Saddle-shaped six-coordinate iron(III) porphyrin complexes showing a novel spin crossover between S = 1/2 and S = 3/2 spin states

The field strength of the axial ligands determines the spin state of saddled iron(III) porphyrin complexes (see picture). Strong axial ligands (L), such as imidazole and 4-dimethylaminopyridine, lead to the formation of complexes with a pure S = 1/2 state, while weak ligands, such as THF, give complexes with a pure S = 3/2 state. Intermediate strength ligands, such as pyridine and 4-cyanopyridine, give complexes that show a novel spin crossover between the S = 1/2 and S = 3/2 states.