Structural Study of Acidic Metallocavitands and Characterization of their Interactions with Lewis Bases

We report the synthesis and characterization of tantalum-boronate trimetallic clusters of the general formula {[Cp*Ta](3)(mu(2)-RB(O)(2))(3)(mu(2)-OH)(mu(2)-O)(2)(mu(3)-OH)} (R = i-Bu (Ta(3)-iBu), C(6)H(5) (Ta(3)-Ph), 2,5-(CH(3))(2)(C(6)H(3)) (Ta(3)-2,5Me), 3,5-(CH(3))(2)(C(6)H(3)) (Ta(3)-3,5Me), and 3,5-(CF(3))(2)(C(6)H(3)) (Ta(3)-3,5CF(3))). Three of these complexes feature a Lewis acid type cavity allowing for substrate binding in both the solid and the liquid state using a unique electrostatic interaction and a hydrogen bond. We also report the synthesis of {[Cp*Ta](2)(mu(2)-MesB(O)(2))(2)[MesB(OH)(O)](2)(mu(2)-OH)(2)} (Mes = 2,4,6-(CH(3))(3)C(6)H(2) (Ta(2)-Mes)). All complexes have been characterized by NMR and X-ray diffraction studies, and the steric and electronic effects on the boronate ligands have been investigated. The physical properties of the interaction between the tantalum-boronate clusters and tetrahydrofuran and acetone have been studied by thermogravimetric analysis, FT-IR, and Density Functional Theory calculations to characterize the molecular interactions in the resulting adducts.