Immobilization of Iron(II)-based Hydrogenation Catalysts

The catalytic reduction of polar multiple bonds such as carbonyl functionalities via molecular hydrogen plays a significant role in modern synthetic organic chemistry. This reaction is excellently performed by many transition metal complexes containing noble metals such as ruthenium, rhodium, or iridium. These precious metals are expensive and toxic, therefore, they are undesirable for catalysis and more environmentally friendly alternatives have to be found. In this respect, the preparation of well-defined iron-based catalysts of comparable activity would be desirable and is thus a major challenge for the development of more sustainable environmentally friendly reduction reactions. Iron is the most abundant transition metal in the earth crust, and ubiquitously available. Moreover, it has to be noted that nature often uses iron-based catalysts, so called hydrogenases, for hydrogenations. The main goals of the proposed project are (i) the design, synthesis, and immobilization of iron(II) pincer complexes onto mesoporous silica via anchoring, (ii) the characterization of the catalysts, and (iii) the application of the immobilized complexes for the hydrogenation of aldehydes, ketones, and CO 2 under continuous flow conditions.

In sum, this project generated fundamental knowledge in the area of Immobilization of Iron(II)-based Hydrogenation Catalysts. This led to the development of efficient heterogeneous iron-based catalysts and resulted in the development of new, sustainable, and 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. These 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 of multiple bonds via molecular hydrogen. This plays a significant role in modern synthetic organic chemistry for the production of flavors, fragrances, and 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. Thus, more economical and environmentally friendly alternatives have to be found which are in line with green chemistry guidelines. In this context, it is important to mention that iron is the most abundant transition metal in the earth crust, ubiquitously available, and non-toxic. 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. In the course of this project, we were able to develop heterogeneous iron-based catalysts for the hydrogenation of aldehydes to alcohols, which displayed comparable or even higher activity than precious metals. Moreover, these catalysts could easily be separated and reused after the reactions making the processes more economical.