Why design matters: From decorated metal-oxide clusters to functional metal-organic frameworks

The opportunity to generate functional solids with defned properties by deliberate design has not been materialized in traditional solid-state chemistry over many decades. The emergence of metal–organic frameworks (MOFs), permanently porous, crystalline solids with defned metrics, has allowed for studying design, synthesis, and properties, which then translated into new applications. Aggregates of metal ions stitched together by multidentate functional groups form such metal oxide clusters and represent the nodes of MOFs. These clusters, termed secondary building units (SBUs), are decorated with organic moieties that provide directionality and can be linked through geometric principles into extended nets using organic molecules (spacers). This concept of reticular chemistry has aforded permanently porous MOFs, and has resulted in over 20,000 structures over the past 20 years. However, there are still only a limited number of symmetric, discrete SBUs commonly used to design and synthesize MOFs. We herein introduce the most important SBUs that have emerged over time together with prototypal MOF structures and their fundamental applications. Both the discovery and the scientifc impact will be highlighted alongside advantages and/or drawbacks. In addition, an outlook will be given on how the combination of multiple SBUs can lead to heterogeneous but ordered materials with higher complexity and functionality.