Homogenous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions

Dioxygen Reduction to Hydrogen Peroxide by a Molecular Mn Complex: Mechanistic Divergence between Homogeneous and Heterogeneous Reductants

The selective electrocatalytic reduction of dioxygen (O₂) to hydrogen peroxide (H₂O₂) could be an alternative to the anthraquinone process used industrially, as well as enable the on-demand production of a useful chemical oxidant, obviating the need for long-term storage. There are challenges associated with this, since the two-proton/two-electron reduction of H₂O₂ to two equivalents of water (H₂O) or disproportionation to O₂ and H₂O can be competing reactions. Recently, we reported a Mn(III) Schiff base-type complex, Mn(^tbudhbpy)Cl, where 6,6'-di(3,5-di- tert-butyl-2-phenolate)-2,2'-bipyridine = [^tbudhbpy]^2-, which is active for the electrocatalytic reduction of O₂ to H₂O₂ (ca. 80% selectivity). The less-than-quantitative selectivity could be attributed in part to a thermal disproportionation reaction of H₂O₂ to O₂ and H₂O. To understand the mechanism in greater detail, spectrochemical stopped-flow and electrochemical techniques were employed to examine the catalytic rate law and kinetic reaction parameters. Under electrochemical conditions, the catalyst produces H₂O₂ by an ECCEC mechanism with appreciable rates down to overpotentials of 20 mV and exhibits a catalytic response with a strong dependence on proton donor p K_a. Mechanistic studies suggest that under spectrochemical conditions, where the homogeneous reductant decamethylferrocene (Cp*₂Fe) is used, H₂O₂ is instead produced via a disproportionation pathway, which does not show a strong acid dependence. These results demonstrate that differences in mechanistic pathways can occur for homogeneous catalysts in redox processes, dependent on whether an electrode or homogeneous reductant is used.

Small-molecule activation with iron porphyrins using electrons, photons and protons: some recent advances and future strategies

Substituted tetraphenyl Fe porphyrins are versatile molecular catalysts for the activation of small molecules (such as O₂, H⁺ or CO₂), which could lead to renewable energy storage, the direct production of fuels or new catalytic relevant processes.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.

Role of 2nd sphere H-bonding residues in tuning the kinetics of CO2 reduction to CO by iron porphyrin complexes

Iron porphyrins are potential catalysts for the electrocatalytic and photocatalytic reduction of CO2. It has been recently established that the reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates: a Fe(ii)-CO22- and a Fe(ii)-COOH species. A distal hydrogen bonding interaction was found to be key in determining the stability of these intermediates and affecting both the selectivity and rate of CO2 reduction. In this report, a series of iron porphyrins that vary only in the distal H-bonding network are further investigated and these exhibit turnover frequencies (TOFs) ranging from 1.0 s-1 to 103 s-1. The experimental TOFs correlate with the H-bonding ability of the distal superstructure of these iron porphyrin complexes and analysis suggests that H-bonding alone can tune the rate of CO2 reduction by as much as 1000 fold. DFT calculations provide a detailed insight into how the, apparently weak, 2nd sphere interactions lead to efficient CO2 activation for reduction. The ability to tune CO2 reduction rates by changing the H-bonding residue instead of the acid source is a convenient way to tune CO2 reduction electrocatalysis without compromising selectivity by introducing competitive hydrogen evolution reaction or formate generation.

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e-/4H+ O2 Reduction: Activation of O-O Bond by 2nd Sphere Hydrogen Bonding

Facile and selective 4e^-/4H⁺ electrochemical reduction of O₂ to H₂O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e^-/4H⁺ electrochemical O₂ reduction to H₂O is possible with rate constants as fast as 10⁵ M^-1 s^-1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of O-O bond cleavage determines the overall rate of O₂ reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient O-O bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O₂ to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O₂ with rate constants >10⁷ M^-1 s^-1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst.

