Tuning the electronic properties of tetradentate iron-NHC complexes: Towards stable and selective epoxidation catalysts

• Published in: Journal of catalysis 2020-11, Vol.391, p.548-561
• Main Authors: Bernd, Marco A., Dyckhoff, Florian, Hofmann, Benjamin J., Böth, Alexander D., Schlagintweit, Jonas F., Oberkofler, Jens, Reich, Robert M., Kühn, Fritz E.
• Format: Article
• Language: English
• Subjects: Electronic properties; Epoxidation catalysis; Lewis acids; N-heterocyclic carbene; Non-heme iron complexes; Trans-labile coordination
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2020.08.037

[Display omitted] •Synthesis and characterization of novel macrocyclic tetradentate iron-NHC complexes.•In-depth catalytic studies in the epoxidation of olefins.•Tolerance of elevated temperatures in catalytic reactions.•DFT calculation investigation of the influence of different ligand substituents on catalytic properties. Two sets of bio-inspired non-heme iron complexes, each comprising of an FeII and FeIII species, bearing 16-membered macrocyclic tetradentate N-heterocyclic carbene ligands are reported. The complexes exhibit trans labile coordination sites, are characterized by means of NMR, ESI-MS, elemental analysis, SC-XRD and cyclic voltammetry and assessed as olefin epoxidation (pre-)catalysts applying H2O2 as oxidant. Sc(OTf)3 and AcOH are evaluated as Lewis and Brønsted acidic additives, respectively, resulting in partially noticeable improvement in catalytic performance. Hereby, complex 2b shows high stability (TON = 1000 at 20 °C), high temperature tolerance and advances in the more challenging epoxidation of terminal and functionalized olefins. Furthermore, in-depth DFT calculations are conducted to put the catalysts’ structural and electronic features into relation with the catalytic results.