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.