Oriented Electrostatic Effects on O2 and CO2 Reduction by a Polycationic Iron Porphyrin

Unveiling the potential of tailored β-substituted iron-porphyrins for highly efficient oxygen reduction reactions (ORR)

A series of β-substituted iron porphyrins, including Fe(III)ClTPPMe4, Fe(III)ClTPPPh4, and Fe(III)ClTPPBr4, featuring electron-withdrawing and electron-donating groups, were synthesized, characterized, and evaluated for their catalytic activity in the oxygen reduction reaction (ORR). These catalysts exhibited onset potentials of 0.77 V, 0.98 V, and 0.79 V (RHE), comparable to platinum-based electrodes, along with high stability over 11 000 s and a 4e-/4H+ transfer pathway. Electron-withdrawing β-substituents combined with steric effects enhance ORR performance, with the phenyl-substituted porphyrin exhibiting unique catalytic behavior due to steric distortions in the porphyrin ring. DFT calculations revealed that a reduced HOMO-LUMO gap facilitates electron transfer which in turn improves catalytic efficiency.

Using Host-Guest Chemistry to Examine the Effects of Porosity and Catalyst-Support Interactions on CO2 Reduction

Bis-porphyrin nanocages (M2BiCage, M = FeCl, Co, Zn) and their host-guest complexes with C60 and C70 were used to examine how molecular porosity and interactions with carbon nanomaterials affect the CO2 reduction activity of metalloporphyrin electrocatalysts. The cages were found to adsorb on carbon black to provide electrocatalytic inks with excellent accessibilities of the metal sites (≈50%) even at high metal loadings (2500 nmol cm-2), enabling good activity for reducing CO2 to CO. A complex of C70 bound inside (FeCl)2BiCage achieves high current densities for CO formation at low overpotentials (|jCO| >7 mA cm-2, η = 320 mV; >13.5 mA cm-2, η = 520 mV) with ≥95% Faradaic efficiency (FECO), and Co2BiCage achieves high turnover frequencies (≈1300 h-1, η = 520 mV) with 90% FECO. In general, blocking the pore with C60 or C70 improves the catalytic performance of (FeCl)2BiCage and has only small effects on Co2BiCage, indicating that the good catalytic properties of the cages cannot be attributed to their internal pores. Neither enhanced electron transfer rates nor metal-fullerene interactions appear to underlie the ability of C60/C70 to improve the performance of (FeCl)2BiCage, in contrast to effects often proposed for other carbon nanosupports.

Bimetallic Synergy in Oxygen Reduction: How Tailored Metal-Metal Interactions Amplify Cooperative Catalysis

Bimetallic cooperative catalysis, inspired by cytochrome c oxidase and multicopper oxidase, plays a crucial role in the development of four-electron oxygen reduction catalysts. The distance between metals is a crucial factor affecting the cooperative effect, but its precise influence on bimetallic cooperativity in selective oxygen reduction catalysis still awaits an in-depth understanding. Herein, we employ a series of dicopper complexes with varying linkers to systematically adjust the Cu···Cu distance for electrocatalytic oxygen reduction. Structure-activity relationship analyses reveal that catalysts with a shorter dicopper center exhibited significantly higher four-electron selectivity (approaching 100% for BPMPDCu2 and BPMANCu2) than that with a longer distance (below 80% for 6-HPACu2) in an aqueous solution (pH 7.0). Notably, the catalytic activity of BPMPDCu2 is 11 times and 237 times faster than those of 6-HPACu2 and BPMANCu2, respectively, which does not correlate directly with their Cu···Cu distances. Further investigations into low-valent LCuI2 intermediates, supported by DFT calculations, indicate that the oxygen binding process is the rate-determining step under electrocatalytic conditions and is sensitive to the CuI···CuI distance. The closest BPMANCuI2 characterized by strong CuI-CuI interactions and the more distant 6-HPACuI2 with its separated dicopper sites both hinder effective O2 binding. In contrast, BPMPDCuI2 maintains an optimal Cu···Cu distance that facilitates O2 binding and ensures robust bimetallic cooperativity throughout the catalytic cycle. This work underscores the significance of metal-metal distance regulation in bimetallic cooperatively selective oxygen reduction and provides valuable insights for the rational design of high-performance oxygen reduction catalysts.

Selective two-electron and four-electron oxygen reduction reactions using Co-based electrocatalysts

The oxygen reduction reaction (ORR) can take place via both four-electron (4e-) and two-electron (2e-) pathways. The 4e- ORR, which produces water (H2O) as the only product, is the key reaction at the cathode of fuel cells and metal-air batteries. On the other hand, the 2e- ORR can be used to electrocatalytically synthesize hydrogen peroxide (H2O2). For the practical applications of the ORR, it is very important to precisely control the selectivity. Understanding structural effects on the ORR provides the basis to control the selectivity. Co-based electrocatalysts have been extensively studied for the ORR due to their high activity, low cost, and relative ease of synthesis. More importantly, by appropriately designing their structures, Co-based electrocatalysts can become highly selective for either the 2e- or the 4e- ORR. Therefore, Co-based electrocatalysts are ideal models for studying fundamental structure-selectivity relationships of the ORR. This review starts by introducing the reaction mechanism and selectivity evaluation of the ORR. Next, Co-based electrocatalysts, especially Co porphyrins, used for the ORR with both 2e- and 4e- selectivity are summarized and discussed, which leads to the conclusion of several key structural factors for ORR selectivity regulation. On the basis of this understanding, future works on the use of Co-based electrocatalysts for the ORR are suggested. This review is valuable for the rational design of molecular catalysts and material catalysts with high selectivity for 4e- and 2e- ORRs. The structural regulation of Co-based electrocatalysts also provides insights into the design and development of ORR electrocatalysts based on other metal elements.

