X-ray Photoelectron Spectra of JV-Methyltetraphenylporphyrins: Evidence for a Correlation of Binding Energies with Metal-Nitrogen Bond Distances

X-ray photoelectron spectroscopic data are reported for N-methyltetraphenylporphyrin, its dicationic salt, and a number of metal complexes, with special attention given to the N Is binding energies. The free-base N Is energies are 397.6 and 399.9 eV, with the N-CH₃nitrogen atom being perturbed to essentially the same extent as the N-H nitrogen atom. The spectrum of the acid shows an increase in the relative area of the peak at 399.9 eV, indicating protonation of the nonmethylated nitrogen atoms. The N Is energies for the metal complexes (398.3 and 400.4 eV for Mn(JV-CH₃TPP)Cl, 398.2 and 400.2 eV for Fe(N-CH₃TPP)C1, 398.4 and 400.2 eV for Co(N-CH₃TPP)Cl, and 398.5 and 400.0 eV for Zn(N-CH₃TPP)Cl, with the lower energies due to the nonmethylated nitrogen atoms in each case) correspond to changes in the size of the metal atom. The metal atom perturbs the nonmethylated nitrogen atoms to a greater extent than the methylated nitrogen atom. The difference in these energies correlates in a linear fashion with differences in the respective metal-nitrogen bond lengths determined crystallographically. In the ease of greatest difference in binding energies, Mn(N-CH₃TPP)Cl, the XPS data indicate a relatively strong interaction with the methylated nitrogen atom while in the case of the greatest bond length difference, Zn(N-CH₃TPP)Cl. very little interaction is evident.