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 p 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 kcalmol(-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 O-16-mCPBA in the presence of (H2O)-O-18 gave O-18-labeled cyclohexanol with 100% inclusion of the O-18 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.