Halide binding in laterally non-symmetric aza-oxa cryptands through N/O/C-H··· halide interactions with characterization of small water clusters

Abstract

The structural study of halide complexes has been performed on two laterally non-symmetric aza-oxa cryptands L_o and L_m having different cavity dimensions. For the fluoride complex of L_o, the 'oxa' end bridgehead N atom gets protonated to pull the internal fluoride towards the 'oxa' end. While bromide remains outside the cavity either by being sandwiched between the opposite-side directed arms of the two different moieties or by being incorporated into the space provided by the two same-side directed arms of a single L_o. In comparison, L_m having a higher cavity dimension can accommodate chloride and bromide in addition to fluoride in its cavity. While Lo changes its conformation for its bromide complex attaining a 'Y'-shaped architecture, cryptand L_m retains its symmetrical pinwheel shape viewed down the C₃ axis joining the two bridgehead amines throughout all of its halide complexes. While encapsulated fluoride resides at the 'oxa' end of L_o, the encapsulated fluoride/chloride/bromide of L_m is displaced far away from the 'oxa' end to the 'aza' end. However, iodide remains outside the cavity for both cryptands. Two discrete water heptamers of acyclic and cyclic/acyclic configurations have been characterised by X-ray diffraction. Discrete water trimer, tetramer and adamantanoid clusters have also been identified within these organic crystal hosts.