Cavity Effect on Perrhenate Recognition by Polyammonium Cages

The affinity of azacryptands towards perrhenate has been investigated by potentiometric, 1H NMR spectroscopic and ITC studies in aqueous solutions. The association constants could only be determined for the p-xylyl and m-xylyl azacryptands in the hexaprotonated form. The experimental results showed the outstanding affinity of the p-xylyl cryptand for perrhenate, attributable to the geometric complementarity between the anion and the cavity of the receptor. Single crystals of the inclusion complex could also be obtained. The X-ray diffraction studies indicate that the perrhenate anion is included in the cavity of the receptor and interacts with the ammonium groups by means of both direct and water-mediated H-bonding interactions. The receptors were also investigated in the presence of different anions (i.e. chloride, bromide, iodide, perchlorate and nitrate). The affinity trend demonstrates that the p-xylyl cryptand cavity is more suitable to host large anionic guests such as perrhenate, perchlorate and iodide, despite their low density charge. On the contrary, nitrate, bromide and chloride fit better into the smaller cavity of the m-xylyl cryptand. The higher affinity of the m-xylyl cage towards chloride over that of perrhenate was also demonstrated by the crystal structure of the 1:1 adduct.