# Main-group element-mediated activation of dinitrogen

Main-group element-mediated activation of dinitrogen is the N$_2$ activation facilitated by reactive main group element centered molecules (e.g., low valent main group metal calcium, dicoordinate borylene, boron radical, carbene, etc.).

## Background

Dinitrogen fixation is essential for human life. Currently, the industry uses the Haber–Bosch process to convert N$_2$ and H$_2$ to NH$_3$ based on the metal catalysis under very high pressure and temperature conditions. Alternative strategies that realize the transformation from N$_2$ to NH$_3$ 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) N$_2$. 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 N$_2$. 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 N$_2$ activation by main group elements.
Lithium can also react with N$_2$ at room temperature to give an isolable product Li$_3$N. However, it was until recently that the controllable, stepwise N$_2$ activation by main group element began to thrive, especially for those whose key intermediates were well structurally characterized and even isolated.

## N$_2$ activation by calcium

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)]$_2$. 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 N$_2$ 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 N$_2$$^{2-}$. The N$_2$$^{2ˉ}$ anion could also be protonated to diazene (N$_2$H$_2$) with the intramolecular deprotonation of THF under the heating condition.

## N$_2$ activation by boron

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. [(CAAC)BDurBr$_2$] could smoothly undergo one-electron reduction with the limited amount of KC$_8$ (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 N$_2$. The empty sp$^2$ orbital, which acted as a Lewis acid, accepted the electrons from N$_2$ through σ donation. Following the further reduction by KC$_8$ and stabilization by another borylene molecule, the dipotassium complex {[(CAAC)DurB]$_2$(μ$^2$-N$_2$K$_2$)} 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]$_2$(μ$^2$-N$_2$)} and a paramagnetic diradical complex {[(CAAC)DurB]$_2$(μ$^2$ -N$_2$H$_2$)}. Further protonation and reduction of {[(CAAC)DurB]$_2$(μ$^2$ -N$_2$H$_2$)} could lead to the cleavage of central N-N bond, which could finally lead to the formation of ammonium chloride in one-pot reaction.
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 KC$_8$, affording the complex {[(CAAC)-TipB]$_2$(μ$^2$-N$_4$K$_2$)}, a product with catenation of two N$_2$ molecules, forming a N4 chain. It should be mentioned that this kind of coupling reaction was never found in the transition-metal-mediated N$_2$ activation processes.
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 N$_2$. In 2022, Mézailles et al. reported the N$_2$ activation by in situ generated boron-centered radicals. Though key intermediate which activated N$_2$ is unclear, DFT calculation suggested that the coordination of N$_2$ occurs prior to the second chloride elimination. Following the further reduction and coordination of boron, N$_2$ was finally reduced to its lowest oxidation state and a mixture of two borylamine compounds, N(BCy$_2$)$_3$ and NH(BCy$_2$)$_2$, were generated.

## N$_2$ activation by carbon

Carbene species have also been considered a good choice to activate N$_2$. The decomposition of diazoalkanes with the release of N$_2$ is one of the most widely used strategies to produce carbenes. Its reverse reaction could be considered as the activation of N$_2$ with carbenes. 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 N$_2$ photochemically in matrix to form the corresponding diazo compound.