Nickel-macrocycle interaction in nickel(II) porphyrins and porphyrazines bearing alkylthio β-substituents: A combined DFT and XPS study

An electronic structure analysis of two nickel(II) tetrapyrrole complexes bearing beta-alkylthio substituents, NiOMTP and NiOETPz, has been carried out through a combination of high-resolution XPS experiments and DFT calculations. The Ni 2p XPS spectra show a 0.5 eV shift to higher energy of the Ni 2p(3/2) and Ni 2p(1/2) binding energies on going from the porphyrin to the porphyrazine complex. This shift, which is well-reproduced by relativistic spin-orbit ZORA calculations, is indicative of a depletion of electron density on the central metal. Such a depletion of electron density is related to the macrocycle-induced changes in the metal-ligand interactions. In the porphyrazine complex both the ligand to metal sigma donation and the metal to ligand p-back donation increase. The latter increases slightly more than the former, however, leading to a decrease of electron density on the central metal.