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

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-butylthiacalix[4]arene, 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.