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The synthesis and characterization of a series of bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN) 2 ] -where R is H, Me, Et, and i Pr, are reported. The 1 H NMR spectrum of the unsubstituted [Fe(THP)(CN) 2 ] -shows a pyrrole signal at δ ) -23.19 ppm (-25 °C) in CD 2 Cl 2 , which is quite typical as a low spin ferric complex. As the bulkiness of the meso substituent increases, the pyrrole signal moves to lower magnetic field; 0.34, -2.26, and 11.94 ppm for [Fe(TMeP)(CN) 2 ] -, [Fe(TEtP)(CN) 2 ] -, and [Fe(T i PrP)(CN) 2 ] -, respectively. Corresponding to the pyrrole proton signal, the cyanide carbon signal also exhibits a large downfield shift. The difference in chemical shifts between [Fe(THP)(CN) 2 ] -and [Fe(T i PrP)(CN) 2 ] -reaches as much as 1443 ppm at -25 °C. The substituent dependent phenomena are also observed in EPR spectra taken in frozen CH 2 Cl 2 solution at 4.2 K. While the unsubstituted complex gives a so called large g max type signal at 3.65, the alkyl substituted complexes exhibit axial type spectra; the EPR parameters for [Fe(T i PrP)(CN) 2 ] -are g ⊥ ) 2.43 and g | ) 1.73. These results clearly indicate that the electronic ground state changes from the usual (d xy ) 2 (d xz , d yz ) 3 to the unusual (d xz , d yz ) 4 (d xy ) 1 as the substituent becomes bulkier. Analysis of the EPR g values reveals that the orbital of the unpaired electron has more than 90% d xy character in the alkyl substituted complexes. The unusual electron configuration is ascribed to the destabilization of d xy orbital and/or stabilization of d xz and d yz orbitals caused by the S 4 ruffled structure of the alkyl substituted porphyrin ring. Thus, in a strongly ruffled low spin complex such as [Fe(T i PrP)(L) 2 ] ( , electron configuration of iron is presented by (d xz , d yz ) 4 (d xy ) 1 regardless of the kind and basicity of the axial ligand (L). In fact, low spin bis(pyridine) complex [Fe(T i PrP)(Py) 2 ] + gives a pyrrole signal at quite a low field, δ ) +16.4 ppm at -87 °C, which is actually the lowest pyrrole signal ever reported for the low spin ferric porphyrin complexes. Correspondingly, the EPR spectrum taken at 77 K showed a clear axial type spectrum, g ⊥ ) 2.46 and g | ) 1.59. In every case examined, (d xz ,d yz ) 4 (d xy ) 1 ground state is more or less stabilized by the addition of methanol as exemplified by the further downfield shift of the pyrrole proton and cyanide carbon signals together with the smaller EPR g ⊥ values. The methanol effect is explained in terms of the stabilization of d xz and d yz relative to d xy due to the hydrogen bond formation between coordinated cyanide and methanol.

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.

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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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