Beyond scaling relations in electrocatalysis: unifying concepts from molecular systems and metallic surfaces

Transitioning the chemical industry away from fossil fuels is a critical goal that requires the adoption of alternative, non-fossil carbon feedstocks. The electrochemical CO2 reduction reaction, driven via renewable-derived electricity, represents an unparalleled technology that uses CO2 as a C1-building block to generate industrially relevant products. Although many electrocatalytic systems have demonstrated promising activities in producing a wide range of products, challenges remain in controlling the product selectivity and reducing the operating overpotential for large-scale applications. This Perspective outlines recent efforts in designing tailored microenvironments in electrocatalytic systems to boost their selectivity and energy efficiency. We review examples from homogeneous and heterogeneous systems, emphasizing mechanistic studies that elucidate how the modulation of the space surrounding catalytic active sites can control the outcome of electrocatalysis. Lastly, we carry out a thermodynamic-kinetic analysis to identify existing scaling relationships that govern the electrocatalytic performance of molecular catalysts, and we highlight examples of catalysts that circumvent these relations through the functionalization of their secondary coordination sphere.

Origins of HCOOH Selectivity Over CO Mediated by an Unusual Fe(I)-Porphyrin Bearing a β-Substituted Cation

Molecular metalloporphyrins have been commonly reported to efficiently catalyze electrochemical CO2-to-CO conversion. Unconventionally, Dey and coworkers reported that an iron-porphyrin analogue bearing a pendant amine binds with CO2 at the Fe(I) state and reduces CO2 into formic acid using water molecules as proton sources. However, the origins of HCOOH selectivity over the conventional CO product, as well as fundamental mechanistic details, are lacking. In the work, theoretical computations were employed to fundamentally investigate the reaction mechanisms. Our calculations reconfirmed that the formal Fe(I)-porphyrin would proceed with a direct CO2-binding step, and this behavior could be ascribed to the significant hydrogen bonding and through-space electrostatic interactions between the cationic N-H and [CO22-]-coordinated species. A two-electron transfer process in the key CO2-binding step is found, which is estimated to proceed consecutively with protonation and 1e-reduction to give rise to an Fe(III)-COOH and Fe(II)-COOH intermediate, respectively. The cationic N-H plays vital roles in the stabilization of C-protonation species to yield HCOOH. Moreover, the cationic N-H terminal could hinder the dissociation of CO. Our computational results are consistent with experimental observations. The origins of HCOOH selectivity are elucidated, and an insightful mechanistic understanding of the cooperative roles of second-sphere hydrogen bonding and cationic effects is provided.

Improving Electrocatalytic CO2 Reduction over Iron Tetraphenylporphyrin with Triethanolamine as a CO2 Shuttle

Delivering CO2 molecules to catalyst sites is a vital step in the CO2 reduction reaction (CO2RR). Achievements have been made to develop efficient catalysts, but few efforts have been dedicated to improving CO2 delivering in solutions. Herein, we report on electrocatalytic CO2-to-CO conversion using Fe tetraphenylporphyrin (FeTPP) as a catalyst and triethanolamine as a CO2 shuttle. Compared to ethanol, the electrocatalytic CO2RR current with triethanolamine increases by more than three times. We show that triethanolamine can effectively capture a CO2 molecule to form a zwitterionic alkylcarbonate through the collaboration between its tripodal alcohol and amine units. This alkylcarbonate can release the bound CO2 molecule for activation at the Fe site upon its interaction with FeTPP. In addition to shuttling CO2, alkylcarbonates can also provide protons to assist the C-O bond cleavage. Therefore, this work is significant to demonstrate a new strategy to improve electrocatalytic CO2RR by shuttling CO2.

Breaking the Scaling Relationship for Oxygen Reduction Reaction Using Molecular Cobalt Complexes

Developing catalysts that achieve a higher turnover frequency (TOF) with a lower effective overpotential (ηeff) in electrocatalytic reactions is an emerging focus of research. A promising approach to this end is modifying the secondary coordination sphere (SCS) that can facilitate hydrogen bonding interactions or electrostatic effects, thereby lowering the activation energy barrier in the rate-determining step (rds). Herein, we designed and synthesized a series of CoIII complexes (1-8) featuring a bis-pyridine-dioxime framework, each with a distinct SCS (-C6H5 (1), o-NHMe2+-C6H4- (2), o-OMe-C6H4- (3), o-OH-C6H4- (4), p-NHMe2+-C6H4- (5), p-OMe-C6H4- (6), pyridine (7), and pyrimidine (8)). We investigated their electrocatalytic oxygen reduction reaction (ORR) in acetonitrile, both with CF3COOH and in a 1:1 CF3COOH/CF3COO- buffer solution. All complexes demonstrated selective 4e-/4H+ reduction of O2. A linear free energy relationship (LFER) analysis revealed a trend of increasing the TOF with ηeff, aligning with molecular scaling expectations. However, 2 and 3 with the o-NHMe2+-C6H4- and o-OMe-C6H4- substituents diverged from this trend, exhibiting TOF values over 1000 and 250 times higher than predicted based on their positions in the log(TOF)/ηeff correlation within the buffer solution. Kinetic investigations indicate that protonation of the CoIII(O2•) adduct is the rds for all catalysts, suggesting that the functional groups in the SCS of 2 and 3 facilitate proton transfer, acting as proton relay sites. We propose that this effect reduces the activation energy barrier in the rds, accounting for the observed deviation from the LFER. This study underscores the critical role of designing an appropriate SCS to enhance catalyst efficiency beyond LFER expectations.

Synthesis of ethane from CO2 by a methyl transferase-inspired molecular catalyst

Molecular catalysts with a single metal center are reported to reduce CO2 to a wide range of valuable single-carbon products like CO, HCOOH, CH3OH, etc. However, these catalysts cannot reduce CO2 to two carbon products like ethane or ethylene and the ability to form C-C from CO2 remains mostly limited to heterogeneous material-based catalysts. We report a set of simple iron porphyrins with pendant thiol group can catalyze the reduction of CO2 to ethane (C2H6) with H2O as the proton source with a Faradaic yield >40% the rest being CO. The mechanism involves a CO2-derived methyl group transfer to the pendant thiol akin to the proposal forwarded for methyl transferases and a follow-up C-C bond formation of the thioether thus formed and a Fe(II)-CH3 species generated by the reduction of a second molecule of CO2. The availability of a "parking space" in the molecular framework for the first reduced C1 product from CO2 reduction allows C-C bond formation resulting in a unique case where a component of natural gas can be generated from direct electrochemical reduction of CO2.

