Ka Cl - Ka H2PO4 - Ka H2P2O7 The crystal structure of 2•Cl -showed in the asymmetric unit the presence of two macrocycles adopting a flat conformation (Figure ). In each macrocycle, two acylhydrazone groups coordinate to one chloride ion, whereas a water molecule fills the remanent space of the cavity (Figure ). In the complex 2•H2PO4 -, the macrocycle shows a flat conformation similar to the previous one for 2•Cl -. However, in this case, two molecules of H2PO4 -anions are observed in the macrocycle cavity (Figure ), which corroborates the 1:2 host-guest stoichiometry observed in the binding studies by NMR and UV-Vis titration experiments (Figures and ). Finally, for the 2•H2P2O7 2-complex, a 1:1 ratio between the macrocycle and the anion is observed, favored by the size of the H2P2O7 2-anion, allowing its interaction with all the four acylhydrazone groups. The four different N-H•••O hydrogen bonds have a N•••O distance in the range of 2.683 Å to 2.825 Å. Once again, the hydrogen atoms in the H2P2O7 2-anion allow interactions between anions. Nevertheless, this association was observed for only two anions, forming a dimer rather than a linear network as observed for H2PO4 -. The crystal lattice of 2•H2P2O7 2-can be described as a sandwich-type complex in which two macrocycle molecules surround two anions (Figure ). Figure . The crystal structure obtained for macrocycle 1 in the presence of Cl -. The macrocycle is shown in capped stick representation and H2O and Cl -as space filled representation. Hexyl side chains, Tetrabutylammonium, solvent molecules not found within the binding cavities as well as non H-bonding hydrogens are omitted for clarity.