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

Abstract

Pyridine solutions of ClFeIII(meso-NH2-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFeIII(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, ClFeIII(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)ClFeIII(meso-NH2-OEP)}, and the six-coordinate, low spin (S = 1/2 with the less common (dxzdyz)4(dxy)1 ground state)) complex, [(py)2FeIII(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 ClFeIII(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, ClFeIII(meso-NH2-OEP), is compared with that of the iron(II) complex, (py)2FeII(meso-NH2-OEP); both converge to form identical products.