Catalytic C-H bond amination from high-spin iron imido complexes

Area of Science:

• Organometallic Chemistry
• Catalysis
• Synthetic Chemistry

Background:

• Dipyrromethene ligands are versatile scaffolds for transition metal complexes.
• Iron complexes are increasingly explored as catalysts for organic transformations.
• Understanding reaction mechanisms, especially those involving radical intermediates, is crucial for catalyst design.

Purpose of the Study:

• To synthesize novel iron complexes with sterically demanding dipyrromethene ligands.
• To investigate the catalytic activity of these complexes in C-H amination and olefin aziridination.
• To elucidate the mechanism of catalysis, including the identification of key intermediates.

Main Methods:

• Synthesis of 1,9-disubstituted-5-mesityldipyrromethene ligands with bulky aryl or alkyl groups.
• Preparation and characterization of four- and three-coordinate iron complexes using techniques like NMR, X-ray crystallography, and Mössbauer spectroscopy.
• Catalytic reactions including C-H amination and olefin aziridination, with kinetic studies and isotope effect measurements.

Main Results:

• Successfully synthesized three new dipyrromethene ligands and their corresponding iron complexes.
• Isolated a unique three-coordinate, high-spin iron complex with a sterically encumbered aryl-substituted ligand.
• Demonstrated catalytic activity in C-H amination (12 turnovers for toluene) and olefin aziridination (>85% conversion for styrene) at room temperature.
• Identified a high-spin Fe(III) imido radical intermediate, evidenced by spectroscopic data and kinetic isotope effects (12.8(5) and 24(3)).

Conclusions:

• Sterically demanding dipyrromethene ligands enable the formation of unusual iron coordination complexes.
• The isolated high-spin Fe(III) imido radical is proposed as a key intermediate in the catalytic amination and aziridination pathways.
• The unique electronic structure of these iron imido complexes contributes to their observed catalytic reactivity.