Topology Exploration in Highly Connected Rare-Earth Metal-Organic Frameworks via Continuous Hindrance Control

The structural diversity of highly connected metal-organic frameworks (MOFs) has long been limited due to the scarcity of highly connected metal clusters and the corresponding available topology. Herein, we deliberately chose a series of tritopic linkers with multiple substituents to construct a series of highly connected rare-earth (RE) MOFs. The steric hindrance of these substituents can be systematically tuned to generate various linker rotamers with tunable configurations and symmetries. For example, the methyl-functionalized linker (L-CH₃) with C_{2 v} symmetry exhibits larger steric hindrance, forcing two peripheral phenyl rings perpendicular to the central one. The combination of C_{2 v} linkers and 9-connected RE₆ clusters leads to the formation of a new fascinating (3,9)-c sep topology. Unlike Zr-MOFs exhibiting Zr₆ clusters in various linker configurations and corresponding different structures, the adaptable RE₆ clusters can undergo metal insertion and rearrange into new RE₉ clusters when connected to an unfunctionalized linker (L-H) with C₁ symmetry, giving rise to a new (3,3,18)-c ytw topology. More interestingly, by judiciously combining the linkers with both small and bulky substituents through mixed-linker strategies, an RE₉-based MOF with an engaging (3,3,12)-c flg topology could be obtained as a result of continuous steric hindrance control. In this case, the two mixed linkers adopt configurations with moderate steric hindrances. Molecular simulation demonstrates that the combination of substituents with various steric hindrances dictates the resulting MOF structures. This work provides insights into the discovery of unprecedented topologies through systematic and continuous steric tuning, which can further serve as a blueprint for the design and construction of highly complicated porous structures for sophisticated applications.