Sulfate recognition by persistent crystalline capsules with rigidified hydrogen-bonding cavities
Sulfate recognition by persistent crystalline capsules with rigidified hydrogen-bonding cavities
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DOI：
10.1002/anie.200704937
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Moyer, Bruce A.
中科院分区：
文献类型：
--
作者：
Custelcean, Radu;Remy, Priscilla;Moyer, Bruce A.
Selectivity is a fundamental property of pervasive importance in chemistry and biology as reflected in phenomena as diverse as membrane transport, catalysis, sensing, adsorption, complexation, and crystallization. Although the key principles of complementarity and preorganization governing the binding interactions underlying such phenomena were delineated long ago,[1] truly profound designed selectivity has proven elusive, in part because synthetic molecular architectures are neither maximally complementary for binding target species nor sufficiently rigid. Even if a host molecule possesses a high degree of complementarity for a guest species, with almost no exceptions it can distort its structure or even rearrange its conformation altogether to accommodate competing guests. One approach taken to overcome this challenge has been to devise three-dimensional rigid molecules that bind species within complementary cavities. Although rare examples have been reported to demonstrate the principle,[1b] such cases are not generally of practical utility, because of synthetic inaccessibility and often poor kinetics. Alternatively, more readily synthesized flexible architectures are generally employed,[2] but then the challenge becomes one of locking their structures in place. This locking has been somewhat easier to accomplish in self-assembled capsules,[3, 4] though other challenges emerge, such as finding means to internally functionalize the cavities of the capsules.[5] Taking a cue from natural binding agents [6] that derive their rigidity from a network of molecular interactions, especially hydrogen bonding, we present herein an example of a crystalline capsule that recognizes sulfate by a highly complementary array of rigidified hydrogen bonds. We have been employing crystalline host environments functionalized with anion-coordinating groups as a means to obtain maximal three-dimensional complementarity and rigidity.[7] In the present study, we focused on the problem of sulfate recognition and separation,[8, 9] motivated in part by its high relevance to environmental remediation and nuclear-waste cleanup [10] but also by the challenge of overcoming the Hofmeister bias that disfavors transfer of densely charged, highly hydrated anions.[7d] The crystalline capsules described herein display hydrogen-bonding cavities that are highly complementary to tetrahedral divalent oxoanions, and remarkably persistent in the presence of other oxoanions of various shapes and basicities, despite being made from rather simple and flexible building blocks. This system thus presents a rare opportunity to examine anion selectivity under the ideal condition of a constant hydrogen-bonding host environment, and, at the same time offering a potentially practical solution for sulfate separation from highly competitive aqueous salt solutions. The tripodal tris-urea ligand L1 was selected for this study based on analogous urea derivatives of tris (2-aminoethyl)-amine that exhibited recognition of sulfate in solution and the crystalline state.[9a, 11] Notably, the m-CN-Ph substituted trisurea ligand encapsulated sulfate in a 2: 1 complex when incorporated into a silver coordination polymer, with the resulting formation of 12 hydrogen bonds representing the highest coordination number reported for SO4 2À ions in a synthetic receptor.[9a] However, selective and efficient separation of sulfate from aqueous solutions with this or other analogous crystalline frameworks proved elusive so far owing to the low stability and relatively high solubility of the frameworks in water. We reasoned that functionalization of this tripodal scaffold with stronger coordinating groups, such as pyridines, would …
