First sphere coordination of divalent metal cations by cyclodextrin: structure of the β-cyclodextrin–calcium chloride–water (1/2/11.25) compound

The structure of the title compound provides the first illustration of direct coordination of a polyvalent cation, as well as an inorganic anion, by β-cyclodextrin (β-CD). Crystal data: P2₁2₁2₁, Z = 4, a = 15.875 (1), b = 17.583 (1), c = 24.270 (2) Å, V = 6775 (1) ų, R = 0.059 for 4744 unique observed reflections with I > 3σ(I). There are two CaCl₂ per β-CD, with no direct contacts between the ions. None of the ions are included within the cyclodextrin cavity. The cations are in direct interaction simultaneously with two or three β-CD molecules and an eightfold coordination occurs for both. No disorder was found except for the two water molecules situated inside the cavity. The crystalline form is a novel monomer-type structure. The β-CD molecules are stacked as monomer entities inclined at ~50° along a twofold screw axis, such that they are arranged in a new herringbone-like scheme. The overall packing is stabilized by intermolecular interactions between cyclodextrin monomers and by direct coordination to the ions. Moreover, the cohesion is reinforced by hydrogen bonds involving water molecules arranged in infinite chain motifs traversing the whole structure.

-1