New secondary building units (SBUs) in metal–organic frameworks (MOFs) open opportunities for the rational design of new MOF structures and their emerging properties. Zr-based MOFs are one of the representative classes of MOFs, featuring high chemical stability, various topological structures, and practical applications. Recently, the number of new Zr SBUs is rising, while the first type of Zr SBU (Zr6) has been exclusive for a long time. Several new Zr SBUs exhibit higher performance than that of Zr6 in catalytic and adsorptive properties. Before the discovery of several new Zr MOF SBUs, their molecular counterparts (Zr molecular clusters) have been reported. A survey on unlooked Zr molecular clusters unveils their diverse structural types. Understanding Zr molecular clusters will provide important clues to access future Zr-based MOFs. The predefined geometries of metal–organic framework (MOF) building blocks, which consist of secondary building units (SBUs) and organic linkers, are integral to the rational design of new structures. In contrast to that of organic linkers, access to new SBUs is a daunting challenge because of their complex chemistry during MOF synthesis. Here, we present novel opportunities for molecular clusters to bring future SBUs in MOF chemistry. This opinion article summarizes recently emerging structural types of Zr SBUs and provides an understanding of Zr molecular clusters, suggesting them as new SBU candidates. A structural library of Zr molecular clusters will accelerate the discovery of new Zr-based MOFs and motivate similar analysis on various classes of MOFs.
Metal-organic frameworks (MOFs) have been gaining attention as promising carbon-neutral porous materials, thanks to their high performance in gas storage, separation, and conversion. The geometric building blocks of MOFs, metal clusters and organic linkers, allow chemists to predict and synthesize new structures like assembling LEGO®. However, finding new metal building blocks is still a daunting challenge due to the complex nature of metal ions in synthesis.
A research team, led by Professor Wonyoung Choe at Ulsan National Institute of Science and Technology (UNIST), South Korea, was inspired by the molecular metal clusters previously synthesized before realized in porous materials. This implies one can predict future MOFs by looking closely at their metal building blocks.
Figure 1. Zr-based secondary building units (SBUs) and their metal–organic frameworks (MOFs).
The research team compared zirconium metal clusters found in both MOFs and molecules. Zirconium-based MOFs are one of the representative metal-organic porous materials with remarkable stability and a broad range of applications. The researchers identified seven types of zirconium building blocks in MOFs and discovered additional fourteen types of potential metal building blocks.
The research team provided a library of those various molecular clusters for future metal building blocks of MOFs, and suggested practical synthetic strategies. This research is expected to greatly accelerate the discovery of new MOFs by presenting metal cluster candidates whose structures were previously difficult to predict.
Figure 2. Synthetic strategies for future Zr secondary building units (SBUs).
“Zirconium metal clusters feature outstanding molecular adsorptive and catalytic properties depending on their structures. Very excited to see what properties the new zirconium cluster will show,” said Dr. Dongsik Nam, the first author of the study.
“This research can be extended to various metal clusters in MOFs beyond zirconium,” noted Professor Wonyoung Choe. “New metal building blocks will provide a big hint to accelerate the discovery of future carbon-neutral porous materials.”
The findings of this research have been published as an Opinion in Trends in Chemistry, a sister journal to Cell, on April 4, 2023. This study has been supported by the National Research Foundation (NRF) of Korea via the Mid-Career Researcher Program, Hydrogen Energy Innovation Technology Development Project, Basic Science Research Program, Science Research Center (SRC), and Global Ph.D. Fellowship (GPF), as well as Ulsan National Institute of Science and Technology (UNIST).
Dongsik Nam, Jiyeon Kim, and Wonyoung Choe, “Evolution of Zr nodes in metal-organic frameworks,” Trends in Chemistry, (2023).