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Electronic Structure of Highly Ruffled Low-Spin Iron(III) Porphyrinates with Electron Withdrawing Heptafluoropropyl Groups at the meso Positions

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journal contribution
posted on 15.11.2010, 00:00 by Akira Tozuka, Yoshiki Ohgo, Akira Ikezaki, Miyoko Taniguchi, Mikio Nakamura
Bis(pyridine)[meso-tetrakis(heptafluoropropyl)porphyrinato]iron(III), [Fe(THFPrP)Py2]+, was reported to be the low-spin complex that adopts the purest (dxz, dyz)4(dxy)1 ground state where the energy gap between the iron dxy and dπ(dxz, dyz) orbitals is larger than the corresponding energy gaps of any other complexes reported previously (Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999, 121, 5196−5209). Although the highly ruffled porphyrin core expected for this complex contributes to the stabilization of the (dxz, dyz)4(dxy)1 ground state, the strongly electron withdrawing C3F7 groups at the meso positions should stabilize the (dxy)2(dxz, dyz)3 ground state. Thus, we have reexamined the electronic structure of [Fe(THFPrP)Py2]+ by means of 1H NMR, 19F NMR, and electron paramagnetic resonance (EPR) spectroscopy. The CD2Cl2 solution of [Fe(THFPrP)Py2]+ shows the pyrrole-H signal at −10.25 ppm (298 K) in 1H NMR, the CF2(α) signal at −74.6 ppm (298 K) in 19F NMR, and the large gmax type signal at g = 3.16 (4.2 K) in the EPR. Thus, contrary to the previous report, the complex is unambiguously shown to adopt the (dxy)2(dxz, dyz)3 ground state. Comparison of the spectroscopic data of a series of [Fe(THFPrP)L2]+ with those of the corresponding meso-tetrapropylporphyrin complexes [Fe(TPrP)L2]+ with various axial ligands (L) has shown that the meso-C3F7 groups stabilize the (dxy)2(dxz, dyz)3 ground state. Therefore, it is clear that the less common (dxz, dyz)4(dxy)1 ground state can be stabilized by the three major factors: (i) axial ligand with low-lying π* orbitals, (ii) ruffled porphyrin ring, and (iii) electron donating substituent at the meso position.

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