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

Abstract

The iron-based porphyrin complex containing a bispyridine-based hanging unit termed Py 2 XPFe was previously used as an effective catalyst for the reduction of protons to molecular hydrogen in solution. Here, the molecular compound was immobilized on a modified gold electrode surface and investigated by spectroelectrochemical methods under catalytic conditions. Immobilization of the Py 2 XPFe was facilitated using a pyridine-based amine linker molecule grafted to the gold electrode by electrochemical amine oxidation. The linker molecule denoted in this report as Pyr-1 allows for effective coordination of the iron porphyrin compound to the modified gold surface through axial coordination of the pyridine component to the Fe center. Resonance Raman spectroelectrochemistry was performed on the immobilized catalyst in pH 7 buffer at increasing cathodic potentials. This facilitates the electrochemical hydrogen evolution reaction (HER) while concurrently allowing for the observation of the v 4 , v 3 , and v 2 porphyrin marker bands, which are sensitive to oxidation and spin state changes at the metal center. The observed changes in these bands at decreasing potential indicate that the immobilized Py 2 XPFe exists in the formal highspin Fe 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 the HER reaction. Surfaced enhanced Raman spectroelectrochemistry was also conducted on the system as the gold electrode provides a sufficient surface enhancement effect so as to observe the bonding nature of the pyridine substituents within the second coordination sphere. As the potential is lowered cathodically, the pyridine ring breathing modes at 999 cm −1 are shown to increase in intensity due to protonation, which reach an intensity saturated limit whereat HER is conducted. This suggests that in pH 7 buffer, the increase in cathodic potentials facilitates protonation of the pyridine-based second coordination sphere. The extent to which protonation occurs can be viewed as a function of decreasing potential due to an increase in proton flux at the immobilized catalyst which, at the required onset potential for catalysis, aids in the reduction of protons to molecular hydrogen.

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

“…Naturally occurring heme enzymes usually perform catalysis via oxygen binding, whereas synthetic iron porphyrins have been shown to additionally show activity in hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO 2 RR), or even aminations . All of these reactions are not only influenced by the redox chemistry of the iron center but also by the second coordination sphere of the catalyst. , Possible influences include the additional supply (or uptake) of protons and the (de)­stabilization of certain intermediates that control the course of the reaction. In hangman complexes, functional groups are placed “above” the porphyrin ring via covalently attached linker/spacer units.…”

Iron Dioxygen Adduct Formed during Electrochemical Oxygen Reduction by Iron Porphyrins Shows Catalytic Heme Dioxygenase Reactivity

“…Unlike normal spectro-electrochemistry where the electrode is static and electrode processes are dominated by diffusion from bulk solvent, the forced convection inherent to the rotating disc technique used in SERRS-RDE allows investigation of catalytic mechanism under steady state (Scheme ). − This has allowed the characterization of intermediates involved in ORR catalyzed by different iron porphyrins and obtain insights into factors that control the rate and selectivity of ORR for both molecular and biosynthetic catalysts. − Normally SERRS-RDE is performed at potentials where the current becomes potential independent, i.e., mass transfer controlled. In this potential region, for most synthetic and biosynthetic Fe and Fe/Cu porphyrins, an Fe III –OOH species was found to accumulate on the electrode during ORR (Scheme ).…”

“…Based on these observations, it was proposed that the cleavage of the O–O bond (homolytic or heterolytic) was the rate-determining species in ORR and the site of protonation of this Fe III –OOH species determined reactivity (Scheme ). , …”

Electrocatalytic hydrogen evolution with a copper porphyrin bearing meso-(o-carborane) substituents

“…8–10 Therefore, developing efficient electrocatalysts made of cheap and earth-abundant metal elements has attracted increasing interest. 11–17 Recently, a variety of first-row transition metal complexes, including Mn, 18,19 Fe, 20–22 Co, 23–30 Ni, 31–35 and Cu, 36,37 have been identified to be active for the electrocatalytic HER. Despite these achievements, developing electrocatalysts with new structures for improved efficiency is still required.…”

“…3 Third, porphyrin ligands can be modified by installing various functional groups, making porphyrin an excellent platform for investigating structure–activity relationships. 22,32,43,51 Therefore, metal porphyrins have been shown to be active and stable by functioning as electrocatalysts for the HER.…”