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An in-depth analysis of the distribution of anions about planar six-membered rings in the Cambridge Structural Database reveals anion−π interactions with charge-neutral π systems to be extremely rare in crystal structures. The predominant interaction motif is C−H···anion hydrogen bonding. [ABSTRACT FROM AUTHOR]

Herein we report crystallization of self-assembled capsules functionalized with urea hydrogen-bonding groups as a means for selective separation of sulfate anion. The high complementarity and the rigid environment found in such crystalline systems impart strong discrimination between anions of different shape, like sulfate and sulfite, or anions of the same shape but slightly different size, like sulfate and selenate, with selectivity that exceeds that observed in sulfate-binding protein. Similar to natural receptors, these crystalline capsules completely isolate the anions from the aqueous solvent by encapsulating them inside rigid cavities lined with complementary hydrogen-bonding groups. Furthermore, the capsules are made from flexible building blocks, whose structure and relative orientation in the crystal can be allosterically regulated to fine-tune the anion selectivity. These characteristics suggest that crystallization of such urea-functionalized capsules from simple and flexible components represents a particularly promising approach for selective anion separation from highly competitive aqueous environments. [ABSTRACT FROM AUTHOR]

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 −, NO 3−, and ClO 4−) 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. [ABSTRACT FROM AUTHOR]

The utilization of N,N′-disubstituted ureas and thioureas as design elements in the synthesis of crystalline organic solids is reviewed. These hydrogen-bonding units are versatile yet predictable building blocks that can be rationally employed in both crystal assembly and functionalization. [ABSTRACT FROM AUTHOR]

A bis(thiourea) containing the 1,3-dimethyl-adamantane linker and t-Bu end groups self-assembles in the solid state into crystalline columnar aggregates made of hydrogen-bonded cyclic thiourea octamers with 2 nm in diameter. [ABSTRACT FROM AUTHOR]

A series of metal−organic frameworks (MOFs) functionalized with urea hydrogen-bonding groups has been synthesized and structurally analyzed by single-crystal X-ray diffraction to evaluate the efficacy of anion coordination by urea within the structural constraints of the MOFs. We found that urea-based functionalities may be used for anion binding within metal−organic frameworks when the tendency for urea···urea self-association is decreased by strengthening the intramolecular CH···O hydrogen bonding of N-phenyl substituents to the carbonyl oxygen atom. Theoretical calculations indicate that N,N‘-bis(m-pyridyl)urea (BPU) and N,N‘-bis(m-cyanophenyl)urea (BCPU) should have enhanced hydrogen-bonding donor abilities toward anions and decreased tendencies to self-associate into hydrogen-bonded tapes compared to other disubstituted ureas. Accordingly, BPU and BCPU were incorporated in MOFs as linkers through coordination of various Zn, Cu, and Ag transition metal salts, including Zn(ClO4)2, ZnSO4, Cu(NO3)2, Cu(CF3R.3)2, AgNO3, and AgSO3CH3. Structural analysis by single-crystal X-ray diffraction showed that these linkers are versatile anion binders, capable of chelate hydrogen bonding to all of the oxoanions explored. Anion coordination by the urea functionalities was found to successfully compete with urea self-association in all cases except for that of charge-diffuse perchlorate. [ABSTRACT FROM AUTHOR]

Encapsulation of SO42− into a Ni coordination framework functionalized with urea anion-binding groups allows selective separation of this strongly hydrophilic anion from a highly competitive aqueous environment. [ABSTRACT FROM AUTHOR]

Self-assembly of a tris(urea) anion receptor with Na2SO4or K2SO4yields crystalline capsules held together by coordinating Na K熫抢 and hydrogen-bonding water bridges, with the sulfate anions encapsulated inside urea-lined cavities. The sodium-based capsules can be selectively crystallized in excellent yield from highly competitive aqueous alkaline solutions (∼6 M Na, pH 14), thereby providing for the first time a viable approach to sulfate separation from nuclear wastes. [ABSTRACT FROM AUTHOR]