Stabilizing intermediate-spin state in iron(III) porphyrins

Abstract

Hemoproteins serve many diverse biological functions through the nearly identical heme prosthetic group. The elucidation of the electronic structures of synthetic hemes is quite important in understanding the function and catalytic processes of naturally occurring heme proteins. For example, an important step in cytochrome P450 catalysis has been the spin-state crossover upon substrate binding which thereby facilitates the reduction of the iron centre. Iron(III) porphyrins having intermediate-spin state have attained much of attention since Maltempo’s discovery of its crucial contributions in several bacterial heme proteins such as Cytochromes c′. We have analyzed here various factors responsible for stabilizing such intermediate-spin states of iron using the same iron(III)porphyrinato platform. We report the synthesis, X-ray structure and spectroscopic characterizations of saddle distorted six-coordinated iron(III) porphyrins having weak axial coordination such as THF and dioxane. Through combined solid as well as solution phase studies, it has been found that the complexes demonstrate iron(III) in pure intermediate-spin state. A brief comprehensive personal account for stabilizing intermediate-spin state of iron using the same iron(III) porphyrins has also been presented. Our work highlights the possible spin state switching of iron through external perturbations such as H-bonding interactions, weak axial coordination etc which are indeed important steps in the functioning of various heme proteins.