The internal functionalization of the Keplerate-type capsule Mo132 has been carried out by ligand exchange leading to the formation of glutarate and succinate containing species isolated as ammonium or dimethylammonium salts. Solution NMR analysis is consistent with asymmetric inner dicarboxylate ions containing one carboxylato group grafted onto the inner side of the spheroidal inorganic shell while the second hangs toward the center of the cavity. Such a disposition has been confirmed by the single-crystal X-ray diffraction analysis of the glutarate containing {Mo132 } species. A detailed NMR solution study of the ligand-exchange process allowed determining the binding constant KL of acetate (AcO(-) ), succinate (HSucc(-) ) or glutarate (HGlu(-) ) ligands at the 30 inner coordinating sites, which vary such as K AcO -<K HGlu -<K HSucc --supported by the associated thermodynamic parameters Δr S* and Δr H*. Such a variation is mainly explained by a positive entropic gain attenuated by unfavorable steric effect. Furthermore, these results are completed by (1) H DOSY and (1) H EXSY NMR experiments which are in agreement with bulky guests firmly trapped within the cavity. At last, variable temperature (1) H NMR study below 290 K revealed a striking line broadening occurring abruptly within a 5 K range. Such an effect appears closely related to the presence of the ammonium cations suspected to be present within the cavity and then has been interpreted as an inner-phase transition leading to a frozen state.
Keywords: Keplerate; NMR spectroscopy; confinement effect; dicarboxylate; polyoxometalate.
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