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.
Delivering protons with electrons
Many chemical reactions involve concurrent transfer of a proton and an electron. In electrochemical synthesis, this mechanism could prove useful in lowering the ...energy necessary for cathodic electron transfer alone, but it is hindered by competing direct coupling of the protons and electrons to make hydrogen instead. Chalkley
et al.
now report a molecular mediator consisting of a dimethylaniline base tethered to a cobaltocenium electron acceptor. This construct can deliver both a proton and an electron to a substrate from an acid and a cathode while skirting the hydrogen pathway.
Science
, this issue p.
850
A cobaltocene complex tethered to dimethylaniline transfers electrons from a cathode to a substrate concurrently with protons.
Electrocatalytic approaches to the activation of unsaturated substrates via reductive concerted proton-electron transfer (CPET) must overcome competing, often kinetically dominant hydrogen evolution. We introduce the design of a molecular mediator for electrochemically triggered reductive CPET through the synthetic integration of a Brønsted acid and a redox mediator. Cathodic reduction at the cobaltocenium redox mediator substantially weakens the homolytic nitrogen-hydrogen bond strength of a Brønsted acidic anilinium tethered to one of the cyclopentadienyl rings. The electrochemically generated molecular mediator is demonstrated to transform a model substrate, acetophenone, to its corresponding neutral α-radical via a rate-determining CPET.
Biological N2 fixation to NH3 may proceed at one or more Fe sites in the active-site cofactors of nitrogenases. Modeling individual e–/H+ transfer steps of iron-ligated N2 in well-defined synthetic ...systems is hence of much interest but remains a significant challenge. While iron complexes have been recently discovered that catalyze the formation of NH3 from N2, mechanistic details remain uncertain. Herein, we report the synthesis and isolation of a diamagnetic, 5-coordinate FeNNH2 + species supported by a tris(phosphino)silyl ligand via the direct protonation of a terminally bound Fe-N2 – complex. The FeNNH2 + complex is redox-active, and low-temperature spectroscopic data and DFT calculations evidence an accumulation of significant radical character on the hydrazido ligand upon one-electron reduction to S = 1/2 FeNNH2. At warmer temperatures, FeNNH2 rapidly converts to an iron hydrazine complex, Fe-NH2NH2 +, via the additional transfer of proton and electron equivalents in solution. Fe-NH2NH2 + can liberate NH3, and the sequence of reactions described here hence demonstrates that an iron site can shuttle from a distal intermediate (FeNNH2 +) to an alternating intermediate (Fe-NH2NH2 +) en route to NH3 liberation from N2. It is interesting to consider the possibility that similar hybrid distal/alternating crossover mechanisms for N2 reduction may be operative in biological N2 fixation.
The two-coordinate (CAAC)2Fe complex CAAC = cyclic (alkyl)(amino)carbene binds dinitrogen at low temperature (T<-80 °C). The resulting putative three-coordinate N2 complex, (CAAC)2Fe(N2), was trapped ...by one-electron reduction to its corresponding anion (CAAC)2FeN2(-) at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β-nitrogen atom. The redox-linked complexes (CAAC)2Fe(I)BAr(F)4, (CAAC)2Fe(0), and (CAAC)2Fe(-I)N2(-) were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC8 and HBAr(F)4⋅2 Et2O at -95 °C up to (3.4±1.0) equivalents of ammonia per Fe center. The N2 reduction activity is highly temperature dependent, with significant N2 reduction to NH3 only occurring below -78 °C. This reactivity profile tracks with the low temperatures needed for N2 binding and an otherwise unavailable electron-transfer step to generate reactive (CAAC)2FeN2(-) .
The reduction of nitrogen (N2) to ammonia (NH3) is a requisite transformation for life. Although it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this ...transformation, how they do so remains poorly understood. A central element of debate has been the exact site or sites of N2 coordination and reduction. In synthetic inorganic chemistry, an early emphasis was placed on molybdenum because it was thought to be an essential element of nitrogenases and because it had been established that well-defined molybdenum model complexes could mediate the stoichiometric conversion of N2 to NH3 (ref. 9). This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature molybdenum centres. However, it is now thought that iron is the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data have implicated iron instead of molybdenum as the site of N2 binding in the FeMo-cofactor. Here we describe a tris(phosphine)borane-supported iron complex that catalyses the reduction of N2 to NH3 under mild conditions, and in which more than 40 per cent of the proton and reducing equivalents are delivered to N2. Our results indicate that a single iron site may be capable of stabilizing the various NxHy intermediates generated during catalytic NH3 formation. Geometric tunability at iron imparted by a flexible iron-boron interaction in our model system seems to be important for efficient catalysis. We propose that the interstitial carbon atom recently assigned in the nitrogenase cofactor may have a similar role, perhaps by enabling a single iron site to mediate the enzymatic catalysis through a flexible iron-carbon interaction.
