Synthesis and Characterization of Site-Isolated Hexarhodium Clusters on Titania Powder

[Rh 6 (CO) 16 ] was prepared on the surface of TiO 2 (calcined at 200 or 400 °C) by deposition from n -hexane solution and by a surface-mediated synthesis from TiO 2 -supported [Rh(CO) 2 (acac)] in the presence of CO at 1 atm and 100 °C. The cluster preparation and subsequent decarbonylation by treatment in He or H 2 were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. Deposition from solution gave aggregated [Rh 6 (CO) 16 ] on TiO 2; removal of the carbonyl ligands led to destruction of the Rh 6 frame and sintering to give rhodium aggregates. In contrast, the reductive carbonylation of TiO 2 -supported [Rh(CO) 2 (acac)] gave site-isolated TiO 2 -supported [Rh 6 (CO) 16 ] in high yield, paralleling the chemistry of rhodium carbonyls in neutral solutions and on neutral surfaces. Removal of the carbonyl ligands from the site-isolated clusters by treatment in H 2 at 300 °C led to rhodium aggregates, but decarbonylation in He at 300 °C gave site-isolated Rh 6 clusters on the TiO 2 . The first-shell Rh−Rh coordination number of these clusters was 4.4 ± 0.4 with a bond distance of 2.64 ± 0.03 Å. Thus, the clusters formed by decarbonylation of site-isolated TiO 2 -supported [Rh 6 (CO) 16 ] are represented as octahedral Rh 6 (which has a Rh−Rh first-shell coordination number of 4). EXAFS spectroscopy indicates that the decarbonylated Rh 6 clusters on TiO 2 calcined at 200 °C have a small amount of carbon bonded to them, but no such ligands were indicated in the spectra of the Rh 6 clusters on TiO 2 calcined at 400 °C.