(455f) Understanding The Positive Effect Of Anchoring Upon Both The Intrinsic Activity And Enantioselectivity Of Single Site Chiral Hydrogenation Catalysts: The Case Of Ru-MAB Complexes Supported On Mesoporous Silica

Contrary to the standard belief, supporting a homogeneous single site catalyst has the potential of improving not only the ease of separation, but the intrinsic nature of the catalytic step. This is particularly interesting in the case of chiral catalysis, where the issues of activity and molecular selectivity merge. In a noteworthy example, we reported previously1 that it was possible to reach >97 % of enantiomeric excess during room temperature chiral hydrogenation of the double bond of substrates such as itaconic acid and -acetamidocinnamic acid, in all cases with a 100% yield. This was accomplished by covalent bonding of a Ruthenium-based organometallic complex containing an amine-based ligand, (S)-6,6'-dimethyl-2,2'-diaminodiphenyl (MAB), to the surface of an MCM-41 material. By comparison, the homogeneous reaction resulted in 65-70 % ee. In order to understand the underlying reason behind this phenomenon, it is necessary to properly characterize the dynamics of the reaction itself, as well as the catalyst. In this work, we report kinetic studies with the homogeneous complex, as well as with the complex supported on MCM-41 or SBA-15. We discuss also detailed characterization studies using MAS-NMR, XAS, FTIR and UV-Vis, together with DFT molecular simulation. Contrary to what we expected, there was a significant variation in the kinetic behavior of the catalyst. Use of the complex under homogeneous conditions resulted in a sigmoidal conversion vs. time shape, and an ee of roughly 65-80% depending on the substrate used. By comparison, the tethered chiral catalyst had practically no induction period and followed a first order reaction rate equation with an ee of practically 100%. The ee was constant throughout the reaction. The rate of reaction followed the sequence: (S or R)-MAB-Ru-SBA-15 > (S or R)-MAB-Ru-MCM-41 > homogeneous (S or R)-MAB-Ru complex. A question that arises is whether or not the structure of the complex is retained during the preparation procedure. In general, this question is not frequently asked or answered in the literature, and it is of importance in order to improve the strategies and to attain high efficiency in the use of expensive complexes, and in order to control the support effect. Characterization of our catalysts with the different techniques mentioned before showed that the structure of the complex was preserved upon anchoring. The analysis of the EXAFS spectra gathered at the Campinas synchrotron in Brazil provided an excellent match between the simulated structures of the homogeneous and supported complexes. The EXAFS simulations were based on the spectra of model compounds coupled with DFT simulations. The spectra of spent catalysts did not show the presence of Ru-Ru bonds, and differed slightly from the spectra of fresh samples, apparently as a result of some changes in the ligand sphere. To conclude, we present a structural model that explains the observed enhancements in both ee and reactivity as a result of anchoring, and that also provides a framework for further use of this effect.

1.C. Pérez, S. Pérez, G.A. Fuentes, A. Corma, J. Mol. Catal. A: Chemical, 197 (2003) 275-281.