Pendant Acid–Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays? Mechanisms of the Conversion of CO2 to CO by Electrogenerated Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities

Circumventing Kinetic Barriers to Metal Hydride Formation with Metal-Ligand Cooperativity

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [CoIIICp(PPh2NBn2)(CH3CN)]2+ to [HCoIIICp(PPh2NBn2)]+. This complex catalytically converts CO2 to formate under CO2 reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the PPh2NBn2 ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the PPh2NBn2 ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pKa values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. This work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.

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.

Examining the Importance of Hydrogen Bonding and Proton Transfer in Iron Porphyrin-Mediated Carbon Dioxide Upconversion

ConspectusThe title should give a sense of the "big picture" of this Account, but what is it really about? An unexpected change in research direction? A series of courageous and creative students? A team taking on challenging problems in chemistry? The answer is a definite "yes" to all of the above. More specifically, the problem in which we are interested is the upconversion or valorization of carbon dioxide. This problem has captured the attention of a great many chemists in earnest following the gas crisis of the 1970s and more recently galvanized due to climate concerns arising from the ongoing release of anthropogenic carbon. Addressing the problem of atmospheric carbon accumulation requires effort in two very broad areas: capture and conversion. Storage is an alternative to conversion, but this eliminates the opportunity to use what might be otherwise a waste product. Our group has investigated a series of modified versions of iron(III)-5,10,15,20-tetraphenylporphyrin (FeTPP) that can convert CO2 to carbon monoxide, which is a versatile and useful precursor for other syntheses. Following pioneering work from Savéant and his colleagues in the 1990s and thereafter, we started with a simple question: how many pendent ancillary groups that can donate H-bonds or protons are needed to support efficient CO2-to-CO conversion? Using a molecule with only one 2-hydroxylphenyl group, we demonstrated that the single prepositioned -OH group gave rise to efficient turnover, but only when experiments were carried out in a weakly H-bond-accepting solvent system. In other words, the ability of a solvent to accept H-bonds can impede CO2 reduction. We followed up with a deeper investigation of the influence of H-bonding interactions with external acids in FeTPP-mediated CO2 reduction. Savéant's framework mechanism appears to be independent of solvent, and rate differences can be approximated by considering H-bonding equilibria. Following that work, we sought to better understand the minimum catalyst design requirements with respect to internal H-bond/proton donors. To that end, we produced all possible isomers of tetraarylpoprhyrins with 2,6-dihydroxyphenyl + phenyl groups. All else being equal, the complexes with a formally trans orientation of the 2,6-dihydroxyphenyl groups performed the best. Most recently, we surveyed the roles of internal and external Brønsted acids with different pKa values. Surprisingly, the best-performing catalysts have more weakly acidic internal groups. Overall, our work has demonstrated that CO2 reduction mediated by porphyrin catalysts can be improved by considering solvent H-bonding, the orientation of internal H-bonding groups, and the balance of the pKa values of internal and external acids. The future for molecular electrocatalysts is promising as more ideas emerge about how to design molecules and conditions for CO2 reduction.

Mononuclear Iron Pyridinethiolate Complex Promoted CO2 Photoreduction via Rapid Intramolecular Electron Transfer

Designing earth-abundant metal complexes as efficient molecular photocatalysts for visible light-driven CO2 reduction is a key challenge in artificial photosynthesis. Here, we demonstrated the first example of a mononuclear iron pyridine-thiolate complex that functions both as a photosensitizer and catalyst for CO2 reduction. This single-component bifunctional molecular photocatalyst efficiently reduced CO2 to formate and CO with a total turnover number (TON) of 46 and turnover frequency (TOF) of 11.5 h-1 in 4 h under visible light irradiation. Notably, the quantum yield was determined to be 8.4 % for the generation of formate and CO at 400 nm. Quenching experiments indicate that high photocatalytic activity is mainly attributed to the rapid intramolecular quenching protocol. The mechanism investigation by DFT calculation and electrochemical studies revealed that the protonation of Febpy(pyS)2 is indispensable step for photocatalytic CO2 reduction.

