Synthesis and characterization of site-isolated hexarhodium clusters on titania powder

[Rh-6(CO)(16)] was prepared on the surface of TiO2 (calcined at 200 or 400 degreesC) by deposition from n-hexane solution and by a surface-mediated synthesis from TiO2-supported [Rh(CO)(2)(acac)] in the presence of CO at 1 atm and 100 degreesC. 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 TiO2; 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 TiO2-supported [Rh(CO)(2)(acac)] gave site-isolated TiO2-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 degreesC led to rhodium aggregates, but decarbonylation in He at 300 degreesC gave site-isolated Rh-6 clusters on the TiO2. The first-shell Rh-Rh coordination number of these clusters was 4.4 +/- 0.4 with a bond distance of 2.64 +/- 0.03 W. Thus, the clusters formed by decarbonylation of site-isolated TiO2-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 TiO2 calcined at 200 degreesC have a small amount of carbon bonded to them, but no such ligands were indicated in the spectra of the Rh-6 clusters on TiO2 calcined at 400 degreesC.