The Effect of trans Axial Isocyanide Ligands on Iron(II) Tetra-NHC Complexes and their Reactivity in Olefin Epoxidation

The performance of trans axially substituted mono-(2 a) and bis(tert-butylisocyanide) (2 b) complexes derived from the highly active bio-inspired iron(II) (pre- )catalyst 2 containing an equatorial macrocyclic tetra N-heterocyclic carbene in homogenous olefin epoxidation catalysis is reported. H2O2 is used as oxidant in combination with the Lewis acid Sc(OTf)(3) as additive resulting in a considerable improvement of catalytic activity. In contrast to other iron epoxidation catalysts, the introduction of pi-accepting isocyanide ligands does not improve the catalytic performance of 2 a and 2 b posed by cyclic voltammetry. However, besides their lower activity, a high temperature tolerance of both compounds is found as a unique feature for iron NHC epoxidation catalysts.