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

A series of five-coordinate Fe-III octaaryltetraphenyl-porphyrins [Fe-III(por)(X)] (X = Cl, I, I-3, ClO4, or SO3CF3) and their 1 e(-)-oxidized complexes [Fe-III(por)(X)(Y)] (X = Cl, I, ClO4, or SO3CF3; Y = SbCl6, I-3, ClO4, or SO3CF3) have been synthesized and characterized. The electronic structures have been confirmed by using UV/Vis, IR, H-1 NMR, EPR, and Mossbauer 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 = I-3, ClO4, and SO3CF3) depending upon the axial ligand field strength. The average Fe-Np length decreases with decreasing axial ligand strengths, Cl < I < ClO4 < I-3 approximate to 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 Fe-III porphyrin pi-cation radical, whereas the essentially pure intermediate perchlorato, triflato, triiodo iron(III) porphyrinates produce the corresponding six-coordinate high-spin iron(III) porphyrin pi-cation radicals. The oxidation also induces larger separation between the up- and down-field shifted methylene resonances in the H-1 NMR spectra owing to the presence of the pi-cation radical.