Carbon-13 Nuclear Magnetic Resonance Spectroscopic Study of Fe₃(CO)₁₂ and Ru₃(CO)₁₂ Supported on Metal Oxide Surfaces

¹³C-enriched Fe₃(CO)₁₂ and Ru₃(CO)₁₂ supported on a variety of metal oxide surfaces (γ-Al₂O₃, SiO₂, MgO, and HNa-Y zeolite) have been examined by carbon-13 “magic-angle” sample-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Our results show that the γ-Al₂O₃-supported Fe₃(CO)₁₂ cluster is fluxional at room temperature, but low-temperature spectra give no evidence in favor of the unusual bridging carbonyl proposed previously by some workers from infrared (IR) spectroscopy. On HNa-Y, Fe₃(CO)₁₂forms a highly mobile chemisorbed species, and the ¹³C chemical shift is consistent with formation of the dianion [Fe₃(CO)₁₁]^2-. For Ru₃(CO)₁₂ on γ-Al₂O₃ and SiO₂ surfaces, our results are in agreement with IR studies and show that, under anaerobic conditions, highly mobile physisorbed clusters are initially formed. Upon exposure to air or upon thermal activation, divalent tricarbonyl species are generated. The resonances of these chemisorbed species are enhanced by cross polarization and display large chemical shift anisotropies, due to restricted mobility. On MgO, Ru₃(CO)₁₂ behaves quite differently, the¹³C NMR results being consistent with the formation of highly fluxional hexaruthenium carbonyl clusters.