Tris chelating phosphate complexes of bis(thio)urea ligands

Two bisurea (L¹, L²) and one bisthiourea (L ³) ligands were synthesized and their anion coordination behavior was studied. These ligands can readily form the tris chelates [PO ₄(L)₃]^3- (1, 5, and 6) with phosphate ion (PO₄^3-) in the solid state, in which the anion is coordinated by six urea groups through 12 hydrogen bonds. Solution binding studies by ¹H NMR and UV-vis spectroscopy revealed different binding properties of the ligands toward phosphate ion. While the bis(p-nitrophenyl)- substituted bisurea L¹ retains the 3:1 (host to guest) binding ratio in solution, the diethyl derivative L² only forms 1:1 complex with phosphate ion. The more acidic thiourea L³ undergoes deprotonation/decomposition in the presence of phosphate ion. Moreover, the sulfate complex (2) of L¹ and bicarbonate (3) and carbonate (4) complexes of L² have also been obtained, which show lower coordination numbers both in the solid state and in solution.