Use of Intramolecular Quinol Redox Couples to Facilitate the Catalytic Transformation of O2 and O2-Derived Species

ConspectusThe redox reactivity of transition metal centers can be augmented by nearby redox-active inorganic or organic moieties. In some cases, these functional groups can even allow a metal center to participate in reactions that were previously inaccessible to both the metal center and the functional group by themselves. Our research groups have been synthesizing and characterizing coordination complexes with polydentate quinol-containing ligands. Quinol is capable of being reversibly oxidized by either one or two electrons to semiquinone or para-quinone, respectively. Functionally, quinol behaves much differently than phenol, even though the pKa values of the first O-H bonds are nearly identical.The redox activity of the quinol in the polydentate ligand can augment the abilities of bound redox-active metals to catalyze the dismutation of O2-• and H2O2. These complexes can thereby act as high-performing functional mimics of superoxide dismutase (SOD) and catalase (CAT) enzymes, which exclusively use redox-active metals to transfer electrons to and from these reactive oxygen species (ROS). The quinols augment the activity of redox-active metals by stabilizing higher-valent metal species, providing alternative redox partners for the oxidation and reduction of reactive oxygen species, and protecting the catalyst from destructive side reactions. The covalently attached quinols can even enable redox-inactive Zn(II) to catalyze the degradation of ROS. With the Zn(II)-containing SOD and CAT mimics, the organic redox couple entirely substitutes for the inorganic redox couples used by the enzymes. The ligand structure modulates the antioxidant activity, and thus far, we have found that compounds that have poor or negligible SOD activity can nonetheless behave as efficient CAT mimics.Quinol-containing ligands have also been used to prepare electrocatalysts for dioxygen reduction, functionally mimicking the enzyme cytochrome c oxidase. The installation of quinols can boost electrocatalytic activity and even enable otherwise inactive ligand frameworks to support electrocatalysis. The quinols can also shift the product selectivity of O2 reduction from H2O2 to H2O without markedly increasing the effective overpotential. Distinct control of the coordination environment around the metal center allows the most successful of these catalysts to use economic and naturally abundant first-row transition metals such as iron and cobalt to selectively reduce O2 to H2O at low effective overpotentials. With iron, we have found that the electrocatalysts can enter the catalytic cycle as either an Fe(II) or Fe(III) species with no difference in turnover frequency. The entry point to the cycle, however, has a marked impact on the effective overpotential, with the Fe(III) species thus far being more efficient.

Pre-equilibrium reactions involving pendent relays improve CO2 reduction mediated by molecular Cr-based electrocatalysts

Homogeneous earth abundant transition-metal electrocatalysts capable of carbon dioxide (CO2) reduction to generate value-added chemical products are a possible strategy to minimize rising anthropogenic CO2 emissions. Previously, it was determined that Cr-centered bipyridine-based N2O2 complexes for CO2 reduction are kinetically limited by a proton-transfer step during C-OH bond cleavage. Therefore, it was hypothesized that the inclusion of pendent relay groups in the secondary coordination sphere of these molecular catalysts could increase their catalytic activity. Here, it is shown that the introduction of a pendent methoxy group favorably impacts a pre-equilibrium protonation prior to the catalytic resting state, resulting in a significant increase in catalytic activity without a loss of product selectivity for generating carbon monoxide (CO) from CO2. Interestingly, combining the pendent methoxy group with a cationic acid causes a positive shift of the catalytic reduction potential of the system, while maintaining increased activity and quantitative selectivity. This work suggests that tuning the secondary coordination sphere with respect to cationic proton sources can result in activity improvements by modifying the kinetic and thermodynamic aspects of proton transfer in the catalytic cycle.

Fast Catalysis at Low Overpotential: Designing Efficient Dicationic Re(bpy2+)(CO)3I Electrocatalysts for CO2 Reduction

We report a series of isomeric, dicationic Re(bpy2+)(CO)3I complexes with bpy (2,2'-bipyridine) modified by two phenyl-CH2-(NMe3)+ pendants with cations located at variable distances from the active site for electrocatalytic CO2 reduction in CH3CN/2.8 M H2O. The position of the cationic groups dramatically increases the rate of catalysis by ∼800-fold, from 1.2 to 950 s-1, with only a minor increase in overpotential. Acceleration is due to stabilization of the initial CO2 adduct and lowering of ΔG‡ for C-OH bond cleavage by Coulombic stabilization of anionic charges. Performance may be enhanced by accumulation in the electrochemical double layer. Transition state stabilization in the optimized isomer unlocks the low overpotential "protonation-first" pathway, highlighting the sizable effects of subtle structural optimization.

Design of Cr-Based Molecular Electrocatalyst Systems for the CO2 Reduction Reaction

ConspectusHuman influence on the climate system was recently summarized by the sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, which noted that global surface temperatures have increased more rapidly in the last 50 years than in any other 50-year period in the last 2000 years. Elevated global surface temperatures have had detrimental impacts, including more frequent and intense extreme weather patterns like flooding, wildfires, and droughts. In order to limit greenhouse gas emissions, various climate change policies, like emissions trading schemes and carbon taxes, have been implemented in many countries. The most prevalent anthropogenic greenhouse gas emitted is carbon dioxide (CO2), which accounted for 80% of all U.S. greenhouse gas emissions in 2022. The reduction of CO2 through the use of homogeneous electrocatalysts generally follows a two-electron/two-proton pathway to produce either carbon monoxide (CO) with water (H2O) as a coproduct or formic acid (HCOOH). These reduced carbon species are relevant to industrial applications: the Fischer-Tropsch process uses CO and H2 to produce fuels and commodity chemicals, while HCOOH is an energy dense carrier for fuel cells and useful synthetic reagent. Electrochemically reducing CO2 to value-added products is a potential way to address its steadily increasing atmospheric concentrations while supplanting the use of nonrenewable petrochemical reserves through the generation of new carbon-based resources. The selective electrochemical reduction of CO2 (CO2RR) by homogeneous catalyst systems was initially achieved with late (and sometimes costly) transition metal active sites, leading the field to conclude that transition metal complexes based on metals earlier in the periodic table, like chromium (Cr), were nonprivileged for the CO2RR. However, metals early in the table have sufficient reducing power to mediate the CO2RR and therefore could be selective in the correct coordination environment. This Account describes our efforts to develop and optimize novel Cr-based CO2RR catalyst systems through redox-active ligand modification strategies and the use of redox mediators (RMs). RMs are redox-active molecules which can participate cocatalytically during an electrochemical reaction, transferring electrons─often accompanied by protons─to a catalytic active site. Through mechanistic and computational work, we have found that ligand-based redox activity is key to controlling the intrinsic selectivity of these Cr compounds for CO2 activation. Ligand-based redox activity is also essential for developing cocatalytic systems, since it enables through-space interactions with reduced RMs containing redox-active planar aromatic groups, allowing charge transfer to occur within the catalyst assembly. Following a summary of our work, we offer a perspective on the possibilities for future development of catalytic and cocatalytic systems with early transition metals for small molecule activation.

