Redox-active metal-organic cages for multielectron redox processes

Multielectron redox processes play crucial roles in energy storage, catalysis, and electrochemical energy conversion. The mechanisms of multielectron redox processes often involve stepwise redox events, multiple intermediate states, and electronic communication between redox sites. This complexity makes the understanding of these processes challenging. Metal-organic cages (MOCs) serve as an ideal platform to investigate multielectron redox processes because of their following features. MOCs are supramolecules composed of metal nodes and organic ligands, to which multiple redox-active sites can be introduced, and their designability and well-defined structures allow control over the number, spatial arrangement, and degree of electronic communication of the redox-active sites. This review highlights the critical role of the molecular-level behavior of MOCs near the electrode surface in understanding systems with multiple discrete redox-active sites. Redox-active MOCs (RMOCs) can be categorized into four groups based on which parts of the MOC structure possess redox-active components: metal nodes, organic ligands, metalloligands, or guests. The structure and unique electrochemical behavior of RMOCs are discussed by introducing how these MOCs behave in the electrical double layer (EDL), which unveils the characteristic features that distinguish RMOC from conventional redox-active small molecules. We also outline the future applications of RMOC in catalysis, energy storage, and sensing.