Tailoring activity and stability: Effects of electronic variations on iron-NHC epoxidation catalysts

Abstract

A comparative study of three iron(II) NHC epoxidation catalysts with different electronic properties is performed to gain more profound insight into the influence of electronic variations on catalytic performance. One iron complex contains a pyridyl-NHC ligand, the other two are prepared with a modified ligand counterpart. The complexes are comprehensively characterized by various methods including Mössbauer, SQUID and DFT. While a lower electron density at the iron atom can be associated with a decline in epoxidation activity, a more electron rich iron center does not necessarily correspond with higher activity, due to reduced catalyst stability. Addition of Lewis acids increases both activity and stability significantly and is more effective than temperature variations. All three epoxidation catalysts achieve high selectivity, with a maximum TOF of 24500 h−1 and TON of >700 for the unmodified complex. More nucleophilic alkenes promote higher activity and conversion.