Main-group element-mediated activation of dinitrogen

Dinitrogen fixation is essential for human life. Currently, the industry uses the Haber–Bosch process to convert N₂ and H₂ to NH₃ based on the metal catalysis under very high pressure and temperature conditions. Alternative strategies that realize the transformation from N₂ to NH₃ under mild conditions are a long-lasting goal in chemistry. In the past decades, a number of transition-metal species have been found to bind (and even functionalize) N₂. The prevalence of transition metals in dinitrogen activation is attributed to the fact that the unoccupied and occupied d orbitals could be both energetically and symmetrically accessible to accept electron density from and back donate to N₂. Nevertheless, the development of low-valent, low-coordinate main-group elements which mimic the electronic properties of transition metal provides more opportunities to unearth the N₂ activation by main group elements.[8]

Lithium can also react with N₂ at room temperature to give an isolable product Li₃N.[9][10] However, it was until recently that the controllable, stepwise N₂ activation by main group element began to thrive, especially for those whose key intermediates were well structurally characterized and even isolated.

In 2021, Harder et al. achieved dinitrogen activation by a low-valent calcium complex, which was generated by the reduction of a calcium (II) complex [CaI(BDI)]₂.[1] With the presence of THF, the reduction of [CaI(BDI)]₂ with K/KI could afford red-brown crystals. The single crystal X-ray analysis revealed a centrosymmetric dimer with terminal BDI ligands and side-on bridging N₂ units. The N-N distance in complex (1.258(3) and 1.268(3) Å) is remarkably longer than that of dinitrogen triple-bond (1.098 Å) and comparable with N=N double bond character in N₂^2-. The N₂^2ˉ anion could also be protonated to diazene (N₂H₂) with the intramolecular deprotonation of THF under the heating condition.

Dicoordinate borylene has a filled p orbital and an empty sp-hybridized orbital in appropriate symmetry that can interact with inert small molecules like dinitrogen. In 2018, Braunschweig et al. reported the nitrogen fixation and reduction by active borylene species.[2] [(CAAC)BDurBr₂] could smoothly undergo one-electron reduction with the limited amount of KC₈ (1.5 equiv.) and afford a radical complex [(CAAC)BDurBr]·. The radical complex could be further reduced, forming the transient dicoordinate borylene species and thus had the ability to activate dinitrogen. The filled p orbital of borylene, which acted as a Lewis base, donated to the π* antibonding orbital of N₂. The empty sp² orbital, which acted as a Lewis acid, accepted the electrons from N₂ through σ donation. Following the further reduction by KC₈ and stabilization by another borylene molecule, the dipotassium complex {[(CAAC)DurB]₂(μ²-N₂K₂)} was formed in crystalline solid. Exposure of the dipotassium complex with ambient air and distilled water leads to the formation of dinitrogen bis(borylene) compound {[(CAAC)DurB]₂(μ²-N₂)} and a paramagnetic diradical complex {[(CAAC)DurB]₂(μ² -N₂H₂)}. Further protonation and reduction of {[(CAAC)DurB]₂(μ² -N₂H₂)} could lead to the cleavage of central N-N bond, which could finally lead to the formation of ammonium chloride in one-pot reaction.[4]

Repeating the same reaction but replacing Dur (2,3,5,6-tetramethyl-phenyl) group by a bulkier Tip (2,4,6-triisopropylphenyl) group resulted in a very different result: after the dinitrogen was coordinated by the first borylene molecule, the second coordination by another borylene molecule was considerably hindered by steric repulsion in the case of the bulkier 4-Tip. Instead, the reductive dimerization of transient borylene [(CAAC)BTip] could occur in the presence of extra KC₈, affording the complex {[(CAAC)-TipB]₂(μ²-N₄K₂)}, a product with catenation of two N₂ molecules, forming a N4 chain. It should be mentioned that this kind of coupling reaction was never found in the transition-metal-mediated N₂ activation processes.[3]

For borylene molecules, two-electron-filled p orbital and vacant sp2 orbital provide two push–pull channels to activate dinitrogen. Similarly, for boron radicals, one-electron-filled p orbital and vacant sp2 orbital provide two channels to activate N₂. In 2022, Mézailles et al. reported the N₂ activation by in situ generated boron-centered radicals.[5] Though key intermediate which activated N₂ is unclear, DFT calculation suggested that the coordination of N₂ occurs prior to the second chloride elimination. Following the further reduction and coordination of boron, N₂ was finally reduced to its lowest oxidation state and a mixture of two borylamine compounds, N(BCy₂)₃ and NH(BCy₂)₂, were generated.

Carbene species have also been considered a good choice to activate N₂. The decomposition of diazoalkanes with the release of N₂ is one of the most widely used strategies to produce carbenes. Its reverse reaction could be considered as the activation of N₂ with carbenes.[6] For example, in 1992, Dailey et al. reported that the photolysis of 3-bromo-3-(trifluoromethyl)diazirines in an argon matrix could afford bromo(trifluoromethyl)carbene. Bromo(trifluoromethyl)carbene could rebound N₂ photochemically in matrix to form the corresponding diazo compound.[7]