Electronic Fine-tuning of a Bio-inspired Iron(II) tetra-NHC Complex by trans Axial Isocyanide Substitution.

The synthesis of trans axially substituted mono -butylisocyanide) ( 1b ) 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. Multinuclear NMR spectroscopy and SC-XRD indicate a considerable pi-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 pi-accepting properties: E 1/2 = 0.15 V ( 1 ), E 1/2 = 0.35 V ( 1a ), E 1/2 = 0.44 V ( 1b ).