Tandem electrocatalytic N2 fixation via proton-coupled electron transfer.

New electrochemical ammonia (NH3) synthesis technologies are of interest as a complementary route to the Haber-Bosch process for distributed fertilizer generation, and towards exploiting ammonia as a zero-carbon fuel produced via renewably sourced electricity1. Apropos of these goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for the nitrogen reduction reaction (N2RR)2. These systems generally suffer from poor stability and NH3 selectivity; the hydrogen evolution reaction (HER) outcompetes N2RR3. Molecular catalyst systems can be exquisitely tuned and offer an alternative strategy4, but progress has been thwarted by the same selectivity issue; HER dominates. Here we describe a tandem catalysis strategy that offers a solution to this puzzle. A molecular complex that can mediate an N2 reduction cycle is partnered with a co-catalyst that interfaces the electrode and an acid to mediate proton-coupled electron transfer steps, facilitating N-H bond formation at a favourable applied potential (-1.2 V versus Fc+/0) and overall thermodynamic efficiency. Certain intermediates of the N2RR cycle would be otherwise unreactive via uncoupled electron transfer or proton transfer steps. Structurally diverse complexes of several metals (W, Mo, Os, Fe) also mediate N2RR electrocatalysis at the same potential in the presence of the mediator, pointing to the generality of this tandem approach.

Photoinduced copper-catalysed asymmetric amidation via ligand cooperativity.

The substitution of an alkyl electrophile by a nucleophile is a foundational reaction in organic chemistry that enables the efficient and convergent synthesis of organic molecules. Although there has been substantial recent progress in exploiting transition-metal catalysis to expand the scope of nucleophilic substitution reactions to include carbon nucleophiles1-4, there has been limited progress in corresponding reactions with nitrogen nucleophiles5-8. For many substitution reactions, the bond construction itself is not the only challenge, as there is a need to control stereochemistry at the same time. Here we describe a method for the enantioconvergent substitution of unactivated racemic alkyl electrophiles by a ubiquitous nitrogen-containing functional group, an amide. Our method uses a photoinduced catalyst system based on copper, an Earth-abundant metal. This process for asymmetric N-alkylation relies on three distinct ligands-a bisphosphine, a phenoxide and a chiral diamine. The ligands assemble in situ to form two distinct catalysts that act cooperatively: a copper/bisphosphine/phenoxide complex that serves as a photocatalyst, and a chiral copper/diamine complex that catalyses enantioselective C-N bond formation. Our study thus expands enantioselective N-substitution by alkyl electrophiles beyond activated electrophiles (those bearing at least one sp- or sp2-hybridized substituent on the carbon undergoing substitution)8-13 to include unactivated electrophiles.

Molecular tuning of CO2-to-ethylene conversion.

The electrocatalytic reduction of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources1. However, the highly selective generation of economically desirable products such as ethylene from the carbon dioxide reduction reaction (CO2RR) remains a challenge2. Tuning the stabilities of intermediates to favour a desired reaction pathway can improve selectivity3-5, and this has recently been explored for the reaction on copper by controlling morphology6, grain boundaries7, facets8, oxidation state9 and dopants10. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 milliamperes per square centimetre in the best catalyst reported so far9), resulting in a low energy efficiency. Here we present a molecular tuning strategy-the functionalization of the surface of electrocatalysts with organic molecules-that stabilizes intermediates for more selective CO2RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived by electro-dimerization of arylpyridiniums11, adsorbed on copper. We find that the adhered molecules improve the stabilization of an 'atop-bound' CO intermediate (that is, an intermediate bound to a single copper atom), thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO2RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 milliamperes per square centimetre in a liquid-electrolyte flow cell in a neutral medium. We report stable ethylene electrosynthesis for 190 hours in a system based on a membrane-electrode assembly that provides a full-cell energy efficiency of 20 per cent. We anticipate that this may be generalized to enable molecular strategies to complement heterogeneous catalysts by stabilizing intermediates through local molecular tuning.