Fluorinated chlorin chromophores for red-light-driven CO2 reduction

The utilization of low-energy photons in light-driven reactions is an effective strategy for improving the efficiency of solar energy conversion. In nature, photosynthetic organisms use chlorophylls to harvest the red portion of sunlight, which ultimately drives the reduction of CO2. However, a molecular system that mimics such function is extremely rare in non-noble-metal catalysis. Here we report a series of synthetic fluorinated chlorins as biomimetic chromophores for CO2 reduction, which catalytically produces CO under both 630 nm and 730 nm light irradiation, with turnover numbers of 1790 and 510, respectively. Under appropriate conditions, the system lasts over 240 h and stays active under 1% concentration of CO2. Mechanistic studies reveal that chlorin and chlorinphlorin are two key intermediates in red-light-driven CO2 reduction, while corresponding porphyrin and bacteriochlorin are much less active forms of chromophores.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖(N2S2)]Cl2 (NiN2S2, N2S2=2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1, with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

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.

Ligand-Bound CO2 as a Nonclassical Route toward Efficient Photocatalytic CO2 Reduction with a Ni N-Confused Porphyrin

Implementing the synergistic effects between the metal and the ligand has successfully streamlined the energetics for CO2 activation and gained high catalytic activities, establishing the important breakthroughs in photocatalytic CO2 reduction. Herein, we describe a Ni(II) N-confused porphyrin complex (NiNCP) featuring an acidic N-H group. It is readily deprotonated and exists in an anion form during catalysis. Owing to this functional site, NiNCP gave rise to an outstanding turnover number (TON) as high as 217,000 with a 98% selectivity for CO2 reduction to CO, while the parent Ni(II) porphyrin (NiTPP) was found to be nearly inactive. Our mechanistic analysis revealed a nonclassical reaction pattern where CO2 was effectively activated via the attack of the Lewis-basic ligand. The resulting ligand-bound CO2 adduct could be further reduced to produce CO. This new metal-ligand synergistic effect is anticipated to inspire the design of highly active catalysts for small molecule activations.

Unraveling a Concerted Proton-Coupled Electron Transfer Pathway in Atomically Precise Gold Nanoclusters

Metal nanoclusters (MNCs) represent a promising class of materials for catalytic carbon dioxide and proton reduction as well as dihydrogen oxidation. In such reactions, multiple proton-coupled electron transfer (PCET) processes are typically involved, and the current understanding of PCET mechanisms in MNCs has primarily focused on the sequential transfer mode. However, a concerted transfer pathway, i.e., concerted electron-proton transfer (CEPT), despite its potential for a higher catalytic rate and lower reaction barrier, still lacks comprehensive elucidation. Herein, we introduce an experimental paradigm to test the feasibility of the CEPT process in MNCs, by employing Au18(SR)14 (SR denotes thiolate ligand), Au22(SR)18, and Au25(SR)18- as model clusters. Detailed investigations indicate that the photoinduced PCET reactions in the designed system proceed via an CEPT pathway. Furthermore, the rate constants of gold nanoclusters (AuNCs) have been found to be correlated with both the size of the cluster and the flexibility of the Au-S framework. This newly identified PCET behavior in AuNCs is prominently different from that observed in semiconductor quantum dots and plasmonic metal nanoparticles. Our findings are of crucial importance for unveiling the catalytic mechanisms of quantum-confined metal nanomaterials and for the future rational design of more efficient catalysts.

Understanding the Interplay of the Brønsted Acidity of Catalyst Ancillary Groups and the Solution Components in Iron-porphyrin-Mediated Carbon Dioxide Reduction

The rapid and efficient conversion of carbon dioxide (CO2) to carbon monoxide (CO) is an ongoing challenge. Catalysts based on iron-porphyrin cores have emerged as excellent electrochemical mediators of the two proton + two electron reduction of CO2 to CO, and many of the design features that promote function are known. Of those design features, the incorporation of Brønsted acids in the second coordination sphere of the iron ion has a significant impact on catalyst turnover kinetics. The Brønsted acids are often in the form of hydroxyphenyl groups. Herein, we explore how the acidity of an ancillary 2-hydroxyphenyl group affects the performance of CO2 reduction electrocatalysts. A series of meso-5,10,15,20-tetraaryl porphyrins were prepared where only the functional group at the 5-meso position has an ionizable proton. A series of cyclic voltammetry (CV) experiments reveal that the complex with -OMe positioned para to the ionizable -OH shows the largest CO2 reduction rate constants in acetonitrile solvent. This is the least acidic -OH of the compounds surveyed. The turnover frequency of the -OMe derivative can be further improved with the addition of 4-trifluoromethylphenol to the solution. In contrast, the iron-porphyrin complex with -CF3 positioned opposite the ionizable -OH shows the smallest CO2 reduction rate constants, and its turnover frequency is less enhanced with the addition of phenols to the reaction solutions. The origin of this effect is rationalized based on kinetic isotope effect experiments and density functional calculations. We conclude that catalysts with weaker internal acids coupled with stronger external acid additives provide superior CO2 reduction kinetics.

