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-2(1)-(H(2)BTCA)(3)(O2PPh2)(6)]SbF6 (1) and [Ag-21(C (math)CC6H3-3,5-R-2)(6)(O2PPh2)(10)]SbF6 (2) (H(4)BECA=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(2), 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 degrees C in toluene and under oxidative conditions (30% H2O2). 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.