Topological characterization of metal–organic frameworks: a perspective

Metal-organic frameworks (MOFs) began to emerge over two decades ago, resulting in the deposition of 120 000 MOF-like structures (and counting) into the Cambridge Structural Database (CSD). Topological analysis is a critical step toward understanding periodic MOF materials, offering insight into the design and synthesis of these crystals via the simplification of connectivity imposed on the complete chemical structure. While some of the most prevalent topologies, such as face-centered cubic (fcu), square lattice (sql), and diamond (dia), are simple and can be easily assigned to structures, MOFs that are built from complex building blocks, with multiple nodes of different symmetry, result in difficult to characterize topological configurations. In these complex structures, representations can easily diverge where the definition of nodes and linkers are blurred, especially for cases where they are not immediately obvious in chemical terms. Currently, researchers have the option to use software such as ToposPro, MOFid, and CrystalNets to aid in the assignment of topology descriptors to new and existing MOFs. These software packages are readily available and are frequently used to simplify original MOF structures into their basic connectivity representations before algorithmically matching these condensed representations to a database of underlying mathematical nets. These approaches often require the use of in-built bond assignment algorithms alongside the simplification and matching rules. In this Perspective, we discuss the importance of topology within the field of MOFs, the methods and techniques implemented by these software packages, and their availability and limitations and review their uptake within the MOF community.