Electrocatalytic Interconversions of CO2 and Formate on a Versatile Iron-Thiolate Platform

Exploring bidirectional CO2/HCO2- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO2/HCO2- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph2PC6H4S-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L-)2Fe(III)(μ-HCO2)Fe(III)]+, which exhibits remarkable electrocatalytic performance for the HCO2--to-CO2 transformation with a maximum turnover frequency (TOFmax) ∼103 s-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc+/0) and exhibits an impressive FE ∼93 (±3)% for the CO2-to-HCO2- conversion. Mechanism studies revealed that the HCO2--to-CO2 electrocatalysis passes through dicationic [(L2)-•Fe(III)(μ-HCO2)Fe(III)]2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO2-to-HCO2- reduction involves a critical intermediate of [Fe(II)-H]- that was independently synthesized and structurally characterized.

Hydrogenation of CO2 by a Bifunctional PC(sp3 )P Iridium(III) Pincer Complex Equipped with Tertiary Amine as a Functional Group

Reversible hydrogen storage in the form of stable and mostly harmless chemical substances such as formic acid (FA) is a cornerstone of a fossil fuels-free economy. In the past, we have reported a primary amine-functionalized bifunctional iridium(III)-PC(sp3 )P pincer complex as a mild and chemoselective catalyst for the additive-free decomposition of neat formic acid. In this manuscript, we report on the successful application of a redesigned complex bearing tertiary amine functionality as a catalyst for mild hydrogenation of CO2 to formic acid. The catalyst demonstrates TON up to 6×104 and TOF up to 1.7×104  h-1 . In addition to the practical value of the catalyst, experimental and computational mechanistic studies provide the rationale for the design of improved next-generation catalysts.

Introducing proton/electron mediators enhances the catalytic ability of an iron porphyrin complex for photochemical CO2 reduction

A novel iron porphyrin complex with hydroquinone moieties as proton/electron mediators at meso positions was designed and synthesised. The complex serves as an efficient catalyst for photochemical CO2 reduction, and its turnover frequency (TOF = 1.3 × 104 h-1) was the highest among those of comparable systems with sufficient durability.

Linear Free Energy Relationships and Transition State Analysis of CO2 Reduction Catalysts Bearing Second Coordination Spheres with Tunable Acidity

In molecular catalysts, protic functional groups in the secondary coordination sphere (SCS) work in conjunction with an exogenous acid to relay protons to the active site of electrochemical CO2 reduction; however, it is not well understood how the acidity of the SCS and exogenous acid together determine the kinetics of catalytic turnover. To evaluate the relative contributions of proton transfer driving forces, we synthesized a series of modular iron tetraphenylporphyrin electrocatalysts bearing SCS amides of tunable pKa (17.6 to 20.0 in dimethyl sulfoxide (DMSO)) and employed phenols of variable acidity (15.3 to 19.1) as exogenous acids. This system allowed us to (1) evaluate contributions from proton transfer driving forces associated with either the SCS or exogenous acid and (2) obtain mechanistic insights into CO2 reduction as a function of pKa. A series of linear free-energy relationships show that kinetics become increasingly sensitive to variations in SCS pKa when more acidic exogenous acids are used (0.82 ≥ Brønsted α ≥ 0.13), as well as to variations in exogenous acid pKa when SCS acidity is increased (0.62 ≥ Brønsted α ≥ 0.32). An Eyring analysis suggests that the rate-determining transition state becomes more ordered with decreasing SCS acidity, which is consistent with the proposal that SCS acidity modulates charge accumulation and solvation at the rate-limiting transition state. Together, these insights enable the optimization of activation barriers as a function of both SCS and exogenous acid pKa and can further guide the rational design of electrocatalytic systems wherein contributions from all participants in a proton relay are considered.