Molecular Additives Improve the Selectivity of CO2 Photoelectrochemical Reduction over Gold Nanoparticles on Gallium Nitride.

Photoelectrochemical CO2 reduction (CO2R) is an appealing solution for converting carbon dioxide into higher-value products. However, CO2R in aqueous electrolytes suffers from poor selectivity due to the competitive hydrogen evolution reaction that is dominant on semiconductor surfaces in aqueous electrolytes. We demonstrate that functionalizing gold/p-type gallium nitride devices with a film derived from diphenyliodonium triflate suppresses hydrogen generation from 90% to 18%. As a result, we observe increases in the Faradaic efficiency and partial current density for carbon monoxide of 50% and 3-fold, respectively. Furthermore, we demonstrate through optical absorption measurements that the molecular film employed herein, regardless of thickness, does not affect the photocathode's light absorption. Altogether, this study provides a rigorous platform for elucidating the catalytic structure-property relationships to enable engineering of active, stable, and selective materials for photoelectrochemical CO2R.

Catalytic Reduction of Cyanide to Ammonia and Methane at a Mononuclear Fe Site.

Nitrogenase enzymes catalyze nitrogen reduction (N2R) to ammonia and also the reduction of non-native substrates, including the 7H+/6e- reduction of cyanide to CH4 and NH3. CN- and N2 are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN- and N2 reduction. Here, we describe the catalytic reduction of CN- to NH3 and CH4 by a highly selective (P3Si)Fe(CN) catalyst (P3Si represents a tris(phosphine)silyl ligand). Catalysis is driven in the presence of excess acid ([Ph2NH2]OTf) and reductant ((C6H6)2Cr), with turnover as high as 73 demonstrated. This catalyst system is also modestly competent for N2R and structurally related to other tris(phosphine)Fe-based N2R catalysts. The choice of catalyst and reductant is important to observe high yields. Mechanistic studies elucidate several intermediates of CN- reduction, including iron isocyanides (P3SiFeCNH+/0) and terminal iron aminocarbynes (P3SiFeCNH2+/0). Aminocarbynes are isoelectronic to iron hydrazidos (Fe═N-NH2+/0), which have been invoked as selectivity-determining intermediates of N2R (NH3 versus N2H4 products). For the present CN- reduction catalysis, reduction of aminocarbyne P3SiFeCNH2+ is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P3SiFeCNMe2), and associated computational studies, formation of a Fischer carbene (P3SiFeC(H)(NH2)+) intermediate that is on path for either CH4 and NH3 (6 e-) or CH3NH2 (4 e-) products is proposed. From this carbene intermediate, pathways to the observed CH4 and NH3 products (distinct from CH3NH2 formation) are considered to compare and contrast the (likely) mechanism/s of CN- and N2 reduction.

Intermolecular Proton-Coupled Electron Transfer Reactivity from a Persistent Charge-Transfer State for Reductive Photoelectrocatalysis.

Interest in applying proton-coupled electron transfer (PCET) reagents in reductive electro- and photocatalysis requires strategies that mitigate the competing hydrogen evolution reaction. Photoexcitation of a PCET donor to a charge-separated state (CSS) can produce a powerful H-atom donor capable of being electrochemically recycled at a comparatively anodic potential corresponding to its ground state. However, the challenge is designing a mediator with a sufficiently long-lived excited state for bimolecular reactivity. Here, we describe a powerful ferrocene-derived photoelectrochemical PCET mediator exhibiting an unusually long-lived CSS (τ ∼ 0.9 µs). In addition to detailed photophysical studies, proof-of-concept stoichiometric and catalytic proton-coupled reductive transformations are presented, which illustrate the promise of this approach.

Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential and pKa.

