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Factors Affecting the Electronic Ground State of Low-Spin Iron(III) Porphyrin Complexes

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journal contribution
posted on 08.06.2001, 00:00 authored by Takahisa Ikeue, Yoshiki Ohgo, Takashi Saitoh, Tatsuya Yamaguchi, Mikio Nakamura
To determine the factors affecting the ground-state electron configuration of low-spin Fe(III) porphyrin complexes, we have examined the 1H NMR, 13C NMR, and EPR spectra of a series of low-spin bis-ligated Fe(III) porphyrin complexes [Fe(Por)L2]±, in which the positions of porphyrin substituents and the coordination ability of axial ligands are different. The seven porphyrins used in this study are meso-tetraalkylporphyrins (TRP:  R is propyl, cyclopropyl, or isopropyl), meso-tetraphenylporphyrin (TPP), meso-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin, and 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octaalkylporphyrins (ORTPP:  R is methyl or ethyl). The porphyrin cores of TRP are more or less S4-ruffled depending on the bulkiness of the alkyl substituents, while those of ORTPP are highly S4-saddled. Three types of axial ligands are examined which have the following characteristics in ligand field theory:  they are (i) strong σ-donating imidazole (HIm), (ii) strong σ-donating and weak π-accepting cyanide (CN-), and (iii) weak σ-donating and strong π-accepting tert-butyl isocyanide (tBuNC). In the case of the bis(HIm) complexes, only the isopropyl complex, [Fe(TiPrP)(HIm)2]+, has shown the less common (dxz, dyz)4(dxy)1 ground state; the other six complexes have exhibited the common (dxy)2(dxz, dyz)3 ground state. When the axial imidazole is replaced by cyanide, even the propyl and cyclopropyl complexes have shown the (dxz, dyz)4(dxy)1 ground state; the TPP and ORTPP complexes have still maintained the common (dxy)2(dxz, dyz)3 ground state. In the case of the bis(tBuNC) complexes, all the complexes have shown the (dxz, dyz)4(dxy)1 ground state. However, the contribution of the (dxz, dyz)4(dxy)1 state to the electronic ground state differs from complex to complex; the (dxz, dyz)4(dxy)1 contribution is the largest in [Fe(TiPrP)(tBuNC)2]+ and the smallest in [Fe(OETPPP)(tBuNC)2]+. We have then examined the electronic ground state of low-spin [Fe(OEP)(tBuNC)2]+ and [Fe(ProtoIXMe2)(tBuNC)2]+; OEP and ProtoIXMe2 represent 2,3,7,8,12,13,17,18-octaethylporphyrin and protoporphyrin-IX dimethyl ester, respectively. These porphyrins have a1u HOMO in contrast to the other seven porphyrins that have a2u HOMO. The 13C NMR and EPR studies have revealed that the contribution of the (dxz, dyz)4(dxy)1 state in these complexes is as small as that in [Fe(OETPP)(tBuNC)2]+. On the basis of these results, we have concluded that the low-spin iron(III) porphyrins that have (i) strong axial ligands, (ii) highly saddle shaped porphyrin rings, (iii) porphyrins with a1u HOMO, and (iv) electron withdrawing substituents at the meso positions tend to maintain the common (dxy)2(dxz, dyz)3 ground state.