A Carborane-Derived Proton-Coupled Electron Transfer Reagent.

Reagents capable of concerted proton-electron transfer (CPET) reactions can access reaction pathways with lower reaction barriers compared to stepwise pathways involving electron transfer (ET) and proton transfer (PT). To realize reductive multielectron/proton transformations involving CPET, one approach that has shown recent promise involves coupling a cobaltocene ET site with a protonated arylamine Brønsted acid PT site. This strategy colocalizes the electron/proton in a matter compatible with a CPET step and net reductive electrocatalysis.

Photodriven Sm(III)-to-Sm(II) Reduction for Catalytic Applications.

The selectivity of SmI as a one electron-reductant motivates the development of methods for reductive Sm-catalysis. Photochemical methods for SmI regeneration are desired for catalytic transformations. In particular, returning Sm-alkoxides to Sm is a crucial step for Sm-turnover in many potential applications.

Electrochemical CO Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients.

Inspired by recent advances in electrochemical CO reduction (COR) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of COR 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 COR. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for COR, both in the presence and absence of an organic film layer, at variable [M].

Reductive samarium (electro)catalysis enabled by Sm-alkoxide protonolysis.

Samarium diiodide (SmI) is a privileged, single-electron reductant deployed in diverse synthetic settings. However, generalizable methods for catalytic turnover remain elusive because of the well-known challenge associated with cleaving strong Sm-O bonds. Prior efforts have focused on the use of highly reactive oxophiles to enable catalyst turnover.

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.

Electrode Surface Heating with Organic Films Improves CO Reduction Kinetics on Copper.

Management of the electrode surface temperature is an understudied aspect of (photo)electrode reactor design for complex reactions, such as CO reduction. In this work, we study the impact of local electrode heating on electrochemical reduction of CO reduction. Using the ferri/ferrocyanide open circuit voltage as a reporter of the effective reaction temperature, we reveal how the interplay of surface heating and convective cooling presents an opportunity for cooptimizing mass transport and thermal assistance of electrochemical reactions, where we focus on reduction of CO to carbon-coupled (C) products.

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

Nitrogenase enzymes catalyze nitrogen reduction (NR) to ammonia and also the reduction of non-native substrates, including the 7H/6e reduction of cyanide to CH and NH. CN and N are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN and N reduction. Here, we describe the catalytic reduction of CN to NH and CH by a highly selective (P)Fe(CN) catalyst (P represents a tris(phosphine)silyl ligand).

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

Photoelectrochemical CO reduction (COR) is an appealing solution for converting carbon dioxide into higher-value products. However, COR 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%.

Mechanism of a Luminescent Dicopper System That Facilitates Electrophotochemical Coupling of Benzyl Chlorides via a Strongly Reducing Excited State.

Photochemical radical generation has become a modern staple in chemical synthesis and methodology. Herein, we detail the photochemistry of a highly reducing, highly luminescent dicopper system [Cu] (* ≈ -2.7 V vs SCE; ≈ 10 s) within the context of a model reaction: single-electron reduction of benzyl chlorides.

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

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N reduction reaction (NRR) in particular, parameters that dictate selectivity for either the 6H/6e product ammonia (NH) or the 4H/4e product hydrazine (NH) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (), with which NH is typically the major product of NR, to instead favor NH as the sole observed fixed-N product (>99:1).

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 (NRR). Well-defined molecular catalyst systems have much to offer in this context.

Organic Additive-derived Films on Cu Electrodes Promote Electrochemical CO Reduction to C Products Under Strongly Acidic Conditions.

Electrochemical CO reduction (CO R) at low pH is desired for high CO 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 CO R at low pH. Herein we report an alternative approach to selective CO R (>70 % Faradaic efficiency for C products, FE ) at low pH (pH 2; H PO /KH PO ) and low potassium concentration ([K ]=0.

Electrocatalytic Nitrogen Reduction on a Molybdenum Complex Bearing a PNP Pincer Ligand.

Electrocatalytic nitrogen reduction (NR) mediated by well-defined molecular catalysts is poorly developed by comparison with other reductive electrocatalytic transformations. Herein, we explore the viability of electrocatalytic NR mediated by a molecular Mo-PNP complex. A careful choice of acid, electrode material, and electrolyte mitigates electrode-mediated HER under direct electrolysis and affords up to 11.

Light Alters the NH vs N H Product Profile in Iron-catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH ) Intermediate.

Whereas synthetically catalyzed nitrogen reduction (N R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N H ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P ) Fe catalyst, P Fe , significantly alters its product profile to increase N H versus NH ; P Fe is otherwise known to be highly selective for NH . We posit a key terminal hydrazido intermediate, P Fe=NNH , as selectivity-determining.

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 (SmI) results in substantial O-H bond weakening, affording potent proton-coupled electron transfer (PCET) reagents. However, poorly defined speciation of SmI 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.

Catalytic transfer hydrogenation of N to NH via a photoredox catalysis strategy.

Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven transfer hydrogenations toward deep (>2 e) reductions of small molecules are attractive compared to using harsh chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (NRR), where a synthetic photocatalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH) and related organic structures can behave as 2 e/2 H photoreductants, we show here that, when partnered with a suitable catalyst (Mo) under blue light irradiation, HEH facilitates delivery of successive H equivalents for the 6 e/6 H catalytic reduction of N to NH; this catalysis is enhanced by addition of a photoredox catalyst (Ir).

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, hydride transfer from a Ni-H when interfaced with a metallocene-derived proton-coupled electron transfer (PCET) mediator.

Tandem electrocatalytic N fixation via proton-coupled electron transfer.

New electrochemical ammonia (NH) 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 electricity. Apropos of these goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for the nitrogen reduction reaction (NRR). These systems generally suffer from poor stability and NH selectivity; the hydrogen evolution reaction (HER) outcompetes NRR.

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 Fe , Fe , and Fe . This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, P Fe(N) (P =tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the P Fe-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a A ground state electronic structure of this C -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.

Synthesis and Functionalization Reactivity of Fe-Thiocarbonyl and Thiocarbyne Complexes.

The remarkable catalytic transformation of CO to liquid hydrocarbons by Fe and Co catalysts in the industrial Fischer-Tropsch process motivates interest in developing well-defined systems to model aspects of this chemistry. One of the most interesting potential intermediates in this chemistry is a terminally-bound, first row metal carbide, yet a molecular model of this species remains elusive. With this in mind, we targeted the synthesis of highly-activated Fe-thiocarbonyl complexes, as prospective precursors to S-functionalization, C-S bond cleavage, and carbide generation.

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 (CPET) using a Brønsted base-appended cobaltocene mediator ([CpCoCp][OTf]) with keto-olefin substrates that undergo cyclization subsequent to ketyl radical generation via CPET. 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 -lactone and alkene products.