Polyhydride Iron Complexes in Catalysis

Many polyhydride complexes based on Ru, Os, Rh or Ir are well-known to participate in catalytic reactions that involve the activation of H-E (E = H, C, B, Si) s-bonds. Catalytically active polyhydrides based on Earth abundant non-precious metals, however, are comparatively rare. In particular iron, as the most abundant transition metal in the Earth crust, is a very promising candidate, since it is inexpensive and exhibits a low environmental impact. Currently, 1.300 ppm residual levels of Fe are considered acceptable in pharmaceuticals. These facts represent a distinct advantage when compared with the = 10 ppm prescribed for most other transition metals. In the current project, we will develop new polyhydride iron PNP pincer complexes featuring up to five hydrogen ligands (H2, hydride). We will explore the full catalytic potential of newly designed Fe(II) polyhydride systems for the synthesis of fine chemicals. We hope to discover new reactions and intend to apply the catalysts for reactions which thus far have been difficult to achieve with base metals. In particular, we will evaluate their catalytic performance for selective alkene and alkyne hydrogenations and the possibility of intermolecular coupling of other -E-H (E = C, B, Si, N, O) bonds with alkenes and alkynes. Intriguing is the possibility of catalytic C-H activation by Fe pincer complexes including CO2 insertion to obtain carboxylic acids and, thus, we will also perform the above reactions in the presence of CO2. These studies will make use of the full toolbox of techniques available for modern synthetic chemistry: experimental-based catalyst design, catalytic studies and mechanistic studies. This project is expected to generate fundamental understanding of new concepts in bond activation and may lead to efficient new iron catalysis as well as reactions formerly restricted to noble metals, even surpassing these, and will result in the development of new, environmentally benign catalytic processes.

Many polyhydride complexes based on Ru, Os, Rh or Ir are well-known to participate in catalytic reactions that involve the activation of H-E (E = H, C, B, Si) -bonds. Catalytically active polyhydrides based on Earth abundant non-precious metals, however, are comparatively rare. In particular iron, as the most abundant transition metal in the Earth crust, is a very promising candidate, since it is inexpensive and exhibits a low environmental impact. In the current project, we developed new iron PNP and PCP pincer complexes. We explored the catalytic potential of newly designed Fe(II) systems for the synthesis of fine chemicals. Fe(II) PNP pincer complexes featuring dihydrogen silyl and hydride silane ligands which turned out to be active catalysts for the hydrosilylation of alkenes. Moreover, we developed a selective transfer-semi-hydrogenation process of alkynes catalyzed by an iron PCP pincer alkyl complex. It is the first example of an alkyl iron pincer complex which initiates the catalytic reaction by a migratory insert step of an alkyl into a CO ligand. This project generated fundamental understanding of new concepts in bond activation and may lead to efficient new iron catalysis as well as reactions formerly restricted to noble metals, even surpassing these, and will result in the development of new, environmentally benign catalytic processes.