Deciphering the mechanism of O2 reduction with electronically tunable non-heme iron enzyme model complexes

A homologous series of electronically tuned 2,2',2''-nitrilotris(N-arylacetamide) pre-ligands (H3LR ) were prepared (R = NO2, CN, CF3, F, Cl, Br, Et, Me, H, OMe, NMe2) and some of their corresponding Fe and Zn species synthesized. The iron complexes react rapidly with O2, the final products of which are diferric mu-oxo bridged species. The crystal structure of the oxidized product obtained from DMA solutions contain a structural motif found in some diiron proteins. The mechanism of iron mediated O2 reduction was explored to the extent that allowed us to construct an empirically consistent rate law. A Hammett plot was constructed that enabled insightful information into the rate-determining step and hence allows for a differentiation between two kinetically equivalent O2 reduction mechanisms.

Countercations and Solvent Influence CO2 Reduction to Oxalate by Chalcogen-Bridged Tricopper Cyclophanates

One-electron reduction of Cu₃EL (L^3- = tris(β-diketiminate)cyclophane, and E = S, Se) affords [Cu₃EL]^-, which reacts with CO₂ to yield exclusively C₂O₄^2- (95% yield, TON = 24) and regenerate Cu₃EL. Stopped-flow UV/visible data support an A→B mechanism under pseudo-first-order conditions ( k_{obs, 298K} = 115(2) s^-1), which is 10⁶ larger than those for reported copper complexes. The k_obs values are dependent on the countercation and solvent (e.g., k_obs is greater for [K(18-crown-6)]⁺ vs (Ph₃P)₂N⁺, and there is a 20-fold decrease in k_obs in THF vs DMF). Our results suggest a mechanism in which cations and solvent influence the stability of the transition state.

Positional effects of second-sphere amide pendants on electrochemical CO2 reduction catalyzed by iron porphyrins

The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the secondary coordination sphere to direct chemical transformations occurring at redox-active metal centers, we now report the design, synthesis, and characterization of a series of iron tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies (TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP. Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-functionalized porphyrins can utilize the latter second-sphere property to promote CO2 reduction. Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space interaction than its proximal ortho-amide analogue. These data establish that proper positioning of secondary coordination sphere groups is an effective design element for breaking electronic scaling relationships that are often observed in electrochemical CO2 reduction.

Incorporation of redox-inactive cations promotes iron catalyzed aerobic C-H oxidation at mild potentials

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K+ or Ba2+ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen = N,N'-bis(salicylidene)ethylenediamine), and by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect (electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C-H bonds was explored. Prior studies on iron salen complexes modified through conventional electron-donating or withdrawing substituents found that only the most oxidizing derivatives were competent catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active. Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative deactivation through formation of the corresponding Fe2-μ-oxo complexes, which were isolated and characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through traditional substituent effects on the ligand.

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Electrocatalytic Reduction of CO2 to Formate by an Iron Schiff Base Complex

The synthesis, structural characterization, and reactivity of an iron(III) chloride compound of 6,6'-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(^tbudhbpy)Cl), under electrochemically reducing conditions is reported. In the presence of carbon dioxide (CO₂) under anhydrous conditions in N,N-dimethylformamide (DMF), this complex mediates the 2e^- reductive disproportionation of two equivalents of CO₂ to carbon monoxide (CO) and carbonate (CO₃^2-). Upon addition of phenol (PhOH) as a proton source under CO₂ saturation, catalytic current is observed; product analysis from controlled potential electrolysis experiments shows the majority product is formate (68 ± 4% Faradaic efficiency), with H₂ as a minor product (30 ± 10% Faradaic efficiency) and minimal CO (1.1 ± 0.3% Faradaic efficiency). On the basis of data obtained from cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC), the release of CO from intermediate Fe carbonyl species is extremely slow and undergoes competitive degradation, limiting the activity and lifetime of this catalyst. Mechanistic studies also indicate the phenolate moieties coordinated to Fe are sensitive to protonation in the reduced state, suggesting the possibility of cooperative pendent proton interactions being involved in CO₂ reduction.

Electrocatalytic Reduction of Dioxygen to Hydrogen Peroxide by a Molecular Manganese Complex with a Bipyridine-Containing Schiff Base Ligand

The synthesis and electrocatalytic reduction of dioxygen by a molecular manganese(III) complex with a tetradentate dianionic bipyridine-based ligand is reported. Electrochemical characterization indicates a Nernstian dependence on the added proton source for the reduction of Mn(III) to Mn(II). The resultant species is competent for the reduction of dioxygen to H₂O₂ with 81 ± 4% Faradaic efficiency. Mechanistic studies suggest that the catalytically active species has been generated through the interaction of the added proton donor and the parent Mn complex, resulting in the protonation of a coordinated phenolate moiety following the single-electron reduction, generating a neutral species with a vacant coordination site at the metal center. As a consequence, the active catalyst has a pendent proton source in close proximity to the active site for subsequent intramolecular reactions.

