Much research has been devoted to molybdenum octahedral clusters Mo 6 since the discovery of the A x Mo 6 Y 8 solidstate series (Y = S, Se, Te) in the early 1970s.[1] Indeed, their interesting physical properties and potential applicationse.g., superconductivity at high critical field, thermoelectric, catalysis, or redox intercalation processes -have stimulated the research of many groups. [2] (Fig. 1). The physical properties of Mo 6 solid-state compounds are related to the number of electrons available for metal-metal bonding within the cluster (valence electron count, VEC) and to the strength of interaction between the units. Mo-centered electrons are located on twelve metal-metal bonding molecular orbitals of the molecular orbital diagram. Their full occupation leads to a closed-shell configuration with a VEC of 24.[ [8,9] that can be used for the formation and organization of supramolecular assemblies as well as hybrid materials. Hybrids can be synthesized either by the grafting of functional donor ligands in apical position or through the association of anionic cluster units with organic or organometallic cations by cation metathesis or electrochemical techniques.[10]The large emission region of the [Mo 6 X 14 ] 2-anion in the red and near infrared (580-900 nm) is particularly interesting for biotechnology applications as it is selectively transmitted through tissues owing to the relatively low absorption at these wavelengths.[11] Anionic Mo 6 cluster units are usually associated with alkali counter cations within inorganic solids. Indeed, the use of inorganic cluster compounds as luminescent dyes, for instance in bio-imaging strategies, presupposes that both clusters and counter cations are embedded in an inert matrix in order to avoid ionic diffusion, oxidization of the cluster, or apical ligand exchanges in aqueous media, which will precipitate the cluster as a hydroxo species.

Water-soluble salts of anionic [Re Q (CN) ] (Q=S, Se, Te) chalcogenide octahedral rhenium clusters react with γ-cyclodextrin (γ-CD) producing a new type of inclusion compounds. Crystal structures determined through single-crystal X-ray diffraction analysis revealed supramolecular host-guest assemblies resulting from close encapsulations of the octahedral cluster within two γ-CDs. Interestingly, nature of the inner Q ligands influences strongly the host-guest conformation. The cluster [Re S (CN) ] interacts preferentially with the primary faces of the γ-CD while the bulkier clusters [Re Se (CN) ] and [Re Te (CN) ] exhibit specific interactions with the secondary faces of the cyclic host. Furthermore, analysis of the crystal packing reveals additional supramolecular interactions that lead to 2D infinite arrangements with [Re S (CN) ] or to 1D "bamboo-like" columns with [Re Se (CN) ] and [Re Te (CN) ] species. Solution studies, using multinuclear NMR methods, ESI-MS and Isothermal titration calorimetry (ITC) corroborates nicely the solid-state investigations showing that supramolecular pre-organization is retained in aqueous solution even in diluted conditions. Furthermore, ITC analysis showed that host-guest stability increases significantly ongoing from S to Te. At last, we report herein that deep inclusion alters significantly the intrinsic physical-chemical properties of the octahedral clusters, allowing redox tuning and near IR luminescence enhancement.