Secondary-Sphere Effects in Molecular Electrocatalytic CO2 Reduction

Impact of the Surface Microenvironment on the Redox Properties of a Co-Based Molecular Cathode for Selective Aqueous Electrochemical CO2-to-CO Reduction

Electrode-confined molecular catalysts are promising systems to enable the efficient conversion of CO2 to useful products. Here, we describe the development of an original molecular cathode for CO2 reduction to CO based on the noncovalent integration of a tetraazamacrocyclic Co complex to a carbon nanotube-based matrix. Aqueous electrochemical characterization of the modified electrode allowed for clear observation of a change of redox behavior of the Co center as surface concentration was tuned, highlighting the impact of the catalyst microenvironment on its redox properties. The molecular cathode enabled efficient CO2-to-CO conversion in fully aqueous conditions, giving rise to a turnover number (TONCO) of up to 20 × 103 after 2 h of constant electrolysis at a mild overpotential (η = 450 mV) and with a faradaic efficiency for CO of about 95%. Post operando measurements using electrochemical techniques, inductively coupled plasma, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy characterization of the films demonstrated that the catalysis remained of molecular nature, making this Co-based electrode a new promising alternative for molecular electrocatalytic conversion of CO2-to-CO in fully aqueous media.

An Investigation of Allyl-Substituted Bis(Diphosphine) Iron Complexes: Towards Precursors for Cooperative CO2 Activation

In developing homogenous catalysts capable of CO2 activation, interaction with a metal center is often imperative. This work provides primary efforts towards the cooperative activation of CO2 using a Lewis acidic secondary coordination sphere (SCS) and iron via a paired theoretical/experimental approach. Specifically, this study reports efforts towards [Fe(diphosphine)2 (N2 )] as a CO2 -coordinated synthon where diphosphine=1,2-bis(di(3-cyclohexylboranyl)propylphosphino)ethane) (P2 BCy 4 ) or its precursor, 1,2-bis(diallylphosphino)ethane (tape). Initial efforts toward the {Fe(0)-N2 } complex were focused on deprotonation reactions of [Fe(diphosphine)2 (H)(NCCH3 )]+ and reduction of [Fe(tape)2 Cl2 ]. In the latter case, a mixture of intramolecularly π-bonded alkene and associated metallacyclic Fe(II)-H species were produced - heating this mixture provided the hydride as the major product. Notably, the interconversion of this pair counters that of related intermolecular reactions between [Fe(depe)2 ] (depe=1,2-bis(diethylphosphino)ethane) and ethylene, where hydride formation occurs subsequent to π-coordination; this has been probed by theoretical calculations. Finally, reactivity of the metallacyclic {Fe(II)-H} complex with CO2 was probed, resulting in a pair of isomeric ferra(II)lactones.

Controlling the Activation at NiII -CO2 2- Moieties through Lewis Acid Interactions in the Second Coordination Sphere

Nickel complexes with a two-electron reduced CO2 ligand (CO2 2- , "carbonite") are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO2 entity. For this purpose complexes with NiII (CO2 )AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(i Pr)2 . It was found that not only the nature of the AM ions in vicinity to CO2 affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)2 , THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO2 activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the LtBu Ni and carbonite moieties.

Porosity as a Design Element for Developing Catalytic Molecular Materials for Electrochemical and Photochemical Carbon Dioxide Reduction

The catalytic reduction of carbon dioxide (CO2 ) using sustainable energy inputs is a promising strategy for upcycling of atmospheric carbon into value-added chemical products. This goal has inspired the development of catalysts for selective and efficient CO2 conversion using electrochemical and photochemical methods. Among the diverse array of catalyst systems designed for this purpose, 2D and 3D platforms that feature porosity offer the potential to combine carbon capture and conversion. Included are covalent organic frameworks (COFs), metal-organic frameworks (MOFs), porous molecular cages, and other hybrid molecular materials developed to increase active site exposure, stability, and water compatibility while maintaining precise molecular tunability. This mini-review showcases catalysts for the CO2 reduction reaction (CO2 RR) that incorporate well-defined molecular elements integrated into porous materials structures. Selected examples provide insights into how different approaches to this overall design strategy can augment their electrocatalytic and/or photocatalytic CO2 reduction activity.

