Solution and structural binding studies of phosphate with thiophene-based azamacrocycles

Two thiophene-based monocyclic receptors L1 and L2 have been studied for phosphate binding in solutions (D₂O and DMSO-d₆) by ¹H NMR and ³¹P NMR titrations, and in the solid state by single crystal X-ray analysis. Results from ¹H NMR titrations suggest that the ligands bind phosphate anions in a 1:2 binding mode in DMSO-d₆, with the binding constants of 5.25 and 4.20 (in log K), respectively. The binding of phosphate to L1 and L2 was further supported by ³¹P NMR in D₂O at pH = 5.2. The crystal structure of the phosphate complex of L1 reveals unambiguous proof for the formation of a ditopic complex via multiple hydrogen bonds from NH ⋯ O and CH ⋯ O interactions.