Synthesis and anion binding properties of (thio)urea functionalized Ni(II)-salen complexes

Salen ligands (salen = N,N'-ethylenebis(salicylimine)) are well-known for their versatility and widespread utility in chelating metal complexes. However, installation of hydrogen-bonding units on the salen framework, particularly functional groups that require amine-based precursors such as (thio)ureas, is difficult to achieve without the use of protecting group strategies. In this report, we show that the phenylketone analog of salicyladehyde is a stable alternative that enables the facile installation of hydrogen bonding (thio)urea groups on the salen scaffold, thus imparting anion binding abilities to a metal salen complex. Synthesis of symmetric N-phenyl(thio)urea salen ligands functionalized at the 3,3'-position and an unsymmetric salen ligand with N-phenylurea at the 5-position was achieved. Subsequent metalation with nickel(II) acetate afforded the nickel(II) complexes that were investigated for their anion binding properties towards F^-, Cl^-, Br^-, CH₃COO^-, and H₂PO₄^-. Solid-state structures of the nickel(II) complexes as well as the Cl^- bound dimer of the symmetric urea complex were obtained. The unusual acidity of the (thio)urea groups is reflected in the pK_a-dependent anion binding behavior of the nickel(II) complexes, as elucidated by ¹H and ¹⁹F Nuclear Magnetic Resonance (NMR) spectroscopy and Diffusion Ordered Spectroscopy (DOSY) experiments.