Exploring the impact of abnormal coordination in macrocyclic N-heterocyclic carbene ligands on bio-inspired iron epoxidation catalysis

Abstract

The first macrocyclic abnormal N-heterocyclic carbene (aNHC) Fe^III complex, featuring a calix[4]3-methyl-1,2,3-triazole-5-ylidene ligand system is synthesised and characterised inter alia via EA, SC-XRD, NMR and UV/vis spectroscopy. Including Mössbauer spectroscopy, SQUID and DFT calculations, the impact of the aNHC on the Fe^III complex and its corresponding Fe^II derivative is investigated. A comprehensive study of the aNHC Fe complexes in their performance in homogenous epoxidation reactions is reported and compared to the established benchmark catalysts. The complexes demonstrate efficient and selective catalytic activity in the epoxidation of cis-cyclooctene with H₂O₂, with TOFs up to almost 60 000 h⁻¹. Additionally, the epoxidation of more challenging olefinic substrates is possible. The reactivity under oxidative conditions of both complexes is investigated. NMR measurements reveal the formation of a mono-oxidised triazole ligand as degradation product. HR-ESI-MS measurements, supported by DFT calculations indicate the formation of an oxoiron species.