Spin Modulation in Highly Distorted FeIII Porphyrinates by Using Axial Coordination and Their π‐Cation Radicals

Abstract

A series of five‐coordinate FeIII octaaryltetraphenylporphyrins [FeIII(por)(X)] (X = Cl, I, I3, ClO4, or SO3CF3) and their 1 e–‐oxidized complexes [FeIII(por·)(X)(Y)] (X = Cl, I, ClO4, or SO3CF3; Y = SbCl6, I3, ClO4, or SO3CF3) have been synthesized and characterized. The electronic structures have been confirmed by using UV/Vis, IR, 1H NMR, EPR, and Mössbauer spectroscopy as well as signal‐crystal X‐ray diffraction studies. The neutral five‐coordinate complexes exhibit spin states ranging from essentially pure high spin (S = 5/2 with X = Cl), admixed intermediate spin (S = 5/2, 3/2 with X = I) to an essentially pure intermediate spin (S = 3/2 with X = I3, ClO4, and SO3CF3) depending upon the axial ligand field strength. The average Fe–Np length decreases with decreasing axial ligand strengths, Cl < I < ClO4 < I3 ≈ SO3CF3, which happens to be the order of increasing contributions of the intermediate‐spin state to the complexes. DFT calculations demonstrate a dramatic change in the orbital energy levels upon changing the axial ligand strength. Upon 1 e– oxidation, the high‐spin chloro complex forms the five‐coordinate high‐spin FeIII porphyrin π‐cation radical, whereas the essentially pure intermediate perchlorato, triflato, triiodo iron(III) porphyrinates produce the corresponding six‐coordinate high‐spin iron(III) porphyrin π‐cation radicals. The oxidation also induces larger separation between the up‐ and down‐field shifted methylene resonances in the 1H NMR spectra owing to the presence of the π‐cation radical.