Stabilization of Octahedral Metal Halide Clusters by Host–Guest Complexation with γ-Cyclodextrin: Toward Nontoxic Luminescent Compounds

γ-Cyclodextrin (γ-CD) interacts in aqueous solution with octahedral halide clusters Na2[{M6X8}Cl6] (M = Mo, W; X = Br, I) to form robust inclusion supramolecular complexes [{M6X8}Cl6@2γ-CD]2-. Single-crystal X-ray diffraction analyses revealed two conformational organizations within the adduct depending on the nature of the inner halide X within the {M6X8} core. Using 35Cl NMR and UV-vis as complementary techniques, the kinetics of the hydrolysis process were shown to increase with the following order: {W6I8} < {W6Br8} ≈ {Mo6I8} < {Mo6Br8}. The complexation with γ-CD drastically enhances the hydrolytic stability of luminescent [{M6X8}Cl6]2- cluster-based units, which was quantitatively proved by the same techniques. The resulting host-guest complexation provides a protective shell against contact with water and offers promising horizons for octahedral clusters in biology as revealed by the low dark cytotoxicity and cellular uptake.