Rhodium complexes in homogeneous catalytic systems for oxidative functionalization of alkanes: Experiment and quantum-chemical calculations

• Biomimetic approach.
• Activation of methane via formation of CH3–Rh bond or at oxygen atoms in Rh–OI or Rh–O–O–Cu species.
• Two-electron oxidants from dioxygen.
• Redox co-catalysts—iodine or copper (iron) compounds—for activation of dioxygen.
• Experimental and DFT mechanistic studies.

Due to their stability, rhodium halides are attractive for use in redox reactions without additional ligands. In this review, we analyzed the available data on the catalytic systems containing rhodium halides and redox co-catalysts and their implementation to oxidation of methane and other lower alkanes. Based on experimental data and DFT results, we suggest possible mechanisms for oxidative functionalization of methane and propane. Activation of methane may get started with formation of a σ-complex with Rh. Activation of dioxygen takes place at the activated species of redox co-catalysts—iodine, copper or iron compounds—and leads to formation of two-electron oxidants. The redox co-catalysts may also interact with Rh. The activation of dioxygen is favored by the presence of added reducing agent (CO) which returns oxidized redox co-catalysts to their reduced form. The necessity of preliminary dioxygen reduction brings about analogy with biocatalysts for oxidation of alkanes. The oxidative carbonylation of methane to acetic acid most likely proceeds via an intermediate with the CH3–Rh bond. The formation of alcohols and acetone is consistent with formation of intermediate complexes containing the Rh–OI and Rh–O–O–Cu fragments. The esterification of alcohols with ingressed perfluorocarboxylic acids prevents from deep oxidation.

The activation of dioxygen in oxidative functionalization of methane in the presence of Rh complexes is possible only in the presence of redox co-catalysts, such as iodine, copper or iron compounds. During catalysis with rhodium carbonyls, reduced form of the redox co-catalysts is regenerated under the action of CO.Figure optionsDownload as PowerPoint slide