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.