Cavity Effect on Perrhenate Recognition by Polyammonium Cages

The affinity of azacryptands towards perrhenate has been investigated by potentiometric, 1H-NMR and ITC studies in aqueous solution. Only for the p-xylyl and m-xylyl azacryptands, in the hexaprotonated form, the association constants could be determined. The experimental results pointed out the outstanding affinity of the p-xylyl cryptand for perrhenate, attributable to the geometric complementarity between the anion and the receptor’s cavity. Single crystals of the inclusion complex could be also obtained. The X-ray diffraction studies indicate that the perrhenate anion is included into the receptor’s cavity, 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’s 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 the smaller cavity of the m-xylyl cryptand. The higher affinity of the m-xylyl cage towards chloride with respect to perrhenate was also demonstrated by the crystal structure of the 1:1 adduct.