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N2 reduction reaction (N2RR) in particular, parameters that dictate selectivity for either the 6H+/6e- product ammonia (NH3) or the 4H+/4e- product hydrazine (N2H4) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (Fe), with which NH3 is typically the major product of N2R, to instead favor N2H4 as the sole observed fixed-N product (>99:1). This dramatic shift is achieved by replacing moderate reductants and strong acids with a very strongly reducing but weakly acidic SmII-(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (SmII-PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N2H4 per Fe and 67% fixed-N yield per H+). However, by generating N2H4 as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH3-selective conditions with Fe. Mechanistic data support assignment of iron hydrazido(2-) species FeNNH2 as selectivity-determining: we infer that protonation of FeNNH2 at Nß, favored by strong acids, releases NH3, whereas one-electron reduction to FeNNH2-, favored by strong reductants such as SmII-PH, produces N2H4 via reactivity initiated at Nα. Spectroscopic data also implicate a role for SmIII-binding to anionic FeN2- (via an Fe-N2- -SmIII species) with respect to catalytic efficacy.

Advancing electrocatalytic nitrogen fixation: insights from molecular systems.

Nitrogen fixation has a rich history within the inorganic chemistry community. In recent years attention has (re)focused on developing electrocatalytic systems capable of mediating the nitrogen reduction reaction (N2RR). Well-defined molecular catalyst systems have much to offer in this context. This personal perspective summarizes recent progress from our laboratory at Caltech, pulling together lessons learned from a number of studies we have conducted, placing them within the broader context of thermodynamic efficiency and selectivity for the N2RR. In particular, proton-coupled electron transfer (PCET) provides an attractive strategy to achieve enhanced efficiency for the multi-electron/proton reduction of N2 to produce NH3 (or NH4+), and electrocatalytic PCET (ePCET) via an ePCET mediator affords a promising means of mitigating HER such that the N2RR can be achieved in a catalytic fashion.

Light Alters the NH3 vs N2 H4 Product Profile in Iron-catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH2 ) Intermediate.

Whereas synthetically catalyzed nitrogen reduction (N2 R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N2 H4 ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P3 B ) Fe catalyst, P3 B Fe+ , significantly alters its product profile to increase N2 H4 versus NH3 ; P3 B Fe+ is otherwise known to be highly selective for NH3 . We posit a key terminal hydrazido intermediate, P3 B Fe=NNH2 , as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH3 , a low-lying triplet excited state leads to reactivity at Nα and formation of N2 H4 . Associated electrochemical and spectroscopic studies establish that N2 H4 lies along a unique product pathway; NH3 is not produced from N2 H4 . Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.

Organic Additive-derived Films on Cu Electrodes Promote Electrochemical CO2 Reduction to C2+ Products Under Strongly Acidic Conditions.

Electrochemical CO2 reduction (CO2 R) at low pH is desired for high CO2 utilization; the competing hydrogen evolution reaction (HER) remains a challenge. High alkali cation concentration at a high operating current density has recently been used to promote electrochemical CO2 R at low pH. Herein we report an alternative approach to selective CO2 R (>70 % Faradaic efficiency for C2+ products, FEC2+ ) at low pH (pH 2; H3 PO4 /KH2 PO4 ) and low potassium concentration ([K+ ]=0.1 M) using organic film-modified polycrystalline copper (Modified-Cu). Such an electrode effectively mitigates HER due to attenuated proton transport. Modified-Cu still achieves high FEC2+ (45 % with Cu foil /55 % with Cu GDE) under 1.0 M H3 PO4 (pH≈1) at low [K+ ] (0.1 M), even at low operating current, conditions where HER can otherwise dominate.

Sm(II)-Mediated Proton-Coupled Electron Transfer: Quantifying Very Weak N-H and O-H Homolytic Bond Strengths and Factors Controlling Them.

