Hydrogen-Bonded Helices for Anion Binding and Separation

• Published in: Crystal growth & design 2008-06, Vol.8 (6), p.1909-1915
• Main Authors: Custelcean, Radu, Jiang, De-en, Hay, Benjamin P., Luo, Wensui, Gu, Baohua
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Organic compounds; Physics; Solid-solid transitions; Specific phase transitions; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg800137e

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···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.