Metal Azolate Frameworks: From Crystal Engineering to Functional Materials

With a series of selected examples, the recent progress in the chemistry of MAFs has been discussed. As we can see in the previous sections, MAFs indeed represent a unique and interesting class of coordination polymers and the investigations have been very active in recent years. Some important advances in this multifaceted field are summarized and highlighted here. The well-defined coordination geometries of azolates have largely simplified the difficulty of design and synthesis. Compared to other metalligand systems, many MAFs possess highly predictable coordination geometries and connectivities. Nevertheless, some rational molecular design and synthetic strategies have been developed. A particularly interesting and unique one is the use of uncoordinated side groups to direct the relative orientations of adjacent building blocks and extended framework structures, which has led to a series of novel structures not easily obtained from other metalligand systems. Besides intriguing structures, MAFs also exhibit very interesting properties. As a result of their unique bonding characters, MAFs, especially those based on the diazolates, usually have very high thermal and chemical stability, which is remarkable because stability is a very important issue for practical applications. On the other hand, framework flexibility is also widely observed, particularly in those with simple connectivity (e.g., single metal ions and single bonds) and/or flexible, uncoordinated side groups. So far, MAFs have been mostly explored as porous materials, while other properties have also been topics of some investigations. Highly stable porous MAFs usually possess weak adsorption affinity due to their inert pore surface. Another structural feature of many porous MAFs is the combination of large nanometer-sized cages and very small apertures with diameter similar to or smaller than those of the common gas molecules, which is useful for molecular sieving. Therefore, porous MAFs are useful for separation applications. Nevertheless, porous MAFs can also have good storage and delivery performance when their pore surface, pore shape, and/or framework flexibility are properly engineered and the application condition is well selected. For example, the sorption affinity can be improved/tuned by judicious introduction of active sites on the pore surface. While crystal engineering on azolate-based coordination frameworks has been very fruitful, future research will certainly focus more on developing new applications and improving the performance of either already documented or newly designed MAFs. Some intriguing approaches such as the use of mixed ligands and polyazolates deserve further attention. As we anticipated a few years ago,15b research on MAFs has been largely expanded. Nevertheless, further exploration on MAFs will certainly contribute to the chemistry of coordination polymers in the development of functional materials for practical applications.