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

Abstract

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.

Figure 1.

X-ray structure of the FeMo-cofactor active site in the MoFe protein (PDB: 1M1N); blue = N, red = O, yellow = S, brown = Fe, gray = C, purple = Mo.

Figure 2.

Timeline (1930 to present) of selected advances in nitrogen fixation catalysis by synthetically well-defined complexes.^,,–

Figure 3.

Protonation of M(cis-N₂)₂(PMe₂Ph)₄ and M(N_xH_y); M = Mo or W.

Figure 4.

Scheme demonstrating the distal (or Chatt) cycle for nitrogen fixation. In the modern literature, this cycle is also sometimes referred to as the Schrock cycle. Ligands on molybdenum are omitted for clarity.

Figure 5.

An alternative to the Chatt pathway that can account for productive NH₃ formation.

Figure 6.

Protonation of the hydrazido(2−) complex, (Cp*)W(Me)₃(NNH₂), can initially occur at N_α or N_β, but ultimately gives [(Cp*)W(Me)₃(η²-NHNH₂)]⁺—a reversible process in the presence of NEt₃. It remains unclear whether protonation occurs at the η¹- or η²-form of the hydrazido(2−) species.

Figure 7.

N₂ splitting by a Mo tris(anilide) complex; an alternative pathway to the formation of ammonia from μ-N₂ complexes.

Figure 8.

(Top) Reactions of a Mo triamidoamine complex having C₆F₅ anilido groups provided access to complexes of potential relevance to N₂RR; (bottom) reaction of an analogous Mo complex with TMS anilido groups with FeCl₂ afforded a μ₃-Fe bridging via N₂ to three triamidoamine Mo centers.

Figure 9.

Synthesis of alkylated Mo diazenido and hydrazido complexes en route to a Mo nitride compound in a triamidoamine Mo complex with Ar = 3,5-Ph₂C₆H₃ or 3,5-(4-^tBuC₆H₄)₂C₆H₃ ligands.

Figure 10.

Synthesis of a series of monometallic molybdenum N_xH_y complexes on 3,5-(2,4,6-ⁱPr₃C₆H₂)2C₆H₃ (HIPT) ligated Mo; [Mo] = (HIPTN₃N)Mo.

Figure 11.

(Top) Protonation reactions of (HIPTN₃N)Mo(L) (L = CO or N₂). For the CO analogue, pulse EPR (ENDOR) data indicated protonation at an amido arm; for N₂, the site of protonation remains unknown; (middle) protonation/reduction reactions of the L = N₂ and N analogues gave rise to [Mo]-NNH and [Mo]-NH₃⁺; (bottom) reduction of [[Mo]-NH₃]⁺ under N₂ gives [Mo]-N₂ in ~10% yield; [Mo] = (HIPTN₃N)Mo.

Figure 12.

Catalytic N₂RR cycle depicting (HIPTN₃N)Mo intermediates and their efficacy as (pre)catalysts for N₂RR using [LutH]BAr^F₄ (48 equiv) and Cp*₂Cr (36 equiv); [Mo] = (HIPTN₃N)Mo.^, Characterized compounds are shown in purple and unobserved complexes in gray.

Figure 13.

(Top) [Mo]-NNH and [LutH]⁺ are in equilibrium with [Mo]-NNH₂⁺ and Lut (K_eq = 1.6); (bottom) protonation of [Mo]N gives a mixture of the starting material and [Mo]NH⁺, which upon reduction, decayed to give [Mo]N and [Mo]-NH₂; [Mo] = (HIPTN₃N)Mo.

Figure 14.

Use of an alkylating agent provided access to neutral [Mo]-NNEt₂ and [Mo]-NEt; these species serve as structural analogues of the highly reactive R = H variants; [Mo] = (HIPTN₃N)Mo.^,

Figure 15.

β-H elimination from the diazenido derivative, [Mo]–NNH, generated [Mo]–H; [Mo] = (HIPTN₃N)Mo.

Figure 16.

Generation of [Mo]–H₂ and a plausible decay pathway involving intramolecular deprotonation to a terminal (HIPTN₃NH)Mo(H) species; [Mo] = (HIPTN₃N)Mo.

