Iron porphyrin dications with neutral axial ligands: DFT calculations delineate similarities with heme protein compound II intermediates

OLYP/TZP calculations on two symmetrized model complexes [Fe(TPP)(py) ₂]²⁺ and [Fe(TPP)(PhNC)₂]²⁺ (TPP = meso-tetraphenylporphyrin, py = pyridine, PhNC = phenylisocyanide) reveal dense manifolds of low-energy electronic states. For the latter complex, broken-symmetry calculations successfully reproduce the unique S = 0 ground state that is expected on the basis of experimental measurements on a closely related complex; the S = 0 state arises from antiferromagnetic coupling between a low-spin d_xy¹(d_xz,d_yz)⁴ Fe(III) center and a porphyrin "a_2u" radical. Furthermore, the calculations indicate low-energy Fe(IV) states for both complexes. Overall, the results contribute to our deepening understanding of the factors contributing to the stability of iron(IV) centers. Thus, a dianionic π-donor oxo ligand is no longer deemed a requirement for the stability of heme-based Fe(IV) centers; iron(IV) intermediates of heme proteins such as chloroperoxidase, catalase, and MauG, having only monanionic ligands such as hydroxide, thiolate, and phenolate and/or (in the case of MauG) a neutral histidine as axial ligands, are now firmly established.