Electronic structure of low-spin six-coordinate iron(III) meso-tetrapropylchlorin complexes

Low-spin iron(III) tetrapropylchlorins [Fe(TnPrC)L2]± (L = HIm, 1-MeIm, DMAP, CN-, 4-CNPy, tBuNC) adopt the dxy-type ground state regardless of the nature of axial ligands. Among the complexes examined, [Fe(TnPrC)(tBuNC)2]+ has shown quite unique spectroscopic properties as described below. (1) 1H NMR signals were extremely broad as compared with those of other complexes. In particular, 5,20-CH2(α) signal was too broad to detect. (2) No signals except Cγ were observed in 13C NMR spectra. (3) Tetragonal splitting parameter (|Δ|) estimated by the EPR g values at 4.2 K reached as much as 12.4 λ, which is the largest |Δ| value among all the low-spin iron(III) porphyrins and porphyrinoids reported previously. On the basis of these results, we have concluded that [Fe(TnPrC)(tBuNC)2]+ adopts the low-spin iron(III) with (dxz, dyz)4(dxy)1 electronic ground state at 4.2–30 K where the EPR spectra are taken, while it should be expressed as the low-spin Fe(II) chlorin π-radical cation [FeII(TnPrC.)(tBuNC)2]+ at ambient temperature where the NMR spectra are taken.