Polyoxometalate-Based Ag^I-MOF for Promoting Catalytic Conversion of CO₂ and Propargylic Alcohols under Mild Conditions

The conversion of carbon dioxide to high-value chemicals is greatly significant to green chemistry. The cyclization reaction of CO₂ with propargyl alcohol to form α-alkylidene cyclic carbonates has attracted attention due to atomic economy. Silver (Ag^I) has emerged as one of the most effective activators for alkynes, attributed to its distinctive electronic configuration affording a specific affinity with alkynes. Herein, two novel Ag^I-based metal-organic frameworks containing Keggin PMo₁₂O₄₀^3- (abbreviated as {PMo₁₂}), namely, {Ag₃(btap)₃[PMo₁₂O₄₀]}·H₂O (<b>Ag-1</b>) and {Ag(Hbtap)₂[PMo₁₂O₄₀]} (<b>Ag-2</b>, btap is 3,5-bis(1',2',4'-triazol-1'-yl)pyridine), were successfully synthesized. <b>Ag-1</b> exhibits both exceptional thermal and solvent stability, along with high catalytic activity in CO₂ cyclization with various substituted propargylic alcohols into α-alkyl cyclic carbonates at room temperature and atmospheric pressure. The highly dispersed Ag^I and {PMo₁₂} sites are beneficial for the superior performance of <b>Ag-1</b>, by activating the C≡C bond and adsorbing carbon dioxide, respectively. Based on the catalytic results, structural analysis, and classical chemical bond theory, it is speculated that the three-coordinated silver in <b>Ag-1</b> has higher Lewis acidity compared with <b>Ag-2</b>, making it easier to activate the substrate. Notably, the <b>Ag-1</b> catalyst exhibits outstanding stability with negligible activity loss over at least five successive cycles.