Change in Electron Configuration of Ferric Ion in Bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)₂]-, Caused by the Nonplanarity of the Porphyrin Ring

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Abstract

The synthesis and characterization of a series of bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)2]- where R is H, Me, Et, and iPr, are reported. The 1H NMR spectrum of the unsubstituted [Fe(THP)(CN)2]- shows a pyrrole signal at δ = −23.19 ppm (−25 °C) in CD2Cl2, 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(TiPrP)(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(TiPrP)(CN)2]- reaches as much as 1443 ppm at −25 °C. The substituent dependent phenomena are also observed in EPR spectra taken in frozen CH2Cl2 solution at 4.2 K. While the unsubstituted complex gives a so called large gmax type signal at 3.65, the alkyl substituted complexes exhibit axial type spectra; the EPR parameters for [Fe(TiPrP)(CN)2]- are g⊥ = 2.43 and g∥ = 1.73. These results clearly indicate that the electronic ground state changes from the usual (dxy)2(dxz, dyz)3 to the unusual (dxz, dyz)4(dxy)1 as the substituent becomes bulkier. Analysis of the EPR g values reveals that the orbital of the unpaired electron has more than 90% dxy character in the alkyl substituted complexes. The unusual electron configuration is ascribed to the destabilization of dxy orbital and/or stabilization of dxz and dyz orbitals caused by the S4 ruffled structure of the alkyl substituted porphyrin ring. Thus, in a strongly ruffled low spin complex such as [Fe(TiPrP)(L)2]±, electron configuration of iron is presented by (dxz, dyz)4(dxy)1 regardless of the kind and basicity of the axial ligand (L). In fact, low spin bis(pyridine) complex [Fe(TiPrP)(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, (dxz,dyz)4(dxy)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 dxz and dyz relative to dxy due to the hydrogen bond formation between coordinated cyanide and methanol.

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