Atomically Precise Silver Clusters Stabilized by Lacunary Polyoxometalates with Photocatalytic CO₂ Reduction Activity

The syntheses of atomically precise silver (Ag) clusters stabilized by multidentate lacunary polyoxometalate (POM) ligands have been emerging as a promising but challenging research direction, the combination of redox-active POM ligands and silver clusters will render them unexpected geometric structures and catalytic properties. Herein, we report the successful construction of two structurally-new lacunary POM-stabilized Ag clusters, TBA₆H₁₄Ag₁₄(DPPB)₄(CH₃CN)₉[Ag₂₄(Si₂W₁₈O₆₆)₃] ⋅ 10CH₃CN ⋅ 9H₂O ({Ag₂₄(Si₂W₁₈O₆₆)₃}, TBA=tetra-n-butylammonium, DPPB=1,4-Bis(diphenylphosphino)butane) and TBA₁₄H₆Ag₉Na₂(H₂O)₉[Ag₂₇(Si₂W₁₈O₆₆)₃] ⋅ 8CH₃CN ⋅ 10H₂O ({Ag₂₇(Si₂W₁₈O₆₆)₃}), using a facile one-pot solvothermal approach. Under otherwise identical synthetic conditions, the molecular structures of two POM-stabilized Ag clusters could be readily tuned by the addition of different organic ligands. In both compounds, the central trefoil-propeller-shaped {Ag₂₄}¹⁴⁺ and {Ag₂₇}¹⁷⁺ clusters bearing 10 delocalized valence electrons are stabilized by three C-shaped {Si₂W₁₈O₆₆} units. The femtosecond/nanosecond transient absorption spectroscopy revealed the rapid charge transfer between {Ag₂₄}¹⁴⁺ core and {Si₂W₁₈O₆₆} ligands. Both compounds have been pioneeringly investigated as catalysts for photocatalytic CO₂ reduction to HCOOH with a high selectivity.