Dioxygen Reduction at a Cu(I) Complex Supported by a Macrocyclic N₃O Ligand System

Area of Science:

• Coordination Chemistry
• Bioinorganic Chemistry
• Catalysis

Background:

• Copper complexes are crucial in understanding oxygen reduction mechanisms.
• Macrocyclic ligands influence the electronic and steric properties of copper ions.
• Controlling the reduction pathway of molecular oxygen is vital for catalysis.

Purpose of the Study:

• To investigate the effect of macrocyclic ligand structure on copper-mediated oxygen reduction.
• To elucidate the mechanistic differences between 2-electron and 4-electron oxygen reduction pathways.
• To understand how the copper coordination sphere dictates oxygen binding and reactivity.

Main Methods:

• Synthesis and characterization of copper(I) complexes with N3O and N4 macrocycles (Cu(14TMCO), Cu(14-TMC), [Cu(12-TMC)]+).
• Reaction of copper complexes with molecular oxygen (O2) at cryogenic temperatures.
• Spectroscopic analysis and mechanistic studies to determine reaction intermediates and pathways.

Main Results:

• Cu(I)(14TMCO) forms an end-on peroxodicopper(II) species and catalyzes 4e-/4H+ reduction of O2 to H2O.
• Cu(I)(14-TMC) is unreactive towards O2 due to a strong trans-effect inhibiting O2 binding.
• [Cu(I)(12-TMC)]+ binds O2 and catalyzes 2e-/2H+ reduction to H2O2 via ligand-assisted protonation.

Conclusions:

• The first coordination sphere of copper complexes can be tuned to control O2 reduction pathways (2e- vs 4e-).
• Ligand design, specifically the Cu-N bond length and macrocycle size, dictates O2 binding and subsequent reactivity.
• This study provides mechanistic insights into transition metal-catalyzed oxygen reduction.