Project

N-heterocyclic carbenes; platinum group metals; ligand basicity; catalysis; oxidation; hydrogenation; abnormal bonding; alkane activation; ligand tuning; bond activation

Lead
Lay summary
The advancement of homogeneous catalysis is inherently coupled to the development of powerful catalysts, both in terms of activity and selectivity. In this area, metal-based homogeneous catalysis has been greatly spurred by the recent discovery of N-heterocyclic carbenes (NHCs) as a novel class of ligand with unique bonding properties. In particular, NHCs bind to the metal in a covalent fashion and they are reknown for their strong donor properties, thus having a beneficial impact on the stability of the metal center and on the electron density at the metal. High electron density at the metal may provide access to the activation of less reactive bonds and it may therefore be desirable to further increase this electron density without reducing the stability of the complex. Here, we propose an approach to reach these effects in order to activate bonds that are generally considered to be unreactive, such as H-H, C-H and perhaps even C-C bonds.Specifically, we aim at exploiting the impact of abnormally bound NHCs, which bind via C4 (as opposed to the classical C2 bonding mode). The ligand donor strength is considerably enhanced when the heteroatom stabilization is reduced. While synthetic methodologies have emerged recently that allow to form transition metal complexes with some of these unusual, C4-bound NHCs, not many applications have been developed up to now. The project proposed here will investigate the catalytic potential of platinum group transition metals that are bound to such non-classical carbene ligands. Initial work in our laboratories has shown that such palladium(II) complexes are efficient catalyst precursors for the activation of H2. When using ruthenium(III) as the metal center, transfer hydrogenation is catalyzed. Following two parallel approaches, we will investgate the palladium-mediated H-H activation and the rhodium-mediated C-H activation in more detail in order to identify mechanistic key steps and potentical deactivation and decomposition routes. This part of the work should allow us to improve our catalytic systems. In a second approach, we aim at enhancing the electron density at the metal center, either by redox-state changes, or by ligand-induced methods. This should provide catalysts for C-H bond activation. This type of reaction is of particular relevance, as it allows for selective functionalization of otherwise unreactive alkanes via homogeneous catalysis and may therefore provide a new tool for the synthesis of complex molecules.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

The advancement of homogeneous catalysis is inherently coupled to the development of powerful catalysts, both in terms of activity and selectivity. In this area, metal-based homogeneous catalysis has been greatly spurred by the recent discovery of N-heterocyclic carbenes (NHCs) as a novel class of ligand with unique bonding properties. In particular, NHCs bind to the metal in a covalent fashion and they are reknown for their strong donor properties, thus having a beneficial impact on the stability of the metal center and on the electron density at the metal. High electron density at the metal may provide access to the activation of less reactive bonds and it may therefore be desirable to further increase this electron density without reducing the stability of the complex. Here, we propose an approach to reach these effects in order to activate bonds that are generally considered to be unreactive, such as H-H, C-H and perhaps even C-C bonds.Specifically, we aim at exploiting the impact of abnormally bound NHCs, which bind via C4 (as opposed to the classical C2 bonding mode). The ligand donor strength is considerably enhanced when the heteroatom stabilization is reduced. While synthetic methodologies have emerged recently that allow to form transition metal complexes with some of these unusual, C4-bound NHCs, not many applications have been developed up to now. The project proposed here will investigate the catalytic potential of platinum group transition metals that are bound to such non-classical carbene ligands. Initial work in our laboratories has shown that such palladium(II) complexes are efficient catalyst precursors for the activation of H2. When using ruthenium(III) as the metal center, transfer hydrogenation is catalyzed. Following two parallel approaches, we will investgate the palladium-mediated H-H activation and the rhodium-mediated C-H activation in more detail in order to identify mechanistic key steps and potentical deactivation and decomposition routes. This part of the work should allow us to improve our catalytic systems. In a second approach, we aim at enhancing the electron density at the metal center, either by redox-state changes, or by ligand-induced methods. This should provide catalysts for C-H bond activation. This type of reaction is of particular relevance, as it allows for selective functionalization of otherwise unreactive alkanes via homogeneous catalysis and may therefore provide a new tool for the synthesis of complex molecules.