Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity

Area of Science:

• Bioinorganic Chemistry
• Coordination Chemistry
• Catalysis

Background:

• Metal-superoxo species are crucial intermediates in dioxygen activation by metalloenzymes.
• While iron, cobalt, and nickel superoxo complexes are known, well-defined manganese-superoxo complexes are rare.

Purpose of the Study:

• To synthesize and characterize novel mononuclear manganese-superoxo complexes.
• To investigate the reactivity of these complexes in hydrogen atom abstraction.

Main Methods:

• Synthesis of manganese-superoxo complexes via reaction of manganese(II) precursors with O2 at low temperature.
• Characterization using resonance Raman and electron paramagnetic resonance (EPR) spectroscopy.
• Computational studies to determine electronic structure.
• Reactions with TEMPO-H to form manganese-hydroperoxo species.
• Preparation of manganese-hydroperoxo species from manganese(III)-aqua complexes and H2O2.

Main Results:

• Two mononuclear Mn(III)-superoxo complexes, Mn(BDPP)(O2•−) and Mn(BDPBrP)(O2•−), were successfully synthesized.
• Spectroscopic and computational analyses confirmed high-spin Mn(III) centers antiferromagnetically coupled to superoxo ligands, resulting in S = 3/2 ground states.
• The synthesized complexes demonstrated the ability to abstract hydrogen atoms from TEMPO-H, yielding Mn(III)-hydroperoxo species.
• Mn(III)-hydroperoxo complexes were also prepared independently and characterized with high-spin S = 2 ground states.

Conclusions:

• This work provides rare examples of well-defined mononuclear manganese-superoxo complexes.
• The study elucidates the electronic structure and reactivity of these novel manganese complexes.
• The findings contribute to understanding dioxygen activation mechanisms involving manganese centers.