Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

摘要

The iron-based porphyrin complex containing a bispyridine-basedhanging unit termed Py2XPFe was previously used as an effectivecatalyst for the reduction of protons to molecular hydrogen in solution.Here, the molecular compound was immobilized on a modified gold electrodesurface and investigated by spectroelectrochemical methods under catalyticconditions. Immobilization of the Py2XPFe was facilitatedusing a pyridine-based amine linker molecule grafted to the gold electrodeby electrochemical amine oxidation. The linker molecule denoted inthis report as Pyr-1 allows for effective coordination of the ironporphyrin compound to the modified gold surface through axial coordinationof the pyridine component to the Fe center. Resonance Raman spectroelectrochemistrywas performed on the immobilized catalyst in pH 7 buffer at increasingcathodic potentials. This facilitates the electrochemical hydrogenevolution reaction (HER) while concurrently allowing for the observationof the v (4), v (3), and v (2) porphyrin marker bands, whichare sensitive to oxidation and spin state changes at the metal center.The observed changes in these bands at decreasing potential indicatethat the immobilized Py2XPFe exists in the formal high-spinFe(III) state before being reduced to the low-spin Fe-II state resulting from axial interaction with the linker moiety.This Fe-II state likely acts as the precatalyst for theHER reaction. Surfaced enhanced Raman spectroelectrochemistry wasalso conducted on the system as the gold electrode provides a sufficientsurface enhancement effect so as to observe the bonding nature ofthe pyridine substituents within the second coordination sphere. Asthe potential is lowered cathodically, the pyridine ring breathingmodes at 999 cm(-1) are shown to increase in intensitydue to protonation, which reach an intensity saturated limit whereatHER is conducted. This suggests that in pH 7 buffer, the increasein cathodic potentials facilitates protonation of the pyridine-basedsecond coordination sphere. The extent to which protonation occurscan be viewed as a function of decreasing potential due to an increasein proton flux at the immobilized catalyst which, at the requiredonset potential for catalysis, aids in the reduction of protons tomolecular hydrogen. An iron-porphyrincatalyst with two pending pyridineunits was immobilized on a functionalized gold electrode surface andinvestigated via electrochemical resonance and surface enhanced Ramanspectroscopy. Concomitant with the electrode polarization and reductionof the iron site, protonation of the pyridines could be followed.The observed protonation step at the pyridine unit facilitates thereduction of protons to molecular hydrogen at the iron-porphyrin unit.