Engineering an Oxygen-Binding Protein for Photocatalytic CO2 Reductions in Water

While native CO2 -reducing enzymes display remarkable catalytic efficiency and product selectivity, few artificial biocatalysts have been engineered to allow understanding how the native enzymes work. To address this issue, we report cobalt porphyrin substituted myoglobin (CoMb) as a homogeneous catalyst for photo-driven CO2 to CO conversion in water. The activity and product selectivity were optimized by varying pH and concentrations of the enzyme and the photosensitizer. Up to 2000 TON(CO) was attained at low enzyme concentrations with low product selectivity (15 %), while a product selectivity of 74 % was reached by increasing the enzyme loading but with a compromised TON(CO). The efficiency of CO generation and overall TON(CO) were further improved by introducing positively charged residues (Lys or Arg) near the active stie of CoMb, which demonstrates the value of tuning the enzyme secondary coordination sphere to enhance the CO2 -reducing performance of a protein-based photocatalytic system.

Sulphur vs NH Group: Effects on the CO2 Electroreduction Capability of Phenylenediamine-Cp Cobalt Complexes

The cobalt complex (I) with cyclopentadienyl and 2-aminothiophenolate ligands was investigated as a homogeneous catalyst for electrochemical CO₂ reduction. By comparing its behavior with an analogous complex with the phenylenediamine (II), the effect of sulfur atom as a substituent has been evaluated. As a result, a positive shift of the reduction potential and the reversibility of the corresponding redox process have been observed, also suggesting a higher stability of the compound with sulfur. Under anhydrous conditions, complex I showed a higher current enhancement in the presence of CO₂ (9.41) in comparison with II (4.12). Moreover, the presence of only one -NH group in I explained the difference in the observed increases on the catalytic activity toward CO₂ due to the presence of water, with current enhancements of 22.73 and 24.40 for I and II, respectively. DFT calculations confirmed the effect of sulfur on the lowering of the energy of the frontier orbitals of I, highlighted by electrochemical measurements. Furthermore, the condensed Fukui function f ^- values agreed very well with the current enhancement observed in the absence of water.

Photocatalytic CO2 reduction with aminoanthraquinone organic dyes

The direct utilization of solar energy to convert CO₂ into renewable chemicals remains a challenge. One essential difficulty is the development of efficient and inexpensive light-absorbers. Here we show a series of aminoanthraquinone organic dyes to promote the efficiency for visible light-driven CO₂ reduction to CO when coupled with an Fe porphyrin catalyst. Importantly, high turnover numbers can be obtained for both the photosensitizer and the catalyst, which has not been achieved in current light-driven systems. Structure-function study performed with substituents having distinct electronic effects reveals that the built-in donor-acceptor property of the photosensitizer significantly promotes the photocatalytic activity. We anticipate this study gives insight into the continued development of advanced photocatalysts for solar energy conversion.

Non-negligible Axial Ligand Effect on Electrocatalytic CO2 Reduction with Iron Porphyrin Complexes

Iron(III) porphyrin complexes have been demonstrated as one of the efficient molecular catalysts for the electrochemical reduction of CO₂. However, the role of axial ligands coordinated with a metal center in the complex on the electrochemical CO₂ reduction activity has not been fully explored yet. Herein, iron(III) tetraphenylporphyrin thiocyanate (FeTPP-SCN) is synthesized from a commercially available catalyst of FeTPP-Cl by a counteranion exchanging reaction. Cyclic voltammetry measurements showed that the catalytic activity of FeTPP-SCN is noticeably suppressed in the DMF solutions. The structural dynamics of the axial ligand in FeTPP-SCN are further examined by the FTIR and ultrafast IR spectroscopies, where the SCN ligand is employed as the local vibrational probe. Vibrational relaxation measurements showed that the reorientational dynamics of SCN ligands was strongly restricted in DMF solution, suggesting that the subtle electrostatic interaction between the ligands and metal center in the complex can have a non-negligible effect on its catalytic activity.

Multifunctional Charge and Hydrogen-Bond Effects of Second-Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated with imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2 RR), which promotes multiple synergistic effects to enhance CO2 RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2 RR activity by 2000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP). Comparison with monofunctional analogs reveals that through-space charge effects have a greater impact on catalytic CO2 RR performance compared to hydrogen bonding in this context.

