CHARACTERIZATION OF SILICA-SUPPORTED OSMIUM CARBONYL CLUSTERS BY MAGIC-ANGLE-SPINNING C-13 NMR-SPECTROSCOPY

We report the carbon-13 magic-angle-spinning nuclear magnetic resonance (NMR) spectra of silica-supported Os3((CO)-C-13)12 samples prepared in several different ways. Our results yield both the isotropic chemical shift and the principal components of the chemical shift tensor for each carbonyl resonance. Spectra have been assigned by comparison to solution NMR spectra of a variety of osmium carbonyl clusters that are models for the proposed surface-attached structures: HOs3(CO)10(OR) (R = CH3, C6H5, SiEt3), HOs3(CO)10(O2CH), Os3(CO)10(OCH3)2, and [Os(CO)3Cl2]2. Spectra of samples prepared by impregnation of silica with a solution of Os3((CO)-C-13)12 provide evidence for a physisorbed Os3(CO)12 species undergoing fast isotropic motion. For samples prepared by refluxing Os3((CO)-C-13)12 and SiO2 in n-octane, a spectrum containing five distinct resonances is observed, consistent with a HOs3(CO)10(OSi =) species having C(s) symmetry. The chemical shift anisotropy parameters measured for these resonances at 20-degrees-C suggest that this species is undergoing rapid (>> 32 kHz) large-angle (90-120-degrees) rotational jumps. Spectra of samples prepared by vacuum pyrolysis (200-degrees-C) are consistent with Os(II) carbonyl species that are immobile on the 10(-5)-10(-4)-s time scale. These results demonstrate the utility of C-13 NMR spectroscopy for studying the structures and dynamics of supported metal carbonyl clusters.