Magic-angle spinning ¹³C NMR spectroscopy of transition-metal carbonyl clusters

A variety of ¹³C-enriched metal carbonyl clusters (Fe₃(CO)₁₂, Ru₃(CO)₁₂, Os₃(CO)₁₂, Rh₆(CO)₁₆, and Ir₄(CO)₁₂) have been examined by magic-angle spinning ¹³C nuclear magnetic resonance spectroscopy. The results yield both the isotropic chemical shifts and the principal components of the chemical shift tensor for each crystallographically distinct carbonyl site, as well as their spin-lattice relaxation times. With the 0.25-ppm resolution obtained at high field (8.45 T), effects from ¹³C-¹³C and ¹³C-¹⁰³Rh J coupling, as well as incompletely averaged ¹³C-¹⁹¹Ir/¹⁹³Ir dipolar coupling, can be discerned. The chemical shift tensor components, obtained from analysis of spinning sideband intensities, are shown to be in generally good agreement with values previously obtained from broad-line spectra. The motionally averaged chemical shift tensor components for Fe₃(CO)₁₂, which is fluxional at room temperature, are shown to be consistent with a previously proposed exchange process.