Engineering Cavity and Aperture Binding Sites Within a Metal-Organic Cage for Up- and Down-Regulation of Catalysis

Engineering molecular recognition events into catalytic systems to precisely control the up- or down-regulation of catalysis in a biomimetic fashion is a challenging goal in supramolecular chemistry. In this work, we report on the construction of a new metal-organic cage, Zn^II ₄L₄ tetrahedron 1, using a protonated azacalix[3](2,6)pyridine-based ligand as the capping faces. The cage features a large cavity and wide gaps between its faces, enabling the simultaneous binding of anionic guests centrally and peripherally. Encapsulation of α-Mo₈O₂₆ ^4- within the T-symmetric tetrahedron 1 leads to a C₃-symmetric inclusion complex Mo₈O₂₆ ^4-⊂1. The apertures of Mo₈O₂₆ ^4-⊂1 act as secondary binding sites for accommodating tetraarylborate guests or for providing access to the included Mo₈O₂₆ ^4- for catalyzing reactions. Catalytic experiments demonstrated that inclusion within 1 significantly enhances the catalytic activity of Mo₈O₂₆ ^4- for the oxidation of sulfides into sulfoxides. In contrast, peripheral binding of the bulky tetraarylborate anion to the inclusion complex Mo₈O₂₆ ^4-⊂1 effectively inhibits its catalytic activity by obstructing access to the catalytic active sites of Mo₈O₂₆ ^4-.