Equilibrium Geometries and Electronic Structure of Iron−Porphyrin Complexes:  A Density Functional Study

Abstract

We have performed density functional theory (DFT) calculations of iron−porphyrin (FeP) and its complexes with O2, CO, NO, and imidazole (Im). Our fully optimized structures agree well with the available experimental data for synthetic heme models. Comparison with crystallographic data for proteins highlights interesting features of carbon monoxymyoglobin. The diatomic molecule induces a 0.3−0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron−diatomic bond increases in the order Fe−O2 (9 kcal/mol) < Fe−CO (26 kcal/mol) < Fe−NO (35 kcal/mol). The ground state of FeP(O2) is an open shell singlet. The bent Fe−O2 bond can be formally described as FeIII−O2 -, and it is characterized by the anti-ferromagnetic coupling between one of the d electrons of Fe and one of the π* electrons of O2. FeP(CO) is a closed shell singlet, with a linear Fe−C−O bond. The complex with NO has a doublet ground state and a Fe−NO geometry intermediate between that of FeP(CO) and FeP(O2). The bending of the Fe−(diatomic) angle requires a rather low energy for these three complexes, resulting in large-amplitude oscillations of the ligand even at room temperature. The addition of an imidazole ligand to FeP moves the Fe atom out of the porphyrin plane toward the imidazole and decreases significantly the energy differences among the spin states. Moreover, our calculations underline the potential role of the imidazole ligand in controlling the electronic structure of FeP by changing the out-of-planarity of the Fe atom. The presence of the imidazole increases the strength of the Fe−O2 and Fe−CO bonds (15 and 35 kcal/mol, respectively), but does not affect the energy of the Fe−NO bond, while the resulting FeP(Im)(NO) complex exhibits a longer and weaker Fe−Im bond.

Density Functional Theory Study of Transitional Metal Macrocyclic Complexes' Dioxygen-Binding Abilities and Their Catalytic Activities toward Oxygen Reduction Reaction

“…This is consistent with reported theoretical calculations for FeP. 12 For the rest of the systems, only end-on bindings were investigated. The optimized structure parameters and dioxygen-binding energies for end-on adducts are listed in Table 2.…”

How O2 Binds to Heme

“…Already the simple Hartree-Fock formalism correctly predicts the bent form of the O 2 adduct (8), whereas state-of-the-art density functional theory (DFT) pro-vides excellent geometries of porphyrins in general (9 -13). Among these, the B3LYP density functional predicts very closelying quintet and triplet states in deoxyheme models, sometimes with a triplet ground state (14). Recently, DFT was used to compute the electronic spectrum of Fe II -porphine with 2-methylimidazole as the axial ligand, making the quintet state the lowest in energy, but the triplet state only 12 kJ/mol higher (15), in excellent agreement with the experiment.…”

Spectroscopic identification of reactive porphyrin motions

“…86,87 Difficulties in correctly predicting the relative energies of different spin states are a widely acknowledged shortcoming of DFT calculations. 88,89 Ferric porphyrin chlorides, in particular, have been cited as an illustration of this problem.…”