Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes

Area of Science:

• Organometallic Chemistry
• Inorganic Chemistry
• Catalysis

Background:

• Nitrogen fixation, the conversion of atmospheric N2 into reactive nitrogen compounds, is crucial for industrial and biological processes.
• Despite decades of research, efficient and selective functionalization of dinitrogen (N2) complexes remains a significant challenge.
• Previous efforts often resulted in low yields and limited mechanistic understanding of N2 activation.

Purpose of the Study:

• To synthesize and characterize novel chromium-dinitrogen (Cr-N2) complexes.
• To investigate the reactivity of these complexes towards N2 functionalization, focusing on silylation reactions.
• To elucidate the reaction mechanisms and explore the influence of ligand environment on product selectivity.

Main Methods:

• Synthesis of dinuclear, trinuclear, and mononuclear chromium complexes with cyclopentadienyl-phosphine and N-heterocyclic carbene (NHC) ligands.
• Characterization using spectroscopic techniques and electronic structure analysis.
• Reactions with silylating agents (e.g., Me3SiCl) to form hydrazido and diazenido complexes.
• Kinetic studies to determine reaction rates and intermediate lifetimes.

Main Results:

• Dinuclear and trinuclear Cr(I)-N2 complexes underwent oxidative addition at Cr centers, not N2 functionalization.
• Mononuclear Cr(0)-N2 complexes, particularly those with NHC ligands, enabled N2 silylation via distal or alternating pathways, yielding Cr(III) hydrazido complexes.
• Complexes with bisphosphine ligands facilitated a two-electron route, forming Cr(IV) hydrazido species through diazenido intermediates.
• Nearly quantitative preparations of Cr(IV) hydrazido species with diverse substituents were achieved.

Conclusions:

• Ligand design is critical for controlling the pathway and outcome of N2 functionalization in chromium complexes.
• The developed Cr-N2 systems offer effective routes for N2 activation and the synthesis of valuable nitrogen-containing compounds.
• These findings deepen the understanding of electrophilic attack on N2 and inspire further research in nitrogen fixation.