Geometrical isomerism and F-19 NMR spectroscopy of octahedral perfluoroethyl- and perfluoropropyl-substituted beta-diketonates and monothio-beta-diketonates of rhodium(III)
Ga. Stern et Jb. Westmore, Geometrical isomerism and F-19 NMR spectroscopy of octahedral perfluoroethyl- and perfluoropropyl-substituted beta-diketonates and monothio-beta-diketonates of rhodium(III), CAN J CHEM, 77(11), 1999, pp. 1734-1744
Citations number
59
Categorie Soggetti
Chemistry
Journal title
CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE
In solution in acetone at room temperature, tris(perfluoropropanoyl and per
fluorobutanoyl)benzoylmethanate complexes of rhodium(III) are shown by F-19
NMR spectroscopy to exist as nearly statistical 1:3 mixtures of their resp
ective fac and mer octahedral isomers, whereas the analogous perfluoroalkan
oylthiobenzoylmethanate complexes have a fac-octahedral structure. For all
four complexes studied, vicinal F-F coupling constants are too small (0-3 H
z) 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 perfluoropropan
oylthiobenzoylmethanate complex). (These observations are modified by solve
nt. In toluene, one of the AB patterns of a mer-octahedral complex collapse
s to a single peak) The AB spectra, which are subject to substituent and so
lvent effects, can be interpreted in terms of perfluoroalkyl groups rotatin
g rapidly about the C-ring-C-alkyl bond, with an unsymmetric double potenti
al well, giving unequal rotamer populations. Changes in the spectra were no
t noticeable over the temperature range accessible in the acetone and tolue
ne solvents used.