Figure 17.

Reproduced with permission from ref . Copyright 2015 American Chemical Society. Red: Gibbs free enthalpy ΔG° scheme of the Schrock cycle calculated with the B3LYP functional and the def2-TZVP basis set including solvent correction. Gray: calculations by Studt et al.

Figure 18.

(Left) A depiction of a hydrogen bonded complex between (HIPTN₃N)Mo(N₂) and [LutH]⁺ identified in a “one-pot” calculation; (right) a depiction of a crystallographically characterized hydrogen bonded complex between (Ar₂N₃)Mo(N) (Ar = 2,6-diisopropylphenyl) and [LutH]⁺.

Figure 19.

A survey of second generation complexes tested for N₂RR by Schrock and co-workers. Reported yields were found using the optimized conditions for the original catalytic result: [LutH]⁺ (48 equiv), Cp*₂Cr (36 equiv), heptane, room temperature.^–

Figure 20.

A calix[6]azacryptand ligand designed by the Schrock group aimed at protecting the reactive Mo active site for use in N₂RR.

Figure 21.

Synthesis of candidate N_xH_y intermediates on (HIPTN₃N) W relevant to N₂ fixation. Note: this system produces only stoichiometric yields of NH₃ (1.3−1.5 equiv).

Figure 22.

Synthesis of (HIPTN₃N)Cr(N) and (HIPTN₃N)V(NH) and their reductive protonation to give 0.8 and 0.76 equiv of NH₃, respectively. These results demonstrate that catalytic N₂RR is not accessible with these species, but downstream functionalization reactions are productive.^,

Figure 23.

A diamido(pyridine) pincer ligand used by the Schrock group for Mo-catalyzed N₂RR, Ar = 2,6-diisopropylphenyl.

Figure 24.

Reaction of (Ar₂N₃)Mo(N)(O^tBu) with [H²NPh2]⁺ results in protonation of a Mo-N_anilido arm whose product is shown to the right and use of [LutH]⁺ results in hydrogen-bond formation with the nitride shown on the left. Following reduction, each of these complexes releases H2 to provide the starting material shown in the center.

Figure 25.

Nishibayashi’s first generation, [(PNP)Mo(N₂)₂]₂}(μ-N₂) N₂RR catalyst. In these reactions, Cp₂Co was added via syringe pump and the acid counterion had a marked effect on the performance. Protonation of [(PNP)Mo(N₂)₂]₂}(μ-N₂) using [HOEt₂]BF₄ provided a hydrazido(2−) Mo complex.

Figure 26.

Protonation studies of six-coordinate (PNP)Mo(N₂)₂(PMe₂R) complexes gave 1.38 (R = Me) and 0.85 (R = Ph) equiv of NH₃.

Figure 27.

Modification of ligand phosphine substituent gave different Mo₂(μ-N₂) isomers. For R = ^tBu, R′ = Ad or Ph, trans-/trans- is observed, while for R = ^tBu, R′ = ⁱPr or Cy, trans-/cis-is observed. The R = ^tBu, R′ = Ad substituted phosphine provides the highest yield of NH₃ (14 equiv) during N₂RR.

Figure 28.

On-cycle (PNP)Mo(N₂) complexes can be generated through H₂ evolution from their respective halide precursors.

Figure 29.

Arsine analogues of the PNP class of ligands have been prepared, though Mo complexes of these ligands are not active for catalytic N₂RR under the conditions shown.

Figure 30.

Ligand modifications have been targeted by changing the ligand 4-X group. The reaction scheme depicts the first three steps relevant to catalytic N₂RR as calculated by DFT (1) N_β protonation, (2) N₂ dissociation, and (3) triflate coordination.

Figure 31.

Mononuclear PNP-ligated molybdenum nitrides are active for catalytic N₂ fixation using [LutH]⁺ (48 equiv) and Cp₂Co (36 equiv).

Figure 32.

Complete calculated N₂RR cycle for Nishibayashi’s dinuclear Mo₂ complexes. [Mo] = (PNP)Mo(N₂)₃.

Figure 33.

