Factors affecting the electronic ground state of low-spin iron(III) porphyrin complexes

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 ispropyl, 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 (d_xz, d_yz)⁴(d_xy)¹ ground state; the other six complexes have exhibited the common (d_xy)²(d_xz, d_yz)³ ground state. When the axial imidazole is replaced by cyanide, even the propyl and cyclopropyl complexes have shown the (d_xz, d_yz)⁴(d_xy)¹ ground state; the TPP and ORTPP complexes have still maintained the common (d_xy)²(d_xz, d_yz)³ ground state. In the case of the bis(^tBuNC) complexes, all the complexes have shown the (d_xz, d_yz)⁴(d_xy)¹ ground state. However, the contribution of the (d_xz, d_yz)⁴(d_xy)¹ state to the electronic ground state differs from complex to complex; the (d_xz, d_yz)⁴(d_xy)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 a_1u HOMO in contrast to the other seven porphyrins that have a_2u HOMO. The ¹³C NMR and EPR studies have revealed that the contribution of the (d_xz, d_yz)⁴(d_xy)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 a_1u HOMO, and (iv) electron withdrawing substituents at the meso positions tend to maintain the common (d_xy)²(d_xz, d_yz)³ ground state.