Nitrogen Charge Distributions in Free-Base Porphyrins, Metalloporphyrins, and Their Reduced Analogs Observed by X-ray Photoelectron Spectroscopy

The metal, N 1s, and C 1s XPS binding energies for the free-base compounds (H₂OEP, H₂TPrP, H₂TPP, H₂TPC, H₂TPBC), the metalloporphyrins (MgTPP, ZnTPP, CoTPP, CuTPP, NiTPP, AgTPP), and the metallochlorins (MgTPC, ZnTPC, CoTPC, CuTPC, NiTPC, AgTPC) are reported. The N Is binding energies plus additional information from theoretical calculations and NMR, uv-vis, and ir spectroscopy show that reduction of the free bases slightly decreases the electron density of the nitrogen and only subtly changes their XPS spectra. The replacement of the free-base protons by a metal ion to form the metalloporphyrin increases the symmetry of the molecule and also introduces an electron-withdrawing group into the center of the ligand which increases the N 1s binding energy as the metal ion electronegativity increases. Finally these results are used to show that both effects are found in metallochlorins. In both metalloporphyrins and metallochlorins differences in the spectral features of paramagnetic and diamagnetic ions are noted.