Coordination of alcohols to the single-electron reductant samarium diiodide (SmI2) results in substantial O-H bond weakening, affording potent proton-coupled electron transfer (PCET) reagents. However, poorly defined speciation of SmI2 in tetrahydrofuran (THF)/alcohol mixtures limits reliable thermodynamic analyses of such systems. Rigorous determination of bond dissociation free energy (BDFE) values in such Sm systems, important to evaluating their reactivity profiles, motivates studies of model Sm systems where contributing factors can be teased apart. Here, a bulky and strongly chelating macrocyclic ligand ((tBu2ArOH)2Me2cyclam) maintains solubility, eliminates dimerization pathways, and facilitates clean electrochemical behavior in a well-defined functional model for the PCET reactivity of SmII with coordinating proton sources. Direct measurement of thermodynamic parameters enables reliable experimental estimation of the BDFEs in 2-pyrrolidone and MeOH complexes of ((tBu2ArO)2Me2cyclam)SmII, thereby revealing exceptionally weak N-H and O-H BDFEs of 27.2 and <24.1 kcal mol-1, respectively. Expanded thermochemical cycles reveal that this bond weakening stems from the very strongly reducing SmII center and the formation of strong SmIII-alkoxide (and -pyrrolidonate) interactions in the PCET products. We provide a detailed analysis comparing these BDFE values with those that have been put forward for SmI2 in THF in the presence of related proton donors. We suggest that BDFE values for the latter systems may in fact be appreciably higher than the system described herein. Finally, protonation and electrochemical reduction steps necessary for the regeneration of the PCET donors from SmIII-alkoxides are demonstrated, pointing to future strategies aimed at achieving (electro)catalytic turnover using SmII-based PCET reagents.

Photoinduced, Copper-Catalyzed Enantioconvergent Alkylations of Anilines by Racemic Tertiary Electrophiles: Synthesis and Mechanism.

Transition-metal catalysis of substitution reactions of alkyl electrophiles by nitrogen nucleophiles is beginning to emerge as a powerful strategy for synthesizing higher-order amines, as well as controlling their stereochemistry. Herein, we report that a readily accessible chiral copper catalyst (commercially available components) can achieve the photoinduced, enantioconvergent coupling of a variety of racemic tertiary alkyl electrophiles with aniline nucleophiles to generate a new C-N bond with good ee at the fully substituted stereocenter of the product; whereas this photoinduced, copper-catalyzed coupling proceeds at -78 °C, in the absence of light and catalyst, virtually no C-N bond formation is observed even upon heating to 80 °C. The mechanism of this new catalytic enantioconvergent substitution process has been interrogated with the aid of a wide array of tools, including the independent synthesis of proposed intermediates and reactivity studies, spectroscopic investigations featuring photophysical and EPR data, and DFT calculations. These studies led to the identification of three copper-based intermediates in the proposed catalytic cycle, including a chiral three-coordinate formally copper(II)-anilido (DFT analysis points to its formulation as a copper(I)-anilidyl radical) complex that serves as a persistent radical that couples with a tertiary organic radical to generate the desired C-N bond with good enantioselectivity.

Use of a PCET Mediator Enables a Ni-HER Electrocatalyst to Act as a Hydride Delivery Agent.

The generation of metal hydride intermediates during reductive electrocatalysis in the presence of acid most commonly leads to the hydrogen evolution reaction (HER). Redirecting the reactivity profile of such hydride intermediates toward the reduction of unsaturated substrates is an exciting opportunity in catalysis but presents a challenge in terms of catalyst selectivity. In this study, we demonstrate that a prototypical phosphine-supported Ni-HER catalyst can be repurposed toward the electrocatalytic reduction of a model substrate, methyl phenylpropiolate, via hydride transfer from a NiII-H when interfaced with a metallocene-derived proton-coupled electron transfer (PCET) mediator. Key to success is generation of the NiII-H at a potential pinned to that of the PCET mediator which is appreciably anodic of the onset of HER. Electrochemical, spectroscopic, and theoretical data point to a working mechanism where a PCET step from the metallocene-derived mediator to NiII generates NiIII-H and is rate-determining; the latter NiIII-H is then readily reduced to a NiII-H, which is competent for substrate reduction. Additional studies show that this tandem PCET-mediated hydride generation can afford high stereoselectivity (e.g., >20:1 Z/E using a phosphine-cobalt precatalyst with ethyl 2-heptynoate) and can also be used for the reduction of α,ß-unsaturated ketones.

