Iris Chelating Phosphate Complexes of Bis(thio)urea Ligands

Two bisurea (L-1, L-2) and one bisthiourea (L-3) 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 H-1 NMR and UV-vis spectroscopy revealed different binding properties of the ligands toward phosphate ion. While the bis(p-nitrophenyl)-substituted bisurea L-1 retains the 3:1 (host to guest) binding ratio in solution, the diethyl derivative L-2 only forms 1:1 complex with phosphate ion. The more acidic thiourea L-3 undergoes deprotonation/decomposition in the presence of phosphate ion. Moreover, the sulfate complex (2) of L-1 and bicarbonate (3) and carbonate (4) complexes of L-2 have also been obtained, which show lower coordination numbers both in the solid state and in solution.