Electronic Finetuning of a Bio‐inspired Iron(II) tetra‐NHC Complex by trans Axial Isocyanide Substitution

The synthesis of trans axially substituted mono‐ (1 a) and bis(tert‐butylisocyanide) (1 b) derivatives of the highly active homogeneous bio‐inspired iron(II) olefin epoxidation (pre‐)catalyst 1 bearing an equatorial macrocyclic tetra N‐heterocyclic carbene and two trans axial labile acetonitrile ligands is reported. NMR spectroscopy and SC‐XRD indicate a considerable π‐backdonation from the iron(II) centres to the isocyanide ligand(s). The impact of isocyanide substitution on the electronic features of the complexes is studied by cyclic voltammetry revealing a significant increase in half‐cell potential assignable to the reversible Fe(II)/Fe(III) redox couple with an increasing number of isocyanides as a result of their π‐accepting properties: E(1/2)=0.15 V (1), E(1/2)=0.35 V (1 a), E(1/2)=0.44 V (1 b).