Electron Configuration of Ferric Ions in Low-Spin (Dicyano)(meso-tetraarylporphyrinato)iron(III) Complexes

Abstract

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 (dxy)2(dxz,dyz)3 configuration, the ferric ions of the alkyl-substituted complexes [Fe(R-TPP)(CN)2]- have exhibited the preference of a less common (dxz,dyz)4(dxy)1 configuration. Spectroscopic characteristics of the complexes in which ferric ions take the (dxz,dyz)4(dxy)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 (dxz,dyz)4(dxy)1 configuration in [Fe(R-TPP)(CN)2]- has been ascribed to the electronic interaction between iron(dπ) and cyanide(pπ*) orbitals. The interaction s...