Binding of various anions in laterally non-symmetric aza-oxa cryptands through H-bonds: characterization of water clusters of different nuclearity

Abstract

The X-ray crystallographic study of various anionic complexes have been performed on two laterally non-symmetric aza-oxa cryptands L_o and L_m having different cavity dimension. The anions are bound through various N/C/O-H···X (X = anion) bonding interactions with the cryptand receptors. For the chloride complex of L_o (complex 1), the encapsulated chloride resides perfectly on C₃ axis and prefers to stay at the 'oxa' end despite H- bonding interactions with protonated 'aza' N atoms. In case of the perchlorate (complex 2), the anion remains outside the cavity. The anions occupy the cavity created by four arms of two cryptand molecules and also the space provided by the two arms of a single cryptand molecule. Although, L_m has a larger cavity, perchlorate still remains outside (complex 3). To probe preferences for encapsulation, cryptand L_m is allowed to react with three different binary mixtures of acids. For the case of HCl-H₂SO₄ and HCl-HNO₃ binary systems (complex 4 and 5, respectively), Cl^- is incorporated in the cavity leaving SO₄^- or NO₃^- outside. In case of HBr-HNO₃ mixture, NO₃^- is preferred over Br^- in the cavity (complex 6). In each case for L_m, the encapsulated anion is displaced away from 'oxa' end to the 'aza' end of the cavity. Interestingly, in 2-6 the secondary amino N in the bridges and the bridgehead N at the 'aza' end gets protonated. In these compounds, water clusters of various nuclearity have been identified.