Immobilization of Manganese Catalysts Using Ionic Liquids

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 manganese-based catalysts of comparable activity would be desirable since manganese is the third most abundant transition metal in the Earths crust and has low environmental impacts. Moreover, the immobilization of manganese catalysts using the supported ionic liquid phase (SILP) method allows the easy separation of the products from the transition metal complex at the end of the catalytic process. The main goals of the proposed project are (i) the design, synthesis, and characterization of new families of rationally-designed Mn(I) complexes (ii) the application of the new complexes as catalysts for hydrogenation of numerous carbonyl compounds, and (iii) the immobilization of the most promising catalysts using SILP method; characterization and application of the novel SILPs in hydrogenation reactions. This project is expected to gain advancements in the field of base metal based organometallic chemistry by developing novel, efficient catalysts and generating fundamental knowledge in structure- reactivity relationship, bond activation and SILP catalysis which lead to the development of environmentally friendly, sustainable catalytic processes.

In sum, this project generated fundamental knowledge in the area of "Base Metal Catalysis". This led to the development of new, sustainable, and environmentally benign manganese-based 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 manganese is the third 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 manganese 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 a manganese-based catalyst for the hydrogenation of aldehydes and ketones to alcohols and to apply another manganese-based catalyst for the E-selective semihydrogenation of alkynes.