Cation exchange in metal-organic frameworks (MOFs): The hard-soft acid-base (HSAB) principle appraisal

Cation exchange in Metal-Organic Frameworks (MOFs) is an emerging synthetic pathway for modifying chemical compositions and has been expansively investigated. It has been shown to be an important tool for leveraging novel functional MOFs that otherwise have not been achieved via the conventional technique. Yet, the governing physical and chemical factors responsible for this occurrence are not clearly understood. The most encountered interpretations are related with the presence of open metal sites, coordinated solvent or lattice distortion tolerance. There is no unifying concept that gives a detailed insight on some common observations in cation exchanged MOFs, e.g. why some MOFs can undergo complete and reversible cation exchange, complete but irreversible, partial exchange, and in some cases, they do not exchange. Other puzzles include, why certain cation can exchange others while some cannot, why exchange can proceed in certain solvents but fails in others. Herein, we qualitatively demonstrate how the concept of Hard-Soft Acid-Base principle allows a plethora of reported experimental observations dealing with cation exchange at the SBUs of MOFs could be reasonably explained. This review is intended to provide rationalization of cation exchange behaviors in both stable and labile MOFs, based on the hardness or softness of cations and ligands, effect of solvents, temperature and exchange period using the HSAB principle's point of view. We hope that this review may lead to a deeper understanding of the cation exchange behavior in MOFs, which in turns allow chemists to use it as a predictive tool for engineering novel functional MOFs with the required complexities.