To determine the factors affecting the ground-state electron configuration of low-spin Fe(III) porphyrin complexes, we have examined the (1)H NMR, (13)C NMR, and EPR spectra of a series of low-spin bis-ligated Fe(III) porphyrin complexes [Fe(Por)L(2)](+/-), 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 S(4)-ruffled depending on the bulkiness of the alkyl substituents, while those of ORTPP are highly S(4)-saddled. Three types of axial ligands are examined which have the following characteristics in ligand field theory: they are (i) strong sigma-donating imidazole (HIm), (ii) strong sigma-donating and weak pi-accepting cyanide (CN(-)), and (iii) weak sigma-donating and strong pi-accepting tert-butyl isocyanide ((t)BuNC). In the case of the bis(HIm) complexes, only the isopropyl complex, [Fe(T(i)PrP)(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((t)()BuNC) 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(T(i)PrP)((t)()BuNC)(2)](+) and the smallest in [Fe(OETPPP)((t)BuNC)(2)](+). We have then examined the electronic ground state of low-spin [Fe(OEP)((t)BuNC)(2)](+) and [Fe(ProtoIXMe(2))((t)BuNC)(2)](+); OEP and ProtoIXMe(2) 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 (13)C 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)((t)BuNC)(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.

C NMR, and EPR studies of a series of low-spin (meso-tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(L) 2 ]X where R ) n Pr, c Pr, and i Pr and L represents axial ligands such as imidazoles, pyridines, and cyanide, have revealed that the ground-state electron configuration of [Fe(T n PrP)(L) 2 ]X and [Fe(T c PrP)(L) 2 ]X is presented either as the common (d xy ) 2 (d xz ,d yz ) 3 or as the less common (d xz ,d yz ) 4 (d xy ) 1 depending on the axial ligands. The ground-state electron configuration of the isopropyl complexes [Fe(Ti-PrP)(L) 2 ]X is, however, presented as (d xz ,d yz ) 4 (d xy ) 1 regardless of the kind of axial ligands. In every case, the contribution of the (d xz ,d yz ) 4 (d xy ) 1 state to the electronic ground state increases in the following order: HIm < 4-Me 2 NPy < 2-MeIm < CN -< 3-MePy < Py < 4-CNPy. Combined analysis of the 13 C and 1 H NMR isotropic shifts together with the EPR g values have yielded the spin densities at the porphyrin carbon and nitrogen atoms. Estimated spin densities in [Fe(T i PrP)(4-CNPy) 2 ] + , which has the purest (d xz ,d yz ) 4 (d xy ) 1 ground state among the complexes examined in this study, are as follows: meso-carbon, +0.045; R-pyrrole carbon, +0.0088; β-pyrrole carbon, -0.00026; and pyrrole nitrogen, +0.057. Thus, the relatively large spin densities are on the pyrrole nitrogen and meso-carbon atoms. The result is in sharp contrast to the spin distribution in the (d xy ) 2 (d xz ,d yz ) 3 type complexes; the largest spin density is at the β-pyrrole carbon atoms in bis(1methylimidazole)(meso-tetraphenylporphyrinato)iron(III), [Fe(TPP)(1-MeIm) 2 ] + , as determined by Goff. The large downfield shift of the meso-carbon signal, δ +917.5 ppm at -50 °C in [Fe(T i PrP)(4-CNPy) 2 ] + , is ascribed to the large spin densities at these carbon atoms. In contrast, the large upfield shift of the R-pyrrole carbon signal, δ -293.5 ppm at the same temperature, is caused by the spin polarization from the adjacent mesocarbon and pyrrole nitrogen atoms.

The field strength of the axial ligands determines the spin state of saddled iron(III) porphyrin complexes (see picture). Strong axial ligands (L), such as imidazole and 4‐dimethylaminopyridine, lead to the formation of complexes with a pure S=1/2 state, while weak ligands, such as THF, give complexes with a pure S=3/2 state. Intermediate strength ligands, such as pyridine and 4‐cyanopyridine, give complexes that show a novel spin crossover between the S=1/2 and S=3/2 states.

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Highly saddle shaped iron(III) porphyrin complex 1 showing a novel spin crossover process between the S = 3/2 and S = 1/2 has been crystallographically analyzed at 298, 180, and 80 K. As the temperature is lowered, the lattice contraction has occurred specifically along the b-axis. Correspondingly, the iron-pyridine bonds, which tilt slightly from the b-axis, have decreased by 7.3%. In contrast, the lattice contractions along the a- and c-axes are much smaller and the iron-porphyrin bonds, which almost coincide with the a- and c-axes, have shown much smaller contraction, ca. 2.2%. The large contraction of the specific bonds caused by packing force raises the energy level of the d(z)2 orbital and induces the spin transition. The detailed structural and lattice changes in the single crystal, which may be regarded as a superstructure parallel to a protein matrix, have been clarified.

On the basis of the difference in meso-13C chemical shifts, we have concluded that the intermediate-spin iron(III) complexes with highly ruffled and highly saddled porphyrins have different electron configurations. While the latter has a conventional (dxy)2(dxz, dyz)2(dz2)1, the former adopts a novel (dxz, dyz)3(dxy)1(dz2)1.

The electron configuration of a series of low-spin (dicyano){meso-tetrakis(2,4,6-trialkylphenyl)porphyrinato}iron(III) complexes, [Fe(R-TPP)(CN)2]- where R = Me, Et, or iPr, together with the parent [Fe(TPP)(CN)2]-, has been examined in dichloromethane−methanol solution by 1H NMR, 13C NMR, and EPR spectroscopies. While the ferric ion of [Fe(TPP)(CN)2]- has shown a common (d xy )2(d xz ,d yz )3 configuration, the ferric ions of the alkyl-substituted complexes [Fe(R-TPP)(CN)2]- have exhibited the preference of a less common (d xz ,d yz )4(d xy )1 configuration. Spectroscopic characteristics of the complexes in which ferric ions take the (d xz ,d yz )4(d xy )1 configuration are (i) axial type EPR spectra, (ii) downfield shifted pyrrole and meta signals in 1H NMR spectra, and (iii) downfield shifted meso-carbon signals in 13C NMR spectra. Occurrence of the less common (d xz ,d yz )4(d xy )1 configuration in [Fe(R-TPP)(CN)2]- has been ascribed to the electronic interaction between iron(dπ) and cyanide(pπ*) orbitals. The interaction stabilizes the dπ orbitals and induces (d xz ,d yz )4(d xy )1 configuration. Since the electron configuration of (dicyano){meso-tetrakis(2,6-dichlorophenyl)porphyrinato}iron(III), [Fe(Cl-TPP)(CN)2]-, which carries bulky electronegative chlorine atoms at the ortho positions, is presented as a common (d xy )2(d xz ,d yz )3, the less common (d xz ,d yz )4(d xy )1 configuration in [Fe(R-TPP)(CN)2]- can be ascribed, at least partially, to the electron-donating ability of the meso-aryl groups.