Hydrogen-bonded helices for anion binding and separation

Herein we report the competitive crystallization of urea-functionalized hydrogen-bonded helical frameworks as a new approach to separating anions from aqueous mixtures. N,N′-Bis(m-pyridylurea) (1) containing orthogonal pyridine and urea hydrogen-bonding functionalities forms upon monoprotonation with 1 equiv of HX acids (X = Cl^-, Br^-, I^-, NO₃^-, and ClCO₄^- an isomorphous series of crystalline hydrogen-bonded helices assembled by pyridinium ⋯ pyridine hydrogen bonds, with the urea functional groups binding the anions through chelate hydrogen bonding. The helices are further connected in the crystals by CH⋯X^- and pyridinium⋯X^- interactions, as well as π-stacking interactions. Competitive crystallization experiments and lattice energy calculations of the 1 · HX crystals showed the solvation-based Hofmeister bias that typically dominates anion separation selectivities from water was attenuated, but not completely overturned. The observed selectivity is apparently a result of the relatively soft and unspecific hydrogen-bonding environment around the anions in the crystals, combined with the high flexibility of the helices, which expand or contract as necessary to accommodate each anion.