Mechanism of Dinitrogen Reduction in a Borylene Complex by Density Functional Theory

Boron-centered dinitrogen reduction is an emerging field that complements the corresponding transition-metal chemistry. Comprehensive thermodynamic and kinetic analysis by density functional theory (DFT) of the full N₂ reduction process in a cyclic(alkyl)(amino)carbene (CAAC)-stabilized diborylene N₂ complex, ((CAAC)(Dur)B)₂(μ²-N₂), reveals a spontaneous process under the mild conditions employed experimentally. Geometric and natural bond orbital analyses show N-N bond weakening in the early stages of N₂ fixation. Frontier orbital analysis rationalizes the distinct geometries of the two key intermediates. The N₂ adduct, ((CAAC)(Dur)B)₂(μ²-N₂), adopts an orthogonal arrangement of the two borylene fragments, whereas the diazene species, ((CAAC)(Dur)B)₂(μ²-N₂H₂), has nearly coplanar fragments and a triplet ground state. In both, strong donation from the borylenes' highest occupied molecular orbitals (HOMOs) into the N₂ or N₂H₂ π* orbitals weakens the N-N bond, while orbital-symmetry considerations dictate the observed skeletal orientations. Although experiments detect protonation exclusively at nitrogen, our calculations predict that protonation at boron is thermodynamically accessible for tri- and tetraprotonated intermediates by using stronger acids and weaker reductants. However, large kinetic barriers for B-to-N proton migration would prevent these boron-protonated isomers from contributing to productive N₂ reduction. These insights provide clues for design principles for steering future main-group catalysts for ammonia synthesis.