Final Report Summary - ASC3 (Asymmetric Cluster Catalysis & Chemistry)
The main objectives of this proposal were the investigation of the selectivity and enantioselectivity in catalytic reactions on small metal clusters. To this purpose it was necessary to develop an experimental infrastructure for the synthesis of chiral clusters, the surface sensitive detection of catalysts’ chirality as well as the isomer- and enantioselective detection of reaction products. At the beginning of the project this technology and methodology did not exist for this type of experiments.
We succeeded in developing several beyond state-of-the-art experimental methods. The second harmonic generation optical rotation dispersion spectroscopy allows for detecting catalysts’ chirality with highest surface sensitivity. In comparison to conventional methods an increase of seven orders of magnitudes has been reached. The combination of cavity ring-down spectroscopy with second harmonic generation spectroscopy enables the detection of the optical properties of the catalysts over a wide wavelength range (from the IR to the UV), which is a prerequisite for sensitive chirality detection of the catalysts. The circular dichroism spectroscopy, developed in our group, has been improved, adjusted to cluster experiments, as well as used for fundamental investigations of the circular dichroism via the selective excitation of different modes in the excited state of the molecule. For the latter it was found that a strong increase in the CD is observed and can be used for the detection of chirality with significantly higher sensitivity. For the desorbing reaction products a complete new detection method was developed that works as a combination of temperature programmed reaction and resonant multiphoton ionization time-of-flight mass spectrometry. It has been demonstrated that this setup enables the study of chemoselectivity of reactions in the UHV and can even be applied for investigations of the enantioselectivty of cluster reactions by ionization with circularly polarized light.
For the elucidation of isolated metal clusters a completely new setup has been built. It combines spectroscopic studies with the reaction of the clusters in an ion trap under isothermal conditions with the possibility to study chemo- and enantioselectivity of catalytic reactions with clusters in the gas phase. For reaction study a new control unit for ring electrode ion traps has been developed during the during the course of the project to meet the specifications (mass range as well as the simultaneous detection of a maximum of reaction products with high resolution and sensitivity) required.
With this new experimental infrastructure in combination with new methods, several key scientific question of the proposal could have been addressed. It was possible to identify the chirality of bare small mixed metal clusters and to synthesize chiral protected metal clusters by wet chemistry. Furthermore, it has been shown that such chiral clusters are active in asymmetric catalysis. However, the enantiomeric excess are still rather low. The chemoselectivity of catalytic reactions has been investigated on clusters in the UHV and in liquid phase for hydrogenation reactions. In UHV a clear atom-by-atom selectivity has been detected for the hydrogenation vs. the dehydrogenation of ethylene. Similar trends are also observed the hydrogenation of 3-hexyne with an additional competing reaction path yielding the fully hydrogenated alkane. The concept of this reaction has been transferred to liquid phase, where the stability of the clusters is of additional importance and varies with the protective ligand. The grand-holders are convinced that the project has not only been successful from a scientific point of view, but based on the experimental developments important technological and methodological breakthroughs have been achieved.
We succeeded in developing several beyond state-of-the-art experimental methods. The second harmonic generation optical rotation dispersion spectroscopy allows for detecting catalysts’ chirality with highest surface sensitivity. In comparison to conventional methods an increase of seven orders of magnitudes has been reached. The combination of cavity ring-down spectroscopy with second harmonic generation spectroscopy enables the detection of the optical properties of the catalysts over a wide wavelength range (from the IR to the UV), which is a prerequisite for sensitive chirality detection of the catalysts. The circular dichroism spectroscopy, developed in our group, has been improved, adjusted to cluster experiments, as well as used for fundamental investigations of the circular dichroism via the selective excitation of different modes in the excited state of the molecule. For the latter it was found that a strong increase in the CD is observed and can be used for the detection of chirality with significantly higher sensitivity. For the desorbing reaction products a complete new detection method was developed that works as a combination of temperature programmed reaction and resonant multiphoton ionization time-of-flight mass spectrometry. It has been demonstrated that this setup enables the study of chemoselectivity of reactions in the UHV and can even be applied for investigations of the enantioselectivty of cluster reactions by ionization with circularly polarized light.
For the elucidation of isolated metal clusters a completely new setup has been built. It combines spectroscopic studies with the reaction of the clusters in an ion trap under isothermal conditions with the possibility to study chemo- and enantioselectivity of catalytic reactions with clusters in the gas phase. For reaction study a new control unit for ring electrode ion traps has been developed during the during the course of the project to meet the specifications (mass range as well as the simultaneous detection of a maximum of reaction products with high resolution and sensitivity) required.
With this new experimental infrastructure in combination with new methods, several key scientific question of the proposal could have been addressed. It was possible to identify the chirality of bare small mixed metal clusters and to synthesize chiral protected metal clusters by wet chemistry. Furthermore, it has been shown that such chiral clusters are active in asymmetric catalysis. However, the enantiomeric excess are still rather low. The chemoselectivity of catalytic reactions has been investigated on clusters in the UHV and in liquid phase for hydrogenation reactions. In UHV a clear atom-by-atom selectivity has been detected for the hydrogenation vs. the dehydrogenation of ethylene. Similar trends are also observed the hydrogenation of 3-hexyne with an additional competing reaction path yielding the fully hydrogenated alkane. The concept of this reaction has been transferred to liquid phase, where the stability of the clusters is of additional importance and varies with the protective ligand. The grand-holders are convinced that the project has not only been successful from a scientific point of view, but based on the experimental developments important technological and methodological breakthroughs have been achieved.