Exploring Supramolecular Chemistry as an Innovative Strategy for Pyridine Guest Separations When Employing a TADDOL Derivative as the Host Compound

Abstract

The host compound (4R,5R)-bis(diphenylhydroxymethyl)-2-spiro-1′-cyclopentane-1,3-dioxolane (TADDOL5), based on the TADDOL (α,α,α’,α’-tetraaryl-2,2-disubstituted 1,3-dioxolane-4,5-dimethanol) design, was crystallized from each of pyridine (PYR), 2-methylpyridine (2MP), 3-methylpyridine (3MP), and 4-methylpyridine (4MP). TADDOL5 was thus shown to possess host ability for each of these guest solvents since 1H NMR spectroscopy revealed the formation of 1:1 host:guest (H:G) inclusion compounds in each instance. Crystallization of TADDOL5 from pyridyl mixtures, both equimolar and nonequimolar in nature, provided remarkable results, with extremely high affinities toward 3MP being observed. In the absence of 3MP, PYR was then overwhelmingly favored. It was demonstrated that TADDOL5 has the ability to separate very many of these pyridyl mixtures (which are challenging to separate by means of more conventional fractional distillations). In fact, in many of the nonequimolar binary pyridyl experiments, selectivity coefficients (K) were extremely high, oftentimes being infinite. The single solvent complexes were, furthermore, subjected to single-crystal X-ray diffraction (SCXRD) analyses, and it was shown that the host selectivity behavior was facilitated by significantly shorter (host)O-H···N(guest) hydrogen bonding in the 3MP-containing complex (3MP being favored in the guest competition experiments). Additionally, the guest next preferred, PYR, also experienced a much shorter hydrogen bond with the host molecule compared to disfavored 2MP and 4MP, but which was longer than the complex with 3MP, in direct accordance with the host selectivity behavior for these guest species. Finally, thermal analyses on the single solvent complexes confirmed that 3MP formed a more stable complex with TADDOL5, followed by PYR, while the 2MP- and 4MP-containing complexes were less stable.