O2 Activation by a Coordinated -NH- Function: Hydrogen Atom Transfer and Aromatic Ring Oxidation
Souvik Mukherjee1, Sandip Mondal2, Prasanta Ghosh1
1Department of Chemistry, Ramakrishna Mission Residential College (Autonomous), Narendrapur, Kolkata 700 103, India.
Inorganic Chemistry
|December 4, 2023
Summary
Ruthenium(II) and osmium(II) complexes catalyze the oxidation of 1,4-naphthoquinone rings using molecular oxygen. This process involves hydrogen atom transfer and generates novel oxidized species and metal complexes.
Area of Science:
- Inorganic Chemistry
- Organic Chemistry
- Catalysis
Background:
- The oxidation of organic molecules is crucial in synthetic chemistry.
- Metal-catalyzed oxidation reactions offer efficient pathways for functional group transformations.
- Developing novel oxidation methods using readily available oxidants like molecular oxygen is of significant interest.
Purpose of the Study:
- To disclose a unique method for the air oxidation of a 1,4-naphthoquinone ring.
- To investigate the mechanism of oxygen activation and subsequent oxidation of the naphthoquinone ligand coordinated to ruthenium(II) and osmium(II) ions.
- To synthesize and characterize novel metal complexes involved in the oxidation process.
Main Methods:
- Coordination of a 1,4-naphthoquinone derivative (H3L) to octahedral ruthenium(II) and osmium(II) ions.
- Spontaneous activation of molecular oxygen (3O2) by the coordinated metal complex.
- Isolation and characterization of intermediate and final metal complexes using techniques such as UV-vis spectroscopy and cyclic voltammetry.
- Kinetic studies using time-driven UV-vis spectroscopy to determine reaction rate constants.
Main Results:
- A novel oxidation pathway for the 1,4-naphthoquinone ring in the presence of ruthenium(II) and osmium(II) complexes was established.
- The oxidation proceeds via hydrogen atom transfer (HAT) from the ligand to O2, forming hydroperoxy intermediates and subsequently oxidized products.
- Novel ruthenium(II) and osmium(II) complexes of the types [MII(H2L-)(PPh3)2X], [MII(HLOOH-)(PPh3)2X], and trans-[MII(LOX-)(PPh3)2X] were successfully isolated.
- The reaction rate is dependent on the O2 concentration but not on the metal complex concentration.
Conclusions:
- Ruthenium(II) and osmium(II) complexes can effectively activate molecular oxygen for the oxidation of coordinated 1,4-naphthoquinone ligands.
- The mechanism involves HAT and formation of hydroperoxy intermediates, leading to the fully oxidized ligand.
- The study provides insights into metal-ligand cooperative effects in oxidation reactions.
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