Electronic Effects of Electron-Donating and -Withdrawing Groups in Model Complexes for Iron-Tyrosine-Containing Metalloenzymes

Three new iron(III) complexes with the ligand N,N‘-bis(2-hydroxybenzyl)-N,N‘-bis(pyridin-2-ylmethyl)ethylenediamine, H2bbpen, containing electron-donating and -withdrawing groups (Me, Br, NO2) in the 5-position of the phenol rings were synthesized and fully characterized by IR spectroscopy, ESI mass spectrometry, and CHN elemental analyses. X-ray structures of the iron(III) complexes containing NO2 and Me groups were determined. The effects of the substituents on the electronic properties of the complexes were detected by UV−visible spectroscopy, cyclic voltammetry, and X-ray crystallography. Linear correlations between the Hammett parameter for the substituents (σp) and the FeIII/FeII redox potentials or ligand−metal charge-transfer (LMCT) processes of the complexes were obtained. A theoretical study using DFT is presented and shows good agreement between the experimental and calculated data.