Inclusion ability and selectivity of ethylenediamine derivatives for pyridine in the presence of methylpyridine isomers

The inclusion behaviour of compounds N,N '-bis(9-cyclohexyl-9-xanthenyl)ethylenediamine (OED) and N,N '-bis(9-cyclohexyl-9-thioxanthenyl)ethylenediamine (SED) was assessed in the presence of pyridine (PYR) and its three methylpyridine isomers (2MP, 3MP and 4MP). PYR, 3MP and 4MP were each enclathrated by OED when it was recrystallized independently from each guest solvent, but failed to include 2MP. The thio host derivative, SED, was less efficient, forming a complex only with PYR. When these guests were mixed in equimolar amounts and each host recrystallized from the mixture, OED constantly displayed a significantly enhanced preference for 4MP (near-complete in many instances), while complexation failed under these circumstances for SED, even when PYR was present in the guest mixture (despite PYR having been included in the single solvent experiment). A selectivity order of 4MP (92.8%) >> PYR (6.0%) > 3MP (0.9%) > 2MP (0.3%) was noted for OED when it was recrystallized from the equimolar quaternary mixed solvent system. The selectivity of OED towards 4MP was investigated using single crystal diffraction (SCXRD) and thermal (TA) analyses: interestingly, only 4MP experienced a strong classical hydrogen bond with OED, in direct relation to the enhanced selectivity of OED for 4MP while, additionally, this complex displayed an increased thermal stability relative to the other two complexes with OED.