Extremely effective separations of pyridine/picoline mixtures through supramolecular chemistry strategies employing (4R,5R)-bis(diphenylhydroxymethyl)-2-spiro-1′-cyclohexane-1,3-dioxolane as the host compound†

Abstract

The present investigation focussed on assessing the ability of (4R,5R)-bis(diphenylhydroxymethyl)-2-spiro-1′-cyclohexane-1,3-dioxolane (TADDOL6) to separate pyridine/methylpyridine (picoline) mixtures through supramolecular chemistry protocols. At the outset, TADDOL6 was revealed to possess the ability to form 1 : 1 host : guest inclusion compounds with each of pyridine (PYR) and 2-, 3- and 4-methylpyridine (2MP, 3MP and 4MP) in single solvent crystallization experiments. This host compound, furthermore, demonstrated enhanced selectivities in PYR/MP mixtures: preferred guests were PYR and 3MP (in the absence of PYR), followed by 4MP and then 2MP. Subsequent binary guest competition experiments showed that TADDOL6 may be employed in order to effectively separate very many of these mixtures in this way, and significant selectivity coefficients (K) were calculated in numerous instances. Single crystal X-ray diffraction (SCXRD) experiments showed that the only significant (host)π⋯π(guest) stacking interactions were those between TADDOL6 and the preferred PYR and 3MP guest molecules, while a consideration of Hirshfeld surfaces demonstrated that these preferred guests were involved in a tighter packing motif with TADDOL6 than those with 2MP and 4MP. Results from thermal analyses, more specifically when determining the guest release onset temperatures (T_on) and the enthalpies associated with these release processes, also agreed with the host selectivity order in the mixed guest competition experiments.