyl-9,9′-diol (H) as a host compound for the separation, through supramolecular chemistry strategies, of mixed pyridines (pyridine (PYR) and its methylated derivatives, 2-, 3- and 4-methylpyridine (2MP, 3MP and 4MP)). Initial single-solvent crystallization experiments demonstrated that H formed 1 : 1 host–guest inclusion complexes with each of PYR, 3MP and 4MP while, in 2MP, no crystallization occurred, and a gel remained in the glass vessel. Equimolar guest competition experiments revealed a clear host preference for 3MP and 4MP, while selectivity profiles employing binary mixed guest solutions indicated that H possessed remarkable separation potential for PYR/4MP, 2MP/3MP and 3MP/4MP mixtures, amongst numerous others. Single crystal X-ray diffraction (SCXRD) analyses corroborated these findings, revealing that 3MP and 4MP engaged in more linear hydrogen bonding interactions with H, which likely contributed to their preferential inclusion. Furthermore, these two guests also formed complexes with greater crystal density compared to H·PYR. Hirshfeld surface analyses substantiated these observations, denoting a greater percentage of hydrogen atom interactions in the H complexes with 3MP and 4MP. Further support was provided by thermal analyses, where the H·4MP complex possessed the highest thermal stability, followed by H·3MP, while H·PYR was the least stable one. These results underscore 9,9′-bifluorenyl-9,9′-diol to be a highly effective host compound for the selective separation of various PYR/MP mixtures, offering an alternative separatory strategy that is efficient and, moreover, environmentally friendly, compared with more conventional approaches.