Identifying the Rate-Limiting Elementary Steps of Nitrogen Fixation with Single-Site Fe Model Complexes

Area of Science:

• Computational chemistry
• Catalysis
• Green chemistry

Background:

• The Haber-Bosch process, essential for ammonia production, is energy-intensive.
• Biomimetic nitrogen fixation offers a sustainable alternative but faces performance limitations.
• Understanding catalytic cycles is crucial for designing efficient biomimetic catalysts.

Purpose of the Study:

• To elucidate the catalytic cycle of homogeneous dinitrogen reduction to ammonia using iron complexes.
• To identify dominant mechanisms and rate-determining steps in biomimetic nitrogen fixation.
• To guide the rational design of improved biomimetic catalysts.

Main Methods:

• Calculation of Gibbs free energies for elementary reaction steps.
• Investigation of single-site iron complexes with EPPP tetradentate ligands (E = B, Si).
• Analysis of all possible reaction mechanisms to determine dominant pathways.

Main Results:

• The catalytic mechanism is ligand-dependent, with the distal pathway (E=B) being most favorable.
• Rate-determining steps were identified within the catalytic cycle.
• A lack of thermodynamic driving force in later steps contributes to low catalytic activity.

Conclusions:

• Computational insights provide a foundation for computer-aided rational design of biomimetic catalysts.
• Future catalyst design should aim for a steadily decreasing Gibbs free energy profile, per the Sabatier principle.
• Optimizing ligand structure is key to enhancing catalytic performance in biomimetic nitrogen fixation.