Modification of bio-inspired tetra-NHC iron complexes with axial nitrile ligands

[Display omitted] •Axial ligand substitution of Fe(II) and Fe(III) NHC complexes with t-BuCN and PhCN.•Similar electrochemical properties to the original MeCN complexes.•Significantly higher stability against C–H oxidation. In this work, axial MeCN ligands of highly active iron(II) and iron(III) tetra N-heterocyclic carbene (NHC) olefin epoxidation (pre-)catalysts are substituted with t-BuCN and PhCN, bearing less activated C–H bonds. The obtained complexes are characterized by NMR spectroscopy, electrospray mass spectrometry (ESI-MS), single crystal X-ray diffraction (SC-XRD), cyclic voltammetry (CV), UV/Vis spectroscopy and elemental analysis. The novel compounds exhibit significantly more stable axial ligands against C–H oxidation, as demonstrated by UV/Vis kinetics and depict similar electrochemical properties to their MeCN derivatives. Thus, the new complexes pave the way towards more robust catalysts by enhancing the stability against oxidation of the axial ligand, which competes with substrate oxidation.