Molecularly dispersed nickel complexes on N-doped graphene for electrochemical CO2 reduction

In this work, new hybrid catalysts based on molecularly dispersed nickel complexes on N-doped graphene were developed for electrochemical CO2 reduction (ECR). Nickel(II) complexes (1-Ni, 2-Ni), and a new crystal structure ([2-Ni]Me), featuring N4-Schiff base macrocycles, were synthesized and investigated for their potential in ECR. Cyclic voltammetry (CV) in NBu4PF6/CH3CN solution demonstrated that the nickel complexes bearing N-H groups (1-Ni and 2-Ni) showed a substantial current enhancement in the presence of CO2, while the absence of N-H groups ([2-Ni]Me) resulted in an almost unchanged voltammogram. This indicated the necessity of the N-H functionality towards ECR in aprotic media. All three nickel complexes were successfully immobilized on nitrogen-doped graphene (NG) via non-covalent interactions. All three Ni@NG catalysts exhibited satisfactory CO2-to-CO reduction in aqueous NaHCO3 solution with the faradaic efficiency (FE) of 60-80% at the overpotential of 0.56 V vs. RHE. The ECR activity of [2-Ni]Me@NG also suggested that the N-H moiety from the ligand is less important in the heterogeneous aqueous system owing to viable hydrogen-bond formation and proton donors from water and bicarbonate ions. This finding could pave the way for understanding the effects of modifying the ligand framework at the N-H position toward fine tuning the reactivity of hybrid catalysts through molecular-level modulation.

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.

Exploring the Parameters Controlling Product Selectivity in Electrochemical CO2 Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes

Selective reduction of CO₂ is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental-computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO₂ to HCOOH, CO, and H₂. In contrast to previous Mn(bpy-R)(CO)₃Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H₂. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)₃ moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H₂ and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO₂ to the metal hydride favoring the formation of H₂ over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO₂ adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

NiII and CuII complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere

Herein, we report the synthesis, spectroscopic characterization and electrochemical investigation of the Ni^II and Cu^II complexes of a novel Sal ligand bearing two ferrocene moieties attached at its diimine linker, M(Sal)^Fc. The electronic spectra of M(Sal)^Fc are near identical to its phenyl-substituted counterpart, M(Sal)^Ph, indicating the ferrocene moieties exist in the secondary coordination sphere of M(Sal)^Fc. The cyclic voltammograms of M(Sal)^Fc exhibit an additional two-electron wave in comparison to M(Sal)^Ph, which is assigned to the sequential oxidation of the two ferrocene moieties. The chemical oxidation of M(Sal)^Fc, monitored by low temperature UV-vis spectroscopy, supports the formation of a mixed valent Fe^IIFe^III species followed by a bis(ferrocenium) species upon sequential addition of one and two equivalents of chemical oxidant. The addition of a third equivalent of oxidant to Ni(Sal)^Fc yielded intense near-IR transitions that are indicative of the formation of a fully delocalized Sal-ligand radical (Sal˙), while the same addition to Cu(Sal)^Fc yielded a species that is currently under further spectroscopic investigation. These results suggest the oxidation of the ferrocene moieties of M(Sal)^Fc does not affect the electronic structure of the M(Sal) core, and these are thus in the secondary coordination sphere of the overall complex.

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.

