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 Lo and Lm having different cavity dimensions. For the fluoride complex of Lo, 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 Lo. In comparison, Lm 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 Lm retains its symmetrical pinwheel shape viewed down the C(3) axis joining the two bridgehead amines throughout all of its halide complexes. While encapsulated fluoride resides at the 'oxa' end of Lo, the encapsulated fluoride/chloride/bromide of Lm 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.