Rational Design of Heterogeneous Catalysts by Controlled Immobilization of Organometallic Compounds

Although heterogenization has been a topic of research for many decades, the immobilization of organometallic catalysts with precise spatial control cannot be achieved with common technology. The spatial control of the reactive sites on solid surfaces is a prerequisite for many advanced applications, such as heterogeneous catalysis, molecular sensors, nano-bio-devices, and multi-step catalytic surfaces. Among the different methods developed to heterogenize catalysts, immobilization using covalent tethering techniques is currently the most favored approach to designing stable heterogeneous asymmetric catalysts. In this contribution we present methods for controlled and covalent heterogenization of Group 4 metallocenes as well as palladium-based catalysts on solid supports. In particular, the project focuses on (1) Synthesis of tethered ethylenebis(indenyl) (=EBI) metallocenes [1,2]. The development of the new metallocenes is supported by molecular modeling approaches using Density Functional Theory (DFT) methods. In addition to mechanistic investigations, the structure-function-relationship and the influences of certain structural variations on geometrical and electronic parameters are investigated [3]. (2) Covalent immobilization of the functional EBI-based metallocenes onto (a) H-terminated Si(111) wafers [3] as well as H-terminated Si-particles and (b) functionalized silica gel particles [4] (3) Application of the new homogeneous and heterogeneous metallocenes for the hydrosilylation of imines [5] in order to synthesize important intermediates for the pharmaceutical industry. The immobilization on hydrogen-terminated silicon surfaces is carried out via UV-mediated hydrosilylation. Since this method does not require any chemicals or catalysts, side reactions, as well as post-contamination of the surface, can be prevented. The immobilized Group 4 metallocenes have proven to be active for the asymmetric catalytic hydrosilylation of imines. The products of this reaction are chiral amines, which can be found in more than 85% of all active pharmaceutical ingredients. Finally, we present the immobilization of Pd-based catalysts using the same approaches as for the Group 4 metallocenes. The activity of the Pd-catalysts is tested for Buchwald-Hartwig aminations. The products of these X-coupling reactions are arylpiperazines, which include structural motives that are prevalent among pharmaceutical substances.