Chloro-containing host compounds with fused tricyclic moieties and diamino linkers: host ability and affinity behaviour in pyridine/methylpyridines

Abstract

N,N′-Bis(9-(4-chlorophenyl)-9-thioxanthenyl)ethylenediamine (H1) and N,N′-bis(9-(4-chlorophenyl)-9-xanthenyl)ethylenediamine (H2) were investigated as possible separation agents for mixtures of pyridine (PYR) and methylpyridine isomers (2MP, 3MP, and 4MP) through supramolecular chemistry. MPs are present as mixtures in the chemical industry, but fractional distillation is challenging since they have a narrow boiling range, and so alternative and greener separation strategies are necessary. Initially, the host ability of H1 and H2 was assessed for these solvents; each pyridine was enclathrated by both host compounds. When guests competed, H1 and H2 behaved selectively: host affinities were in the order 3MP > PYR > 4MP ≫ 2MP (H1) and 2MP > 3MP > PYR ≫ 4MP (H2). Moreover, H1 was able to separate the 80 : 20, 60 : 40 and 50 : 50 PYR/2MP, 20 : 80 3MP/PYR and 60 : 40 PYR/4MP mixtures: high selectivity coefficients were calculated (K ≥ 10). H2 fared even better: each of the 20 : 80 2MP/PYR, 40 : 60 and 20 : 80 3MP/PYR, all mixtures of PYR/4MP except 20 : 80, all solutions of 2MP/4MP and 20 : 80 3MP/4MP mixtures may be purified in this fashion. Thermal experiments demonstrated that the favoured guest solvents formed the more stable complexes with H1 and H2. Furthermore, the SCXRD analyses provided reasons for the host affinity of H1 for 3MP, relative to the least preferred 2MP, when presented with guest mixtures. 3MP experienced three C–H⋯π close contacts with the host molecule, while this type of interaction was not observed in the 2MP-containing complex. Finally, 4MP was consistently disfavoured by H2 as a result of the fact that this guest solvent was accommodated in wide open multidirectional channels and, as a result, was the least stable complex, confirmed by thermal analysis, while the favoured 2MP formed the most stable complex of the four, being housed in unidirectional channels.