Geometrical isomerism and 19F NMR spectroscopy of octahedral perfluoroethyl- and perfluoropropyl-substituted β-diketonates and monothio-β-diketonates of rhodium(III)

In solution in acetone at room temperature, tris(perfluoropropanoyl and perfluorobutanoyl)benzoylmethanate complexes of rhodium(III) are shown by F-19 NMR spectroscopy to exist as nearly statistical 1:3 mixtures of their respective fac and mer octahedral isomers, whereas the analogous perfluoroalkanoylthiobenzoylmethanate complexes have a fac-octahedral structure. For all four complexes studied, vicinal F-F coupling constants are too small (0-3 Hz) to lead to resolved multiplet peaks, so that perfluoroethyl groups show singlet resonances for their CF3 groups. Since four-bond coupling constants are significantly larger, similar to 9 Hz, pefluoropropyl groups show the expected multiplet resonances for their CF3 groups. In acetone, AB patterns, with geminal coupling constants equal to similar to 270 Hz, are observed for the CF2 groups next to the chelate ring (except for the perfluoropropanoylthiobenzoylmethanate complex). (These observations are modified by solvent. In toluene, one of the AB patterns of a mer-octahedral complex collapses to a single peak) The AB spectra, which are subject to substituent and solvent effects, can be interpreted in terms of perfluoroalkyl groups rotating rapidly about the C-ring-C-alkyl bond, with an unsymmetric double potential well, giving unequal rotamer populations. Changes in the spectra were not noticeable over the temperature range accessible in the acetone and toluene solvents used.