An iron(II) complex with a tetradentate peptide ligand, cis-C6H10(CO-Cys-Pro-Leu-Cys-OMe)2-1,2, as a model of reduced rubredoxin

Abstract

The complexes [NEt₄]₂[Fe{cis-C₆H₁₀(CO-Cys-OMe)₂-1,2}₂]1 and [NEt₄]₂[Fe{cis-C₆H₁₀(CO-Cys-Pro-Leu-Cys-OMe)₂-1,2}]2 were synthesised by ligand-exchange reaction of [NEt₄]₂[Fe(SBu^t)₄] with the corresponding free peptide. In the ¹H NMR spectra the Cys C_βH₂ protons of 1 and 2 were observed between δ 200 and 270 in CD₃CN at 30 °C. The temperature dependence of these signals indicate that high-spin mononuclear iron(II) complexes are formed. Complexes 1 and 2 exhibit redox potentials at –0.60 and –0.78 V respectively vs. saturated calomel electrode in dimethyl sulfoxide. These negatively shifted redox potentials are explained by the difficulty in forming an intramolecular NH ⋯ S hydrogen bond involving the Leu residue in 2, confirmed by the ²H NMR spectrum.