In situ exfoliation of a copper-based metal–organic framework for boosting the synergistic photoactivation of inert C(sp3)–H bonds and oxygen

Abstract

Metal–organic frameworks (MOFs) with ordered organic linkers and inorganic nodes/clusters possess laudable catalytic properties and underpin the reactivity and selectivity that are not observed in other manifolds by integrating various active components into one network. However, MOFs with a three-dimensional (3D) structure suffer from pore size limitation, slow electron communication and substrate diffusion, which limit their photocatalytic performance and application. Ultrathin two-dimensional (2D) monolayer metal–organic framework nanosheets with more accessible active sites, faster electron communication and substrate diffusion provide a tunable platform to overcome these limitations. In this work, by in situ exfoliation of 3D layered MOFs, an ultrathin 2D copper-based MOF nanosheet containing photoactive anthraquinone groups was successfully constructed for activating the inert C(sp³)–H bonds and oxygen by integrating the energy transfer (EnT), ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) events together into one network. A large number of Cu₂(Ac)₄ paddlewheel clusters are uniformly distributed on the surface of the ultrathin 2D copper-based MOF nanosheets, which provide a wealth of coordination sites with the highly electrophilic radical precursor chloride ions to form Cu–Cl chromophores. Under multiphoton excitation, in situ formed Cu–Cl chromophores and anthraquinone ligands are triggered simultaneously to form the highly active chlorine radical (Cl˙) and reactive oxygen species (ROS) by LMCT and EnT events for the activation and oxidation of C(sp³)–H bonds through a HAT process. This ultrathin 2D MOF nanosheet with a short electron transfer distance and rich active sites not only inherits the versatility of MOFs, but also combines the unique advantages of 2D catalytic materials. Benefiting from the synergistic effects of two active sites and the flexibility of the ultrathin 2D MOF-based nanosheets with high surface utilization, the observed activities are obviously superior to that of bulk 3D MOF crystals in photocatalysis. This work not only highlights the potential of 2D MOF-based nanosheets in various photocatalytic transformations, but also presents opportunities to use nanostructured MOFs to handle the synergy of multiple molecular catalysts.