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 gamma-Al2O3 that had been calcined at 400 degreesC 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 degreesC and held for 2 h, as shown by infrared and C-13 NMR data. Treatment in He at 300 degreesC 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 degreesC. The infrared and 13C NMR data show that when the fully decarbonylated Rh6 clusters were exposed to CO at 25-200 degreesC, they were partially recarbonylated. Subsequent treatment of the partially recarbonylated clusters in He at 300 degreesC 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.