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(μ2-RB(O)2)3(μ2-OH)(μ2-O)2(μ3-OH)} (R = i-Bu (Ta3-iBu), C6H5 (Ta3-Ph), 2,5-(CH3)2(C6H3) (Ta3-2,5Me), 3,5-(CH3)2(C6H3) (Ta3-3,5Me), and 3,5-(CF3)2(C6H3) (Ta3-3,5CF3)). 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(μ2-MesB(O)2)2[MesB(OH)(O)]2(μ2-OH)2} (Mes = 2,4,6-(CH3)3C6H2 (Ta2-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.