Urea-functionalized M ₄L ₆ cage receptors: Anion-templated self-assembly and selective guest exchange in aqueous solutions

We present an extensive study of a novel class of de novo designed tetrahedral M ₄L ₆ (M = Ni, Zn) cage receptors, wherein internal decoration of the cage cavities with urea anion-binding groups, via functionalization of the organic components L, led to selective encapsulation of tetrahedral oxoanions EO ₄ ^n- (E = S, Se, Cr, Mo, W, n = 2; E = P, n = 3) from aqueous solutions, based on shape, size, and charge recognition. External functionalization with tBu groups led to enhanced solubility of the cages in aqueous methanol solutions, thereby allowing for their thorough characterization by multinuclear ( ¹H, ¹³C, ⁷⁷Se) and diffusion NMR spectroscopies. Additional experimental characterization by electrospray ionization mass spectrometry, UV-vis spectroscopy, and single-crystal X-ray diffraction, as well as theoretical calculations, led to a detailed understanding of the cage structures, self-assembly, and anion encapsulation. We found that the cage self-assembly is templated by EO ₄ ^n- oxoanions (n ≥ 2), and upon removal of the templating anion the tetrahedral M ₄L ₆ cages rearrange into different coordination assemblies. The exchange selectivity among EO ₄ ^n- oxoanions has been investigated with ⁷⁷Se NMR spectroscopy using ⁷⁷SeO ₄ ^2- as an anionic probe, which found the following selectivity trend: PO ₄ ^3- ≫ CrO ₄ ^2- > SO ₄ ^2- > SeO ₄ ^2- > MoO ₄ ^2- > WO ₄ ^2-. In addition to the complementarity and flexibility of the cage receptor, a combination of factors have been found to contribute to the observed anion selectivity, including the anions' charge, size, hydration, basicity, and hydrogen-bond acceptor abilities.