Carbonylation and Decarbonylation of γ-Al2O3-Supported Hexarhodium Clusters: Characterization by Infrared, 13C NMR, and Extended X-ray Absorption Fine Structure Spectroscopies

Rh 6 (CO) 16 adsorbed on γ-Al 2 O 3 that had been calcined at 400 °C was decarbonylated in steps by treatment in He at increasing temperatures. The metal frame in the supported clusters remained largely intact during the decarbonylation process as evidenced by extended X-ray absorption fine structure (EXAFS) spectroscopy. The face-bridging CO ligands were removed first when the temperature was raised to 100 °C and held for 2 h, as shown by infrared and 13 C NMR data. Treatment in He at 300 °C led to complete removal of the carbonyl ligands and the formation of supported rhodium clusters with structures modeled on the basis of EXAFS data as Rh 6 (along with some other species inferred to be cationic rhodium). These clusters were stable in He at temperatures as high as 400 °C. The infrared and 13 C NMR data show that when the fully decarbonylated Rh 6 clusters were exposed to CO at 25−200 °C, they were partially recarbonylated. Subsequent treatment of the partially recarbonylated clusters in He at 300 °C again led to complete decarbonylation. The reversibility of the carbonylation and decarbonylation is suggestive of the behavior of extended metal surfaces, and the restriction of the reversibility to only a fraction of the CO ligands that can bond to Rh 6 indicates a role of the support and a contrast in reactivity between the supported metal clusters and extended metal surfaces.