Solution-State Anion Recognition, and Structural Studies, of a Series of Electron-Rich meta-Phenylene Bis(phenylurea) Receptors and Their Self-Assembled Structures.

meta-Phenylene bis(phenylurea) receptors 1-4 were designed and synthesized to investigate the association between receptor shape, anion-selective binding and anion-directed self-assembly processes. Solution studies, performed through 1H NMR titrations with a variety of tetra- N-butylammonium salts, demonstrated strong binding of 2 equiv of H2PO4-, AcO-, BzO- anions and comparatively weak binding of Cl-, HSO4-, and SO42- anions. Binding modes and stability constants (log β) were determined by regression analysis of the obtained 1H NMR titration data in DMSO- d6, and the cooperativities of the binding interactions were probed. Host-guest complexes of receptors 1 and 2 were studied in the crystalline phase to further probe the anion-binding behavior of this motif. This included a triple-stranded helicate consisting of three strands of receptor 2 arranged around a mixed-phosphate anionic core, which was characterized by using X-ray crystallography.