Electron configuration of ferric ions in low-spin (dicyano)(meso-tetraarylporphyrinato)iron(III) complexes

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 Pr-i, together with the parent [Fe(TPP)(CN)(2)](-), has been examined in dichloromethane-methanol solution by H-1 NMR, C-13 NMR , and EPR spectroscopies. While the ferric ion of [Fe(TPP)(CN)(2)](-) has s hown 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 pr eference of a less common (d(xz),d(yz))(4)(d(xy))(1) configuration. Spectro scopic 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) do wnfield shifted pyrrole and meta signals in H-1 NMR spectra, and (iii) down field shifted meso-carbon signals in C-13 NMR spectra. Occurrence of the le ss 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 (d(pi)) and c yanide (p(pi)*) orbitals. The interaction stabilizes the d(pi) orbitals and induces (d(xz),d(yz))(4)(d(xy))(1) configuration. Since the electron confi guration of (dicyano){meso-tetrakis(2,6-dichlorophenyl)porphyrinato}iron(II I), [Fe(Cl-TPP)(CN)(2)](-), which carries bulky electronegative chlorine at oms 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.