Steric Control over the Assembly of Zirconium Metal–Organic Cages Using Zirconium Clusters with Bulky Capping Ligands

Zirconium metal–organic cages are an emerging class of highly tunable supramolecular materials that feature porous structures as well as relatively high thermal and chemical stability. Self-assembly of zirconium cages with ditopic organic linkers can result in the formation of porous tetrahedral or less porous lantern-shaped geometries, posing a key challenge in the synthesis and application of these materials. Herein, we demonstrate steric control in the synthesis of tetrahedral zirconium cages ([((CpZr(μ2-OH))3(μ3-O))4(L)6]X4; Cp– = cyclopentadienide; L2– = dicarboxylate linker; X– = OTf– or Cl–) by tuning the steric profile of zirconium clusters ([(CpZr(μ2-OH))3(μ3-O)(RCO2)3]X; RCO2– = carboxylate capping ligand) used as precursors for cage synthesis. Specifically, zirconium clusters with bulky carboxylate capping ligands hinder the assembly of the more compact lantern geometry, which ultimately leads to selective formation of the tetrahedral structure. Using this approach, we developed a general method for the selective synthesis tetrahedral zirconium metal–organic cages as either triflate or chloride salts. This method was used to synthesize six different zirconium cages, which were characterized by 1H NMR spectroscopy, high-resolution mass spectrometry, gas adsorption measurements, powder X-ray diffraction, and thermogravimetric analysis. We further show that the method enables synthesis of tetrahedral cages with the same selectivity at the gram scale. This work provides a facile route for the synthesis of a wide range of zirconium cages and highlights precursor design as a powerful tool to realize synthetic control over supramolecular cage assembly.