CHARACTERIZATION OF RU(CO)X SPECIES ON SILICA WITH C-13 NMR-SPECTROSCOPY

The nature and dynamics of Ru multicarbonyl sites (Ru(CO)x) are studied through a series of 13C NMR experiments. Interpretation of the NMR spectra suggests two types of multicarbonyls: motionally averaged species that produce a narrow Lorentzian line and immobile species that produce a broad powder pattern characteristic of linearly bonded CO. Evolution of CO site distributions, primarily the conversion of mobile to rigid multicarbonyls, is observed with the aging of samples. The 13C-13C dipolar coupling of the species comprising the broad powder pattern, obtained through spin-echo spectra, support the separation into Ru(CO)x species and CO adsorbed on Ru particles. Exchange between CO in motionally averaged multicarbonyl sites and CO on Ru particles is not observed through selective inversion of the Lorentzian line. Spectra observed under various CO pressures indicate that exchange with gas-phase CO is not a dominant line-narrowing mechanism. An activation energy of 0.7 kcal/mol for the motionally averaged multicarbonyls, which is consistent with hindered rotation or wagging, is obtained from a variable-temperature study. © 1990 American Chemical Society.