Oxo-transfer catalysis from t-BuOOH with C–H bond insertion using tridentate Schiff-base-chelate complexes of ruthenium(III)

Mixed-chelate complexes of ruthenium have been synthesized using tridentate Schiff-base ligands (TDLs) derived by condensation of aldehydes (salicyldehyde, 2-pyridinecarboxaldehyde) with 2-aminobenzoic acid, and bidentate ligands (2,2′-bipyridine or picolinic acid). [RuIII(cpsd)(bipy)(H2O)]+ (1), [RuIII(cpsd)(pic)(H2O)] (2), [RuIII(cppc)(bipy)(H2O)]2+ (3) and [RuIII(cppc)(pic)(H2O)]+ (4) complexes (where, cpsd2−=(N-(2-carboxyphenyl)salicylaldiminato); cppc−=N-2-carboxyphenylpyridine-2-carboxaldiminato; bipy=2,2′-bipyridine and pic−=picolinate) were characterized by analytical, spectral (IR and UV–Vis), conductance, magnetic moment and electrochemical studies. Catalysis of hydrocarbon oxidations for cyclohexene, cyclohexane, cyclohexanol, toluene, benzyl alcohol, and tetrahydrofuran have been studied using various O-atom transfer agents (t-BuOOH, H2O2, NaOCl, KHSO5 and pyridinium-N-oxide). The influence of product yield as a function of solvent was evaluated for CH2Cl2, CH3CN, and 1,4-dioxane. Coordinating solvents suppress the reactivity by inhibiting coordination of t-BuOOH, and compete for the RuVO group through their own intrinsic C–H reactivity. The main pathway transfers the oxo group from the [RuO(TDL)(XY)] intermediate, TDL=cpsd2− and cppc2−; XY=bipy or pic−, with insertion of the oxo group into a C–H bond of all substrates tested (rather than olefin epoxidation for cyclohexene). A mechanism involving intermediacy of a high valent Ru(V)-oxo species is proposed for the catalytic oxidation processes.