Synthesis and Characterization of Mononuclear Ruthenium(III) Pyridylamine Complexes and Mechanistic Insights into Their Catalytic Alkane Functionalization with m‐Chloroperbenzoic Acid

Abstract

A series of mononuclear Ru^III complexes [RuCl₂(L)]⁺, where L is tris(2‐pyridylmethyl)amine (TPA) or one of four TPA derivatives as tetradentate ligand, were prepared and characterized by spectroscopic methods, X‐ray crystallography, and electrochemical measurements. The geometry of a Ru^III complex having a non‐threefold‐symmetric TPA ligand bearing one dimethylnicotinamide moiety was determined to show that the nicotine moiety resides trans to a pyridine group, but not to the chlorido ligand. The substituents of the TPA ligands were shown to regulate the redox potential of the ruthenium center, as indicated by a linear Hammett plot in the range of 200 mV for Ru^III/Ru^IV couples with a relatively large ρ value (+0.150). These complexes act as effective catalysts for alkane functionalization in acetonitrile with m‐chloroperbenzoic acid (mCPBA) as terminal oxidant at room temperature. They exhibited fairly good reactivity for oxidation of cyclohexane (CH bond energy 94 kcal mol⁻¹), and the reactivity can be altered significantly by the electronic effects of substituents on TPA ligands in terms of initial rates and turnover numbers. Catalytic oxygenation of cyclohexane by a Ru^III complex with ¹⁶O‐mCPBA in the presence of H₂¹⁸O gave ¹⁸O‐labeled cyclohexanol with 100 % inclusion of the ¹⁸O atom from the water molecule. Resonance Raman spectra under catalytic conditions without the substrate indicate formation of a Ru^IVO intermediate with lower bonding energy. Kinetic isotope effects (KIEs) in the oxidation of cyclohexane suggest that hydrogen abstraction is the rate‐determining step and the KIE values depend on the substituents of the TPA ligands. Thus, the reaction mechanism of catalytic cyclohexane oxygenation depends on the electronic effects of the ligands.