Dramatic Improvement of Homogeneous Carbon Dioxide and Bicarbonate Electroreduction Using a Tetracationic Water-Soluble Cobalt Phthalocyanine

Electrochemical conversion of carbon dioxide (CO2) offers the opportunity to transform a greenhouse gas into valuable starting materials, chemicals, or fuels. Since many CO2 capture strategies employ aqueous alkaline solutions, there is interest in catalyst systems that can act directly on such capture solutions. Herein, we demonstrate new catalyst designs where the electroactive molecules readily mediate the CO2-to-CO conversion in aqueous solutions between pH 4.5 and 10.5. Likewise, the production of CO directly from 2 M KHCO3 solutions (pH 8.2) is possible. The improved molecular architectures are based on cobalt(II) phthalocyanine and contain four cationic trimethylammonium groups that confer water solubility and contribute to the stabilization of activated intermediates via a concentrated positive charge density around the active core. Turnover frequencies larger than 103 s-1 are possible at catalyst concentrations of down to 250 nM in CO2-saturated solutions. The observed rates are substantially larger than the related cobalt phthalocyanine-containing catalysts. Density functional theory calculations support the idea that the excellent catalytic properties are attributed to the ability of the cationic groups to stabilize CO2-bound reduced intermediates in the catalytic cycle. The homogeneous, aqueous CO2 reduction that these molecules perform opens new frontiers for further development of the CoPc platform and sets a greatly improved baseline for CoPc-mediated CO2 upconversion. Ultimately, this discovery uncovers a strategy for the generation of platforms for practical CO2 reduction catalysts in alkaline solutions.

Porphyrin Atropisomerism as a Molecular Engineering Tool in Medicinal Chemistry, Molecular Recognition, Supramolecular Assembly, and Catalysis

Porphyrin atropisomerism, which arises from restricted σ-bond rotation between the macrocycle and a sufficiently bulky substituent, was identified in 1969 by Gottwald and Ullman in 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrins. Henceforth, an entirely new field has emerged utilizing this transformative tool. This review strives to explain the consequences of atropisomerism in porphyrins, the methods which have been developed for their separation and analysis and present the diverse array of applications. Porphyrins alone possess intriguing properties and a structure which can be easily decorated and molded for a specific function. Therefore, atropisomerism serves as a transformative tool, making it possible to obtain even a specific molecular shape. Atropisomerism has been thoroughly exploited in catalysis and molecular recognition yet presents both challenges and opportunities in medicinal chemistry.

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O2 to H2O at High Rates and Low Overpotentials

We prepare iron(II) and iron(III) complexes with polydentate ligands that contain quinols, which can act as electron proton transfer mediators. Although the iron(II) complex with N-(2,5-dihydroxybenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (H2qp1) is inactive as an electrocatalyst, iron complexes with N,N'-bis(2,5-dihydroxybenzyl)-N,N'-bis(2-pyridinylmethyl)-1,2-ethanediamine (H4qp2) and N-(2,5-dihydroxybenzyl)-N,N'-bis(2-pyridinylmethyl)-1,2-ethanediamine (H2qp3) were found to be much more active and more selective for water production than a previously reported cobalt-H2qp1 electrocatalyst while operating at low overpotentials. The catalysts with H2qp3 can enter the catalytic cycle as either Fe(II) or Fe(III) species; entering the cycle through Fe(III) lowers the effective overpotential. On the basis of their TOF0 values, the successful iron-quinol complexes are better electrocatalysts for oxygen reduction than previously reported iron-porphyrin compounds, with the Fe(III)-H2qp3 arguably being the best homogeneous electrocatalyst for this reaction. With iron, the quinol-for-phenol substitution shifts the product selectivity from H2O2 to water with little impact on the overpotential, but unlike cobalt, this substitution also greatly improves the activity, as assessed by TOFmax, by hastening the protonation and oxygen binding steps. The addition of a second quinol further enhances the activity and selectivity for water but modestly increases the effective overpotential.

Electrocatalytic formate and alcohol oxidation by hydride transfer at first-row transition metal complexes

The electrocatalytic oxidation of carbon-based liquid fuels, such as formic acid and alcohols, has important applications for our renewable energy transition. Molecular electrocatalysts based on transition metal complexes provide the opportunity to explore the interplay between precise catalyst design and electrocatalytic activity. Recent advances have seen the development of first-row transition metal electrocatalysts for these transformations that operate via hydride transfer between the substrate and catalyst. In this Frontier article, we present the key contributions to this field and discuss the proposed mechanisms for each case. These studies also reveal the remaining challenges for formate and alcohol oxidation with first-row transition metal systems, for which we provide perspectives on future directions for next-generation electrocatalyst design.

Porphyrin Aggregation under Homogeneous Conditions Inhibits Electrocatalysis: A Case Study on CO2 Reduction

Metalloporphyrins are widely used as homogeneous electrocatalysts for transformations relevant to clean energy and sustainable organic synthesis. Metalloporphyrins are well-known to aggregate due to π-π stacking, but surprisingly, the influence of aggregation on homogeneous electrocatalytic performance has not been investigated previously. Herein, we present three structurally related iron meso-phenylporphyrins whose aggregation properties are different in commonly used N,N-dimethylformamide (DMF) electrolyte. Both spectroscopy and light scattering provide evidence of extensive porphyrin aggregation under conventional electrocatalytic conditions. Using the electrocatalytic reduction of CO2 to CO as a test reaction, cyclic voltammetry reveals an inverse dependence of the kinetics on the catalyst concentration. The inhibition extends to bulk performance, where up to 75% of the catalyst at 1 mM is inactive compared to at 0.25 mM. We additionally report how aggregation is perturbed by organic additives, axial ligands, and redox state. Periodic boundary calculations provide additional insights into aggregate stability as a function of metalloporphyrin structure. Finally, we generalize the aggregation phenomenon by surveying metalloporphyrins with different metals and substituents. This study demonstrates that homogeneous metalloporphyrins can aggregate severely in well-solubilizing organic electrolytes, that aggregation can be easily modulated through experimental conditions, and that the extent of aggregation must be considered for accurate catalytic benchmarking.

