Various coordination modes of sulfate by cyanuric acid platform-based first- and second-generation urea receptors

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 SO₄^2– to the pentafluorophenyl analogue of the tris-urea receptor L3 to evaluate the various modes of SO₄^2– binding in this new generation of cyanuric acid platform-based anion receptors. The crystallographic results show a 1:1 binding pattern between L1 and SO₄^2– 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 SO₄^2–, which is structurally similar to that of the sulfate-binding protein (SBP). Interestingly, L2 shows a 2:1 host/guest binding pattern with SO₄^2– 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 SO₄^2– 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 SO₄^2–. In contrast, L3 shows 11 hydrogen-bonding interactions with the encapsulated SO₄^2– 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 SO₄^2– complex of L4, complex 4 shows 1:1 encapsulation of SO₄^2– in its inner cavity with 10 N–H···O interactions. A binding study of SO₄^2– in solution was performed with L4 in dmso by isothermal titration calorimetry (ITC). This study also showed a 1:1 binding pattern of SO₄^2– with an association constant of approximately 5.11 (log K). Furthermore, the binding of SO₄^2– in the gas phase was examined by ESI mass spectrometry.