Fighting Fenton Chemistry: A Highly Active Iron(III) Tetracarbene Complex in Epoxidation Catalysis

Organometallic Fe complexes with exceptionally high activities in homogeneous epoxidation catalysis are reported. The compounds display FeII and FeIII oxidation states and bear a tetracarbene ligand. The more active catalyst exhibits activities up to 183 000 turnovers per hour at room temperature and turnover numbers of up to 4300 at −30 °C. For the FeIII complex, a decreased Fenton‐type reactivity is observed compared with FeII catalysts reported previously as indicated by a substantially lower H2O2 decomposition and higher (initial) turnover frequencies. The dependence of the catalyst performance on the catalyst loading, substrate, water addition, and the oxidant is investigated. Under all applied conditions, the advantageous nature of the use of the FeIII complex is evident. Iron out the creases: Two iron complexes are presented that have the highest turnover frequencies in homogeneous epoxidation catalysis reported to date (up to 183 000 h−1 at room temperature, and turnover numbers up to 4300 at −30 °C). The compounds are in the FeII and FeIII oxidation states, bear a tetracarbene ligand, and are compared in terms of their performance in epoxidation. If we compare the reactivity of the FeIII complex to FeII catalysts, a decreased Fenton‐type reactivity is observed.