Facile Ring Opening of Iron(III) and Iron(II) Complexes of meso-Amino-octaethylporphyrin by Dioxygen

Pyridine solutions of ClFe^III(meso-NH₂-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFe^III(meso-NH₂-OEP), has been isolated and characterized by X-ray crystallography. In the solid state, it has a five-coordinate structure typical for high-spin (S = ⁵/₂) iron(III) complex. In chloroform-d solution, ClFe^III(meso-NH₂-OEP) displays an ¹H NMR spectrum characteristic of a high-spin, five-coordinate complex and is unreactive toward dioxygen. However, in pyridine-d₅ solution a temperature-dependent equilibrium exists between the high-spin (S = ⁵/₂), six-coordinate complex, {(py)ClFe^III(meso-NH₂-OEP)}, and the six-coordinate, low spin (S = ¹/₂ with the less common (d_xzd_yz)⁴(d_xy)¹ ground state)) complex, [(py)₂Fe^III(meso-NH₂-OEP)]⁺. Such pyridine solutions are air-sensitive, and the remarkable degradation has been monitored by ¹H NMR spectroscopy. These studies reveal a stepwise conversion of ClFe^III(meso-NH₂-OEP) into an open-chain tetrapyrrole complex in which the original amino group and the attached meso carbon atom have been converted into a nitrile group. Additional oxidation at an adjacent meso carbon occurs to produce a ligand that binds iron by three pyrrole nitrogen atoms and the oxygen atom introduced at a meso carbon. This open-chain tetrapyrrole complex itself is sensitive to attack by dioxygen and is converted into a tripyrrole complex that is stable to further oxidation and has been isolated. The process of oxidation of the Fe(III) complex, ClFe^III(meso-NH₂-OEP), is compared with that of the iron(II) complex, (py)₂Fe^II(meso-NH₂-OEP); both converge to form identical products.