Construction of a Lanthanide Cage with a Hollow-Walled Cavity and Large Windows to Promote Nucleophilic Additions

Abstract

Metal–organic cages (MOCs) as artificial mimic enzymes can allow small organic molecules to freely move in and out of the cavity with confined space, which often can increase intermolecular collisions and accelerate those reactions that are difficult to occur. This kind of homogeneous catalyst possesses high catalytic activity and regioselectivity, attracting growing interest recently. However, the rational design of MOCs with large openings as well as rich active sites for efficient chemical conversions remains a great challenge. Herein, we report a decanuclear 3d-4f MOC, Zn₂Yb₈, with a hollow-walled cavity and four large windows self-assembled cooperatively by bridging ligands, Zn-based metalloligands, and lanthanide Yb³⁺ ions. The lantern-like Zn₂Yb₈ not only exposes unblocked passageways for allowing more guest molecules to penetrate the cage smoothly but also provides rich Lewis centers within the cavity, which could promote nucleophilic additions to effectively boost Friedel–Crafts alkylation and the three-component Strecker reaction. With the Zn₂Yb₈ catalyst, more than 13 bis(indolyl)methane derivatives could be synthesized easily in 53–98% through Friedel–Crafts alkylation, and the conversion of the Strecker reaction for aniline, benzaldehyde, and trimethylsilyl cyanide could achieve approximately 98% in 3 h. Furthermore, host–guest relationship investigations confirmed that the catalytic function of the Zn₂Yb₈ cage could be mainly attributed to the synergy of the inherent confinement effect, multiple Lewis catalytic sites, and host–guest electrostatic interactions in the coordination cage. The construction of the discrete 3d-4f MOC with large windows and its catalytic applications in nucleophilic additions may represent a potential approach for developing enzyme-like supramolecular nanoreactors.