Investigation of the separation potential of xanthenyl- and thioxanthenyl-based host compounds for pyridine and isomeric picoline mixtures

Abstract

Here, the potential of employing trans-N,N′-bis(9-phenyl-9-xanthenyl)cyclohexane-1,2-diamine (1,2-DAX) and trans-N,N′-bis(9-phenyl-9-thioxanthenyl)cyclohexane-1,2-diamine (1,2-DAT) for the purification of various pyridine (PYR), 2-methylpyridine (2MP), 3-methylpyridine (3MP) and 4-methylpyridine (4MP) mixtures, through host–guest chemistry, was investigated. All four solvents were independently clathrated by these host compounds, with a 1:1 host:guest ratio being most favoured. The recrystallization of 1,2-DAX from equimolar mixtures of these guests revealed it to possess an enhanced affinity for 4MP. Furthermore, it was shown that 1,2-DAX may be useful for the separation of solutions of 3MP/PYR, 2MP/PYR and 4MP/2MP; here, host selectivities ranged between 72.9 and 76.6%. The alternative host compound, 1,2-DAT, on the other hand, behaved contrastingly and discriminated significantly against 4MP, whilst preferring the 2-alkylated analogue. Additionally, the selectivity behaviour of 1,2-DAT far exceeded that of 1,2-DAX, and when the solutions were 2MP/4MP, 3MP/4MP and PYR/4MP, very high host selectivities were noted (86.0–94.9%), alluding to the plausibility of using this host compound for these guest separations. Varying the guest amounts in competition experiments confirmed observations from the equimolar investigations. Single crystal X-ray diffraction and thermal analyses were also employed to further analyse the complexes, the reasons for guest retention as well as their relative thermal stabilities. All of our findings are presented here.