Mechanochemical nitrogen fixation catalysed by molybdenum complexes

Mechanochemical reactions driven by ball milling enable solvent-free or low-solvent transformations with often higher yields and selectivity than conventional homogeneous reactions. While many solid–solid mechanochemical reactions are known, gas–solid reactions using molecular (transition metal) catalysts remain scarce despite the prevalence of gaseous substrates in industrial catalysis. Nitrogen fixation (conversion of N2 to NH3) is a prime target, typically realized industrially by the Haber–Bosch process (Fe catalysts, 150–250 atm, 400–500 °C), which is energy intensive and CO2-emitting. Recent mechanochemical studies using Fe or Ti metals produced NH3 from N2 and H2 under milder conditions but only in sub-stoichiometric amounts relative to the metal, i.e., not catalytically. Under homogeneous conditions, molybdenum complexes bearing pincer-type ligands can catalyse N2-to-NH3 conversion at 1 atm and ambient temperatures with samarium diiodide (SmI2) as reductant and H2O or alcohols as proton sources, achieving turnover numbers up to 60,000. However, the large volumes of organic solvent (e.g., THF) limit practicality. This study redesigns the homogeneous Mo-catalysed system for solvent-free mechanochemical conditions at the gas–solid interface, aiming to achieve catalytic NH3 formation using SmI2(THF)2 as reductant and solid proton sources (including insoluble biopolymers) and to elucidate the underlying gas–solid activation and solid-state proton-coupled electron transfer (PCET) steps.

• Numerous transition metal-catalysed mechanochemical reactions are established for solid–solid substrates, often outperforming solution methods in yield or selectivity.
• Gas–solid mechanochemical catalysis is far less explored; a few examples involve gaseous reactants (e.g., hydroformylation in ball mills), but direct use of gaseous substrates with molecular catalysts remains limited.
• Prior mechanochemical ammonia synthesis mediated by Fe or Ti metals with N2 and H2 under mild conditions did not achieve catalysis (NH3 amounts below stoichiometric relative to metal).
• Homogeneous catalytic N2 fixation with molecular catalysts is well developed: Mo and other complexes convert N2 to NH3 at 1 atm and ambient temperature using reductants/proton sources; the Mo PCP-type systems with SmI2 and H2O/alcohols have achieved up to 60,000 turnovers in THF.
• Practical drawbacks of homogeneous systems include large solvent volumes and energy-intensive separation of NH3 from solvent and water.
• These works motivate adapting molecular N2 reduction catalysis to solvent-free mechanochemical conditions to harness gas–solid reactivity and mitigate solvent-related costs and energy demands.

Reaction setup and conditions:

• Mechanochemical reactions performed in a Retsch MM400 shaker mill using a 5 mL stainless-steel milling jar and a single 10 mm stainless-steel stainless ball; frequency 30 Hz; no external heating.
• Atmosphere: 1 atm N2 unless otherwise noted; reagents handled in an Ar-filled glovebox; jars filled with N2 in a N2-filled glovebox and tightly sealed prior to milling.
• Typical reagent loadings (screening): SmI2(THF)2 180 equivalents relative to molybdenum catalyst (e.g., 0.36 mmol vs 2 µmol catalyst), proton source 180 equivalents (water or alcohols; for solids such as pentaerythritol, glucose, or cellulose), catalyst Mo(PCP)-type complexes (triiodide precursors 1a or 1b; additional catalysts 2–5).
• Standard milling time 1 h; time-course experiments up to 2 h.

Proton source scope and protocols:

• Liquid proton sources tested: H2O, MeOH, EtOH, ethylene glycol; solids tested: pentaerythritol (tetravalent), neopentyl glycol (divalent), neopentyl alcohol (monovalent), D-glucose, cellulose.
• Observations: liquids increased H2 evolution; solids improved NH3 selectivity and yield; higher alcohol functionality (multivalency) correlated with higher NH3.
• Pre-activation experiments: milling SmI2(THF)2 with cellulose under Ar for 2 h (prior to catalysis) reduced induction period and increased early NH3 formation.