Photoinduced Processes in Rhenium(I) Terpyridine Complexes Bearing Remote Amine Groups: New Insights from Transient Absorption Spectroscopy

Photophysical properties of two Re(I) complexes [ReCl(CO)3(R-C6H4-terpy-κ2N)] with remote amine groups, N-methyl-piperazinyl (1) and (2-cyanoethyl)methylamine (2), were investigated. The complexes show strong absorption in the visible region corresponding to metal-to-ligand charge transfer (1MLCT) and intraligand-charge-transfer (1ILCT) transitions. The energy levels of 3MLCT and 3ILCT excited-states, and thus photoluminescence properties of 1 and 2, were found to be strongly affected by the solvent polarity. Compared to the parent chromophore [ReCl(CO)3(C6H5-terpy-κ2N)] (3), both designed complexes show significantly prolonged (by 1–2 orders of magnitude) phosphorescence lifetimes in acetonitrile and dimethylformamide, contrary to their lifetimes in less polar chloroform and tetrahydrofuran, which are comparable to those for 3. The femtosecond transient absorption (fsTA) measurements confirmed the interconversion between the 3MLCT and 3ILCT excited-states in polar solvents. In contrast, the emissive state of 1 and 2 in less polar environments is of predominant 3MLCT nature.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)₂bpy vs mes₂bpy ligands) and Lewis basic ((Ph)₂bpy vs (MeOPh)₂bpy vs (MeSPh)₂bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO₂ reduction mechanisms with [Mn^I(R₂bpy)(CO)₃(CH₃CN)]⁺ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)₂bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO₂ binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes₂bpy-based precatalysts. Cyclic voltammetry conducted in CO₂-saturated electrolyte for the (Ph)₂bpy-based precatalyst [2-CH₃CN]⁺ confirms that CO₂ binding occurs at the two-electron-reduced activated catalyst [2]^- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO₂ binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]⁰ dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO₂ binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH₃CN]⁺ maintains a lower overpotential than [2-CH₃CN]⁺ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOF_max is observed at its reduction-first pathway with an almost identical overpotential as [2-CH₃CN]⁺. Notably, excessive steric bulk in the mes₂bpy-based precatalyst [3-CH₃CN]⁺ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH₃CN]⁺ and [2-CH₃CN]⁺. In fact, [3-CH₃CN]⁺ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO₂ with 2.8 M (5%) H₂O establishes [2-CH₃CN]⁺ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]⁰ dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH₃CN]⁺ possesses an increased steric presence relative to [2-CH₃CN]⁺, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

A Mesoionic Carbene-Pyridine Bidentate Ligand That Improves Stability in Electrocatalytic CO2 Reduction by a Molecular Manganese Catalyst

Tricarbonyl Group 7 complexes have a longstanding history as efficacious CO₂ electroreduction catalysts. Typically, these complexes feature an auxiliary 2,2'-bipyridine ligand that assists in redox steps by delocalizing the electron density into the ligand orbitals. While this feature lends to an accessible redox potential for CO₂ electroreduction, it also presents challenges for electrocatalysis with Mn because the electron density is removed from metal-ligand bonding orbitals. The results presented here thus introduce a mesoionic carbene (MIC) as a potent ligand platform to promote Mn-based electrocatalysis. The strong σ donation of the N,C-bidentate MIC is shown to help centralize the electron density on the Mn center while also maintaining relevant redox potentials for CO₂ electroreduction. Mechanistic investigation supports catalytic turnover at two operative potentials separated by 400 mV. In the low operating potential regime at -1.54 V, Mn(0) species catalyze CO₂ to CO and CO₃^2-, which has a maximum rate of 7 ± 5 s^-1 and is stable for up to 30.7 h. At higher operating potential at -1.94 V, "Mn(-1)" catalyzes CO₂ to CO and H₂O with faster turnovers of 200 ± 100 s^-1, with the trade-off being less stability at 6.7 h. The relative stabilities of Mn complexes bearing MIC and 4,4'-di-tert-butyl-2,2'-bipyridine were compared by evaluation under the same electrolysis conditions and therefore elucidated that the MIC promotes longevity for CO evolution throughout a 5 h period.

