The single crystal X-ray structure of receptor 2c•Cl -complex (Figure , a) shows the rigid macrocyclic scaffold forming four N-H•••Cl -hydrogen bonding interactions with a Cl -anion with bond lengths between 3.251-3.341 Å and two C-H•••Cl -hydrogen bonds with lengths between 3.411-3.427 Å. Rather than being bound within the cavity, the anion is situated just above the plane, suggesting that the ionic radius of Cl -is slightly too large for the macrocyclic cavity to accommodate. The solid-state structure of 3c is reasonably planar (Figure ). With no guest bound within the central cavity, two of the amide protons point up while the other two point down. Extended stacked structures were also observed in the solid state where the amide protons of one 3c molecule participated in N-H•••O intermolecular hydrogen bonding interactions with the urea oxygen atom of an adjacent receptor with interaction lengths between 2.846-3.138 Å. In contrast, the inclusion of the F -atom directly within the binding cavity of 2a resulted in a greater out-of-plane curvature of the 2a•F -complex (Figure ). All six directional hydrogen bond donors within the binding site are coordinated to the F -anion with N-H•••F -interaction lengths between 2.827-3.013 Å and C-H•••F -hydrogen bonding interaction lengths between 2.953-2.961 Å.