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO2 Reduction Using Mn, Re, and Ru Catalysts

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO₂ hydrogenation using renewable H₂. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.g., Mn, Re, and Ru), which offer versatile platforms for mechanistic studies of photo- and electrochemical CO₂ reduction. The interactions of catalytic intermediates with Lewis or Brønsted acids, hydrogen-bonding moieties, solvents, cations, etc., that function as promoters or cofactors have become increasingly important for efficient catalysis. These interactions may have dramatic effects on selectivity and rates by stabilizing intermediates or lowering transition state barriers, but they are difficult to elucidate and challenging to predict. We have been carrying out experimental and theoretical studies of CO₂ reduction using molecular catalysts toward addressing mechanisms of efficient CO₂ reduction systems with emphasis on those containing intramolecular (or pendent) and intermolecular (solution phase) additives. This Account describes the identification of reaction intermediates produced during CO₂ reduction in the presence of triethanolamine or ionic liquids, the benefits of hydrogen-bonding interactions among intermediates or cofactors, and the complications of pendent phenolic donors/phenoxide bases under electrochemical conditions.Triethanolamine (TEOA) is a common sacrificial electron donor for photosensitizer excited state reductive quenching and has a long history of use in photocatalytic CO₂ reduction. It also functions as a Brønsted base in conjunction with more potent sacrificial electron donors, such as 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Deprotonation of the BIH^•+ cation radical promotes irreversible photoinduced electron transfer by preventing charge recombination. Despite its wide use, most research to date has not considered the broader reactions of TEOA, including its direct interaction with CO₂ or its influence on catalytic intermediates. We found that in acetonitrile, TEOA captures CO₂ in the form of a zwitterionic adduct without any metal catalyst. In the presence of ruthenium carbonyl catalysts bearing α-diimine ligands, it participates in metal hydride formation, accelerates hydride transfer to CO₂ to form the bound formate intermediate, and assists in the dissociation of formate anion from the catalyst ( J. Am. Chem. Soc. 2020, 142, 2413-2428).Hydrogen bonding and acid/base promoters are understood to interact with key catalytic intermediates, such as the metallocarboxylate or metallocarboxylic acid during CO₂ reduction. The former is a high energy species, and hydrogen-bonding or Lewis acid-stabilization are beneficial. We have found that imidazolium-based ionic liquid cations can stabilize the doubly reduced form of the [ReCl(bpy)(CO)₃] (bpy = 2,2'-bipyridine) electrocatalyst through both hydrogen-bonding and π-π interactions, resulting in CO₂ reduction occurring at a more positive potential with a higher catalytic current ( J. Phys. Chem. Lett. 2014, 5, 2033-2038). Hydrogen bonding interactions between Lewis basic methoxy groups in the second coordination sphere of a Mn-based catalyst and the OH group of the Mn-COOH intermediate in the presence of a Brønsted acid were also found to promote C-(OH) bond cleavage, enabling access to a low-energy protonation-first pathway for CO₂ reduction ( J. Am. Chem. Soc. 2017, 139, 2604-2618).The kinetics of forming the metallocarboxylic acid can be enhanced by internal acids, and its proton-induced C-OH bond cleavage to the metallocarbonyl and H₂O is often the rate-limiting step. Therefore, proton movement organized by pendent hydrogen-bonding networks may also accelerate this step. In contrast, during electrolysis, OH groups in the second coordination sphere are deprotonated to the oxyanions, which deter catalytic CO₂ reduction by directly binding CO₂ to form the carbonate or by making an M-O bond in competition with CO₂ binding ( Inorg. Chem. 2016, 55, 4582-4594). Our results emphasize that detailed mechanistic research is critical in discovering the design principles for improved catalysts.

Selectivity in Electrochemical CO2 Reduction

The electrocatalytic CO₂ reduction reaction (CO₂RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO₂ emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO₂RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO₂ can be partially reduced to produce several C₁ products like CO, HCOOH, CH₃OH, etc. before its complete 8e^-/8H⁺ reduction to CH₄. In addition to that, the low-valent electron-rich metal centers deployed to activate CO₂, a Lewis acid, are prone to reduce protons, which are a substrate for CO₂RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O₂ (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO₂RR. Not only is the requirement of O₂-free reaction conditions impractical, but it also leads to the release of partially reduced O₂ species such as O₂^-, H₂O₂, etc., which are reactive and result in oxidative degradation of the catalyst.In this Account, mechanistic investigations of CO₂RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO₂RR. The spectroscopic data obtained from different intermediates have been identified in different CO₂RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e^-/2H⁺ CO₂RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO₂RR (producing only CO, only HCOOH, or only CH₄) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO₂RR and HER as well as CO₂RR and ORR could be skewed overwhelmingly in favor of CO₂RR in both cases.

