Observing Anion Binding in Single Charge-Neutral Metal-Organic Frameworks through C-H Hydrogen-Bonding Interactions.

Achieving anion capture with metal-organic frameworks (MOFs) usually relies on anion exchange reactions. Here, we report the direct visual imaging of the anion binding process within a charge-neutral Bi-based MOF (UU-200) in water at the single-particle level using in situ dark-field optical microscopy. Notably, an unexpected anion-induced structural shrinkage of UU-200 is mapped, and concentration-dependent responses are applied to determine the association constants. The resulting anion affinity is correlated with its basicity, demonstrating that charge-dense anions such as F^-, SO₃^2-, and SO₄^2- feature strong binding with the UU-200 framework. Moreover, the unusual anion binding processes are identified as the C-H hydrogen-bonding interactions between electron-deficient hydrogen atoms on the channel wall and negatively charged anions by combining imaging results, nuclear magnetic resonance spectroscopy, and theoretical simulation. These discoveries reshape and strengthen our fundamental understanding of the anion capture within MOFs, favoring the rational design of MOF-based anion receptors.