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The selectivity of SmI2as a one electron-reductant motivates the development of methods for reductive Sm-catalysis. Photochemical methods for SmI2regeneration are desired for catalytic transformations. In particular, returning SmIII-alkoxides to SmIIis a crucial step for Sm-turnover in many potential applications. To this end, photochemical conditions for reduction of both SmI3and a model SmIII-alkoxide to SmI2(THF)nare described here. The Hantzsch ester can serve either as a direct photoreductant or as the reductive quencher for an Ir-based photoredox catalyst. In contrast to previous SmIIIreduction methodologies, no Lewis acidic additives or byproducts are involved, facilitating selective ligand coordination to Sm. Accordingly, SmIIspecies can be generated photochemically from SmI3in the presence of protic, chiral, and/or Lewis basic additives. Both the photoreductant and photoredox methods for SmI2generation translate to intermolecular ketone-acrylate coupling as a proof-of-concept demonstration of a photodriven, Sm-catalyzed reductive cross-coupling reaction.

Inspired by recent advances in electrochemical CO2reduction (CO2R) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of CO2R at low pH. Using attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and fluorescent confocal laser scanning microscopy, we investigate the role that alkali cations (M+) play on electrochemical CO2R. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for CO2R, both in the presence and absence of an organic film layer, at variable [M+]. We show that in an acidic electrolyte, an appropriate current density can enable CO2R in the absence of metal cations. In situ local pH measurements suggest the local [H+] must be sufficiently depleted to promote H2O reduction as the competing reaction with CO2R. Incrementally incorporating [K+] leads to increases in the local pH that promotes CO2R but only at proton consumption rates sufficient to drive the pH up dramatically. Stark tuning measurements and analysis of surface water structure reveal no change in the electric field with [M+] and a desorption of interfacial water, indicating that improved CO2R performance is driven by suppression of H+mass transport and modification of the interfacial solvation structure. In situ pH measurements confirm increasing local pH, and therefore decreased local [CO2], with [M+], motivating alternate means of modulating proton transport. We show that an organic film formed via in situ electrodeposition of an organic additive provides a means to achieve selective CO2R (FECO2R∼ 65%) over hydrogen evolution reaction in the presence of strong acid (pH 1) and low cation concentrations (≤0.1 M) at both low and high current densities.

Ionomers have shown promise as organic coatings on Cu electrodes to increase the CO2reduction (CO2R) selectivity toward multi-carbon (C2+) products. However, the effects of systematic polymer structure modification on electrocatalytic performance have been seldom reported. Herein, we report on a series of polystyrene-based ionomers to probe the effect of local [K+] in the Cu electrode microenvironment on CO2R performance. Partial current density toward C2+products (|jC2+|) increases with [K+] in ionomers, up to 225 mA cm−2. Replacing K+with [Me4N]+lowers performance to the level of bare Cu, highlighting the crucial role of K+in improving C2+product selectivity. Molecular dynamics simulations show that CO2diffusivity increases with [K+], implicating CO2transport to the electrode as a potential mechanism for improved CO2R performance. Our results highlight the intersection of synthetic polymer chemistry and electrocatalysis as a promising strategy in electrode modification toward achieving high selectivity of value-added chemicals.

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 (t ~ 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.

Photoelectrochemical CO2reduction (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.

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 SmI2in 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 SmIIwith 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 SmIIcenter 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 SmI2in 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.

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 NH3selectivity; 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 N2reduction 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.

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.

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.

Boundary conditions for catalyst performance in the conversion of common precursors such as N2, O2, H2O, and CO2are governed by linear free energy and scaling relationships. Knowledge of these limits offers an impetus for designing strategies to alter reaction mechanisms to improve performance. Typically, experimental demonstrations of linear trends and deviations from them are composed of a small number of data points constrained by inherent experimental limitations. Herein, high-throughput experimentation on 14 bulk copper bimetallic alloys allowed for data-driven identification of a scaling relationship between the partial current densities of methane and C2+products. This strict dependence represents an intrinsic limit to the Faradaic efficiency for C–C coupling. We have furthermore demonstrated that coating the electrodes with a molecular film breaks the scaling relationship to promote C2+product formation.