Investigation of the C-N Bond-Forming Step in a Photoinduced, Copper-Catalyzed Enantioconvergent N-Alkylation: Characterization and Application of a Stabilized Organic Radical as a Mechanistic Probe.

Whereas photoinduced, copper-catalyzed couplings of nitrogen nucleophiles with alkyl electrophiles have recently been shown to provide an attractive approach to achieving a variety of enantioselective C-N bond constructions, mechanistic studies of these transformations have lagged the advances in reaction development. Herein we provide mechanistic insight into a previously reported photoinduced, copper-catalyzed enantioconvergent C-N coupling of a carbazole nucleophile with a racemic tertiary α-haloamide electrophile. Building on the isolation of a copper(II) model complex whose EPR parameters serve as a guide, we independently synthesize two key intermediates in the proposed catalytic cycle, a copper(II) metalloradical (L*CuII(carb')2) (L* = a monodentate chiral phosphine ligand; carb' = a carbazolide ligand), as well as a tertiary α-amide organic radical (R·); the generation and characterization of R· was guided by DFT calculations, which suggested that it would be stable to homocoupling. Continuous-wave (CW) and pulse EPR studies, along with corresponding DFT calculations, are among the techniques used to characterize these reactive radicals. We establish that these two radicals do indeed combine to furnish the C-N coupling product in good yield and with significant enantiomeric excess (77% yield, 55% ee), thereby supporting the chemical competence of these proposed intermediates. DFT calculations are consistent with R· initially binding to copper(II) via a dative interaction from the closed-shell carbonyl oxygen atom of the radical, which positions the α-carbon for direct reaction with the copper(II)-bound carbazole N atom, to generate the C-N bond with enantioselectivity, without the formation of an alkylcopper(III) intermediate.

Electrocatalytic Ketyl-Olefin Cyclization at a Favorable Applied Bias Enabled by a Concerted Proton-Electron Transfer Mediator.

Recent studies showcase reductive concerted proton-electron transfer (CPET) as a powerful strategy for transferring a net hydrogen atom to organic substrates; however, direct application of CPET in the context of C-C bond formation beyond homocoupling is underexplored. We report herein the expansion of electrocatalytic CPET (eCPET) using a Brønsted base-appended cobaltocene mediator ([CpCoCpNMe2][OTf]) with keto-olefin substrates that undergo cyclization subsequent to ketyl radical generation via eCPET. Using acetophenone-derived substrates with tethered acrylates as radical acceptors, in the presence of tosylic acid, we demonstrate that ketyl-olefin cyclization is achieved by characterization of cis-lactone and alkene products. Mechanistic analysis of this 2 H+/2 e- process reveals a mixed order in substrate and acid and a Hammett plot with a modest negative slope, highlighting the contribution of sequential CPET and ET/PT steps involved in the overall rate of the reaction and providing support for initial O-H bond formation. The ability to access ketyl radicals at comparatively mild reduction potentials via controlled potential electrolysis enables functional group tolerance across a range of substrates.

Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes.

Nitrogen fixation, the six-electron/six-proton reduction of N2, to give NH3, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N2 into its bioavailable form, NH3. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N2 functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.

Characterization of a Proposed Terminal Iron(III) Nitride Intermediate of Nitrogen Fixation Stabilized by a Trisphosphine-Borane Ligand.

