Space Exploration of Metal–Organic Frameworks in the Mesopore Regime

The past decades have witnessed the proliferation of porous materials offering high surface areas and the revolution in gas storage and separation, where metal–organic frameworks (MOFs) stand out as an important family. Alongside the pursuit of higher surface area, the increase in the size of guests, such as nanoparticles and biomolecules, has also led to the demand for larger space defined by the pores and cages within the MOF structure, from the conventional micropore regime (<2 nm) toward the mesopore regime (2–50 nm). Among the essential elements in the design of MOFs, molecular building blocks, their coordination and spatial arrangement, the chemistry for molecular design, and coordination bonds have become relatively mature, offering precise control of the shape and environment of the molecularly defined 3D cages; however, the correlation between the geometrical parameters and the size of polyhedrons describing the cages, concerning the spatial arrangement of building blocks, is much less explored.

In this Account, we made efforts to associate actual cage size with the critical geometrical components, vertices, edges, connectivity, rings, and underlying polyhedrons, as well as the combination of components of various types in the design of MOFs. Several trends were found, such as influence from connectivity and expansion efficiency, offering insights into the construction of 3D cages in MOFs. This enables the creation of extremely large mesoporous cages in MOFs with an internal diameter up to 11.4 nm from relatively small building blocks. Furthermore, we discuss a strategy of partial removal or replacement of organic linkers to construct mesoporous cages from readily known topologies.

All of the above efforts urged us to ask the following questions: Is there any limit in the sculpting of the 3D space from molecules? How large an area can one chemical bond support? The answer to these questions will deepen the knowledge of efficient utilization of chemical bonds in the sculpting of 3D spaces and guide the design of larger mesopores. Several general geometrical principals emerged: (1) Expansion efficiency and radius are positively correlated with the number of vertices. (2) Increase in the number of vertices and decrease of their connectivity favor the construction and expansion of large cages. (3) The boundary of the 3D space constructed by the chemical bonds is related to the polyhedron type and determined by the energy involved in crystallinity. Such principals are likely to be applicable also in the design of isolated cages in supramolecular chemistry. In addition to the structural design and synthesis, the applications of these mesoporous cages in MOFs are also summarized.