Catalytic Nitrogen Fixation Using Molybdenum–Dinitrogen Complexes as Catalysts

Silylamido supported dinitrogen heterobimetallic complexes: syntheses and their catalytic ability

Molybdenum dinitrogen complexes supported by monodentate arylsilylamido ligand, [Ar(Me₃Si)N]₃MoN₂Mg(THF)₂[N(SiMe₃)Ar] (5) and [Ar(Me₃Si)N]₃MoN₂SiMe₃ (6) (Ar = 3,5-Me₂C₆H₃) were synthesized and structurally characterized, and proved to be effective catalysts for the disproportionation of cyclohexadienes and isomerization of terminal alkenes. The ¹H NMR spectrum suggested that the bridging nitrogen ligand remains intact during the catalytic reaction, indicating possible catalytic ability of the Mo-N=N motif.

Determination of the N-H Bond Dissociation Free Energy in a Pyridine(diimine)molybdenum Complex Prepared by Proton-Coupled Electron Transfer

The pyridine(diimine)molybdenum bis(imido) complex (^iPrPDI)Mo(═NTol)₂ (Tol = 4-methylphenyl) was synthesized by the addition of 2 equiv of 4-methylphenylazide to the corresponding molybdenum benzene derivative, (^iPrPDI)Mo(η⁶-C₆H₆) [^iPrPDI = 2,6-(2,6-iPr₂C₆H₃N═CMe)₂C₅H₃N]. Protonation of (^iPrPDI)Mo(═NTol)₂ with 2,6-lutinidum triflate yielded a cationic molybdenum amido complex, [(^iPrPDI)Mo(NHTol)(═NTol)][OTf], which was further transformed into the neutral molybdenum amido (^iPrPDI)Mo(NHTol)(═NTol) by reduction with zinc powder. A series of spectroscopic, synthetic, and pK_a determination studies along with electrochemical measurements by the protonation-reduction pathway were used to establish an N-H bond dissociation free energy (BDFE) between 65 and 69 kcal/mol for the molybdenum imido-amido compound, (^iPrPDI)Mo(NHTol)(═NTol). Full-molecule density functional theory studies provided a computed value of 61 kcal/mol. By contrast, reduction of (^iPrPDI)Mo(═NTol)₂ with KC₈ afforded the corresponding anionic molybdenum complex K[(^iPrPDI)Mo(═NTol)₂], which has a potassium cation intercalated with the pyridine and tolyl groups. Protonation of K[(^iPrPDI)Mo(═NTol)₂] with the weak amidinium acid [TBD(H)][BArF₂₄] (TBD = triazabicyclodecene; BArF₂₄ = B[3,5-(CF₃)₂C₆H₃]₄) also produced the neutral molybdenum amido complex (^iPrPDI)Mo(NHTol)(═NTol). Measurement of the pK_a and oxidation potential of K[(^iPrPDI)Mo(═NTol)₂] provided a range of 69-73 kcal/mol for the N-H BDFE of (^iPrPDI)Mo(NHTol)(═NTol), in good agreement with the protonation-reduction route and completing the square scheme. The similar pK_a and redox potentials obtained from each pathway demonstrate that both sequences are energetically feasible for proton-coupled electron-transfer (PCET) events. This study on the determination of N-H BDFE of the molybdenum amido complex renders fundamental insight into the N₂ reduction cycle by PCET.

Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H₂ activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (η⁵-C₅Me₅)Rh(^MePhI)H (^MePhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η⁵-C₅Me₅)Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh-H bond dissociation free energies of 51.8 kcal mol^-1 for (η⁵-C₅Me₅)Rh(^MePhI)H and 51.1 kcal mol^-1 for (η⁵-C₅Me₅)Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H₂ in THF, the manganese nitride (^tBuSalen)Mn≡N underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH₃ or 18 turnovers of H^• transfer. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.

Steric Switching From Photochemical to Thermal N2 Splitting: A Computational Analysis of the Isomerization Reaction {(Cp*)(Am)Mo}2(μ-η1:η1-N2) → {(Cp*)(Am)Mo}2(μ-N)2

A μ-η1:η1-N2-bridged Mo dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N2), cleaves dinitrogen thermally resulting in a crystallographically characterized bis-μ-N-bridged dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N)2. A structurally related Mo dimer with a bulkier amidinate ligand, ([N(iPr)C(Me)N(iPr)]), is only capable of photochemical dinitrogen activation. These opposing reactivities were rationalized as steric switching between the thermally and photochemically active species. A computational analysis of the geometric and electronic structures of intermediates along the isomerization pathway from Mo2(μ-η1:η1-N2) to Mo2(μ-η2:η1-N2) and Mo2(μ-η2:η2-N2), and finally Mo2(μ-N)2, is presented here. The extent to which dispersion affects the thermodynamics of the isomers is evaluated, and it is found that dispersion interactions play a significant role in stabilizing the product and making the reaction exergonic. The concept of steric switching is further explored with theoretical models with sterically even less demanding ligands, indicating that systematic ligand modifications could be used to rationally design the N2 activation energy landscape. An analysis of electronic excitations in the computed UV-vis spectra of the two complexes shows that a particular type of asymmetric excitations is only present in the photoactive complex.

Enhanced Activation of Coordinated Dinitrogen with p-Block Lewis Acids

This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.

Development of catalytic nitrogen fixation using transition metal-dinitrogen complexes under mild reaction conditions

This paper describes our recent progress in catalytic nitrogen fixation using transition metal-dinitrogen complexes as catalysts. Our research group has recently developed novel reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and hydrazine using molybdenum-, iron-, cobalt- and vanadium-dinitrogen complexes under mild reaction conditions. The new findings presented in this paper may provide a new approach to the development of economical nitrogen fixation to replace the energy-consuming Haber-Bosch process.

Dinitrogen photoactivation: status quo and future perspectives

New perspectives for dinitrogen activation: an overview of photochemical pathways to cleave the strong N–N bond.