Silver Clusters as Robust Nodes and π–Activation Sites for the Construction of Heterogeneous Catalysts for the Cycloaddition of Propargylamines

The use and transformation of carbon dioxide as a C1 source into valuable chemical products using cheap industrial chemicals under mild reaction conditions fulfill the requirements of atom economy in the chemical industry. In this paper, two new silver-cluster-based frameworks were synthesized by incorporating thiourea on the backbone of the tripodal ligands. Given their large pores, high density of catalytic sites, and appropriate coordination geometries, these heterogeneous materials could be used as high-efficiency π-activation catalysts in the atom-economical cycloaddition of propargylamines with carbon dioxide. The loading of 0.1 mol % catalysts enabled the almost complete transformation of the propargylamines into oxazolidinones at atmospheric pressure and room temperature. DFT calculations show significant charge accumulation regions with π* symmetry on C atoms associated with the acetylene bond along the Ag–C directions and charge depletion regions with σ symmetry along the C–C direction, further supporting the important role of silver-cluster-based catalysts in π* activation of substrates for subsequent reactions with approaching CO2. The crystallinity of the frameworks allowed the structure of the encapsulated substrates and the interactions of the C≡C bonds in the active intermediates to be clearly observed. The recyclability and high turnover number of the cycloaddition reaction demonstrate the broad potential applications of these designed materials as π-activation catalysts for practical applications in the chemical industry.