Substituent effects on geometric and electronic properties of iron tetraphenylporphyrin: a DFT investigation

To investigate the effects of the substituents, substituent positions and axial chloride ligand on the geometric and electronic properties of the iron tetraphenylporphyrin (FeTPP), a series of the substituented iron tetraphenylporphyrins and their chlorides, FeT(o/p-R)PP and FeT(o/p-R)PPCl (R = -H, -Cl, -NO2, -OH, -OCH3), were systematically calculated without any symmetry constraint by using DFT method. For geometric structure, the substituent position and axial Cl ligand change the configuration of the iron porphyrin obviously. The ortho-substituents prefer making the phenyls perpendicular to the porphyrin ring; the axial chloride draws the central Fe ion similar to 0.500 out of the porphyrin plane toward the ligand. With regard to electronic properties, it is found that E-LUMO could be related to the catalytic activity. The electron-withdrawing group always lowers the energies of both frontier orbitals, while the electron-donating one heightens them simultaneously, but they affect the E-HOMO and E-LUMO in the same sequence, -NO2 < -Cl < -H < -OH < -OCH3. The substituent effects on the central Fe ion were explored by calculating NBO charge distribution, spin density and natural electron configuration