Electrostatic Secondary-Sphere Interactions That Facilitate Rapid and Selective Electrocatalytic CO2 Reduction in a Fe-Porphyrin-Based Metal-Organic Framework

Metal–organic frameworks (MOFs) are promising platforms for heterogeneous tethering of molecular CO₂ reduction electrocatalysts. Yet, to further understand electrocatalytic MOF systems, one also needs to consider their capability to fine‐tune the immediate chemical environment of the active site, and thus affect its overall catalytic operation. Here, we show that electrostatic secondary‐sphere functionalities enable substantial improvement of CO₂‐to‐CO conversion activity and selectivity. In situ Raman analysis reveal that immobilization of pendent positively‐charged groups adjacent to MOF‐residing Fe‐porphyrin catalysts, stabilize weakly‐bound CO intermediates, allowing their rapid release as catalytic products. Also, by varying the electrolyte's ionic strength, systematic regulation of electrostatic field magnitude was achieved, resulting in essentially 100 % CO selectivity. Thus, this concept provides a sensitive molecular‐handle that adjust heterogeneous electrocatalysis on demand.

Bimetallic Cobalt–Copper Nanoparticle-Decorated Hollow Carbon Nanofibers for Efficient CO2 Electroreduction

Bimetallic materials are one of the most promising catalysts for the electrochemical reduction of CO₂, but there are still many challenges to be overcome on the route to industrialization. Herein, a series of carbon nanofiber-supported bimetallic cobalt–copper catalysts (Co_xCu_y/CFs) are designed and constructed through the electrospinning technique and a subsequent pyrolysis procedure. Small-sized Co–Cu nanoparticles are homogenously distributed on the porous carbon nanofibers, which can significantly improve the utilization rate of metal sites and greatly reduce the loading amount of metals. Moreover, different product distributions and catalytic performance can be obtained in CO₂ reduction via adjusting the metal proportion of Co_xCu_y/CFs. Especially, Co₃Cu/CFs can bring forth a 97% total faradaic efficiency (FE) of CO (68%) and HCOOH (29%) at –0.8 V_RHE cathode potential in 0.5 M KHCO₃ electrolyte. Furthermore, the hierarchical pores can firmly confine the small Co–Cu nanoparticles and keep them from easy agglomeration during electrolysis, eventually leading to 60 h of stability for Co₃Cu/CFs in CO₂ electroreduction. This study might provide a facile and economic method to fabricate efficient bimetallic catalysts for CO₂ electroreduction and other electrocatalysis applications.

Construction of C–N bonds from small-molecule precursors through heterogeneous electrocatalysis

Energy-intensive thermochemical processes within chemical manufacturing are a major contributor to global CO₂ emissions. With the increasing push for sustainability, the scientific community is striving to develop renewable energy-powered electrochemical technologies in lieu of CO₂-emitting fossil-fuel-driven methods. However, to fully electrify chemical manufacturing, it is imperative to expand the scope of electrosynthetic technologies, particularly through the innovation of reactions involving nitrogen-based reactants. This Review focuses on a rapidly emerging area, namely the formation of C-N bonds through heterogeneous electrocatalysis. The C-N bond motif is found in many fertilizers (such as urea) as well as commodity and fine chemicals (with functional groups such as amines and amides). The ability to generate C-N bonds from reactants such as CO₂, NO₃^- or N₂ would provide sustainable alternatives to the thermochemical routes used at present. We start by examining thermochemical, enzymatic and molecular catalytic systems for C-N bond formation, identifying how concepts from these can be translated to heterogeneous electrocatalysis. Next, we discuss successful heterogeneous electrocatalytic systems and highlight promising research directions. Finally, we discuss the remaining questions and knowledge gaps and thus set the trajectory for future advances in heterogeneous electrocatalytic formation of C-N bonds.

