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 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;...