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Capsule Formation, Carboxylate Exchange, and DFT Exploration of Cadmium Cluster Metallocavitands: Highly Dynamic Supramolecules

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posted on 24.03.2010, 00:00 by Peter D. Frischmann, Glenn A. Facey, Phuong Y. Ghi, Amanda J. Gallant, David L. Bryce, Francesco Lelj, Mark J. MacLachlan
A family of molecular heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temperature solution and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT) calculations. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host−guest and supramolecular phenomena. Specifically, dimerization of these metallocavitands yields capsules with vacant coordination and hydrogen-bonding sites accessible to encapsulated guests. Strong host−guest interactions explain the exceptionally high packing coefficient (0.80) observed for encapsulated N,N-dimethylformamide (DMF). The guest-accessible hydrogen-bonding sites arise from an unusual μ3-OH ligand bridging three cadmium ions. Thermodynamic and kinetic studies show that dimerization is an entropy-driven process with a highly associative mechanism. In DMF the exchange rate of peripheral cluster supporting carboxylate ligands is intrinsically linked to the rate of dimerization and these two seemingly different events have a common rate-determining step. Investigation of guest dynamics with solid-state 2H NMR spectroscopy revealed 3-fold rotation of an encapsulated DMF molecule. These studies provide a solid understanding of the host−guest and dynamic properties of a new family of metallocavitands and may help in designing new supramolecular catalysts and materials.