Main-group element-mediated activation of dinitrogen

Main-group element-mediated activation of dinitrogen is the N2 activation facilitated by reactive main group element centered molecules (e.g., low valent main group metal calcium, dicoordinate borylene, boron radical, carbene, etc.).

N2 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 N2 units. The N-N distance in complex (1.258(3) and 1.268(3) ÃÂ) is remarkably longer than that of dinitrogen triple-bond (1.098 ÃÂ) and comparable with N=N double bond character in N22-. The N22ÃÂ anion could also be protonated to diazene (N2H2) with the intramolecular deprotonation of THF under the heating condition.

N2 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)BDurBr2] could smoothly undergo one-electron reduction with the limited amount of KC8 (1.5 equiv.) and afford a radical complex [(CAAC)BDurBr]ÃÂ·. 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 ÃÂ* antibonding orbital of N2. The empty sp2 orbital, which acted as a Lewis acid, accepted the electrons from N2 through ÃÂ donation. Following the further reduction by KC8 and stabilization by another borylene molecule, the dipotassium complex {[(CAAC)DurB]2(ÃÂ¼2-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(ÃÂ¼2-N2)} and a paramagnetic diradical complex {[(CAAC)DurB]2(ÃÂ¼2 -N2H2)}. Further protonation and reduction of {[(CAAC)DurB]2(ÃÂ¼2 -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.

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(ÃÂ¼2-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.

For borylene molecules, two-electron-filled p orbital and vacant sp2 orbital provide two pushÃ¢ÂÂpull 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, MÃÂ©zailles et al. reported the N2 activation by in situ generated boron-centered radicals. 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.

N2 activation by carbon

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. 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.

Main-group element-mediated activation of dinitrogen

N<sub>2</sub> activation by calcium

N<sub>2</sub> activation by boron

N<sub>2</sub> activation by carbon

References