Influence of the protonation state on the binding mode of methyl orange with cucurbiturils

• Three crystal structures of methyl orange and cucurbituril were obtained.
• Hirshfeld surface analysis was used to study the intermolecular interactions.
• Protonation state of methyl orange determines the binding mode of the complex.
• Water molecule forms hydrogen bonds with both host and guest in MO@CB8.

Binding modes of methyl orange (MO) with cucurbiturils (CBs) have been investigated by Single Crystal X-ray Diffraction and NMR Spectroscopy. Detailed study of intermolecular interactions was supported by the Hirshfeld surface analysis. Protonation state of the anionic part of methyl orange has greatly influenced the binding mode of the complex. Stabilized by hydrogen bonding at the portal, hydrophobic and dispersion interactions in the cavity, the protonated methyl orange was deeply inserted into the cavity. On the contrary, the anionic methyl orange has been pushed towards the outside of the cavity by the electrostatic repulsion between the azo group and the portal oxygen. A “water bridge” was found in MO@CB8 linking both host and guest via hydrogen bonds.

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