Sulfate encapsulation by hydrogen bonding in organic clefts based on biphenyl-bridged oligourea ligands

Sulfate encapsulation faces great challenges due to the large hydration energy. Herein, a sulfate co-crystal [1, TBA4L(SO4)2, C104H174N16O20S2],was synthesized and characterized by 1H NMR, elemental analysis and Fourier transform infrared spectroscopy (FTIR). The X-ray single-crystal diffraction analysis showed that the sulfate anion was encapsulated by multiple hydrogen bonds in organic clefts containing biphenyl-bridged oligourea ligands. In the extended structure, a macrocycle including four ligands L and eight SO42- anions was constructed by N-H & sdot;& sdot;& sdot;O and C-H & sdot;& sdot;& sdot;O hydrogen bonds, which was further extended to two-dimensional hydrogen bonding network. DFT calculations showed that two sulfate anions were encapsulated by two clefts constructed by biphenyl bridged oligourea ligand, respectively, with the four corresponding tetrabutylammonium cations locating on both sides of the sulfate anions. This encapsulation mode is consistent with that of the crystal structure of the sulfate anion co-crystal 1. X-ray powder diffraction was used to study the phase purity of the bulk sample of co-crystal 1. In addition, the thermal stability of co-crystal 1 was explored by TG-DTG method, and the apparent activation energy Ea of co-crystal 1 was calculated during the main thermal decomposition process. Furthermore, the sulfate binding properties of L was investigated using UV/Vis spectroscopy method in CH3CN/ DMSO solution. The binding ratio of ligand L to sulfate anion was derived as 1:2 by Job's plot, with the binding constants K1 = 2.45 x 105 M-1 and K2 = 2.46 x 105 M-1, obtained by sulfate titration experiments. The study provides a new method for sulfate encapsulation and relative application.