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

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Catalysis

Background:

• Dinitrogen (N₂) reduction is crucial for ammonia synthesis.
• Transition metal catalysts dominate N₂ reduction, but main-group alternatives are emerging.
• Boron-based complexes offer a complementary approach to N₂ fixation.

Purpose of the Study:

• To investigate the thermodynamics and kinetics of N₂ reduction by a CAAC-stabilized diborylene complex.
• To elucidate the structural and electronic factors governing N₂ fixation and subsequent reduction steps.
• To provide design principles for novel main-group catalysts for ammonia synthesis.

Main Methods:

• Density Functional Theory (DFT) calculations were employed for comprehensive thermodynamic and kinetic analysis.
• Geometric and Natural Bond Orbital (NBO) analyses were performed to study N-N bond weakening and electronic structure.
• Frontier orbital analysis was used to rationalize the geometries of key intermediates.

Main Results:

• The N₂ reduction process in the ((CAAC)(Dur)B)₂ complex is thermodynamically spontaneous under mild conditions.
• N-N bond weakening occurs early in N₂ fixation due to strong donation from borylene HOMOs to N₂ π* orbitals.
• Distinct intermediate geometries (orthogonal N₂ adduct, coplanar diazene) arise from orbital interactions and symmetry considerations.
• While boron-protonation is accessible, kinetic barriers prevent its contribution to productive N₂ reduction.

Conclusions:

• Boron-centered N₂ reduction is a viable pathway with potential for ammonia synthesis.
• Understanding electronic and geometric factors is key to designing efficient main-group catalysts.
• DFT analysis provides valuable insights for steering catalyst development, complementing experimental findings.