Zirconium Metal-Organic Cages: Synthesis and Applications

For the last two decades, materials scientists have contributed to a growing library of porous crystalline materials. These synthetic materials are typically extended networks, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), or discrete materials like metal-organic cages (MOCs) and porous organic cages (POCs). Advanced porous materials have shown promise for various applications due to their modular nature and structural tunability. MOCs have recently garnered attention because of their molecularity that bestows them with many unique possibilities (e.g., solution-processability, structural diversity, and postsynthetic processability). MOCs are discrete molecular assemblies of organic ligands coordinated with either metal cations or metal oxide clusters of different nuclearities, resulting in architectures with inherent porosity. Notably, the molecular nature of MOCs endows them with easy solution-processability unattainable with traditional framework materials. To date, a number of stable MOCs have been reported, such as those based on Rh (Rh-O bond energy: 405 +/- 42 kJ/mol), Fe (Fe-O bond energy: 407.0 +/- 1.0 kJ/mol), Cr (Cr-O bond energy: 461 +/- 8.7 kJ/mol), Ti (Ti-O bond energy: 666.5 +/- 5.6 kJ/mol), and Zr (Zr-O bond energy: 766.1 +/- 10.6 kJ/mol). Paddle-wheel MOCs have also shown great stability in aqueous environments due to their rigid backbones. The zirconium MOC (Zr-MOCs) family emerges as a class of very robust cages for which their high bond energy endows them with high hydrothermal stability. In 2013, we reported the first four zirconocene tetrahedrons assembled from trinuclear zirconium oxide clusters with ditopic or tritopic organic ligands. Since then, significant progress in the rational design of Zr-MOC has led to an assortment of structures dedicated to meaningful applications. In this Account, we highlight the recent progress in synthesizing Zr-MOCs and Zr-MOC-based higher dimensional frameworks and their applications dedicated in our laboratories and beyond. The general Zr-MOC synthetic strategy involves assembling Zr trinuclear clusters with organic ligands (rigid or flexible) containing various functional groups. This chemistry has afforded cages with structural versatility and active sites, e.g., amino groups, for postsynthetic modifications (PSMs). Since the extrinsic porosity of cage-based frameworks is relatively weak, the resulting frameworks are susceptible to structural rearrangement after solvent removal. To circumvent this limitation, increasing the hydrogen bond ratio and strength between interlinked cages and conducting in situ catalytic polymerizations have been reported to afford permanently porous structures amenable to host-guest reactions. To expand their potential applications, multifunctional Zr-MOCs are highly desired. Such multivariate MOCs can be attained by either employing the isoreticular expansion strategy to create MOCs with high surface areas or using mixed-ligand approaches to afford heterogeneous MOCs. In addition, amorphous MOCs, flexible organic ligands, new functionalities, and MOC-based extended networks are exciting new approaches to developing materials with structural versatility and enhanced characteristics. Thereby, we believe the stability and versatility of the Zr-MOC family hold great potential in expanding and addressing challenging applications.