Effect of Axial Ligand on the Electronic Configuration, Spin States, and Reactivity of Iron Oxophlorin

Abstract

Iron-oxophlorin is an intermediate in heme degradation, and the nature of the axial ligand can alter the spin, electron distribution, and reactivity of the metal and the oxophlorin ring. The structure and reactivity of iron-oxophlorin in the presence of imidazole, pyridine, and t-butyl isocyanide as axial ligands was investigated using the B3LYP and OPBE methods with the 6-31+G* and 6-311+G** basis sets. OPBE/6-311+G** has shown that the doublet state of [(Py)(2)Fe(III)(PO)] (where pyridines are in perpendicular planes and PO is the oxophlorin trianion) is 3.45 and 5.27 kcal/mol more stable than the quartet and sextet states, respectively. The ground-state electronic configuration of the aforementioned complex is π(xz)(2) π(yz)(2) a(2u)(2) d(xy)(1) at low temperatures and changes to π(xz)(2) π(yz)(2) d(xy)(2) a(2u)(1) at high temperatures. This latter electronic configuration is consistently seen for the [(t-BuNC)(2)Fe(II)(PO(•))] complex (where PO(•) is the oxophlorin dianion radical). The complex [(Im)(2)Fe(III)(PO)] adopted the d(xy)(2) (π(xz) π(yz))(3) ground state and has low-lying quartet excited state which is readily populated when the temperature is increased.

Electronic properties of the low-lying spin states of dimethylnitrosamine coordinated to Fe(III) heme models: An ab initio study

“…As can be seen from Table , the calculated charges on the Fe atom of both cations are significantly smaller than its expected formal charge (III), indicating a substantial charge transfer to the iron atom. Significant charge transfer to the iron atom of Fe(III) porphyrins has also been reported by other authors …”

“…The electronic configuration for the sextet state of both systems is …(3d xy ) 1 (3d xz ) 1 (3d yz ) 1 (3d z2 ) 1 (3d x2‐y2 ) 1 . The same electronic configuration has been obtained for the high spin state of the iron‐oxophlorin in the presence of pyridine as axial ligands . The small energy difference between the 3d xz and the 3d yz orbitals (less than 0.05 eV) of both cations indicates a small deviation from the D 4h symmetry.…”

Nickel(II)meso‐Hydroxyporphyrin Complexes Revisited: Palladium‐Catalysed Synthesis, Electronic Structures of Derived Oxy Radicals, and Oxidative Coupling to a Dioxoporphodimethene Dyad

“…Other methods have been developed, for example, the nucleophilic substitution of the meso ‐nitro group by the sodium salt of E‐ benzaldoxime . There was a flurry of activity regarding these species in the 1990s in view of the discovery of their relevance to the heme catabolism pathways, and this topic is still being investigated . Free base meso‐ hydroxyporphyrins ( 1 ), the macrocyclic tetrapyrrole analogues of phenols, have a keto tautomer referred to as oxophlorin ( 2 ; Scheme ).…”

A novel mechanism of heme degradation to biliverdin studied by QM/MM and QM calculations

“…16 Pervious density functional theory (DFT) calculations have suggested that iron (ІІI) hydroxyheme is converted to iron(ІІІ) superoxo oxophlorin in the presence of dioxygen. Three major resonances form of hydroxyheme or oxophlorin has been suggested by our group 17,18 (see Scheme3). The reactivity of these isoforms depends on electronic environment and iron coordination.…”