A family of molecular nickel hydrogen evolution catalysts providing tunable overpotentials using ligand-centered proton-coupled electron transfer paths

The overpotential for H₂ evolution from water can be rationally controlled by tuning the ligand-centered proton-coupled electron transfer (PCET) processes.

From Enzymes to Functional Materials-Towards Activation of Small Molecules

The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences.

Surface enhanced resonance Raman spectroscopy of iron Hangman complexes on electrodes during electrocatalytic oxygen reduction: advantages and problems of common drycast methods

Drycast methods have been used frequently in recent decades to adsorb a range of synthetic catalysts on electrodes. The uncoordinated multilayers that are formed via this immobilization method can however have a strong impact on the electrocatalytic reaction pathway as slow electron transfer and intermolecular interactions can alter the chemistry of the catalysts on the surface. To gain insight into the structure of Fe porphyrin Hangman catalysts during electrocatalytic oxygen reduction a combination of electrochemistry and surface enhanced resonance Raman spectroscopy (SERRS) was applied. The Hangman complexes were attached to the electrodes via different methods and the influence of the immobilisation technique on oxygen chemistry was studied. In multilayer systems, new intermediates could be identified via potential dependent SERRS that were not present in solution or in monolayer systems under catalytic conditions. A comparison of Raman spectra obtained either via Soret or Q-band excitation showed that the porphyrin symmetry is strongly distorted under reducing conditions, which was interpreted by the transient formation of dimer complexes during catalysis.

Multielectron, multisubstrate molecular catalysis of electrochemical reactions: Formal kinetic analysis in the total catalysis regime

Cyclic voltammetry responses are derived for two-electron, two-step homogeneous electrocatalytic reactions in the total catalysis regime. The models developed provide a framework for extracting kinetic information from cyclic voltammograms (CVs) obtained in conditions under which the substrate or cosubstrate is consumed in a multielectron redox process, as is particularly prevalent for very active catalysts that promote energy conversion reactions. Such determination of rate constants in the total catalysis regime is a prerequisite for the rational benchmarking of molecular electrocatalysts that promote multielectron conversions of small-molecule reactants. The present analysis is illustrated with experimental systems encompassing various limiting behaviors.

Heterogeneous Molecular Catalysis of Electrochemical Reactions: Volcano Plots and Catalytic Tafel Plots

We analyze here, in the framework of heterogeneous molecular catalysis, the reasons for the occurrence or nonoccurrence of volcanoes upon plotting the kinetics of the catalytic reaction versus the stabilization free energy of the primary intermediate of the catalytic process. As in the case of homogeneous molecular catalysis or catalysis by surface-active metallic sites, a strong motivation of such studies relates to modern energy challenges, particularly those involving small molecules, such as water, hydrogen, oxygen, proton, and carbon dioxide. This motivation is particularly pertinent for what concerns heterogeneous molecular catalysis, since it is commonly preferred to homogeneous molecular catalysis by the same molecules if only for chemical separation purposes and electrolytic cell architecture. As with the two other catalysis modes, the main drawback of the volcano plot approach is the basic assumption that the kinetic responses depend on a single descriptor, viz., the stabilization free energy of the primary intermediate. More comprehensive approaches, investigating the responses to the maximal number of experimental factors, and conveniently expressed as catalytic Tafel plots, should clearly be preferred. This is more so in the case of heterogeneous molecular catalysis in that additional transport factors in the supporting film may additionally affect the current-potential responses. This is attested by the noteworthy presence of maxima in catalytic Tafel plots as well as their dependence upon the cyclic voltammetric scan rate.

Evaluating the role of acidic, basic, and polar amino acids and dipeptides on a molecular electrocatalyst for H2 oxidation

Outer coordination sphere interactions reduce the overpotential for H₂ oxidation catalysts (brown ellipse) compared to those that have –COOH groups but don't have stabilizing interactions (blue ellipse).