Effective alternative host-guest separation strategies for mixed pyridine/methylpyridines with thioxanthenyl- and xanthenyl-derived host molecules

Abstract

N,N’-Bis(9-(4-fluorophenyl)-9-thioxanthenyl)ethylenediamine (H1) and N,N’-bis(9-(4-fluorophenyl)-9-xanthenyl)ethylenediamine (H2) were demonstrated to have inclusion ability for pyridine (PYR) and the methylpyridine isomers (2MP, 3MP and 4MP); H:G ratios were 1:1 or 1:2. When guests competed, the host selectivities were observed to be in the order 4MP > 3MP > 2MP > > PYR (H1) and PYR > 4MP > 2MP > > 3MP (H2) and so substitution of the sulfur atom for oxygen in the central B ring of the host compound resulted in significant changes in the host selectivity behaviour. Owing to the fact that these guests present as mixtures in the chemical industry and are difficult to separate through fractional distillations as a result of their comparable boiling points, we subsequently investigated whether these host compounds have the ability to separate binary pyridyl mixtures in various molar ratios through supramolecular chemistry strategies. As such, it was demonstrated that H1 has the ability to separate the 80:20 4MP/PYR solution (K = infinite, in favour of 4MP), while H2 was even more effective, able to separate numerous of the binary mixtures employed here. Thermal experiments demonstrated that H1·4MP (4MP being preferred by H1) was thermally more stable than the other three complexes; H1·PYR (least favoured guest) was unstable even at ambient conditions. The affinity behaviour of H2, however, could not be as readily explained through thermal analyses. All of the complexes presented here experienced numerous non-covalent short contacts, including a (host)N–H···N(guest) classical hydrogen bond in each one (this was only absent in H1·2MP). This technique also provided the reasons for the affinity of H1 for 4MP and its constant low preference for PYR, and also why H2 favoured PYR and never 3MP.