Product analysis and quantification:

• Gas-phase NH3: initially not detected after single-step reactions (likely trapped by Sm(III)–NH3 Lewis acid-base interactions). For solvent-free gas NH3 generation, a two-step protocol was employed: (1) conduct standard mechanochemical reaction (e.g., with 0.1 µmol 1b, 0.36 mmol SmI2(THF)2, 0.36 mmol pentaerythritol under N2); (2) add solid KOH (2.7 mmol) to the same jar and mill for 1 h to liberate NH3; gas identified by FT-IR and quantified colorimetrically.
• Solid-phase NH3: reaction mixture transferred post-milling into THF or water, treated with aqueous KOH, and volatiles distilled and trapped in dilute H2SO4; NH3 quantified by the indophenol (phenol–hypochlorite) method (A634 nm). Hydrazine checked and not detected by the p-(dimethylamino)benzaldehyde method (A458 nm).
• H2 quantified by GC from gas sampled after opening jar in a sealed bag.

Spectroscopic and mechanistic studies:

• UV–vis diffuse reflectance: SmI2(THF)2 with cellulose or pentaerythritol after milling showed loss of ~700 nm band, indicating formation of Sm(II)–proton-source adducts akin to SmI2(H2O).
• Formation of Mo–nitride intermediate (2): milling 1a with 5 equiv SmI2(THF)2 under N2 for 10 min; post-dissolution NMR (1H, 31P{1H}) showed 2 in 34% NMR yield; ESI-TOF MS corroborated. Solid-state Mo K-edge XAS (XANES/EXAFS) consistent with significant conversion to 2 and loss of Mo–I scattering.
• Stoichiometric N–H formation: milling 2 with SmI2(THF)2 and pentaerythritol for 1 h gave quantitative NH3; 2 also catalysed NH3 formation similarly to 1a.
• PCET probe reactions: anthracene and trans-stilbene reduced under SmI2(THF)2/pentaerythritol milling to 9,10-dihydroanthracene (87% NMR yield) and 1,2-diphenylethane (37%), respectively; deuterated pentaerythritol gave KIE ~1.3, supporting PCET in the solid state.

Representative “Methods” quantities (typical run):

• SmI2(THF)2 198 mg (0.36 mmol), pentaerythritol 49.0 mg (0.36 mmol), Mo(PCP) catalyst 1.9 mg (0.002 mmol), 10 mm steel ball, 5 mL steel jar, 30 Hz, 1 h milling under 1 atm N2. Post-reaction workup for NH3 as above.

• First catalytic gas–solid mechanochemical nitrogen fixation using molecular molybdenum complexes under solvent-free, near-ambient conditions (1 atm N2, room temperature, ball milling).
• High turnovers: up to 864±30 equivalents of NH3 per Mo catalyst (Table 2, entry 5; catalyst 1b at 0.1 µmol; H2 byproduct 76.2±19.2 equiv/Mo, 4%). Additional results: 1a at 0.25 µmol gave 362 equiv/Mo NH3 (H2 45.1 equiv/Mo); 1a at 0.1 µmol gave 449±102 equiv/Mo (H2 160±17 equiv/Mo). Less active catalysts (3–5) gave much lower NH3; Mo(0) or MoO3 powders were inactive.
• Proton source effects: solid, multivalent alcohols strongly enhanced NH3 formation and suppressed H2 versus liquids. Pentaerythritol enabled ~49 equiv NH3 at 2 µmol 1a; neopentyl glycol/neopentyl alcohol gave 31/18 equiv, indicating multivalency benefit. Sugars (D-glucose) worked well without H2 formation. Insoluble cellulose served as an effective proton source with high NH3 yield and excellent selectivity, whereas cellulose was ineffective in homogeneous THF.
• Kinetics: with cellulose, an induction period (~30 min) was observed; pre-milling SmI2(THF)2 with cellulose accelerated initial NH3 formation, implicating formation of an active Sm(II)–proton-source complex. UV–vis diffuse reflectance supported modification of SmI2 upon milling with solid proton sources.
• Mechanism: direct N–N triple bond cleavage at the gas–solid interface forms a Mo–nitride intermediate (2), evidenced by NMR/MS and solid-state XAS on milled mixtures (e.g., linear combination fitting indicating ~67% 2, ~33% unreacted 1a). Subsequent N–H bond formation proceeds via proton-coupled electron transfer (PCET) in the solid phase with SmI2(THF)2 and pentaerythritol, converting nitride through imide/amide/ammine stages to NH3. Nitride 2 gave quantitative NH3 under these conditions and was catalytically competent.
• Generality of solid-state PCET: anthracene and trans-stilbene reductions proceeded under SmI2(THF)2/pentaerythritol milling to 9,10-dihydroanthracene (87%) and 1,2-diphenylethane (37%); deuterated pentaerythritol gave KIE 1.3, affirming PCET.
• NH3 collection: a solvent-free, two-step milling protocol (standard reaction then addition of solid KOH) liberated 520 equivalents of gas-phase NH3 per Mo catalyst directly from the jar; NH3 identified by FT-IR and quantified colorimetrically. In single-step reactions, NH3 remained in the solid phase, likely bound to Sm(III).
• Selectivity: no hydrazine detected; H2 was the only gaseous byproduct detected (amounts depend on conditions and proton source).

