Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex

A new ruthenium polypyridyl complex, [Ru(bpy)(2)(acpy)](+) (acpy = 2-pyridylacetate, bpy = 2,2 '-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)(2)(pic)](+) (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding alpha-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru-II center. The reaction is one of the few examples of C-H activation at mild conditions using O-2 as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the alpha-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10(-2) h(-1). The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O-2 binding to [Ru(bpy)(2)(acpy)](+) is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting Ru-III-OO center dot- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (Delta G(double dagger) = 41.1 kcal mol(-1)), thus the formation of a diruthenium mu-peroxo intermediate, [(Ru-III(bpy)(2)(O-acpy))(2)O-2](2+)via interaction of a second [Ru(bpy)(2)(acpy)](+) was examined as an alternative pathway. The dimer yields two Ru-IV=O centers with a low Delta G(double dagger) of 2.3 kcal mol(-1). The resulting Ru-IV O species can activate C-H bonds in acpy (Delta G(double dagger) = 23.1 kcal mol(-1)) to produce the coordinated alpha-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the alpha-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O-2 as the sole oxygen source.

Automated summary

A new ruthenium polypyridyl complex was synthesized and characterized, showing instability and oxidation under aerobic conditions. The reaction provides insights into C-H activation with important implications for catalysis. Theoretical and experimental investigations suggest a two-step aerobic oxidative transformation with the formation of a diruthenium mu-peroxo intermediate as an alternative pathway for C-H activation.