Electronic effects of para-substituents on the electron configuration of dicyano[meso-tetrakis(p-substituted phenyl)porphyrinato]iron(III) complexes

There are two types of electron configuration, (dxy)2(dxz, dyz)3 and (dxz, dyz)4(dxy)1, in low spin iron(III) porphyrin complexes. In order to reveal how the electronic effect of substituents affects the electron configuration of low spin iron(III) porphyrin complexes, we have examined the 13C NMR, 1H NMR, and EPR spectra of a series of tetrabutylammonium (dicyano)[meso-tetrakis(p-substituted phenyl)porphyrinato]ferrate(III), [Fe(p-X–TPP)(CN)2]−Bu4N+, in both CD2Cl2 and CD3OD solutions. The chemical shifts of the meso carbon signals, which sharply reflect the electron configuration of low spin iron(III), have changed to a great extent depending on the electron-donating or electron-withdrawing ability of the para-substituent. The isotropic shifts of the meso-carbon signals of [Fe(p-X–TPP)(CN)2]−Bu4N+ are determined on the basis of the meso-carbon chemical shifts of the corresponding diamagnetic cobalt(III) complexes, [Co(p-X–TPP)(CN)2]−Bu4N+. The plots of the isotropic shifts against Hammett σp values have yielded good linear lines with the slopes, −22 and −69 ppm, in CD2Cl2 and CD3OD, respectively. On the basis of these results, we have concluded that the electron-withdrawing groups at the phenyl para-positions stabilize the (dxy)2(dxz, dyz)3 state while the electron-donating groups at the same positions stabilize the (dxz, dyz)4(dxy)1 state. The conclusion is further supported by the 1H NMR and EPR results.