Deciphering Distinct Overpotential-Dependent Pathways for Electrochemical CO2 Reduction Catalyzed by an Iron-Terpyridine Complex

[Fe(tpyPY2Me)]²⁺ ([Fe]²⁺) is a homogeneous electrocatalyst for converting CO₂ into CO featuring low overpotentials of <100 mV, near-unity selectivity, and high activity with turnover frequencies faster than 100 000 s^-1. To identify the origins of its exceptional performance and inform future catalyst design, we report a combined computational and experimental study that establishes two distinct mechanistic pathways for electrochemical CO₂ reduction catalyzed by [Fe]²⁺ as a function of applied overpotential. Electrochemical data shows the formation of two catalytic regimes at low (η_TOF/2 of 160 mV) and high (η_TOF/2 of 590 mV) overpotential plateaus. We propose that at low overpotentials [Fe]²⁺ undergoes a two-electron reduction, two-proton-transfer mechanism (electrochemical-electrochemical-chemical-chemical, EECC), where turnover occurs through the dicationic iron complex, [Fe]²⁺. Computational analysis supports the importance of the singlet ground-state electronic structure for CO₂ binding and that the rate-limiting step is the second protonation in this low-overpotential regime. When more negative potentials are applied, an additional electron-transfer event occurs through either a stepwise or proton-coupled electron-transfer (PCET) pathway, enabling catalytic turnover from the monocationic iron complex ([Fe]⁺) via an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism. Comparison of experimental kinetic data obtained from variable controlled potential electrolysis (CPE) experiments with direct product detection with calculated rates obtained from the energetic span model supports the PCET pathway as the most likely mechanism. Moreover, we build upon this mechanistic understanding to propose the design of an improved ligand framework that is predicted to stabilize the key transition states identified in our study and explore their electronic structures using an energy decomposition analysis. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.

Dissection of Light-Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO₂ reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO₂ substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h^-1 , respectively. While the Fe⁰ redox state has been widely accepted as the catalytically active species, we show here that the Fe^I species is already involved in the CO₂ activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO₂ upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO₂ .

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.

A guide to secondary coordination sphere editing

This tutorial review showcases recent (2015-2021) work describing ligand construction as it relates to the design of secondary coordination spheres (SCSs). Metalloenzymes, for example, utilize SCSs to stabilize reactive substrates, shuttle small molecules, and alter redox properties, promoting functional activity. In the realm of biomimetic chemistry, specific incorporation of SCS residues (e.g., Brønsted or Lewis acid/bases, crown ethers, redox groups etc.) has been shown to be equally critical to function. This contribution illustrates how fundamental advances in organic and inorganic chemistry have been used for the construction of such SCSs. These imaginative contributions have driven exciting findings in many transformations relevant to clean fuel generation, including small molecule (e.g., H⁺, N₂, CO₂, NO_x, O₂) reduction. In most cases, these reactions occur cooperatively, where both metal and ligand are requisite for substrate activation.