A survey of Nishibayashi’s Mo complexes for catalytic N₂RR Fc = ferrocene, EtFc = ethylferrocene.^,,–

Figure 34.

Proposed pathway for reductive cleavage of N₂ to form a terminal nitride, (PNP)Mo(N)(I), of relevance to catalytic N₂RR. The observed reactivity profile is I (50.7 equiv of NH₃) > Br (40.5 equiv of NH₃) > Cl (24.4 equiv of NH₃).

Figure 35.

PCP-type pincer ligands featuring N-heterocyclic carbene donors are useful for N₂RR in the presence of [ColH]OTf (96 equiv) and reductant (72 equiv); Cp*₂Cr was found to be most effective for both classes of catalyst.

Figure 36.

Efforts toward using H₂O as a proton source in N₂RR. Water oxidation was performed using [Ru(bpy)₃]OTf₂ as a photooxidant, peroxydisulfate as a sacrificial reductant, and a Ru complex, [Ru(bda)-(isoq)₂] (bda = 2,2′-bipyridine-6,6′-dicarboxylate, isoq = isoquinoline), as the water oxidation catalyst. The generated acid was trapped using lutidine to give [LutH]⁺, which enabled N₂RR catalysis by (PNP)Mo.

Figure 37.

Remarkable efficiencies were achieved using a Sm/alcohol mixture for N₂RR. This reaction is proposed to occur via PCET. Use of H₂O (14400 equiv) and SmI₂ (14 400 equiv). gave ca. 4350 equiv of NH₃.

Figure 38.

Structure of a recently reported, proposed intermediate of VFe-nitrogenase, featuring removal of a bridging sulfide as SH⁻ and the identification of a μ₂-bridging light atom in its place. The light atom (X) has been hypothesized to be the N atom of an imido ligand. Further studies are needed to validate this assignment.

Figure 39.

(Left) Outer cycle depicts the distal or Chatt-cycle for N₂RR. Central intermediate (highlighted in blue) represents a “hybrid” mechanism for N₂RR, which may be relevant in both biological and synthetic^, nitrogenases; (right) the alternating mechanism for the reduction of N₂ to NH₃ or N₂H₄.^,

Figure 40.

Homogeneous Fe complexes reported to mediate catalytic N₂-to-NH₃ conversion; Dipp = 2,6-diisopropylphenyl.^,,,,,,– **(depe)₂Fe(N₂) (depe = diethylphosphinoethane) is instead selective for N₂H₄.

Figure 41.

Interconversion of different nitrogen fixation intermediates on the (P₃^B)Fe platform.^,,,

Figure 42.

Reaction scheme showing synthetic conversions of the catalytic resting state, (P₃^μ-B-H)Fe(H)(N₂), including an α-hydride elimination step that leads to release of a hydrazine surrogate following acidic workup.^,,

Figure 43.

Representative tetrahedral Fe(IV) nitride complexes.^,–

Figure 44.

Comparison of reactivity of (A) [(P₃^B)Fe(N₂)]⁻ and (B) [(^ArP₃^B)Fe(N₂)]⁻ with acid. The bottom figure was taken from ref and shows the reaction progress of [(^ArP3^B)Fe(N₂)]⁻ with excess HBAr^F₄ in 2-Me-THF, as monitored by CW X-band EPR. Simulations for discrete species are shown in red. Spectra show the progression from (A) pure [(^ArP₃^B)Fe(N₂)]⁻. (B) Mixture of[(^ArP₃^B)Fe(N₂)]⁻ and (^ArP₃^B)Fe(NNH), on addition of 1 equiv of HBAr^F₄ for 15 min at 138 K. (C) Same as in spectrum B, but instead with 2.3 equiv of HBAr^F₄ present, showing full conversion to (^ArP₃^B)Fe(NNH). An identical spectrum was obtained on mixing for 30 min in the presence of 1 equiv of HBAr^F₄. (D) Reaction mixture from trace C, after warming to 195 K for 30 s and then rapidly freeze quenching in liquid N₂. Trace shows a mixture of (^ArP₃^B)Fe(NNH) and [(^ArP₃^B)Fe(NNH₂)]⁺. (E) Reaction mixture from trace D after warming to 195 K for 90 s, showing complete conversion to [(^ArP₃^B)Fe(NNH₂)]⁺. Reactions with up to 5 equiv of HBAr^F₄ provided identical spectra. Acquisition parameters for all spectra: temperature = 77 K; MW frequency = 9.44 GHz; MW power = 6.44 mW; modulation amplitude = 0.1 mT; conversion time = 5.12 ms·E.^, EPR spectrum reproduced with permission from ref . Copyright 2017 American Chemical Society.

