Environmentally Benign Catalytic Reactions Based on Iron

In view of concerns regarding economy, environment and sustainable energy, there is a constant need for the discovery of new catalytic reactions. A process we are interested in is the catalytic hydrogenation of polar multiple bonds via molecular hydrogen. This plays a significant role in modern synthetic organic chemistry and is excellently performed by many transition metal hydride complexes containing noble metals such as ruthenium, rhodium, or iridium. The limited availability of precious metals, their high price, and their toxicity diminish their attractiveness in the long run and more economical and environmentally friendly alternatives have to be found which are in line with green chemistry guidelines. In this respect, the preparation of well-defined iron-based hydride catalysts of comparable or even higher activity is desirable. Iron is the most abundant transition metal in the earth crust, and ubiquitously available. The main objective of this program is the discovery of novel catalytic reactions based on our findings with iron hydride complexes with new PNP and CNC pincer ligand combinations and architectures with andwithout the possibility of metal-ligand cooperation (based on dearomatization/aromatization processes of the pyridine backbone). Secondly, CO as a co-ligand is present in all iron PNP pincer systems. Replacing CO by stronger ligands with less or no -accepting tendencies will make hydride complexes more basic and thus facilitate reactions particularly in the second coordination sphere. We plan to build on these concepts for the development of reactions catalyzed by iron compounds. This project is expected to generate fundamental understanding of new concepts in bond activation and may lead to efficient iron catalysis of reactions formerly restricted to noble metals and result in the development of new, environmentally benign catalytic processes.

In sum, this project generated a fundamental understanding of new concepts in the area Sustainability through Base Metal Catalysis. This led to the development of efficient iron-based catalysis in reactions formerly restricted to noble metals, and resulted in the development of new, environmentally benign catalytic processes. Driven by both public demand and government regulations, pharmaceutical and fine chemical manufacturers are increasingly seeking to replace stoichiometric reagents as well as precious metal based catalysts. This modifications used in synthetic transformations will develop greener, safer, and more cost-effective chemical processes. A process we were interested in was and still is the catalytic hydrogenation and dehydrogenation reactions involving molecular hydrogen and alcohols, respectively. This plays a significant role in modern synthetic organic chemistry for the production of pharmaceuticals and is excellently performed by many transition metal complexes containing noble metals such as ruthenium, rhodium, or iridium. The limited availability of precious metals, their high cost, and their toxicity diminish their attractiveness in the long run and more economical and environmentally friendly alternatives have to be found which are in line with green chemistry guidelines. This project aimed at the discovery, development, and implementation of new catalytic methodologies based on iron catalysts which open the door to the sustainable production of pharmaceuticals and fine chemicals (Sustainability through Base Metal Catalysis). In the course of this project, we were able to develop well-defined iron-based catalysts for the selective alkylation of amines with alcohols, hydrogenation of aldehydes to alcohols at silca and carbon surfaces (SILP catalysts) as well as Z-selective couplings of alkynes to and boranes to give enynes and olefins, respectively. Moreover, these catalysts were also very active for the dehydrogenation of formic acid to yield carbon dioxide and molecular hydrogen. In addition, we also started to develop related chemistry with other base metals such as copper, cobalt, nickel, and group six elements. The outcome of this project is documented in 21 vpublications in highly ranked peer-reviewed journals.