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

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 NH center dot center dot center dot O interactions with the three urea moieties of the receptor and one CH center dot center dot center dot 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 angstrom 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 NH center dot center dot center dot 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 NH center dot center dot center dot O and five CH center dot center dot center dot 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 NH center dot center dot center dot 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 (logK). Furthermore, the binding of SO42 in the gas phase was examined by ESI mass spectrometry.