Unveiling the Activity and Mechanism Alterations by Pyrene Decoration on a Co(II) Macrocyclic Catalyst for CO2 Reduction

Mechanistic studies involving characterization of crucial intermediates are desirable for rational optimization of molecular catalysts toward CO2 reduction, while fundamental challenges are associated with such studies. Herein we present the systematic mechanistic investigations on a pyrene-appended CoII macrocyclic catalyst in comparison with its pyrene-free prototype. The comparative results also verify the reasons of the higher catalytic activity of the pyrene-tethered catalyst in noble-metal-free CO2 photoreduction with various photosensitizers, where a remarkable apparent quantum yield of 36±3 % at 425 nm can be obtained for selective CO production. Electrochemical and spectroelectrochemical studies in conjunction with DFT calculations between the two catalysts have characterized the key CO-bound intermediates and revealed their different CO-binding behavior, demonstrating that the pyrene group endows the corresponding CoII catalyst a lower catalytic potential, a higher stability, and a greater ease in CO release, all of which contribute to its better performance.

The meso-substituent electronic effect of Fe porphyrins on the electrocatalytic CO2 reduction reaction

We report Fe porphyrins bearing different meso-substituents for the electrocatalytic CO2 reduction reaction (CO2RR). By replacing two and four meso-phenyl groups of Fe tetraphenylporphyrin (FeTPP) with strong electron-withdrawing pentafluorophenyl groups, we synthesized FeF10TPP and FeF20TPP, respectively. We showed that FeTPP and FeF10TPP are active and selective for CO2-to-CO conversion in dimethylformamide with the former being more active, but FeF20TPP catalyzes hydrogen evolution rather than the CO2RR under the same conditions. Experimental and theoretical studies revealed that with more electron-withdrawing meso-substituents, the Fe center becomes electron-deficient and it becomes difficult for it to bind a CO2 molecule in its formal Fe0 state. This work is significant to illustrate the electronic effects of catalysts on binding and activating CO2 molecules and provide fundamental knowledge for the design of new CO2RR catalysts.

Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt-porphyrin complexes

A CoII-porphyrin complex (1) with an appended aza-crown ether for Lewis acid (LA) binding was synthesized and characterized. NMR spectroscopy and electrochemistry show that cationic group I and II LAs (i.e., Li+, Na+, K+, Ca2+, Sr2+, and Ba2+) bind to the aza-crown ether group of 1. The binding constant for Li+ is comparable to that observed for a free aza-crown ether. LA binding causes an anodic shift in the CoII/CoI couple of between 10 and 40 mV and also impacts the CoIII/CoII couple. The magnitude of the anodic shift of the CoII/CoI couple varies linearly with the strength of the LA as determined by the pKa of the corresponding metal-aqua complex, with dications giving larger shifts than monocations. The extent of the anodic shift of the CoII/CoI couple also increases as the ionic strength of the solution decreases. This is consistent with electric field effects being responsible for the changes in the redox properties of 1 upon LA binding and provides a novel method to tune the reduction potential. Density functional theory calculations indicate that the bound LA is 5.6 to 6.8 Å away from the CoII ion, demonstrating that long-range electrostatic effects, which do not involve changes to the primary coordination sphere, are responsible for the variations in redox chemistry. Compound 1 was investigated as a CO2 reduction electrocatalyst and shows high activity but rapid decomposition.

Breaking a Molecular Scaling Relationship Using an Iron-Iron Fused Porphyrin Electrocatalyst for Oxygen Reduction

The design of efficient electrocatalysts is limited by scaling relationships governing trade-offs between thermodynamic and kinetic performance metrics. This ″iron law″ of electrocatalysis arises from synthetic design strategies, where structural alterations to a catalyst must balance nucleophilic versus electrophilic character. Efforts to circumvent this fundamental impasse have focused on bioinspired applications of extended coordination spheres and charged sites proximal to a catalytic center. Herein, we report evidence for breaking a molecular scaling relationship involving electrocatalysis of the oxygen reduction reaction (ORR) by leveraging ligand design. We achieve this using a binuclear catalyst (a diiron porphyrin), featuring a macrocyclic ligand with extended electronic conjugation. This ligand motif delocalizes electrons across the molecular scaffold, improving the catalyst's nucleophilic and electrophilic character. As a result, our binuclear catalyst exhibits low overpotential and high catalytic turnover frequency, breaking the traditional trade-off between these two metrics.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Bioinspired Binickel Catalyst for Carbon Dioxide Reduction: The Importance of Metal-ligand Cooperation

Catalyst design for the efficient CO2 reduction reaction (CO2RR) remains a crucial challenge for the conversion of CO2 to fuels. Natural Ni-Fe carbon monoxide dehydrogenase (NiFe-CODH) achieves reversible conversion of CO2 and CO at nearly thermodynamic equilibrium potential, which provides a template for developing CO2RR catalysts. However, compared with the natural enzyme, most biomimetic synthetic Ni-Fe complexes exhibit negligible CO2RR catalytic activities, which emphasizes the significance of effective bimetallic cooperation for CO2 activation. Enlightened by bimetallic synergy, we herein report a dinickel complex, NiIINiII(bphpp)(AcO)2 (where NiNi(bphpp) is derived from H2bphpp = 2,9-bis(5-tert-butyl-2-hydroxy-3-pyridylphenyl)-1,10-phenanthroline) for electrocatalytic reduction of CO2 to CO, which exhibits a remarkable reactivity approximately 5 times higher than that of the mononuclear Ni catalyst. Electrochemical and computational studies have revealed that the redox-active phenanthroline moiety effectively modulates the electron injection and transfer akin to the [Fe3S4] cluster in NiFe-CODH, and the secondary Ni site facilitates the C-O bond activation and cleavage through electron mediation and Lewis acid characteristics. Our work underscores the significant role of bimetallic cooperation in CO2 reduction catalysis and provides valuable guidance for the rational design of CO2RR catalysts.