Copper(II) tetrakis(pentafluorophenyl)porphyrin: highly active copper-based molecular catalysts for electrochemical CO2 reduction

We report a highly active copper-based catalyst for electrochemical CO₂ reduction. Electrochemical analysis revealed that the maximum turnover frequency for CO₂ to CO conversion reached 1 460 000 s^-1 at an overpotential (η) of 0.85 V. Surprisingly, this value is more than 1 000 000 times higher than those of other reported copper-based molecular catalysts.

Switching Catalyst Selectivity via the Introduction of a Pendant Nitrophenyl Group

The conversion of abundant small molecules to value-added products serves as an attractive method to store renewable energy in chemical bonds. A family of macrocyclic cobalt aminopyridine complexes was previously reported to reduce CO₂ to CO with 98% faradaic efficiency through the formation of hydrogen-bonding networks and with the number of secondary amines affecting catalyst performance. One of these aminopyridine macrocycles, (NH)₁(NMe)₃-bridged calix[4]pyridine (L⁵), was modified with a nitrophenyl group to form L^NO2 and metalated with a cobalt(II) precursor to generate CoL^NO2, which would allow for probing the positioning and steric effects on catalysis. The addition of a nitrophenyl moiety to the ligand backbone results in a drastic shift in selectivity. Large current increases in the presence of added protons and CoL^NO2 are observed under both N₂ and CO₂. The current increases under N₂ are ∼30 times larger than the ones under CO₂, suggesting a change in the selectivity of CoL^NO2 to favor H₂ production versus CO₂ reduction. H₂ is determined to be the dominant reduction product by gas chromatography, reaching faradaic efficiencies up to 76% under N₂ with TFE and 71% under CO₂ with H₂O, in addition to small amounts of formate. X-ray photoelectron spectroscopy (XPS) reveals the presence of a cobalt-containing heterogeneous deposit on the working electrode surface, indicating the addition of the nitrophenyl group reduces the electrochemical stability of the catalyst. These observed catalytic behaviors are demonstrably different relative to the tetra-NH bridged macrocycle, which shows 98% faradaic efficiency for CO₂-to-CO conversion with TFE, highlighting the importance of pendant hydrogen bond donors and electrochemically robust functional groups for selective CO₂ conversion.

Electronic structure analysis of electrochemical CO2 reduction by iron-porphyrins reveals basic requirements to design catalysts bearing non-innocent ligands

Electrocatalytic CO2 reduction is a possible solution to the increasing CO2 concentration in the earth atmosphere, because it enables storage of energy while using the harmful CO2 feedstock as starting...

Secondary Coordination Effect on Monobipyridyl Ru(II) Catalysts in Photochemical CO2 Reduction: Effective Proton Shuttle of Pendant Brønsted Acid/Base Sites (OH and N(CH3)2) and Its Mechanistic Investigation

While the incorporation of pendant Brønsted acid/base sites in the secondary coordination sphere is a promising and effective strategy to increase the catalytic performance and product selectivity in organometallic catalysis for CO₂ reduction, the control of product selectivity still faces a great challenge. Herein, we report two new trans(Cl)-[Ru(6-X-bpy)(CO)₂Cl₂] complexes functionalized with a saturated ethylene-linked functional group (bpy = 2,2'-bipyridine; X = -(CH₂)₂-OH or -(CH₂)₂-N(CH₃)₂) at the ortho(6)-position of bpy ligand, which are named Ru-bpy^OH and Ru-bpy^diMeN, respectively. In the series of photolysis experiments, compared to nontethered case, the asymmetric attachment of tethering ligand to the bpy ligand led to less efficient but more selective formate production with inactivation of CO₂-to-CO conversion route during photoreaction. From a series of in situ FTIR analyses, it was found that the Ru-formate intermediates are stabilized by a highly probable hydrogen bonding between pendent proton donors (-diMeN⁺H or -OH) and the oxygen atom of metal-bound formate (Ru^I-OCHO···H-E-(CH₂)₂-, E = O or diMeN⁺). Under such conformation, the liberation of formate from the stabilized Ru^I-formate becomes less efficient compared to the nontethered case, consequently lowering the CO₂-to-formate conversion activities during photoreaction. At the same time, such stabilization of Ru-formate species prevents the dehydration reaction route (η¹-OCHO → η¹-COOH on Ru metal) which leads toward the generation of Ru-CO species (key intermediate for CO production), eventually leading to the reduction of CO₂-to-CO conversion activity.

