Cobalt-Cluster-Triggered Exfoliation for Ultrathin Metal–Covalent Organic Framework Nanosheets

Metal-covalent organic frameworks (MCOFs), an emerging type of porous material, have the combined advantages of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). Of particular interest are MCOF nanosheets with two-dimensional morphology, whose fully exposed active sites and quasi-molecular size are expected to exhibit maximum functionality for a wide range of applications. However, due to their strong interlayer coupling and the potential structural damage during the synthesis process, the preparation of MCOF nanosheets has rarely been reported. Here, we demonstrate a cobalt-cluster-triggered chemical exfoliation strategy to directly prepare ultrathin MCOF nanosheets from bulk COFs. By the reaction of cobalt carbonyl with alkynyl-based COFs at room temperature, ultrathin MCOF nanosheets are constructed under mild conditions. The mechanism for direct growth of MCOF nanosheets includes the formation of stable cobalt clusters within the COF skeletons, which alters the alkynyl bonds and their related dihedral angle and disrupts the interlayer interactions. Consequently, the bulk COFs are directly converted into MCOF nanosheets, with a yield of 40%, an average lateral size of 0.8 μm, and an average thickness of 1.6 nm. This strategy lays the foundation for the preparation and application of MCOF nanosheets by treating alkynyl-based COFs with cobalt carbonyl.