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 Na 2 [{M 6 X 8 }Cl 6 ] (M = Mo, W; X = Br, I) to form robust inclusion supramolecular complexes [{M 6 X 8 }Cl 6 @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 {M 6 X 8 } core. Using 35 Cl NMR and UV-vis as complementary techniques, the kinetics of the hydrolysis process were shown to increase with the following order: {W 6 I 8 } < {W 6 Br 8 } ≈ {Mo 6 I 8 } < {Mo 6 Br 8 }. The complexation with γ-CD drastically enhances the hydrolytic stability of luminescent [{M 6 X 8 }Cl 6 ] 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.