Nitrogen Fixation via Splitting into Nitrido Complexes

Abstract

The large carbon footprint of the Haber–Bosch process, which provides ammonia for fertilizers but also the feedstock for all nitrogenous commercial products, has fueled the quest for alternative synthetic strategies to nitrogen fixation. Owing to the extraordinarily strong NN triple bond, the key step of the Haber–Bosch reaction, i.e., the dissociative adsorption of N2, requires high temperatures. Since the first report in 1995, a wide variety of molecular transition metal and f-block compounds have been reported that can fully cleave N2 at ambient conditions and form well-defined nitrido complexes. We here provide a comprehensive survey of the current state of N2 splitting reactions in solution and follow-up nitrogen transfer reactivity. Particular emphasis is put on electronic structure requirements for the formation of suitable molecular precursors and their N–N scission reactivity. The prospects of N2 splitting for the synthesis of nitrogen containing products will be discussed, ranging from ammonia and heterocumulenes to organic amines, amides or nitriles via proton coupled electron transfer, carbonylation, or electrophilic functionalization of N2 derived nitrido complexes. Accomplishments and challenges for nitrogen fixation via N2 splitting are presented to offer guidelines for the development of catalytic platforms.

Cyanate Formation via Photolytic Splitting of Dinitrogen

“…After the seminal report of Laplaza and Cummins in 1995, the splitting of dinitrogen into molecular nitrido complexes has evolved as a synthetic strategy to nitrogen fixation at ambient conditions. − Catalytic ammonia formation that commences with full N–N bond rupture, followed by proton-coupled electron transfer (PCET) steps, resembles the mechanism of the heterogeneously catalyzed Haber–Bosch process. , Such a dissociative mechanism was recently proposed by Nishibayashi and co-workers for the currently most active class of homogeneous catalysts, which are Mo pincer complexes that mediate N 2 fixation with activities up to TON max = 4350 and TOF max = 117 min –1 using SmI 2 /H 2 O as a PCET reductant. , Alternatively, nitride formation potentially offers an entry to subsequent C–N bond formation. ,, Several groups demonstrated the suitability of dissociative mechanistic scenarios, e.g., to synthesize organic nitriles from N 2 , within stepwise, cyclic reaction schemes (“synthetic cycles”). − However, truly catalytic protocols that allow for the direct transformation of N 2 to organic products remain unknown to date.…”

“…The thermochemical challenge of the dissociative approach to N 2 fixation arises from the extraordinarily strong N–N triple bond (BDE = 941 kJ·mol –1 ), , which needs to be counterbalanced by the formed M–N bonds. In consequence, C–N bond formation and N-transfer requires quite reactive reagents, such as strong electrophiles that are often incompatible with the reductive conditions of N 2 activation.…”

“…Transfer to idealized D 4 h symmetry expands this picture to the emerging class of metal-pincer complexes that mediate N 2 splitting (Figure ). , As a general feature, the precursors to N 2 splitting exhibit electronic ground state configurations with 10 electrons in the {MNNM} π-MO manifold, such as [(N 2 )­{ReCl­(PNP)} 2 ] ( D , π 1 4 π 2 4 δ 1 2 δ 2 2 π 3 2 ) or [(N 2 )­{MCl­(HPNP)} 2 ] 2+ (π 1 4 π 2 4 δ 1 1 δ 2 1 π 3 2 ), respectively (M = Mo, W; PNP = N­(CH 2 CH 2 P t Bu 2 ) 2 ). − Full N–N cleavage is associated with an electronic rearrangement from the π*−π–π* to the N–N σ* MO via a zigzag transition state of the {MNNM} core. So far, this simple picture has proven fully consistent with computational electronic structure treatment of related systems. ,,− ,, However, several aspects of thermally and photochemically driven N–N cleavage are yet to be addressed: Both strongly activated N 2 complexes and the terminal nitride products can exhibit high degrees of covalent M–N bonding. , The actual extent of charge transfer that is associated with cleavage of the N–N bond is surprisingly ill-defined.…”

Theoretical Study on Reaction Mechanisms of Dinitrogen Activation and Coupling by Carbene‐Stabilized Borylenes in Comparison with Intramolecular C−H Bond Activation

“…The significant bond strength (bond dissociation energy: 945 kJ/mol) and HOMO‐LUMO gap (10.8 eV; HOMO: the highest occupied molecular orbital; LUMO: the lowest unoccupied molecular orbital) in dinitrogen (N 2 ) [1] make its activation particularly challenging. Due to the advantage of the diversity of oxidation‐state and orbital, transition metals have been used for N 2 activation and conversion into ammonia and other useful nitrogen compounds [2–14] . Note that the industrial Haber‐Bosch process [15] for ammonia production requires relatively harsh conditions (350° to 500 °C and 150 to 200 atm), leading to consumption of more than 1% of the world‘s annual energy supply.…”

Elaboration on the Electronics of Salen Manganese Nitrides: Investigations into Alkoxy-Substituted Ligand Scaffolds

“…Metal nitride complexes are critical reactive species for a variety of synthetic and catalytic processes. − In particular, manganese nitride complexes featuring salen ligands are well-established systems, with early work investigating the ability of these complexes to participate in N-atom-transfer reactivity with a variety of substrates. − Salen manganese nitride complexes are currently experiencing a renaissance as isolable intermediates in both ammonia production and dinitrogen formation. For instance, the Chirik group has recently published a series of papers investigating salen manganese nitride species as nitrogen sources in the stoichiometric production of ammonia using proton-coupled electron transfer. − Metal nitrides are also proposed to be important intermediates in the opposite transformation, ammonia oxidation to dinitrogen .…”