Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems

Abstract

Activation and reduction of O2 and H2O2 by synthetic and biosynthetic iron porphyrin models have proved to be a versatile platform for evaluating second-sphere effects deemed important in naturally occurring heme active sites. Advances in synthetic techniques have made it possible to install different functional groups around the porphyrin ligand, recreating artificial analogues of the proximal and distal sites encountered in the heme proteins. Using judicious choices of these substituents, several of the elegant second-sphere effects that are proposed to be important in the reactivity of key heme proteins have been evaluated under controlled environments, adding fundamental insight into the roles played by these weak interactions in nature. This review presents a detailed description of these efforts and how these have not only demystified these second-sphere effects but also how the knowledge obtained resulted in functional mimics of these heme enzymes.

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

“…Estimates of the O 2 binding rate to synthetic iron porphyrins on the electrode place it around ∼10 6–8 M –1 s –1 , which is 2 orders of magnitude faster than that of the O–O bond cleavage rds of the ORR, consistent with the proposal based on the population of the species observed on the electrode. In the presence of basic distal residues, the population of the LS-Fe III –OOH species under steady state is greatly decreased and the rate of the ORR increased to ∼10 7 M –1 s –1 , i.e., 2 orders of magnitude faster relative to a simple iron porphyrin, which is in the same range of the O 2 binding step. ,, …”

“…In the presence of basic distal residues, the population of the LS-Fe III −OOH species under steady state is greatly decreased and the rate of the ORR increased to ∼10 7 M −1 s −1 , i.e., 2 orders of magnitude faster relative to a simple iron porphyrin, which is in the same range of the O 2 binding step. 29,35,58 In the present case with Fe−Bisphen, the SERRS-RDE data implies a switch in the rds of the O 2 reduction cycle with a change in pH. At pH 4, O−O bond heterolysis of the LS -Fe III −OOH (compound 0) species is the rds (rate of ORR 10 6 M −1 s −1 ), while at pH 9, the rds becomes the reduction of the Fe IV �O species, showing accumulation of this species on the electrode in SERRS data (rate of the ORR is 10 7 M −1 s −1 ).…”

“…Analogously, in synthetic systems at neutral pH, amine/guanidine pendant groups, presumably protonated, can only act as hydrogen-bond donors while groups like ethers, neutral tertiary amines, and deprotonated carboxylates can only act as hydrogen-bond acceptors (Scheme 1). 58 Alternatively, water, primary and secondary amines, phenols, quinols, etc., can act as both hydrogen-bond donors and acceptors (Scheme 1). Despite several reports of iron porphyrins with different pendant second-sphere residues, how hydrogen-bond donors and acceptors affect the rate and selectivity of the 4e − /4H + ORR is not yet understood as it is difficult to access synthetic systems where these effects can be investigated systematically in the same molecular framework.…”

Role‐Specialized Division of Labor in CO2Reduction with Doubly‐Functionalized Iron Porphyrin Atropisomers

“…Enzymes usually have many amino acid residues at their active sites [1–5] . These residues function differently to promote chemical reactions, including providing hydrogen‐bonding and electrostatic interactions, [6, 7] assisting electron and proton transfer, [8] and tuning the size and hydrophobicity of active sites [5] .…”

Role‐Specialized Division of Labor in CO2Reduction with Doubly‐Functionalized Iron Porphyrin Atropisomers

“…Enzymes usually have many amino acid residues at their active sites. [1][2][3][4][5] These residues function differently to promote chemical reactions, including providing hydrogenbonding and electrostatic interactions, [6,7] assisting electron and proton transfer, [8] and tuning the size and hydrophobicity of active sites. [5] Through the close cooperation between these active site residues, enzymes show exceptional high activity and selectivity for various chemical reactions.…”