Catalytic reduction of oxygen to water by non-heme iron complexes: exploring the effect of the secondary coordination sphere proton exchanging site

In this study, we prepared non-heme FeIII complexes (1, 2, and 3) of an N4 donor set of ligands (H2L, Me2L, and BPh2L). 1 is supported by a monoanionic bispyridine-dioxime ligand (HL). In 2 and 3, the primary coordination sphere of Fe remained similar to that in 1, except that the oxime protons of the ligand were replaced with two methyl groups and a bridging -BPh2 moiety, respectively. X-ray structures of the FeII complexes (1a and 3a) revealed similar Fe-N distances; however, they were slightly elongated in 2a. The FeIII/FeII potential of 1, 2, and 3 appeared at -0.31 V, -0.25 V, and 0.07 V vs. Fc+/Fc, respectively, implying that HL and Me2L have comparable donor properties. However, BPh2L is more electron deficient than HL or Me2L. 1 showed electrocatalytic oxygen reduction reaction (ORR) activity in acetonitrile in the presence of trifluoroacetic acid (TFAH) as the proton source at Ecat/2 = -0.45 V and revealed selective 4e-/4H+ reduction of O2 to H2O. 1 showed an effective overpotential (ηeff) of 0.98 V and turnover frequency (TOFmax) of 1.02 × 103 s-1. Kinetic studies revealed a kcat of 2.7 × 107 M-2 s-1. Strikingly, 2 and 3 remained inactive for electrocatalytic ORR, which established the essential role of the oxime scaffolds in the electrocatalytic ORR of 1. Furthermore, a chemical ORR of 1 has been investigated using decamethylferrocene as the electron source. For 1, a similar rate equation was noted to that of the electrocatalytic pathway. A kcat of 6.07 × 104 M-2 s-1 was found chemically. Complex 2, however, underwent a very slow chemical ORR. Complex 3 chemically enhances the 4e-/4H+ reduction of O2 and exhibits a TOF of 0.24 s-1 and a kcat value of 2.47 × 102 M-1 s-1. Based on the experimental observations, we demonstrate that the oxime backbone of the ligand in 1 works as a proton exchanging site in the 4e-/4H+ reduction of O2. The study describes how the ORR is affected by the tuning of the ligand scaffold in a family of non-heme Fe complexes.

Insight into the Role of Entropy in Promoting Electrochemical CO2 Reduction by Imidazolium Cations

The electroreduction of CO2 plays an important role in achieving a net-zero carbon economy. Imidazolium cations can be used to enhance the rate of CO2 reduction reactions, but the origin of this promotion remains poorly understood. In this work, we show that in the presence of 1-ethyl-3-methylimidazolium (EMIM+), CO2 reduction on Ag electrodes occurs with an apparent activation energy near zero, while the applied potential influences the rate through the pre-exponential factor. Our findings suggest that the CO2 reduction rate is controlled by the initial state entropy, which depends on the applied potential through the organization of cations at the electrochemical interface. Further characterization shows that the C2-proton of EMIM+ is consumed during the reaction, leading to the collapse of the cation organization and a decrease in the catalytic performance. Our results have important implications for understanding the effect of potential on reaction rates, as they indicate that the common picture based on vibrational activation of electron transfer reactions is insufficient for describing the impact of potential in complex systems, such as CO2 reduction in the presence of imidazolium cations.

Redox-Active Ligand Assisted Multielectron Catalysis: A Case of Electrocatalyzed CO2-to-CO Conversion

The selective reduction of carbon dioxide remains a significant challenge due to the complex multielectron/proton transfer process, which results in a high kinetic barrier and the production of diverse products. Inspired by the electrostatic and H-bonding interactions observed in the second sphere of the [NiFe]-CODH enzyme, researchers have extensively explored these interactions to regulate proton transfer, stabilize intermediates, and ultimately improve the performance of catalytic CO2 reduction. In this work, a series of cobalt(II) tetraphenylporphyrins with varying numbers of redox-active nitro groups were synthesized and evaluated as CO2 reduction electrocatalysts. Analyses of the redox properties of these complexes revealed a consistent relationship between the number of nitro groups and the corresponding accepted electron number of the ligand at -1.59 V vs. Fc+/0. Among the catalysts tested, TNPPCo with four nitro groups exhibited the most efficient catalytic activity with a turnover frequency of 4.9 × 104 s-1 and a catalytic onset potential 820 mV more positive than that of the parent TPPCo. Furthermore, the turnover frequencies of the catalysts increased with a higher number of nitro groups. These results demonstrate the promising design strategy of incorporating multielectron redox-active ligands into CO2 reduction catalysts to enhance catalytic performance.

Selective Four-Electron Reduction of Oxygen by a Nonheme Heterobimetallic CuFe Complex

We report herein the first nonheme CuFe oxygen reduction catalyst ([CuII (bpbp)(μ-OAc)2 FeIII ]2+ , CuFe-OAc), which serves as a functional model of cytochrome c oxidase and can catalyze oxygen reduction to water with a turnover frequency of 2.4×103  s-1 and selectivity of 96.0 % in the presence of Et3 NH+ . This performance significantly outcompetes its homobimetallic analogues (2.7 s-1 of CuCu-OAc with %H2 O2 selectivity of 98.9 %, and inactive of FeFe-OAc) under the same conditions. Structure-activity relationship studies, in combination with density functional theory calculation, show that the CuFe center efficiently mediates O-O bond cleavage via a CuII (μ-η1  : η2 -O2 )FeIII peroxo intermediate in which the peroxo ligand possesses distinctive coordinating and electronic character. Our work sheds light on the nature of Cu/Fe heterobimetallic cooperation in oxygen reduction catalysis and demonstrates the potential of this synergistic effect in the design of nonheme oxygen reduction catalysts.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Coordinatively Unsaturated Metallates of Cobalt(II), Nickel(II), and Zinc(II) Guarded by a Rigid and Narrow Void

Both natural enzymatic systems and synthetic porous material catalysts utilize well-defined and uniform channels to dictate reaction selectivities on the basis of size or shape. Mimicry of this design element in homogeneous systems is generally difficult owing to the flexibility inherent in most small molecular species. Herein, we report the synthesis of a tripodal ligand scaffold that orients a narrow and rigid cavity atop accessible metal coordination space. The permanent void is formed through a macrocyclization reaction whereby the 3,5-dihydroxyphenyl arms are covalently linked through methylene bridges. Deprotonative metallation leads to anionic and coordinatively unsaturated complexes of divalent cobalt, nickel, and zinc. An analogous series of trigonal monopyramidal complexes bearing a nonmacrocyclized variant of the tripodal ligand are also reported. Physical characterization of the coordination complexes has been carried out using multiple spectroscopic techniques (NMR, EPR, and UV-vis), cyclic voltammetry, and X-ray diffraction. Complexes of the macrocyclized [L^OCH2O]^3- ligand retain a rigid cavity upon metallation, with this cavity guarding the entrance to the open axial coordination site. Through a combination of spectroscopic and computational studies, it is shown that acetonitrile entry into the void is sterically precluded, disrupting anticipated coordination at the intracavity site.

