Abstract
Homogeneous catalysis in room-temperature ionic liquids (ILs)
constitutes a most interesting field of research with high potential in
technical applications. As concerns the hydrogenation of unsaturated
hydrocarbons, Wilkinson's compound RhCl(PPh3)(3) represents a catalyst
that provides high selectivity and activity. Herein, we demonstrate the
application of infrared spectroscopy to the quantitative analysis of
the Wilkinson catalyst in the IL 1-ethyl-3-methylimidazolium acetate
(EMIMOAc). Our study demonstrates for the first time the
quantitative, accurate and reproducible determination of the
concentration of a rhodium catalyst by means of IR spectroscopy and,
moreover, allows the investigation of intermolecular interactions.
Spectral features, located mainly in the fingerprint region of the IR
spectrum, are identified revealing the influence of the dissolved
catalyst on the IL's vibrational structure. In particular, the
ring-bending mode of the imidazolium ring shows a frequency shift as a
function of catalyst concentration, probably due to hydrogen-bond
formation between the IL cation and the Rh complex. The results show
the potential of IR spectroscopy both for application as a quick
process control technology in catalytic processes and as a toot for
better understanding of IL-catalyst interactions.
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