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)8–13to include unactivated electrophiles.

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.

This study demonstrates the synergies and limits of multiple measurement types for the detection of smectite chemistry and oxidation state on planetary surfaces to infer past geochemical conditions. Smectite clay minerals are common products of water-rock interactions throughout the solar system, and their detection and characterization provides important clues about geochemical conditions and past environments if sufficient information about their composition can be discerned. Here, we synthesize and report on the spectroscopic properties of a suite of smectite samples that span the intermediate compositional range between Fe(II), Fe(III), Mg, and Al end-member species using bulk chemical analyses, X‑ray diffraction, Vis/IR reflectance spectroscopy, UV and green-laser Raman spectroscopy, and Mössbauer spectroscopy. Our data show that smectite composition and the oxidation state of octahedral Fe can be reliably identified in the near infrared on the basis of combination and fundamental metal-OH stretching modes between 2.1–2.9 μm, which vary systematically with chemistry. Smectites dominated by Mg or Fe(III) have spectrally distinct fundamental and combination stretches, whereas Al-rich and Fe(II)-rich smectites have similar fundamental minima near 2.76 μm, but have distinct combination M-OH features between 2.24 and 2.36 μm. We show that with expanded spectral libraries that include intermediate composition smectites and both Fe(III) and Fe(II) oxidation states, more refined characterization of smectites from MIR data is now possible, as the position of the 450 cm–1absorption shifts systematically with octahedral Fe content, although detailed analysis is best accomplished in concert with other characterization methods. Our data also provide the first Raman spectral libraries of smectite clays as a function of chemistry, and we demonstrate that Raman spectroscopy at multiple excitation wavelengths can qualitatively distinguish smectite clays of different structures and can enhance interpretation by other types of analyses. Our sample set demonstrates how X‑ray diffraction can distinguish between dioctahedral and trioctahedral smectites using either the (02,11) or (06,33) peaks, but auxiliary information about chemistry and oxidation state aids in specific identifications. Finally, the temperature-dependent isomer shift and quadrupole splitting in Mössbauer data are insensitive to changes in Fe content but reliability differentiates Fe within the smectite mineral structure.

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 E1net H-atom abstraction step that furnishes the cisamide/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.

CO2electroreduction provides a route to convert waste emissions into chemicals such as ethylene (C2H4). However, the direct transformation of CO2-to-C2H4suffers from CO2loss to carbonate, consuming up to 72% of energy input. A cascade approach—coupling a solid-oxide CO2-to-CO electrochemical cell (SOEC) with a CO-to-C2H4membrane electrode assembly (MEA)—would eliminate CO2loss to carbonate. However, this approach requires a CO-to-C2H4MEA with energy efficiency well beyond demonstrations to date. Focusing on the MEA, we find that an N-tolyl substituted tetrahydro-bipyridine film improves the stabilization of key reaction intermediates, while an SSC ionomer enhances CO transport to the Cu surface, enabling a C2H4faradaic efficiency of 65% at 150 mA cm−2for 110 h. Demonstrating a cascade SOEC-MEA approach, we achieve CO2-to-C2H4with a ~48% reduction in energy intensity compared with the direct route. We further reduce the energy intensity by coupling CO electroreduction (CORR) with glucose electrooxidation.

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P3Xligands (X = B, C, Si) have previously been shown to mediate catalytic N2-to-NH3conversion (N2RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P3B) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and N2RR activity, Fe–N2complexes supported by analogous group 13 tris(phosphino)alane (P3Al) and tris(phosphino)gallane (P3Ga) ligands are synthesized. The series of P3XFe–N2[0/1−]compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N2activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)2][P3XFe–N2] (X = Al, Ga) with excess acid/reductant in the form of HBArF4/KC8generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH3per Fe, respectively. Similarly, the use of [H2NPh2][OTf]/Cp*2Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH3. Preliminary reactivity studies confirming P3XFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus N2RR may be responsible for the attenuated yields of NH3observed for P3AlFe and P3GaFe relative to P3BFe.

Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. This work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*FeIII–H species, [FeIII(η5-Cp*)(dppe)H]+(Cp* = C5Me5–, dppe = 1,2-bis(diphenylphosphino)ethane), to a highly reactive, S= 1/2 ring-protonated endo-Cp*H–Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [FeIII(η5-Cp*)(dppe)(CO)H]+. Herein, pulse EPR spectroscopy (1,2H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [FeI(endo-η4-Cp*H)(dppe)(CO)]+. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe0(exo-η4-Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFEC–H≈ 29 kcal mol–1and 25 kcal mol–1, respectively) cf. BDFEFe–Hof 56 kcal mol–1for [FeIII(η5-Cp*)(dppe)H]+. These weak C–H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [FeII(η5-Cp*)(dppe)CO]+byproduct.

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8–10complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/−. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ligand throughout with strong Fe−NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe−B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]−complex, an example of a ls-{FeNO}10species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8system, with two additional electrons “stored” on site in an Fe−B single bond. The outlier in this series is the ls-{FeNO}9complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe−NO bond. These data are further contextualized by comparison with a related N2complex, [Fe(TPB)(N2)]−, which is a key intermediate in Fe(TPB)-catalyzed N2fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

Terminal NiIIIhydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S= 1/2, terminally bound thiolate–NiIII–H complex. This species and its terminal hydride ligand in particular have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corresponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine coupling to the terminal hydride ligand of the thiolate–NiIII–H species is comparable to that of the hydride ligand proposed for the Ni–C hydrogenase intermediate (NiIII–H–FeII). Upon warming, the featured thiolate–NiIII–H species undergoes bimolecular reductive elimination of H2. Associated kinetic studies are discussed and compared with a structurally related FeIII–H species that has also recently been reported to undergo bimolecular H–H coupling.

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.

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 state9and 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.

Electrocatalytic CO2reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO2reduction by copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C–C coupled products relative to CH4and H2remains an impediment. Herein we report a simple yet highly selective catalytic system for CO2reduction to C≥2hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70–80% for C2and C3products with a hydrocarbon ratio of C≥2/CH4significantly greater than 100 have been observed with several additives. 13C-labeling studies verify CO2to be the sole carbon source in the C≥2hydrocarbons produced. Upon electroreduction, the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C≥2species to H2(∼90%) while suppressing methane with certain N-heterocyclic additives.

Nitrogenase enzymes catalyze nitrogen reduction (N2R) to ammonia and also the reduction of non-native substrates, including the 7H+/6e–reduction of cyanide to CH4and NH3. CN–and N2are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN–and N2reduction. Here, we describe the catalytic reduction of CN–to NH3and CH4by a highly selective (P3Si)Fe(CN) catalyst (P3Sirepresents 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 (NH3versus N2H4products). 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 CH4and NH3(6 e–) or CH3NH2(4 e–) products is proposed. From this carbene intermediate, pathways to the observed CH4and NH3products (distinct from CH3NH2formation) are considered to compare and contrast the (likely) mechanism/s of CN–and N2reduction.

We report a unique class of dinitrogen complexes of iron featuring sulfur donors in the ancillary ligand. The ligands utilized are related to the recently studied tris(phosphino)silyl ligands (2-R2PC6H4)3Si (R = Ph, iPr) but have one or two phosphine arms replaced with thioether donors. Depending on the number of phosphine arms replaced, both mononuclear and dinuclear iron complexes with dinitrogen are accessible. These complexes contribute to a desirable class of model complexes that possess both dinitrogen and sulfur ligands in the immediate iron coordination sphere.

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. “Fischer–Tropsch” type coupling of non-native C1substrates to higher-order C≥2products is also known for nitrogenase, though potential metal–carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S= 1/2 FeI(η2-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP3)Fe≡CCH3and the paramagnetic cation S= 1/2 [(SiP3)Fe≡CCH3]+. Both alkylcarbyne compounds possess short Fe–C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH3protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH3unit. The electronic structures of (SiP3)Fe≡CCH3and [(SiP3)Fe≡CCH3]+are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP3)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.

Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N2-to-NH3conversion (N2RR) mediated by synthetic iron complexes and relatedly as intermediates of N2RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR3), and also of further downstream intermediates (e.g., Fe(NNH2)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N2–species, supported by a bulky trisphosphinoborane ligand (ArP3B), generates an S= 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The 1H-hyperfine for ArP3BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom (aiso= 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH2)]+in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH2)]+intermediate of N2RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N2-to-NH3conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N2RR in a synthetic system.

Metallocenes, including their permethylated variants, are extremely important in organometallic chemistry. In particular, many are synthetically useful either as oxidants (e.g., Cp2Fe+) or as reductants (e.g., Cp2Co, Cp*2Co, and Cp*2Cr). The latter have proven to be useful reagents in the reductive protonation of small-molecule substrates, including N2. As such, understanding the behavior of these metallocenes in the presence of acids is paramount. In the present study, we undertake the rigorous characterization of the protonation products of Cp*2Co using pulse electron paramagnetic resonance (EPR) techniques at low temperature. We provide unequivocal evidence for the formation of the ring-protonated isomers Cp*(exo/endo-η4-C5Me5H)Co+. Variable temperature Q-band (34 GHz) pulse EPR spectroscopy, in conjunction with density functional theory (DFT) predictions, are key to reliably assigning the Cp*(exo/endo-η4-C5Me5H)Co+species. We also demonstrate that exo-protonation selectivity can be favored by using a bulkier acid and suggest this species is thus likely a relevant intermediate during catalytic nitrogen fixation given the bulky anilinium acids employed. Of further interest, we provide physical data to experimentally assess the C–H bond dissociation free energy (BDFEC–H) for Cp*(exo-η4-C5Me5H)Co+. These experimental data support our prior DFT predictions of an exceptionally weak C–H bond (<29 kcal mol–1), making this system among the most reactive (with respect to C–H bond strength) to be thoroughly characterized. These data also point to the propensity of Cp*(exo-η4-C5Me5H)Co to mediate hydride (H–) transfer. Our findings are not limited to the present protonated metallocene system. Accordingly, we outline an approach to rationalizing the reactivity of arene-protonated metal species, using decamethylnickelocene as an additional example.

The intermediacy of metal–NNH2complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N2) fixation. In this context, we have shown that triphosphine-supported Fe(N2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH2) complexes over an array of charge and oxidation states. Upon exposure to further H+/e–equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N–H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe2) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the “NNMe2” moiety. Specifically, although two closed-shell configurations of the “NNR2” ligand have been commonly considered in the literature—isodiazene and hydrazido(2−)—we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR2]•–ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N2fixation activity.

The biomimetic diiron complex 4-(N2)2, featuring two terminally bound Fe–N2centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N2. One is the Janus intermediate E4(4H), which has accumulated 4[e–/H+], stored as two [Fe–H–Fe] bridging hydrides, and is activated to bind and reduce N2through reductive elimination (RE) of the hydride ligands as H2. The second is a possible RE intermediate. 1H and 14N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N2)2complex exhibits a fully delocalized, Robin–Day type-III mixed valency. The two bridging hydrides exhibit a fully rhombic dipolar tensor form, T≈ [−t, +t, 0]. The rhombic form is reproduced by a simple point-dipole model for dipolar interactions between a bridging hydride and its “anchor” Fe ions, confirming validity of this model and demonstrating that observation of a rhombic form is a convenient diagnostic signature for the identification of such core structures in biological centers such as nitrogenase. Furthermore, interpretation of the 1H measurements with the anchor model maps the gtensor onto the molecular frame, an important function of these equations for application to nitrogenase. Analysis of the hyperfine and quadrupole coupling to the bound 14N of N2provides a reference for nitrogen-bound nitrogenase intermediates and is of chemical significance, as it gives a quantitative estimate of the amount of charge transferred between Fe and coordinated N, a key element in N2activation for reduction.

