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A Balancing Act: Stability versus Reactivity of Mn(O) Complexes
Heather M Neu1, Regina A Baglia1, David P Goldberg1
1Department of Chemistry, The Johns Hopkins University , Baltimore, Maryland 21218, United States.
Accounts of Chemical Research
|September 10, 2015
Summary
Synthetic chemists create stable high-valent manganese-oxo porphyrinoid complexes to study their reactivity in oxidation reactions. These manganese-oxo complexes show tunable reactivity for selective organic substrate oxidation.
Area of Science:
- Bioinorganic Chemistry
- Organometallic Chemistry
- Synthetic Chemistry
Background:
- Metalloenzymes use high-valent metal-oxo intermediates for selective oxidations.
- These reactive intermediates are difficult to characterize due to their short lifespan.
- Synthetic chemists aim to create stable analogues for studying reactivity.
Purpose of the Study:
- To generate and stabilize high-valent manganese-oxo porphyrinoid complexes.
- To tune the reactivity of these complexes in organic substrate oxidation.
- To investigate the mechanisms of oxidation reactions mediated by these complexes.
Main Methods:
- Generation of high-valent Mn(V)(O) corrolazine complexes using visible light irradiation of Mn(III)(TBP8Cz) with O2 and a substrate.
- Characterization using spectroscopy (Mn K-edge X-ray absorption, resonance Raman, NMR, EPR, UV-vis, MS).
- Tuning reactivity by addition of proton donors, anionic donors, and Lewis acids.
Main Results:
- Selective hydroxylation of hexamethylbenzene using a light/O2/substrate system.
- Catalytic oxidation achieved with H+ donors, while anionic donors enhanced oxygen atom transfer (OAT) rates.
- Formation of Mn(V)(O) π-radical cation and Mn(IV)(O) π-radical cation complexes with Lewis acids, showing enhanced hydrogen atom transfer (HAT) but reduced OAT.
Conclusions:
- Stable high-valent manganese-oxo porphyrinoid complexes can be synthesized and characterized.
- Reactivity (OAT and HAT) is tunable through modifications like anionic donors and Lewis acid coordination.
- These complexes serve as valuable models for understanding metalloenzyme-catalyzed oxidation mechanisms.
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