The role of the counter-ion in metal-organic frameworks’ chemistry and applications

A review of the synthesis, structure and specifically the effect of counter-ions on the properties of ionic metal organic frameworks (ionic-MOFs) is presented, highlighting the important advances in this regard over the past decade. Most focus is on ionic porous metal–organic frameworks (MOFs) materials consisting of porous anionic or cationic frameworks and cationic or anionic guests. The structure and performance of cationic and anionic MOFs are influenced by several factors, such as organic ligands, metal ions and charge-balancing anions. These types of anionic and cationic materials have distinct advantages in comparison with neutral MOFs in the design of functional materials and their synthesis affords several distinct advantages over that for routine neutral frameworks by virtue of the isolated charged species in confined nano-spaces. The nanosized and charged pores in these ionic structures generate a strong interaction between the host and guest molecules, including enhanced adsorption towards small gases and solvent molecules. Ionic-MOFs can be easily modified via ion exchange and accommodate other charged guest molecules, making them an ideal platform for different applications, such as functional materials. The development of synthesis methods by ion exchange and the potential applications of such cationic or anionic structures have been discussed thoroughly in this review.