Polyoxometalates (POMs) are inorganic metal-oxygen anionic clusters of nanometric size. They represent a wide family of molecules with versatile properties, which have been shown to be potentially attractive for catalysis, medicine, magnetism, or supramolecular chemistry.

We present DFT calculations and geometry optimizations carried out on three host-guest complexes labelled [Mo₁₂Adip]^2-, [Mo₁₂Pim]^2- and [Mo₁₂Sub]^2-. The host is a neutral, ring-like oxothiomolybdate with formula Mo₁₂O₁₂S₁₂(OH)₁₂.^[1] This structure was shown to encapsulate dicarboxylate anions with linear, flexible chains with formula [O₂C-(CH₂)_n-CO₂]^2-, such as adipate (n=4), pimelate (n=5), and suberate (n=6). The stability of the host-guest systems decreases as the alkyl chain becomes longer, due to the increasing constraints affecting the guest.^[2]

Car-Parrinello molecular dynamics (CPMD) simulations have also been carried out on these three host-guest complexes, in order to visualize the mutual adjustment of the host and guest fragments. Simulations have been carried out on both the free dicarboxylates and the whole host-guest complexes in order to point out and visualize the rotations occurring around the C-C bonds of the alkyl chains, resulting in conformational changes in the carboxylate.

Such changes could be characterized at relatively high temperatures (500 K), without leading however to the decomposition of the complex. They seem to occur more easily for suberate, the dicarboxylate with the longest carbon chain, illustrating the higher flexibility of this “floppy” molecule.

[1] E. Cadot, F. Sécheresse, Chem Commun, 2002, 2189.

[2] J.-F. Lemonnier, S. Floquet, A. Kachmar, M.-M. Rohmer, M. Bénard, J. Marrot, E. Terazzi, C. Piguet, E. Cadot, Dalton Trans., 2007, 28, 3043.

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