The study addresses whether molecular catalysts can mediate true catalytic nitrogen fixation under mechanochemical, gas–solid conditions. By adapting a high-performance homogeneous Mo/SmI2 system to solvent-free ball milling, the authors realize catalytic NH3 production at 1 atm N2 and near room temperature, overcoming the previously non-catalytic outcomes of metal-mediated mechanochemical approaches. The key enabling factors are (1) gas–solid activation of N2 at Mo catalysts via direct N–N cleavage to a nitride at the jar–gas interface and (2) solid-state PCET steps that efficiently forge N–H bonds from SmI2(THF)2 and solid proton sources. The solid proton source choice proved crucial: multivalent alcohols (pentaerythritol, sugars) and insoluble cellulose both enhance NH3 yields and suppress H2 versus liquid alcohols or water, demonstrating a unique advantage of mechanochemical conditions for insoluble substrates. Mechanistic data (solution NMR of dissolved milled mixtures, MS, solid-state XANES/EXAFS) substantiate the nitride intermediate, while stoichiometric conversion of the nitride to NH3 and solid-state PCET probe reactions strengthen the proposed catalytic cycle. Practically, the method avoids bulk solvent and enables direct gas-phase NH3 collection via a secondary base-milling step, mitigating energy-intensive separations intrinsic to solution processes (dehydration of NH3 and vaporization of THF and water). Although the achieved turnovers (≤864) are lower than the best homogeneous values, the results validate mechanochemical gas–solid molecular catalysis for N2 reduction and reveal design principles (proton source phase and multivalency; PCET compatibility) that can guide further optimization.

Catalytic mechanochemical nitrogen fixation has been demonstrated with molybdenum pincer complexes using SmI2(THF)2 and solid proton sources under 1 atm N2 and solvent-free ball milling. Solid, multivalent alcohols—including insoluble biomass-derived cellulose—enable efficient NH3 formation with suppressed H2, and a nitride intermediate formed by direct N–N cleavage at the gas–solid interface undergoes PCET-driven N–H bond formation in the solid phase. The approach allows direct, solvent-free generation of gas-phase NH3 via a two-step milling procedure with solid KOH. These findings showcase the feasibility and advantages of molecular gas–solid catalysis and solid-state PCET under mechanochemical conditions. Future work can aim to (i) increase turnover numbers and catalyst stability, (ii) broaden reductant and proton source chemistries (including greener and recyclable reagents), (iii) deepen mechanistic understanding of gas–solid activation and PCET in the solid state, and (iv) scale and engineer continuous/mechanochemical processes for practical NH3 production.

• Gas-phase NH3 is not produced directly in single-step reactions; NH3 is retained in the solid phase, likely coordinated to Sm(III), necessitating a second milling step with solid base (KOH) to release NH3 without solvents.
• An induction period (~30 min) occurs with cellulose as a proton source, indicating slower formation of active Sm(II)–proton-source species; pre-activation mitigates but does not eliminate this.
• The observed turnovers (up to 864 equiv/Mo) are well below the best homogeneous systems (up to 60,000), indicating room for catalyst and process optimization.
• Kinetic behavior for benchmark PCET reductions (e.g., trans-stilbene) differs from expectations based on solution-state BDFE values; the precise factors controlling PCET rates in the solid state remain unclear.
• Ball milling is essential; simple stirring without solvent yields negligible NH3, which may limit applicability to systems requiring mechanical energy input.