Considering the Influence of Polymer-Catalyst Interactions on the Chemical Microenvironment of Electrocatalysts for the CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is an attractive method for capturing intermittent renewable energy sources in chemical bonds, and converting waste CO₂ into value-added products with a goal of carbon neutrality. Our group has focused on developing polymer-encapsulated molecular catalysts, specifically cobalt phthalocyanine (CoPc), as active and selective electrocatalysts for the CO₂RR. When CoPc is adsorbed onto a carbon electrode and encapsulated in poly(4-vinylpyridine) (P4VP), its activity and reaction selectivity over the competitive hydrogen evolution reaction (HER) are enhanced by three synergistic effects: a primary axial coordination effect, a secondary reaction intermediate stabilization effect, and an outer-coordination proton transport effect. We have studied multiple aspects of this system using electrochemical, spectroscopic, and computational tools. Specifically, we have used X-ray absorption spectroscopy measurements to confirm that the pyridyl residues from the polymer are axially coordinated to the CoPc metal center, and we have shown that increasing the σ-donor ability of nitrogen-containing axial ligands results in increased activity for the CO₂RR. Using proton inventory studies, we showed that proton delivery in the CoPc-P4VP system is controlled via a proton relay through the polymer matrix. Additionally, we studied the effect of catalyst, polymer, and graphite powder loading on CO₂RR activity and determined best practices for incorporating carbon supports into catalyst-polymer composite films.In this Account, we describe these studies in detail, organizing our discussion by three types of microenvironmental interactions that affect the catalyst performance: ligand effects of the primary and secondary sphere, substrate transport of protons and CO₂, and charge transport from the electrode surface to the catalyst sites. Our work demonstrates that careful electroanalytical study and interpretation can be valuable in developing a robust and comprehensive understanding of catalyst performance. In addition to our work with polymer encapsulated CoPc, we provide examples of similar surface-adsorbed molecular and solid-state systems that benefit from interactions between active catalytic sites and a polymer system. We also compare the activity results from our systems to other results in the CoPc literature, and other examples of molecular CO₂RR catalysts on modified electrode surfaces. Finally, we speculate how the insights gained from studying CoPc could guide the field in designing other polymer-electrocatalyst systems. As CO₂RR technologies become commercially viable and expand into the space of flow cells and gas-diffusion electrodes, we propose that overall device efficiency may benefit from understanding and promoting synergistic polymer-encapsulation effects in the microenvironment of these catalyst systems.

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.

Boosting CO2-to-CO evolution using a bimetallic diketopyrrolopyrrole tethered rhenium bipyridine catalyst

The use of homogeneous electro- and photo-catalysis involving molecular catalysts offers valuable insight into reaction mechanisms as it relates to the structure–function of these tunable systems.

A Rhenium Single-Atom Catalyst for the Electrocatalytic Oxygen Reduction Reaction

Single atom catalysts (SACs) have received a great deal of attention due to their extremely high active site utilization and superior activities. The exploration of metal SACs has been carried out by screening the elemental periodic table from first-row to second-row, and even third-row transition metals. However, Re SACs have not been reported, even if Re metal sites also play essential roles in catalyzing many important reactions. The construction of Re SACs may maximize Re catalytic sites and provide new Re active sites for higher activity. Herein, we used 1,10-phenanthroline to complex Re cations on carbon black, followed by heat treatment to obtain Re SAC. The Re SAC exhibited an oxygen reduction reaction (ORR) half-wave potential of 0.72 V versus reversible hydrogen electrode (RHE) in 0.1 M KOH, superior to Re nanoparticles catalyst (0.67 V vs. RHE). Re SAC exhibited better stability at 0.5 V vs. RHE than Pt/C, showing potential as a new electrocatalyst for ORR.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives

Urgent solutions are needed to efficiently convert the greenhouse gas CO₂ into higher-value products. In this work, fac-Mn(bpy)(CO)₃Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO₂ conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et₃N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)₃Br and [Mn(bpy)(CO)₃]^- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et₃N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et₃N is replaced with Et₂NH. The origin of this difference lies in the ability of Et₃N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO₂, while with Et₂NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO₂ in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO₂-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO₂ to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

Effects of Protonation State on Electrocatalytic CO2 Reduction by a Cobalt Aminopyridine Macrocyclic Complex

