Electron exchange in pyridine solutions of iron(III) protoporphyrin IX chloride

Proton magnetic resonance and absorption spectroscopy have been used to examine solutions of mixtures of reduced and oxidised iron protoporphyrin IX chloride in deuterated pyridine. The Fe(II) species are low spin but the Fe(III) complex is an equilibrium mixture of high and low spin forms. The movement to high field of the ring protons of the low-spin Fe(III) signals alone increases regularly with the amount of diamagnetic Fe(II) relative to the paramagnetic Fe(III) haem. The low spin Fe(III) must be in rapid exchange with the low-spin Fe(II) complex but not with the high-spin form. The addition of carbon monoxide to the Fe(II)/Fe(III) mixture effectively blocks electron exchange between the complexes as shown by a return of the proton resonances of the Fe(III) complex to positions seen in the absence of any Fe(II).