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

Abstract

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 Cγwere observed in13C 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)1electronic 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.