Protonation of Pendant Pyridine Substituents in an Iron Porphyrin Hangman Complex: Influence on Spectral Visibility and Electrocatalysis

A hangman complex based on a functionalized iron porphyrin with two covalently attached pyridine (hanging) groups (Py2XPFe) was immobilized onto rough silver electrodes. To elucidate the protonation state of the hanging groups, surface-enhanced resonance Raman (SERR) spectroscopy was performed for the complete complex and a truncated molecular version without the porphyrin unit (Py2XBr) in buffers of different pH values (6.0–8.5). Spectra simulations of Py2XBr for different protonation states suggested three different interaction modes: one in which one proton is shared between both functional groups in a Zundel-like way, another one is a proton located at only one of the pyridine substituents, and a third one in which both pyridines are protonated. Comparison with experimental Raman difference spectra indicated that shared protonation predominates in the pH range of 8.0 to 7.5 and the double protonation in more acidic buffers. The local protonation appears as a potential-dependent intermediate at neutral pH. These results were correlated with the spectral data of the hangman complex Py2XPFe as a function of electric potential, demonstrating that only for a single localized protonation state the SERR spectra of the reduced FeII-porphyrin were visible. Using the electrocatalytic oxygen reduction reaction (ORR) as a model reaction, it was furthermore shown that the pH- and potential-dependent protonation of the pyridine groups has an influence on the reaction pathway.