Nonheme Iron Mediated Oxidation of Light Alkanes with Oxone: Characterization of Reactive Oxoiron(IV) Ligand Cation Radical Intermediates by Spectroscopic Studies and DFT Calculations

• Published in: Angewandte Chemie International Edition 2014-01, Vol.53 (3), p.798-803
• Main Authors: Tse, Chun-Wai, Chow, Toby Wai-Shan, Guo, Zhen, Lee, Hung Kay, Huang, Jie-Sheng, Che, Chi-Ming
• Format: Article
• Language: English
• Subjects: Alkanes; Alkanes - chemistry; Cations; Coordination Complexes - chemical synthesis; Coordination Complexes - chemistry; Cyclohexanes - chemistry; Electron Spin Resonance Spectroscopy; Ethane; Iron; Iron - chemistry; Ligands; nonheme iron complexes; Oxidation; Oxidation-Reduction; oxoiron complexes; Propane; Quantum Theory; Spectroscopy; Sulfuric Acids - chemistry
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201305153

The oxidation of light alkanes that is catalyzed by heme and nonheme iron enzymes is widely proposed to involve highly reactive {FeVO} species or {FeIVO} ligand cation radicals. The identification of these high‐valent iron species and the development of an iron‐catalyzed oxidation of light alkanes under mild conditions are of vital importance. Herein, a combination of tridentate and bidentate ligands was used for the generation of highly reactive nonheme {FeO} species. A method that employs [FeIII(Me3tacn)(Cl‐acac)Cl]+ as a catalyst in the presence of oxone was developed for the oxidation of hydrocarbons, including cyclohexane, propane, and ethane (Me3tacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane; Cl‐acac=3‐chloro‐acetylacetonate). The complex [FeIII(Tp)2]+ and oxone enabled stoichiometric oxidation of propane and ethane. ESI‐MS, EPR and UV/Vis spectroscopy, 18O labeling experiments, and DFT studies point to [FeIV(Me3tacn)({Cl‐acac}.+)(O)]2+ as the catalytically active species. Highly reactive {FeO} intermediates, such as [Fe(Tp)2(O)]+ or complex I (see Scheme), are likely to be involved in the oxidation of propane and ethane with oxone that is either mediated by [FeIII(Tp)2]+ (1) or catalyzed by iron complex 2. The cationic intermediate I features an {FeO} moiety and is stabilized by a combination of tridentate and bidentate ligands.