Activating the Fe(I) State of Iron Porphyrinoid with Second-Sphere Proton Transfer Residues for Selective Reduction of CO2 to HCOOH via Fe(III/II)-COOH Intermediate(s)

The ability to tune the selectivity of CO₂ reduction by first-row transition metal-based complexes via the inclusion of second-sphere effects heralds exciting and sought-after possibilities. On the basis of the mechanistic understanding of CO₂ reduction by iron porphyrins developed by trapping and characterizing the intermediates involved ( J. Am. Chem. Soc. 2015, 137, 11214), a porphyrinoid ligand is envisaged to switch the selectivity of the iron porphyrins by reducing CO₂ from CO to HCOOH as well as lower the overpotential to the process. The results show that the iron porphyrinoid designed can catalyze the reduction of CO₂ to HCOOH using water as the proton source with 97% yield with no detectable H₂ or CO. The iron porphyrinoid can activate CO₂ in its Fe(I) state resulting in very low overpotential for CO₂ reduction in contrast to all reported iron porphyrins, which can reduce CO₂ in their Fe(0) state. Intermediates involved in CO₂ reduction, Fe(III)-COOH and a Fe(II)-COOH, are identified with in situ FTIR-SEC and subsequently chemically generated and characterized using FTIR, resonance Raman, and Mössbauer spectroscopy. The mechanism of the reaction helps elucidate a key role played by a closely placed proton transfer residue in aiding CO₂ binding to Fe(I), stabilizing the intermediates, and determining the fate of a rate-determining Fe(II)-COOH intermediate.

Quick and Easy Method to Dramatically Improve the Electrochemical CO2 Reduction Activity of an Iron Porphyrin Complex

The development of artificial molecular catalysts for CO₂ reduction is the key to solving energy and environmental problems. Although chemical modifications can generally improve the catalytic activity of this class of compounds, they often require complicated synthetic procedures. Here, we report a simple procedure that dramatically enhances electrochemical CO₂ reduction activity. A one-step counteranion-exchange reaction increased the solubility of a commercially available catalyst, iron(III) tetraphenylporphyrin chloride, in a variety of solvents, allowing the investigation of its catalytic performance under various conditions. Surprisingly, the turnover frequency for CO evolution in acetonitrile (MeCN) reached 7 300 000 s^-1 , which is the highest among those of current best-in-class molecular catalysts. This excellent catalytic activity originates from the unique reaction between the generated Fe^I species and CO₂ in MeCN during catalysis. The present study offers a "quick and easy" method for obtaining an efficient catalytic system for electrochemical CO₂ reduction.

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

Next-generation energy technologies require improved methods for rapid and efficient chemical-to-electrical energy transformations. One new approach has been to include atomically positioned, electrostatic motifs in molecular catalysts to stabilize high-energy, charged intermediates. For example, an iron porphyrin bearing four cationic, o-N,N,N-trimethylanilinium groups (o-[N(CH₃)₃]⁺) has recently been used to catalyze the complex, multistep O₂ and CO₂ reduction reactions (ORR and CO₂RR) with fast rates and at low overpotentials. The success of this catalyst is attributed, at least in part, to specific charge-charge interactions between the atomically positioned o-[N(CH₃)₃]⁺ groups and the bound substrate. However, by nature of the mono-ortho substitution pattern, there are four possible atropisomers of this metalloporphyrin and thus four unique electrostatic environments. This work reports that each of the four individual atropisomers catalyzes both the ORR and CO₂RR with fast rates and low overpotentials. The maximum turnover frequencies vary among the atropisomers, by a factor of 60 for the ORR and a factor of 5 for CO₂RR. For the ORR, the αβαβ isomer is the fastest and has the highest overpotential, while for the CO₂RR the αααα isomer is the fastest and has the highest overpotential. The role of charge positioning is complex and can affect more than a single step such as CO₂ binding. These data offer a first-of-a-kind perspective on atomically positioned charge and highlight the significance of high charge density, rather than orientation, on the thermodynamics and kinetics of multistep molecular electrochemical transformations.

