Synthesis and characterization of mononuclear ruthenium(III) pyridylamine complexes and mechanistic insights into their catalytic alkane functionalization with m-chloroperbenzoic acid

A series of mononuclear Ru^III complexes [RuCl ₂(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-three-fold-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-chloroper-benzoic 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 turn-over numbers. Catalytic oxygenation of cyclohexane by a Ru ^III complex with ¹⁶O-mCPBA in the presence of H ₂¹⁸O 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.