Electric field effect of positive and negative charges of substituents on electronic structure and reactivity of oxoiron(IV) porphyrin π-cation radical complex

Electric field effect by the positive and negative changes near the active site is an important factor for controlling the reactivity of metalloenzymes. Previously, we reported that the positive charge of the N-methyl-2-pyridinium cation increases the reactivity of oxoiron(IV) porphyrin π-cation radical complex (Compound I), due to the attractive Coulomb interaction with electrons in Compound I. To further investigate the electric field effect, we study here the effect of the negative charge of the sulfonate group on the electronic structure and reactivity using Compound I of meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrin (TMPS-I). Although Compound I has been known as a very unstable complex, TMPS-I is very stable in 0.1 M acetate buffer at pH = 6. The half-life of TMPS-I is estimated to be 6.9 × 10³ s, which is the longest in Compound I previously reported. The redox potential of TMPS-I is estimated to be 0.76 V vs SCE in phosphate buffer, pH = 10. Kinetic analysis with stopped-flow technique indicates TMPS-I is less reactive than Compounds I reported previously. However, ¹H NMR and EPR spectra of TMPS-I are very close to those of Compounds I reported previously. The DFT calculations show that the orbital energy of Compound I is drastically altered by the positive and negative charges on the meso-phenyl group, suggesting the electric field effect. The difference of the reactivity of Compound I can be rationalized with the change of the orbital energy caused by the intramolecular electric field effect of the positive and negative charges.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

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

Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O₂ reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e^-/4H⁺ reduction of oxygen to water at rates that are 2-3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins. In the aqueous medium, the same trend of higher ORR rates at a lower overpotential is observed. In situ resonance Raman data under heterogeneous aqueous conditions show that the presence of one amine group in the second sphere leads to a cleavage of the O-O bond in a Fe^III-OOH intermediate as the rate-determining step (rds). The presence of four such amine groups enhances the rate of O-O bond cleavage such that this intermediate is no longer observed during the ORR; rather, the proton-coupled reduction of the Fe^III-O₂^- intermediate with a H/D isotope effect of 10.6 is the rds. These data clearly demonstrate changes in the rds of the electrochemical ORR depending on the nature of second-sphere residues and explain their deviation from linear scaling relationships.

Electrocatalytic oxygen reduction with cobalt corroles bearing cationic substituents

Recent decades have seen increasing interest in developing highly active and selective electrocatalysts for the oxygen reduction reaction (ORR). The active site environment of cytochrome c oxidases (CcOs), including electrostatic and hydrogen-bonding interactions, plays an important role in promoting the selective conversion of dioxygen to water. Herein, we report the synthesis of three Co^III corroles, namely 1 (with a 10-phenyl ortho-trimethylammonium cationic group), 2 (with a 10-phenyl ortho-dimethylamine group) and 3 (with a 10-phenyl para-trimethylammonium cationic group) as well as their electrocatalytic ORR activities in both acidic and neutral solutions. We discovered that 1 is much more active and selective than 2 and 3 for the electrocatalytic four-electron ORR. Importantly, 1 showed ORR activities with half-wave potentials at E_1/2 = 0.75 V versus RHE in 0.5 M H₂SO₄ solutions and at E_1/2 = 0.70 V versus RHE in neutral 0.1 M phosphate buffer solutions. This work is significant for outlining a strategy to increase both the activity and selectivity of metal corroles for the electrocatalytic ORR by introducing cationic units.

Two-Electron-Induced Reorganization of Cobalt Coordination and Metal-Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO2 Reduction

Electrochemical reorganization of complex structures is directly related to catalytic reactivity; thus, the geometric changes of catalysts induced by electron transfer should be considered to scrutinize the reaction mechanism. Herein, we studied electron-induced reorganization patterns of six-coordinate Co complexes with neutral N-donor ligands. Upon two-electron transfer into a Co center enclosed within a bulky π-acceptor ligand, the catalytic site exhibited different reorganization patterns depending on the ligand characteristics. While a bipyridyl ligand released Co-bound solvent (CH₃CN) to open a reaction site, a phenanthroline ligand caused Co-N_arm (side "arm" of NNN-ligand) bond dissociation. The first electron transfer occurred in the Co(II/I) reduction step and the second electron entered the bulky π-acceptor, of which redox steps were assigned from cyclic voltammograms, magnetic moment measurements, and DFT calculations. In comparison, the Co complex of [NNN^NCH₃-Co(CH₃CN)₃](PF₆)₂ ([1-(CH₃CN)₃](PF₆)₂) showed a high H₂ evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNN^NCH₃-Co(L)(CH₃CN)](PF₆)₂ (L = phen ([2-CH₃CN](PF₆)₂), bpy ([3-CH₃CN](PF₆)₂), [NNN^NCH₃-Co(tpy)](PF₆)₂ ([4](PF₆)₂), and [NNN^CH₂-Co(phen)(CH₃CN)](PF₆)₂ ([5-CH₃CN](PF₆)₂)) suppressed the HER but rather enhanced the CO₂ reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO₂ reduction. Additionally, the amine pendant attached to the NNN^NCH₃-ligand could stabilize the CO₂ reduction intermediate through the hydrogen-bonding interaction with the Co-CO₂H adduct.