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of Fe-mediated nitrogen fixation, and well-defined synthetic iron nitrides have been characterized in high oxidation states, including FeIV , FeV , and FeVI . This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, P3 B Fe(N) (P3 B =tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the P3 B Fe-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a 2 A ground state electronic structure of this C3 -symmetric nitride species, placing the unpaired spin in a sigma orbital along the B-Fe-N vector; this electronic structure is distinct for an iron nitride. The unusual d5 -configuration is stabilized by significant delocalization (≈50 %) of the unpaired electron onto the axial boron and nitrogen ligands, with a majority of the spin residing on boron.

Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron Complex Featuring Cis Coordination Sites.

The goal of using ammonia as a solar fuel motivates the development of selective ammonia oxidation (AO) catalysts for fuel cell applications. Herein, we describe Fe-mediated AO electrocatalysis with [(bpyPy2Me)Fe(MeCN)2]2+, exhibiting the highest turnover number (TON) reported to date for a molecular system. To improve on our recent report of a related iron AO electrocatalyst, [(TPA)Fe(MeCN)2]2+ (TON of 16), the present [(bpyPy2Me)Fe(MeCN)2]2+ system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains cis-labile sites and a dominant low-spin population at the Fe(II) state. The latter is posited to mitigate demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the catalytic conditions. Additionally, the [(bpyPy2Me)Fe(MeCN)2]2+ system exhibits a substantially faster AO rate (ca. 50×) at significantly lower (∼250 mV) applied bias compared to [(TPA)Fe(MeCN)2]2+. Electrochemical data are consistent with an initial E1 net H-atom abstraction step that furnishes the cis amide/ammine complex [(bpyPy2Me)Fe(NH2)(NH3)]2+, followed by the onset of catalysis at E2. Theoretical calculations suggest the possibility of N-N bond formation via multiple thermodynamically plausible pathways, including both reductive elimination and ammonia nucleophilic attack. In sum, this study underscores that Fe, an earth-abundant metal, is a promising metal for further development in metal-mediated AO catalysis by molecular systems.

Electrocatalytic Reduction of C-C π-Bonds via a Cobaltocene-Derived Concerted Proton-Electron Transfer Mediator: Fumarate Hydrogenation as a Model Study.

Reductive concerted proton-electron transfer (CPET) is poorly developed for the reduction of C-C π-bonds, including for activated alkenes that can succumb to deleterious pathways (e.g., a competing hydrogen evolution reaction or oligomerization) in a standard electrochemical reduction. We demonstrate herein that selective hydrogenation of the C-C π-bond of fumarate esters can be achieved via electrocatalytic CPET (eCPET) using a CPET mediator comprising cobaltocene with a tethered Brønsted base. High selectivity for electrocatalytic hydrogenation is observed only when the mediator is present. Mechanistic analysis sheds light on two distinct kinetic regimes based on the substrate concentration: low fumarate concentrations operate via rate-limiting CPET followed by an electron-transfer/proton-transfer (ET/PT) step, whereas high concentrations operate via CPET followed by a rate-limiting ET/PT step.

Molecular enhancement of heterogeneous CO2 reduction.

The electrocatalytic carbon dioxide reduction reaction (CO2RR) addresses the need for storage of renewable energy in valuable carbon-based fuels and feedstocks, yet challenges remain in the improvement of electrosynthesis pathways for highly selective hydrocarbon production. To improve catalysis further, it is of increasing interest to lever synergies between heterogeneous and homogeneous approaches. Organic molecules or metal complexes adjacent to heterogeneous active sites provide additional binding interactions that may tune the stability of intermediates, improving catalytic performance by increasing Faradaic efficiency (product selectivity), as well as decreasing overpotential. We offer a forward-looking perspective on molecularly enhanced heterogeneous catalysis for CO2RR. We discuss four categories of molecularly enhanced strategies: molecular-additive-modified heterogeneous catalysts, immobilized organometallic complex catalysts, reticular catalysts and metal-free polymer catalysts. We introduce present-day challenges in molecular strategies and describe a vision for CO2RR electrocatalysis towards multi-carbon products. These strategies provide potential avenues to address the challenges of catalyst activity, selectivity and stability in the further development of CO2RR.