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 ClO4-) 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 center dot center dot center dot X- and pyridinium center dot center dot center dot X- interactions, as well as pi-stacking interactions. Competitive crystallization experiments and lattice energy calculations of the 1 center dot 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.