Various Coordination Modes of Sulfate by Cyanuric Acid Platform-Based First- and Second-Generation Urea Receptors

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Abstract

Cyanuric acid platform-based first-generation p-nitrophenyl- (L1), p-cyanophenyl- (L2) and pentafluorophenyl- (L3) substituted tris-ureas and second-generation p-nitrophenyl-functionalized hexakis-urea receptor (L4) have been synthesized. Detailed solid-state structural studies on the binding of sulfate to L1, L2 and L4 have been examined thoroughly by single-crystal X-ray diffraction and compared with the binding modes of SO42– to the pentafluorophenyl analogue of the tris-urea receptor L3 to evaluate the various modes of SO42– binding in this new generation of cyanuric acid platform-based anion receptors. The crystallographic results show a 1:1 binding pattern between L1 and SO42– through six N–H···O interactions with the three urea moieties of the receptor and one C–H···O interaction with one of the tetrabutylammonium counter cations in complex 1. Thus, this complex shows an uncommon example of a seven-coordinate SO42–, which is structurally similar to that of the sulfate-binding protein (SBP). Interestingly, L2 shows a 2:1 host/guest binding pattern with SO42– that generates a dimeric capsular assembly with dimensions of approximately 10.398 Å in complex 2. In this case, each of the oxygen atoms of SO42– forms a trifurcated hydrogen bond with the urea NH groups and thus a total of 12 N–H···O hydrogen-bonding interactions can be observed for SO42–. In contrast, L3 shows 11 hydrogen-bonding interactions with the encapsulated SO42– in complex 3 assisted by two nBu4N+ counter cations through six N–H···O and five C–H···O interactions. In the case of the SO42– complex of L4, complex 4 shows 1:1 encapsulation of SO42– in its inner cavity with 10 N–H···O interactions. A binding study of SO42– in solution was performed with L4 in dmso by isothermal titration calorimetry (ITC). This study also showed a 1:1 binding pattern of SO42– with an association constant of approximately 5.11 (log K). Furthermore, the binding of SO42– in the gas phase was examined by ESI mass spectrometry.

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