Figure 45.

A proposed N₂RR cycle for Peters’ (P₃^B)Fe platform. Characterized compounds are shown in green and unobserved complexes in gray. Solid arrows indicate a distal mechanism of nitrogen fixation, while dashed arrows indicate hybrid mechanisms of nitrogen fixation.^,–,, Note that steps are shown as discrete e⁻ and H⁺ transfer steps, though concerted PCET steps may also be operative.

Figure 46.

Comparison of performance for N₂RR by [(P₃^B)Fe]⁺ depending on the acid/reductant.^,, For discussion of overpotential see Section 8.3.

Figure 47.

Bond dissociation enthalpies of the thermodynamically preferred protonation isomer for three commonly used reductants in N₂RR (Cp₂Co, Cp*₂Cr, and Cp*₂Co). The site of protonation and the C–H bond dissociation enthalpy of the resulting product has been confirmed experimentally for Cp*₂Co.^,

Figure 48.

Thermochemistry of protonated Cp*₂Co species that have been hypothesized to be relevant to PCET and hydride transfer in N₂RR.

Figure 49.

Reductive formation of [(C^SiP₃Ph)Fe(N₂)]− and subsequent silylation chemistry.

Figure 50.

Trends in Fe–C bond distance for a redox series of (P₃^C)Fe(N₂) complexes. Related datafor [(P₃^B)Fe(N₂)]⁻ Figure 64.

Figure 51.

Reactivity of (P₃^Si)Fe hydrazido(2−) complexes.

Figure 52.

Overview of predicted bimolecular HER and N₂RR pathways for (P₃^E)Fe(NNH_y) species and estimated BDFE_N–H values. Reproduced with permission from ref . Copyright 2018 American Chemical Society.

Figure 53.

Reproduced with permission from ref . Copyright 2018 American Chemical Society. Gibbs free enthalpy, ΔG°, scheme of the (P₃^Si)Fe cycle calculated with the B3LYP functional and the def2-TZVP basis set.

Figure 54.

Catalytic N₂RR efficiencies for all of Nishibayashi’s (PNP)Fe systems. aThese complexes were only tested under higher loading conditions with KC₈ (200 equiv) and HBAr^F₄ (184 equiv).^,,

Figure 55.

Catalytic conditions for (CAAC)₂Fe. Equilibrium binding of N₂ at −80 °C and access to a (CAAC)₂Fe(NNTMS) via reductive silylation; Dipp = 2,6-diisopropylpheyl.

Figure 56.

Reactivity of (dmpe)₂Fe (dmpe = 1,2-bis(dimethylphosphino)ethane) complexes.^,–

Figure 57.

(Top) Acid/base chemistry that interconverts hydrazine, hydrazido(1−), and diazene ligands on (DMeOPrPE)₂Fe;^, (bottom) potential N₂RR intermediates of the form (dxpe)₂Fe(H)(L) species (x = m or e, L = H₂, N₂, NH₃, and N₂H₄) can be synthesized under aqueous conditions.^,,,

Figure 58.

Treatment of (dmpe)₂Fe(N₂) with electrophiles.

Figure 59.

Catalytic N₂ fixation by (depe)₂Fe(N₂) using Cp*₂Co (270 equiv) and [Ph₂NH₂]OTf (360 equiv).

Figure 60.

Observed reactivity of (depe)₂Fe(N₂) with electrophiles occurred at N_β.^,, The relationship between this reactivity and the catalytic reactionz remains unclear.

Figure 61.

Pathways for HER, N₂RR, and hypothesized origins of the light enhancement for (PP₂)Fe.

Figure 62.

