Tris Chelating Phosphate Complexes of Bis(thio)urea Ligands

Two bisurea (L1, L2) and one bisthiourea (L3) ligands were synthesized and their anion coordination behavior was studied. These ligands can readily form the tris chelates [PO4(L)3]3– (1, 5, and 6) with phosphate ion (PO43–) 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.