Using Substituted [Fe4N(CO)12]- as a Platform To Probe the Effect of Cation and Lewis Acid Location on Redox Potential

The impact of cationic and Lewis acidic functional groups installed in the primary or secondary coordination sphere (PCS or SCS) of an (electro)catalyst is known to vary depending on the precise positioning of those groups. However, it is difficult to systematically probe the effect of that position. In this report, we probe the effect of the functional group position and identity on the observed reduction potentials (E_p,c) using substituted iron clusters, [Fe₄N(CO)₁₁R]ⁿ, where R = NO⁺, PPh₂-CH₂CH₂-9BBN, (^MePTA⁺)₂, (^MePTA⁺)₄, and H⁺ and n = 0, -1, +1, or +3 (9-BBN is 9-borabicyclo(3.3.1)nonane; ^MePTA⁺ is 1-methyl-1-azonia-3,5-diaza-7-phosphaadamantane). The cationic NO⁺ and H⁺ ligands cause anodic shifts of 700 and 320 mV, respectively, in E_p,c relative to unsubstituted [Fe₄N(CO)₁₂]^-. Infrared absorption band data, ν_CO, suggests that some of the 700 mV shift by NO⁺ results from electronic changes to the cluster core. This contrasts with the effects of cationic ^MePTA⁺ and H⁺ which cause primarily electrostatic effects on E_p,c. Lewis acidic 9-BBN in the SCS had almost no effect on E_p,c.

Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction

We present a supramolecular approach to catalyzing photochemical CO2 reduction through second-sphere porosity and charge effects. An iron porphyrin box (PB) bearing 24 cationic groups, FePB-2(P), was made via post-synthetic modification of an alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO2 reduction reaction (CO2 RR) with 97 % selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOFmax ) reaching 1400 min-1 . The cooperativity between porosity and charge results in a 41-fold increase in activity relative to the parent Fe tetraphenylporphyrin (FeTPP) catalyst, which is far greater than analogs that augment catalysis through porosity (FePB-3(N), 4-fold increase) or charge (Fe p-tetramethylanilinium porphyrin (Fe-p-TMA), 6-fold increase) alone. This work establishes that synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.

CO2 Electroreduction on Carbon-Based Electrodes Functionalized with Molecular Organometallic Complexes—A Mini Review

Heterogeneous electrochemical CO2 reduction has potential advantages with respect to the homogeneous counterpart due to the easier recovery of products and catalysts, the relatively small amounts of catalyst necessary for efficient electrolysis, the longer lifetime of the catalysts, and the elimination of solubility problems. Unfortunately, several disadvantages are also present, including the difficulty of designing the optimized and best-performing catalysts by the appropriate choice of the ligands as well as a larger heterogeneity in the nature of the catalytic site that introduces differences in the mechanistic pathway and in electrogenerated products. The advantages of homogeneous and heterogeneous systems can be preserved by anchoring intact organometallic molecules on the electrode surface with the aim of increasing the dispersion of active components at a molecular level and facilitating the electron transfer to the electrocatalyst. Electrode functionalization can be obtained by non-covalent or covalent interactions and by direct electropolymerization on the electrode surface. A critical overview covering the very recent literature on CO2 electroreduction by intact organometallic complexes attached to the electrode is summarized herein, and particular attention is given to their catalytic performances. We hope this mini review can provide new insights into the development of more efficient CO2 electrocatalysts for real-life applications.

Across the Board: Rui Cao on Electrocatalytic CO2 Reduction

In this series of articles, the Board Members of ChemSusChem review recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. Rui Cao, who discusses how tuning the second-sphere environments of Fe porphyrins can improve the activity and selectivity for CO₂ reduction. Substituents with proton relay capability, hydrogen-bonding, and electrostatic features have significant impact on the efficiency of Fe porphyrins for the electrocatalytic CO₂ reduction reaction.

Two-Dimensional Porphyrin-Based Covalent Organic Framework with Enlarged Inter-layer Spacing for Tunable Photocatalytic CO2 Reduction

Two-dimensional (2D) porphyrin-based covalent organic frameworks (COFs) are one of the most promising candidates for photocatalytic carbon dioxide reduction reaction (CO₂RR), which however still suffer from the hindered mass transfer during the catalysis procedure associated with the close packing of 2D COF layers due to the strong axial π-π stacking. Herein, condensation between the porphyrinic aldehydes p-MPor-CHO (M = H₂, Co, and Ni) and 3,8-diamino-6-phenyl-phenanthridine (DPP) affords new porphyrin-based 2D COF architecture MPor-DPP-COFs (M = H₂, Co, and Ni). The bulky phenyl substituent at the phenanthridine periphery of the linking unit reduces the axial π-π stacking, providing an enlarged inter-layer spacing of 6.0 Å according to high-resolution transmission electron microscopy. This, in combination with the large surface area (1021 m² g^-1) revealed by nitrogen sorption measurements at 77 K for CoPor-DPP-COF possessing electroactive Co ions, endows it with excellent photocatalytic activity for CO₂RR with a CO generation rate of 10 200 μmol g^-1 h^-1 and a CO selectivity up to 82%. This work affords new ideas for achieving efficient photocatalytic CO₂RR upon fine-tuning the inter-layer spacing of 2D COFs.

Promotion of Catalytic Oxygen Reduction Reactions: The Utility of Proton Management Substituents on Cobalt Porphyrins

Three ABAB-type cobalt meso-tetraarylporphyrins with fluorine (F-CoPor), acetic acid (AC-CoPor), and cyanoacetic acid (CN-CoPor) groups at the para-positions of phenyl rings at the 10,20-positions are synthesized and evaluated as catalysts for oxygen reduction reactions (ORRs). In density functional theory calculations, the frontier molecular orbitals of these complexes were found to be stabilized relative to model complexes with electron-withdrawing atoms or moieties on the meso-aryl rings. Electrochemical measurements suggest that electrodes with CN-CoPor (CN-CoPor/C) exhibit the most positive ORR potential values and the highest limiting current density in both acidic and alkali electrolytes, while the F-CoPor/C electrocatalyst exhibits extremely low ORR performance. The electron transfer numbers for the electrocatalysts are more than 3.0, indicating that a mixture of 2- and 4-electron transfer pathways occurs. The results demonstrate that coupling the hydrogen bonding properties and electron-withdrawing abilities through rational design of the substituent at the meso-position is an efficient way to modify the ORR performance.