Computational study on the reactivity of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2 and Me4) for CO2-to-CO conversion over H2 formation

We have recently reported a series of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts, [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2, and Me4), for CO2-to-CO conversion in the presence of H2O as the proton source [J. Am. Chem. Soc., 2019, 141, 6569]. These catalysts feature slightly acidic imidazolium moieties in the secondary coordination sphere and reduce CO2 at mild electrochemical potentials. Here, we employ density functional theory (DFT) calculations to understand the electronic structure and reactivity for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) using [Mn[bpyMe(ImMe)](CO)3Br]+ (1+). Our work indicates that, in the absence of water, the imidazolium ligand stabilizes the Mn-CO2 adduct through hydrogen bonding-like interactions, similar to the activated CO2 molecule in the C-cluster of the Ni,Fe-carbon monoxide dehydrogenase II, and assists the protonation steps during CO2RR and HER. More significantly, based on the energy span model, we demonstrate that the selectivity for CO2 fixation over proton reduction results from a higher activation energy for yielding the manganese dihydrogen intermediate before H2 release, which is the TOF determining transition state (TDTS) under an applied potential of Φ = -1.82 V versus Fc0/+. The calculated TOF also reflects the selectivity for CO2RR, which is four orders of magnitude larger than for HER, consistent with the CPE experiments that show no hydrogen was obtained. In the case of CO2 reduction, the TOF determining intermediate (TDI) corresponds to the doubly reduced active catalyst, 1C2(red2), which features a manganese(0) center that couples ferromagnetically with one unpaired electron in the π* orbital of bipyridine. On the other hand, for HER, the metal-hydride intermediate, 1C2(I11-R), is the TDI. Finally, second-order perturbation analyses imply that the strongest hydrogen bonding-like interaction at the C2 position in 1+ contributes to the higher catalytic activity with respect to [Mn[bpyMe(ImMe2)](CO)3Br]+ (2+) and [Mn[bpyMe(ImMe4)](CO)3Br]+ (3+) for CO2 fixation, consistent with the experimental data.

Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO₂-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [Fe^II(TPP)] complex (1; TPP^2- = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin (S = 1) ground state was contradictorily assigned to either a nondegenerate ³A_2g state with a (d_xy)²(d_z²)²(d_xz,yz)² configuration or a degenerate ³E_g^θ state with a (d_xy)²(d_xz,yz)³(d_z²)¹/(d_z²)²(d_xy)¹(d_xz,yz)³ configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex 1. Highly correlated wave-function-based computations predicted that ³A_2g and ³E_g^θ are well-isolated from other triplet states by ca. 4000 cm^-1, whereas their splitting Δ_A-E is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (≈400 cm^-1). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the ³A_2g and ³E_g^θ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field ⁵⁷Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, Δ_A-E, an angle θ that describes the mixing of (d_xy)²(d_xz,yz)³(d_z²)¹ and (d_z²)²(d_xy)¹(d_xz,yz)³ configurations, and the ⟨r_d^-3⟩ expectation value of the iron d orbitals that is necessary to estimate the ⁵⁷Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both ³A_2g (<88%) and ³E_g (>12%), owing to their accidental near-triple degeneracy with Δ_A-E = +950 cm^-1. As a consequence of this peculiar electronic structure, 1 exhibits a huge effective magnetic moment (4.2 μB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization (g_⊥ ≈ 3 and g_∥ ≈ 1.7) and a large and positive internal field at the ⁵⁷Fe nucleus aligned in the xy plane. Further in-depth analyses suggested that g_⊥ ≫ g_∥ is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of 1 for CO₂ reduction are discussed.

Through-Space Electrostatic Interactions Surpass Classical Through-Bond Electronic Effects in Enhancing CO2 Reduction Performance of Iron Porphyrins

In his pioneering work to unravel the catalytic power of enzymes, Warshel has pertinently validated that electrostatic interactions play a major role in the activation of substrates. Implementing such chemical artifice in molecular catalysts may help improve their catalytic properties. In this study, a series of tetra-, di-, and mono-substituted iron porphyrins with cationic imidazolium groups were designed. Their presence in the second coordination sphere helped stabilize the [Fe-CO₂ ] intermediate through electrostatic interactions. It was found herein that the electrocatalytic overpotential is a function of the number of embarked imidazolium. Importantly, a gain of six orders of magnitude in turnover frequencies was observed going from a tetra- to a mono-substituted catalyst. Furthermore, the comparative study showed that catalytic performances trend of through-space electrostatic interaction, a first topological effect reported for iron porphyrins, outperforms the classical through-structure electronic effect.