Ligand Engineering toward the Trade‐Off between Stability and Activity in Cluster Catalysis

Abstract We report the structures, stability and catalysis properties of two Ag 21 nanoclusters, namely Ag 21 (H 2 BTCA) 3 (O 2 PPh 2 ) 6 SbF 6 ( 1 ) and Ag 21 (C≡CC 6 H 3 ‐3,5‐R 2 ) 6 (O 2 PPh 2 ) 10 SbF 6 ( 2 ) (H 4 BTCA= p ‐ tert ‐butylthiacalix4arene, R=OMe). Both Ag 21 structures possess an identical icosahedral kernel that is surrounded by eight peripheral Ag atoms. Single‐crystal structural analysis and ESI‐MS revealed that 1 is an 8‐electron cluster and 2 has four free electrons. Theoretical results show that the P‐symmetry orbitals are found as HOMO‐1 and HOMO states in 1 , and the frontier unoccupied molecular orbitals (LUMO, LUMO+1 and LUMO+2) show D‐character, indicating 1 is a superatomic cluster with an electronically closed shell 1S 2 1P 6 , while 2 has an incomplete shell configuration 1S 2 1P 2 . These two Ag 21 clusters show superior stability under ambient conditions, and 1 is robust even at 90 °C in toluene and under oxidative conditions (30 % H 2 O 2 ). Significantly, 2 exhibits much higher activity than 1 as catalyst in the reduction of 4‐nitrophenol. This work demonstrates that ligands can influence the electronic structures of silver clusters, and further affect their stability and catalytic performance.