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

The synthesis and characterization of a series of bis(cyanide)(meso- tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)₂]^- where R is H, Me, Et, and Pr, 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(T(i)PrP)(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(T(i)PrP)(CN)₂]^- reaches as much as 1443 ppm at -25 °C. The substituent dependent phenomena are also observed in EPR spectra taken in frozen CH'2Cl'2 solution at 4.2 K. While the unsubstituted complex gives a so called large g(max) type signal at 3.65, the alkyl substituted complexes exhibit axial type spectra; the EPR parameters for [Fe(T(i)PrP)(CN)₂]^- are g is perpendicular to = 2.43 and g is parallel with = 1.73. These results clearly indicate that the electronic ground state changes from the usual (d(xy))²(d(xz), d(yz))³ to the unusual (d(xz), d(yz))⁴(d(xy))¹ as the substituent becomes bulkier. Analysis of the EPR g values reveals that the orbital of the unpaired electron has more than 90% d(xy) character in the alkyl substituted complexes. The unusual electron configuration is ascribed to the destabilization of d(xy) orbital and/or stabilization of d(xz) and d(yz) orbitals caused by the S'4 raffled structure of the alkyl substituted porphyrin ring. Thus, in a strongly ruffled low spin complex such as [Fe(T(i)PrP)(L)₂](±), electron configuration of iron is presented by (d(xz), d(yz))⁴(d(xy))¹ regardless of the kind and basicity of the axial ligand (L). In fact, low spin bis(pyridine) complex [Fe(T(i)PrP)(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 is perpendicular to = 2.46 and g is parallel with = 1.59. In every case examined, (d(xz),d(yz))⁴(d(xy))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 is perpendicular to values. The methanol effect is explained in terms of the stabilization of d(xz) and d(yz) relative to d(xy) due to the hydrogen bond formation between coordinated cyanide and methanol.