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

A family of mol. heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temp. soln. and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and d. functional theory (DFT) calcns. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host-guest and supramol. 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 coeff. (0.80) obsd. for encapsulated N,N-dimethylformamide (DMF). The guest-accessible hydrogen-bonding sites arise from an unusual μ3-OH ligand bridging three cadmium ions. Thermodn. 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-detg. step. Investigation of guest dynamics with solid-state 2H NMR spectroscopy revealed 3-fold rotation of an encapsulated DMF mol. 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 supramol. catalysts and materials. [on SciFinder(R)]