Analysing low-spin ferric complexes using pulse EPR techniques: a structure determination of bis (4-methylimidazole)(tetraphenylporphyrinato)iron(iii)

Abstract

Continuous-wave (CW) EPR has been extensively used as a characterization tool for low-spin ferric complexes. Using the bis(4-methylimidazole) complex of iron(III) tetraphenylporphyrin as an example we show how the combination of CW EPR and pulsed EPR techniques allows for a detailed structure analysis of such ferric complexes. Both proton HYSCORE and combination-peak experiments indicate that the imidazole ligand planes are (nearly) parallel to the g_x axis (±20°). Simulations of the porphyrin nitrogen contributions to the HYSCORE spectra allow a determination of the g-axes frame in the molecular frame. Combination of these spectral results and the counter-rotation principle show that the g_x and g_y axes are rotated 5° (±5°) away from the N_p–Fe–N_p axes and that the imidazole ligand planes are counter-rotated over 5° (±5°). The nitrogen hyperfine and nuclear-quadrupole interactions of the pyrrole and imidazole nitrogens are determined and discussed as a function of the electronic structure.