A critical component in the reduction of CO₂ to CO and H₂O is the delivery of 2 equiv of protons and electrons to the CO₂ molecule. The timing and sequencing of these proton and electron transfer steps are essential factors in directing the activity and selectivity for catalytic CO₂ reduction. In previous studies, we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO₂ to CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant (k_obs) and the number of pendant amine hydrogen bond donors minus one, suggesting the presence of a deprotonated active catalytic state. Herein, we investigate the feasibility of these proposed deprotonated complexes toward CO₂ reduction. Two deprotonated derivatives, Co(L^4-) and Co(L^2-), of the tetraamino macrocycle Co(L) were independently synthesized and structurally characterized revealing extensive delocalization of the negative charge upon deprotonation. ¹H nuclear magnetic resonance spectroscopy and ultraviolet-visible titration studies confirm that under catalytic conditions, the active form of the catalyst gradually becomes deprotonated, supporting thus the n_donor - 1 relationship with k_obs. Electrochemical studies of Co(L^4-) reveal that this deprotonated analogue is competent for electrocatalysis upon addition of an exogenous weak acid source, such as 2,2,2-trifluoroethanol, resulting in faradaic efficiencies for CO₂-to-CO conversion identical to those observed with the fully protonated derivative (>98%).

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.

Substrate Specific Metal-Ligand Cooperative Binding: Considerations for Weak Intramolecular Lewis Acid/Base Pairs

Metal-ligand cooperative binding modes were interrogated in a series of zinc bis(thiophenoxide) complexes. A weak B-S binding interaction is observed in solution between the weakly Lewis basic thiophenoxide ligands and an appended trialkylborane. The energy of this binding event is dependent upon the strength of the Lewis acid and its proximity to the zinc thiophenoxide.

Pendent Relay Enhances H2O2 Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co-N2O2 Complexes

Generally, cobalt-N₂O₂ complexes show selectivity for hydrogen peroxide during electrochemical dioxygen (O₂) reduction. We recently reported a Co(III)-N₂O₂ complex with a 2,2'-bipyridine-based ligand backbone which showed alternative selectivity: H₂O was observed as the primary reduction product from O₂ (71 ± 5%) with decamethylferrocene as a chemical reductant and acetic acid as a proton donor in methanol solution. We hypothesized that the key selectivity difference in this case arises in part from increased favorability of protonation at the distal O position of the key intermediate Co(III)-hydroperoxide species. To interrogate this hypothesis, we have prepared a new Co(III) compound that contains pendent -OMe groups poised to direct protonation toward the proximal O atom of this hydroperoxo intermediate. Mechanistic studies in acetonitrile (MeCN) solution reveal two regimes are possible in the catalytic response, dependent on added acid strength and the presence of the pendent proton donor relay. In the presence of stronger acids, the activity of the complex containing pendent relays becomes O₂ dependent, implying a shift to Co(III)-superoxide protonation as the rate-determining step. Interestingly, the inclusion of the relay results in primarily H₂O₂ production in MeCN, despite minimal difference between the standard reduction potentials of the three complexes tested. EPR spectroscopic studies indicate the formation of Co(III)-superoxide species in the presence of exogenous base, with greater O₂ reactivity observed in the presence of the pendent -OMe groups.

Assembly of a Metal-Organic Framework (MOF) Membrane on a Solid Electrocatalyst: Introducing Molecular-Level Control Over Heterogeneous CO2 Reduction

Electrochemically active Metal‐Organic Frameworks (MOFs) have been progressively recognized for their use in solar fuel production schemes. Typically, they are utilized as platforms for heterogeneous tethering of exceptionally large concentration of molecular electrocatalysts onto electrodes. Yet so far, the potential influence of their extraordinary chemical modularity on electrocatalysis has been overlooked. Herein, we demonstrate that, when assembled on a solid Ag CO₂ reduction electrocatalyst, a non‐catalytic UiO‐66 MOF acts as a porous membrane that systematically tunes the active site's immediate chemical environment, leading to a drastic enhancement of electrocatalytic activity and selectivity. Electrochemical analysis shows that the MOF membrane improves catalytic performance through physical and electrostatic regulation of reactants delivery towards the catalytic sites. The MOF also stabilizes catalytic intermediates via modulation of active site's secondary coordination sphere. This concept can be expanded to a wide range of proton‐coupled electrochemical reactions, providing new means for precise, molecular‐level manipulation of heterogeneous solar fuels systems.