Alternative separation strategy for o-/p-dichlorobenzene mixtures through supramolecular chemistry protocols

Abstract

N,N′-Bis(9-phenyl-9-xanthenyl)butane-1,4-diamine (H), a compound bearing two tricyclic fused ring systems linked by means of a four carbon diamino chain, was assessed for its host ability when presented with the three dichlorobenzene (DCB) isomers by means of crystallization experiments from each one. In this manner, it was shown that H was not capable of encapsulating pDCB, whilst both oDCB and mDCB successfully formed inclusion compounds with this host compound; host : guest (H : G) ratios were 1 : 1.5 in both instances. Host crystallization experiments from binary guest mixtures involving oDCB and mDCB demonstrated that H possessed only a moderate selectivity towards oDCB (the selectivity coefficients, K, were low and ranged between 2.1 and 5.4). However, remarkably, the preference of H towards oDCB when mixed in a 40 : 60 molar ratio with pDCB was overwhelming, and 89.3% of oDCB was measured in the crystals; K was significant (12.5). Of even greater prominence was the observation that when oDCB/pDCB were mixed in 80 : 20 molar proportions, only oDCB (100.0%) was observed in the complex, and K was infinite. These results demonstrate that oDCB/pDCB (40 : 60 and 80 : 20) may be separated/purified by means of H through supramolecular chemistry strategies, this being extremely challenging to achieve by means of more conventional fraction distillations due to similarities in the physical properties of these isomers. Meaningful single crystal X-ray diffraction (SCXRD) data were only possible for H·1.5(oDCB) as the crystal quality of complex H·1.5(mDCB) was poor (owing to extreme twinning). This complex (H·1.5(oDCB)) was also subjected to Hirshfeld surface analyses, while both complexes were analysed by means of thermoanalytical experiments as well: H·1.5(oDCB) experienced a multi-stepped guest release process whilst the guest in H·1.5(mDCB) escaped from the crystals of the complex in a single step. Finally, the thermal stability of the complex with preferred oDCB was higher than that with less favoured mDCB.