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

High Resolution Image Download MS PowerPoint Slide 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 N 2 reduction process in a cyclic(alkyl)(amino)carbene (CAAC)-stabilized diborylene N 2 complex, ((CAAC)(Dur)B) 2 (μ 2 -N 2 ), 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 N 2 fixation. Frontier orbital analysis rationalizes the distinct geometries of the two key intermediates. The N 2 adduct, ((CAAC)(Dur)B) 2 (μ 2 -N 2 ), adopts an orthogonal arrangement of the two borylene fragments, whereas the diazene species, ((CAAC)(Dur)B) 2 (μ 2 -N 2 H 2 ), has nearly coplanar fragments and a triplet ground state. In both, strong donation from the borylenes’ highest occupied molecular orbitals (HOMOs) into the N 2 or N 2 H 2 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 N 2 reduction. These insights provide clues for design principles for steering future main-group catalysts for ammonia synthesis.