Frustrated behavior of Lewis/Brønsted pairs inside molecular cages

Abstract

Different endohedrally functionalized cages were designed to investigate the effects of the size and shape of molecular cavities on the frustrated behavior of Lewis/Brønsted acid–base pairs and on catalytic activities. The shape of the inner space above the reactive center was found to strongly affect these properties. When an acidic azaphosphatrane is inserted in the smallest cage and associated with t-BuOK or ⁻CD₂CN bases, a frustrated Brønsted pair is obtained. In contrast, when encapsulated in the medium or large cage, the P–H⁺ azaphosphatrane acid is easily deprotonated under the same conditions. The resulting two supramolecular Verkade's superbases lead to frustrated Lewis pair systems in the presence of TiCl₄. Furthermore, the larger cage displays better catalytic activity in the MBH reaction. Thus, a small change in the cage size, which only differs by one methylene group in the linkers, can influence the frustrated properties of the related systems, and a right balance between the frustrated behavior and cage flexibility has to be reached to obtain optimal systems for catalytic applications.