Second-Sphere Hydrogen-Bond Donors and Acceptors Affect the Rate and Selectivity of Electrochemical Oxygen Reduction by Iron Porphyrins Differently

Abstract

The factors that control the rate and selectivity of 4e − /4H + O 2 reduction are important for efficient energy transformation as well as for understanding the terminal step of respiration in aerobic organisms. Inspired by the design of naturally occurring enzymes which are efficient catalysts for O 2 and H 2 O 2 reduction, several artificial systems have been generated where different second-sphere residues have been installed to enhance the rate and efficiency of the 4e − /4H + O 2 reduction. These include hydrogen-bonding residues like amines, carboxylates, ethers, amides, phenols, etc. In some cases, improvements in the catalysis were recorded, whereas in some cases improvements were marginal or nonexistent. In this work, we use an iron porphyrin complex with pendant 1,10-phenanthroline residues which show a pHdependent variation of the rate of the electrochemical O 2 reduction reaction (ORR) over 2 orders of magnitude. In-situ surface-enhanced resonance Raman spectroscopy reveals the presence of different intermediates at different pH's reflecting different rate-determining steps at different pH's. These data in conjunction with density functional theory calculations reveal that when the distal 1,10-phenanthroline is neutral it acts as a hydrogen-bond acceptor which stabilizes H 2 O (product) binding to the active Fe II state and retards the reaction. However, when the 1,10-phenanthroline is protonated, it acts as a hydrogen-bond donor which enhances O 2 reduction by stabilizing Fe III −O 2 .− and Fe III −OOH intermediates and activating the O−O bond for cleavage. On the basis of these data, general guidelines for controlling the different possible ratedetermining steps in the complex multistep 4e − /4H + ORR are developed and a bioinspired principle-based design of an efficient electrochemical ORR is presented.

Amine Groups in the Second Sphere of Iron Porphyrins Allow for Higher and Selective 4e–/4H+ Oxygen Reduction Rates at Lower Overpotentials

“…This is due to higher solvation of the protonated amines in aqueous solvents, which lowers the Coulombic interaction between the ammonium group and the iron center in the system. The protonated amines are positively charged and, unlike in organic solvents, can hydrogen-bond as well via water molecules trapped in the cavity. , Both of these effects, electrostatic and hydrogen bonding, can raise the Fe III/II redox potential …”

“…The O 2 binding to the Fe II porphyrin leading to the Fe III -O 2 ·– , the reduction of the Fe III -O 2 ·– to the peroxide species, and the O–O bond cleavage of ferric hydroperoxide species (Scheme ) are the potential candidates involved in the rds. Under heterogeneous electrochemical conditions, the rate of O 2 binding to ferrous porphyrins are reported to be ∼10 6 M –1 s –1 for high-spin Fe II species with a rare exception. , The O 2 binding to low-spin Fe II porphyrins is relatively slower as it requires the replacement of a strongly bound axial ligand from the six-coordinated iron center . However, for FeOB, the rate of O 2 binding must be at the least an order of magnitude faster than its PCET reduction to Fe III -OOH 2.2 × 10 7 M –1 s –1 , that is, ∼10 8 M –1 s –1 with the latter being the rds.…”

“…Recently, deviations from the log­(TOF) versus η linear relationship have been reported for iron porphyrins with second-sphere proton transfer residues as well as for iron porphyrins with cationic residues in the second sphere. ,, These non-covalent and, typically, weak second-sphere interactions include hydrogen bonding that may act as a proton shuttle or provide through space electrostatic interactions, which, if understood, allows us to tune the rds of a reaction without altering the reactive center. ,− This entails a clear understanding of how they affect the different steps involved in the 4e – /4H + reduction of O 2 to H 2 O and, in particular, whether these effects change the rds of the ORR. If this is the case, the deviations from the log­(TOF) versus η scaling relationships reported in iron porphyrin complexes with second-sphere residues can be explained. ,, …”

“…The combination of the resonance Raman (rR) effect and SERS effect allows us to obtain high-resolution rR data from the monolayer of iron porphyrins on the electrode surface during RDE under electrocatalytic conditions. , In SERRS-RDE, the intermediate with the slowest decay rate (rds of catalysis) is accumulated and characterized using the high-fidelity spin and oxidation state marker vibrations (ν 4 and ν 2 ) , and metal–ligand vibrations (with isotope labeling) . SERRS-RDE data on synthetic iron porphyrins with different axial ligands and a distal Cu ion and myoglobin-based biosynthetic models of CcO have revealed that the rds of the ORR at neutral pH is the O–O bond cleavage of a Fe III -OOH species. ,,− Past efforts of understanding the ORR mechanism using in situ spectroscopy methods like SERRS-RDE has focused on iron porphyrins having hydrogen bonding, proton transfer, and Cu in the distal site, but the role of electrostatic interactions has not been investigated to date. ,,, …”

“…If this is the case, the deviations from the log(TOF) versus η scaling relationships reported in iron porphyrin complexes with second-sphere residues can be explained. 31,40,43 In organic solvents, the rate of the ORR catalyzed by iron porphyrins is the protonation of the Fe III -O 2 − species. Normally, the pK a of this species scales well with the electron density at the iron center, and as a result, a scaling relation is followed.…”