Künstliche molekulare Anion-Wirte. Die Synthese eines chiralen bicyclischen Guanidinium-Salzes als funktionalisierte Ankergruppe für Oxo-Anionen

Surprisingly selective sulfate extraction by a simple monofunctional di(imino)guanidinium micelle-forming anion receptor

We report a novel di(imino)guanidinium anion extractant with unparalleled selectivity for sulfate in a liquid-liquid separation system. In addition to a 4.4 order-of-magnitude enhancement in affinity compared to a standard benchmark, our alkylated di(imino)guanidinium receptor is economically synthesized and features good compatibility with application-relevant aliphatic solvents. Small-angle X-ray scattering results reveal the formation of reverse-micelles, which together with the significant organic-phase water content challenge traditional notions of selectivity in extraction of superhydrophilic anions.

Probing Binding-Mode Diversity in Guanidinium-Oxoanion Host-Guest Systems

An attempt to experimentally estimate the role of binding-mode diversity (structural fuzziness) on the molecular recognition seen in the prominent guanidinium-oxoanion host-guest pair is described. The global heat response as measured by isothermal titration calorimetry in acetonitrile, which was obtained from the interaction of five different but structurally closely related guanidinium hosts with three rigid phosphinate guests of decreasing accessibility of their binding sites, is correlated to provide a trend analysis. All host-guest associations of 1:1 stoichiometry in this series are strongly enthalpy-driven. The change in complexation entropy can be related to the tightness of the mutual fit of the host-guest partners, which approaches a minimum limit and is interpreted as the unique lock-and-key binding mode. The ordinary host-guest complexation in this ensemble features substantial positive entropy changes that correlate inversely with the binding interface area. This finding excludes desolvation effects as the major cause of entropy production, and provides evidence for the existence of a broad variety of complex configurations rather than a single binding mode to represent the associated host-guest pair. This result bears on the molecular design of systems that vitally depend on structural fidelity, such as nanoassemblies or homogeneous catalysis.