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. Furthermore, H1 was shown to have 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. (These guests do present as mixtures in the chemical industry, which are difficult to separate through fractional distillations owing to comparable boiling points). 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. SCXRD analyses showed numerous noncovalent interactions present in all eight complexes, including a classical (host)N‒H···N(guest) hydrogen bond to each guest molecule (except 2MP in H1·2MP), which assisted in retention of the guest within the crystals of each complex.