Covalent Metal–Organic Frameworks: Fusion of Covalent Organic Frameworks and Metal–Organic Frameworks

Abstract

ConspectusMetal-organic frameworks (MOFs) and covalent organic frameworks (COFs), as emerging porous crystalline materials, have attracted remarkable attention in chemistry, physics, and materials science. MOFs are constructed by metal clusters (or ions) and organic linkers through coordination bonds, while COFs are prepared by pure organic building blocks via covalent bonds. Because of the nature of linkages, MOFs and COFs have their own shortcomings. Typically, the relatively weak bond strengths of coordination bonds lead to poor chemical stability of MOFs, which limits their practical implementations. On the other hand, due to the strong covalent bonds, COFs exhibit rather higher stability under harsh conditions, compared to MOFs. However, the lack of open metal sites restricts their functionalization and application. Therefore, it is hypothesized that the "cream-skimming" of MOFs and COFs would address these drawbacks and produce a new class of crystalline porous material, namely, covalent metal-organic frameworks (CMOFs), with unprecedented structural complexity and advanced functionality. The CMOFs reveal a new synthetic approach for the preparation of reticular materials. Specifically, metal ions are reacted with chelating ligands to assemble metal complexes or clusters with functional reactive sites (e.g., -CHO, and -NH₂), which can be further connected with organic linkers to form networked structures via dynamic covalent chemistry (DCC). The isolated metal complex or cluster precursors show enhanced stability which prevents structural decomposition and rearrangements during the self-assembly process of CMOFs. Since the topology of preassembled metal nodes is well-defined, the CMOFs structure can be readily predicted upon directed networking of covalent bonds. Unaccessible reticular materials from unstable or highly reactive metal ion/clusters under traditional conditions can be prepared via the DCC approach. Moreover, CMOFs synergize the advantages of MOFs and COFs, containing metal active sites ensuring various interesting properties, and covalent linkages that allow rather high chemical stability even under harsh conditions. In the past few years, our group has specifically focused on the development of general synthetic strategies for CMOFs by networking coinage metal (Cu, Ag, and Au)-based cyclic trinuclear units (CTUs) with DCC. The CTUs exhibit trigonal planar structures and can be functionalized with reactive sites, such as -NH₂ and -CHO, that can further react with organic linkers to afford CMOFs. Notably, CTUs also features interesting properties including metallophilic attraction, π-acidity/basicity, luminescence, redox activity and catalytic activity, which can be incorporated into CMOFs. Therefore, we envision that CMOFs would be promising platforms not only for the development of novel reticular materials, but also for potential applications in many research fields including gas absorption/separation, sensing, full-color display, catalysis, energy, and biological applications. In this Account, we summarize the recent studies on CMOFs, starting with linkage and topological design, structural transformation, morphological control, and potential applications in various fields. We also discuss the future opportunities and challenges in this rapidly developed research field of CMOFs. We hope this Account may promote new scientific discoveries and further development of CMOF-based materials and technologies in the future.