last changed: 1.1.2019
Organometallic Chemistry and Catalysis
I am still interested in 1-alkene polymerisation by means of homogeneous, cationic metallocene catalysts. The polymerisation of ethene to polyethene is a large scale industrial process, the world production of polyethene and polypropene alone is estimated at ca. 60 millions of tons. Whereas traditional, heterogeneous Ziegler catalysts are still used, the discovery of homogeneous metallocene complexes in recent years has revolutionised this area. These new catalysts enable the production of tailor made polymers, they are more active than traditional Ziegler catalysts and have opened new markets for polyolefins.
Homogeneous metallocene catalysts usually consist out of a group 4 (Ti, Zr) or group 6 (Cr) halide (catalytic precursor) and an activator which, upon mixture with the precursor, will generate the catalytic active species. Methylaluminoxane (MAO) is the most commonly used activator. However, the exact structure of this compound remains yet to be discovered and hence the nature of the active site and the polymerisation mechanism was obscured for a long time. In order to investigate possible reaction mechanism model compounds have been prepared and investigated, both experimentally and by means of molecular modelling. Right now, I am only doing molecular modelling and have stopped working in the laboratory.
The models I am interested in are group 4 halfsandwich and metallocene compound of the type CpMR3 and Cp2MR2 (Cp=C5H5, C5H5-xR'x, M=Ti, Zr, R=Me, CH2Ph) respectively which, upon activation with cationic generating agents like B(C6F5)3 or [Ph3C]+[B(C6F5)4]- generate highly electrophilic cationic metal alkyl complexes of the general type [Cp2M-R]+. These complexes are generally accepted to be the catalytically active species.
The main focus of my research is the discovery of new reaction intermediates and dormant states of the catalyst. As the metal alkyl complex [Cp2M-R]+ is highly electrophilic, it is forming Lewis Acid-Base adducts, some of which are shown in Scheme 1.
All these research interests led me into the area of NMR spectroscopy, DFT calculations and more recently x-ray and neutron diffraction with the aim to combine experimental results with theoretical calculations. Thus from experimental NMR data we can compare them with computed ones and confirm or reject the exact structure of the observed molecule. We often noticed the non-innocent role of the solvent for example. We can do the same for the electronic structure of a compound as well: from the experimental Bader analysis, obtained from low temperature (30 K) x-ray data combined with neutron data, we can compare the so obtained data with computed ones. This will give us a deeper understanding of catalytic processes, helps us to select the correct level of theory for our calculations which can lead to the development of 'better' catalysts.