Elucidation of the role of the counterion in ionic organometallic catalysts by means of NMR relaxation times

The catalytic activity of ionic enantioselective organometallic compounds is strongly influenced by the type of counterion; parameters, such as turnover frequency, yield, enantioselectivity, vary largely depending upon the anion used. The source of these effects is often completely unknown and may be related to coordination or ion pairing effects, among other explanations. Increasingly, ionic liquids are used in catalytic reactions. However, exactly how these ions interact with substrate or cosolvent is not well documented. Empirically, it has been observed that tetraphenyl-borate derivatives, and especially the BArF anion, seem to be associated with more efficient catalysts. For example, the Ru-catalyzed enantioselective Diels-Alder condensation is significantly faster when BArF is used and the rates decreased according to the order BArF- SbF6- PF6- BF4-. Some attempts have been performed in the last decade aiming at elucidating the role of the counterion. Among the various the approaches used, the NMR spectroscopy, by means of combining NOE and PGSE data, was the most effective. Nevertheless, both have limitations and provided only partial information. In this project we propose the use of NMR spin-lattice relaxation times (T1,) to elucidate the ions molecular dynamics. In particular, we will determine the five contributions to T1, which are Spin Rotation (SR), Chemical Shift Anisotropy (CSA), Quadrupolar Relaxation (QR), Scalar Relaxation (SC) and Dipole-Dipole Relaxation (DD) for the pure counterion and compare these data with those measured when forming an ionic enantioselective organometallic catalyst. Comparison of these values will provide information on the molecular dynamics of the counterion. For example, a large reduction of the SR indicates the presence of tight ion pairs. The project aims at creating a database of the T1 contributions of the most common ions used in organometallic catalysis (BArF-, PF6-, BF4-). As first a simple, stable and fully characterized Ru-catalyst will be consider and then the approach will be tested on an organometallic catalysts used at industrial level.