Identification of the Active Species in Bimetallic Cluster Catalyzed Hydrogenation

Identification of the authentic active species of cluster catalysis is rather challenging, and direct structural evidence is quite valuable and difficult to obtain. Two "isostructural" clusters, Ag25Cu4Cl6(dppb)6(PhC≡C)12(SO3CF3)3 (1) and Ag25Cu4Cl6H8(dppb)6(PhC≡C)12(SO3CF3)3 (2H) (dppb is 1,4-bis(diphenylphosphine)butane), have been successfully isolated and structurally characterized. Both these clusters have a centered icosahedron Ag13 core with the same peripheral composition and structure. The only difference is that 2H has eight hydrides but 1 has none, that is, the kernels are Ag135+ and Ag13H85+ in 1 and 2H, respectively. The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction is assessed with the two clusters. Cluster 2H is very active with 100% yield within 2 h, whereas 1 shows a very low conversion (∼8%) under the same conditions. Interestingly, high catalytic activity was observed when 1 was converted to 2H with the oxidation of H2O2 under catalytic conditions. The unprecedented transformation of a reduced nanocluster to an Ag(I)Cu(I) bimetallic cluster compound provides an excellent platform to determine the real active cluster in terms of metal cluster catalysis. The present work presents clear structural evidence that the catalytic performance of metal nanoclusters can be modulated by properly regulating the oxidation state of their constituted metal atoms.