“Wurster-Type” Ureas as Redox-Active Receptors for Anions

Four redox-active receptors, 1−4, based on the incorporation of p-phenylenediamine(s) within a urea framework, were synthesized, and the affinities of two for a series of anions were quantified through UV−vis and NMR spectroscopic studies. The structure of 1 was confirmed by X-ray crystallography. For the oxoanions studied, complex stabilities approached 106 M−1 in acetonitrile and decreased with the decreasing basicity of the anion (CH3COO− > C6H5COO− > H2PO4− > NO2− > NO3−). The presence of the urea functionality caused an increase in the oxidation potential of the p-phenylenediamine subunit compared to that of free p-phenylenediamine. Electrochemical studies of the anion complexes revealed two-wave behavior with the appearance of a second oxidation wave cathodic to that in the free receptors and characteristic of the bound anion. Ab initio DFT studies of a representative acetate complex revealed the consequences of host oxidation state on complex structure.