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 nucleophiles
, there has been limited progress in corresponding reactions with nitrogen nucleophiles
. 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 sp
-hybridized substituent on the carbon undergoing substitution)
to include unactivated electrophiles.
New approaches toward the generation of late first-row metal catalysts that efficiently facilitate two-electron reductive transformations (e.g., hydrogenation) more typical of noble-metal catalysts ...is an important goal. Herein we describe the synthesis of a structurally unusual S = 1 bimetallic Co complex, ( Cy PBP)CoH 2 (1), supported by bis(phosphino)boryl and bis(phosphino)hydridoborane ligands. This complex reacts reversibly with a second equivalent of H2 (1 atm) and serves as an olefin hydrogenation catalyst under mild conditions (room temperature, 1 atm H2). A bimetallic Co species is invoked in the rate-determining step of the catalysis according to kinetic studies. A structurally related NiINiI dimer, ( Ph PBP)Ni 2 (3), has also been prepared. Like Co catalyst 1, Ni complex 3 displays reversible reactivity toward H2, affording the bimetallic complex ( Ph PBHP)NiH 2 (4). This reversible behavior is unprecedented for NiI species and is attributed to the presence of a boryl–Ni bond. Lastly, a series of monomeric ( tBu PBP)NiX complexes (X = Cl (5), OTf (6), H (7), OC(H)O (8)) have been prepared. The complex ( tBu PBP)NiH (7) shows enhanced catalytic olefin hydrogenation activity when directly compared with its isoelectronic/isostructural analogues where the boryl unit is substituted by a phenyl or amine donor, a phenomenon that we posit is related to the strong trans influence exerted by the boryl ligand.
Although the alkylation of an amine by an alkyl halide serves as a “textbook example” of a nucleophilic substitution reaction, the selective mono-alkylation of aliphatic amines by unactivated, ...hindered halides persists as a largely unsolved challenge in organic synthesis. We report herein that primary aliphatic amines can be cleanly mono-alkylated by unactivated secondary alkyl iodides in the presence of visible light and a copper catalyst. The method operates under mild conditions (–10 °C), displays good functional-group compatibility, and employs commercially available catalyst components. A trapping experiment with TEMPO is consistent with C–N bond formation via an alkyl radical in an out-of-cage process.
The Curtius rearrangement is a classic, powerful method for converting carboxylic acids into protected amines, but its widespread use is impeded by safety issues (the need to handle azides). We have ...developed an alternative to the Curtius rearrangement that employs a copper catalyst in combination with blue-LED irradiation to achieve the decarboxylative coupling of aliphatic carboxylic acid derivatives (specifically, readily available N-hydroxyphthalimide esters) to afford protected amines under mild conditions. This C–N bond-forming process is compatible with a wide array of functional groups, including an alcohol, aldehyde, epoxide, indole, nitroalkane, and sulfide. Control reactions and mechanistic studies are consistent with the hypothesis that copper species are engaged in both the photochemistry and the key bond-forming step, which occurs through out-of-cage coupling of an alkyl radical.
Bridging iron hydrides are proposed to form at the active site of MoFe‐nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N2 via reductive ...elimination of H2. This possibility inspires the investigation of well‐defined molecular iron hydrides as precursors for catalytic N2‐to‐NH3 conversion. Herein, we describe the synthesis and characterization of new P2P′PhFe(N2)(H)x systems that are active for catalytic N2‐to‐NH3 conversion. Most interestingly, we show that the yields of ammonia can be significantly increased if the catalysis is performed in the presence of mercury lamp irradiation. Evidence is provided to suggest that photo‐elimination of H2 is one means by which the enhanced activity may arise.
Light it up: Light‐enhanced N2‐to‐NH3 conversion catalysis is reported. New triphos‐supported Fe(N2)Hx catalysts provide higher ammonia yields for 1 atm N2, and as much as 180 % improvement upon irradiation by a mercury lamp.
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