N₂ silylation reactions have been demonstrated with (PP₂)Fe. The migration of SiMe₃ from N_β to Fe is particularly noteworthy.

Figure 63.

Nitrogen fixation using a cyclohexyl-substituted tris(phosphino) (PP₂)Fe complex.

Figure 64.

Electrochemical and spectroscopic comparison of [(P₃^B)Co(N₂)]⁻ and [(P₃^B)Fe(N₂)]⁻. Maximum N₂RR efficiencies are compared using KC₈/HBAr^F₄ and [Ph₂NH₂]OTf/Cp*₂Co.^,,

Figure 65.

Transformation of (PNP)Co(H) to (PNP)Co(N₂) via protonation, and their respective competency as precatalysts for N₂ fixation using KC₈ (40 equiv) and HBAr^F₄ (38 equiv) in Et₂O at −78 °C.

Figure 66.

Comparison of physical properties and catalytic N₂RR performance for known [(P₃^Si)M(N₂)]ⁿ complexes (M = Co, Fe, Ru, Os; n = 0 or −1).

Figure 67.

Comparison of the reactivity of [(P₃^Si)M(N₂)]⁻ (M = Fe or Os) complexes with excess acid at different temperatures.^,

Figure 68.

Synthesis of pyrrole-based (PNP)V precatalysts for catalytic N₂RR.

Figure 69.

Synthesis and characterization data of triamidoamine-Ti complexes relevant to catalytic nitrogen fixation.

Figure 70.

Electrosynthetic cycle for the formation of NH₃ from N₂ by a (dppe)₂W complex.

Figure 71.

First demonstration of molecular, electrocatalytic N₂RR enabled by the use of cocatalytic [Cp*₂Co]⁺. Conditions: glassy carbon, Et₂O, −35 °C, −2.1 V versus Fc^+/0, 0.1 M [Na]BAr^F₄, X equiv of [Cp*₂Co]BAr^F₄.

Figure 72.

Reproduced with permission from ref . Copyright 2018 American Chemical Society. (A) Cyclic voltammograms of 10 equiv of [Ph₂NH₂]OTf (gray trace), 1 equiv of [Cp*₂Co]BAr^F₄ (Cp*₂Co⁺) (yellow trace), 1 equiv of Cp*₂Co⁺ with 10 equiv of [Ph₂NH₂]OTf (green trace), and [(P₃^B)Fe]⁺ with 1 equiv of Cp*₂Co⁺ and 10 equiv of [Ph₂NH₂]OTf (red trace); (B) Cyclic voltammograms of 10 equiv of [Ph₂NH₂]OTf (gray trace), 1 equiv of [(P₃^B)Fe]⁺ (dark blue trace), 1 equiv of [(P₃^B)Fe]⁺ with 10 equiv of [Ph₂NH₂]OTf (light blue trace), and 1 equiv of [(P₃^B)Fe]⁺ with 1 equiv of [Cp*₂Co]⁺ and 10 equiv of [Ph₂NH₂]OTf (red trace). All voltammograms are collected in 0.1 M [Na]BAr^F₄ solution in Et₂O at −35 °C using a glassy carbon working electrode and externally referenced to the Fc^+/0 couple. Scan rate is 100 mV/s.

Figure 73.

Comparison of conditions and results for the electrosynthesis of NH₃ from N₂ by (Cp)₂Ti(Cl)₂.^–

Figure 74.

(Top) Preliminary, posited mechanism for the electrosynthesis of NH₃ by a (PDl)Al complex; (bottom) stoichiometric N₂ binding and protonation at a (CAAC)B species; Dipp = 2,6-diisopropylphenyl, Dur = 2,3,5,6-tetramethylphenyl.

Figure 75.

N₂ overpotentials calculated as a function of catalyst reagent cocktail using eqs 1 and 2. All of the pK_a values and reduction potentials are literature values for these reagents in acetonitrile.^– Consequently, the C_g and BDFE(H₂) in acetonitrile were also used. The exception is for Sm/H₂O for which aqueous values are used, and the values are approximated due to the inner-sphere nature of the Sm/H₂O interaction. The bottom portion of the figure provides a sample overpotential calculation.