We have recently reported on several Fe catalysts for N2-to-NH3conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC8) and acid ([H(OEt2)2][BArF4]). Here we show that our original catalyst system, P3BFe, achieves both significantly improved efficiency for NH3formation (up to 72% for e–delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH3per Fe site), when employing a significantly weaker combination of reductant (Cp*2Co) and acid ([Ph2NH2][OTf] or [PhNH3][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N2reduction 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 NH3is typically the major product of N2R, to instead favor N2H4as 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 N2H4per Fe and 67% fixed-N yield per H+). However, by generating N2H4as 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 FeNNH2as selectivity-determining: we infer that protonation of FeNNH2at Nβ, favored by strong acids, releases NH3, whereas one-electron reduction to FeNNH2–, favored by strong reductants such as SmII–PH, produces N2H4via reactivity initiated at Nα. Spectroscopic data also implicate a role for SmIII-binding to anionic FeN2–(via an Fe–N2- -SmIIIspecies) with respect to catalytic efficacy.

In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO2catalyzed by the macrocyclic complex [CoIIN4H]2+(N4H= 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [CoIIN4H]2+resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy)3]2+sensitizer yields an infrared band at 1670 cm–1attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN4H(MeCN)]+in CO2saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to CoIin the first reduction, which is consistent with experimental d–d spectra of square Co(I) macrocycle complexes. The energy of the CO2adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN4H(MeCN)]+, which implies that only a fraction of the latter forms an adduct with CO2. By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [CoI(N4H) –•], and show that [CoI(N4H) –•] binds partially reduced CO2fairly strongly. Experimentally accessing [CoI(N4H) –•] with an Ir(bpy)3sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C18O2also shows clear evidence for 18O-substituted byproducts from CO2reduction to CO.

Diiron diazenes: A monomeric Fe(η2‐N2H3) species has been prepared, and exposure to oxygen yields a diiron complex that features five‐coordinate iron centers and an activated bridging diazene ligand (see picture; C gray, N blue, Fe black, P red, O green). Structural, theoretical, and spectroscopic data for the redox pair [Fe2(μ‐N2H2)]2+/+are consistent with 4‐center, 4‐electron π‐delocalized bonding across the Fe‐NH‐NH‐Fe core that finds analogy in butadiene and the butadiene anion.

Adaptable metallaboratranes: A tris‐ (phosphino)borane ligand stabilizes both low‐valent FeN2complexes and a mid‐valent imido species with a FeNAr bond, thanks to its ability to shuttle between trigonal‐bipyramidal and pseudotetrahedral geometries by elongation of the apical iron–boron bond (see picture).

Radikalkomplexe: Wohldefininerte einkernige RuI‐ und OsI‐Komplexe (siehe Schema) haben metalloradikalischen Charakter, wie EPR‐spektroskopische Messungen und DFT‐Rechnungen belegen. Die RuI‐ und OsI‐Metalloradikale zeigen sowohl Einelektronen‐ als auch Zweielektronen‐Redoxreaktivität. Der letztere Prozess liefert ungewöhnliche Imidokomplexe mit beträchtlichem Radikalcharakter der {ArN}‐Einheit.

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, viahydride 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 NiIIgenerates 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/Eusing a phosphine-cobalt precatalyst with ethyl 2–heptynoate) and can also be used for the reduction of α,β-unsaturated ketones.

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.

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Rare examples of monometallic high-spin and low-spin L3Fe(H3BH) complexes have been characterized, where the two L3ligands are [TpPh2] and [PhBP3] ([TpPh2] = [HB(3,5-Ph2pz)3]−and [PhBP3] = [PhB(CH2PPh2)3]−). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(ii) centers. Despite the differences in spin states, short Fe–B distances are observed in both complexes and there is significant theoretical evidence to support a substantial bonding interaction between the iron and boron nuclei. In light of this interaction, we suggest that these complexes can be described as (L3)Fe(η4-H3BH) complexes.

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Divalent complexes of iron and cobalt with new, monoanionic tripodal amido-polyphosphine ligands have been thoroughly characterized, and XRD analysis reveals geometries that are distinct for this class of ligand.

In the presence of moderately strong acids in CH3CN, cobalt complexes with BF2-bridged diglyoxime ligands are active catalysts for the reduction of protons to H2at potentials as positive as −0.28 V vs.SCE.

A homoleptic phosphine adduct of thallium(i) supported by a tris(phosphino)borate ligand has been isolated and structurally characterized.

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