Models for cytochromes c′: Observation of an extremely labile spin state in monoimidazole complexes of saddle-shaped iron(III) porphyrinates

Saddle in a spin: The saddle-shaped octaethyltetraphenylporphyrin-imidazole complex [Fe^III (oetpp)(HIm)]⁺ adopts an essentially pure intermediate-spin state between 298 and 173 K in CD₂Cl₂ solution, the conditions under which the NMR spectra are recorded. This complex has shown, however, a novel spin transition from S=3/2 to S=5/2 and S=1/2 below 70 K. Thus, the complex exists as an equilibrium mixture of three spin isomers at 4 K.