Iron and Cobalt Catalyzed Hydrogenation Reactions

The hydrogenation of unsaturated compounds constitutes one of the most important tools for the industrial synthesis of pharmaceuticals, fine chemicals, and the transformations of oils and fatty acids. With regard to the need for operationally simple, economically and ecologically more advantageous substitutes to the common precious metal complexes we were able to introduce simple iron and cobalt catalysts to the hydrogenation of alkenes, alkynes, imines and ketones.

A special focus was laid on the application of iron-based nano-clusters that combine the benefits of homogeneous and heterogeneous processes. Active catalysts have been formed in situ by reduction of an iron salt, and were successfully applied toward the hydrogenation of styrenes, allylbenzenes and diarylacetylenes. The reaction parameters can be easily adjusted to meet the individual requirements of the different substrate classes. Further, sequential catalysis has been developed involving allylation and hydrogenation starting from an aryl-Grignard reagent.

X-ray absorption spectroscopy (XAS) revealed the formation of iron(0) clusters that are most-probably stabilized by coordination of the solvent. Attempts towards the stabilization of the nanoparticles revealed the potential of different polymers to preserve the reactivity in solution over a short period of time. Further, a biphasic system containing a catalyst phase of iron(0) nanoparticles in an ionic liquid was applied to the semi-hydrogenation of phenylacetylenes. The selectivity for partial or complete hydrogenation of the triple bond as well as the stereo-selectivity were dependent on the substitution pattern of the ionic liquid as catalyst modifier.

Further investigations are directed at the utilization of cobalt and iron complexes for homogeneous hydrogenation reactions. An easily accessible anionic cobalt complex was found to efficiently catalyze the hydrogenation of C=C, C=O and C=N double bonds.