Formation of a Ruthenium(V)Imido Complex and the Reactivity in Substrate Oxidation in Water through the Nitrogen Non-Rebound Mechanism

A Ru^IINH₃ complex, 2, was oxidized through a proton-coupled electron transfer (PCET) mechanism with a Ce^IV complex in water at pH 2.5 to generate a Ru^VNH complex, 5. Complex 5 was characterized with various spectroscopies, and the spin state was determined by the Evans method to be S = 1/2. The reactivity of 5 in substrate C–H oxidation was scrutinized in acidic water, using water-soluble organic substrates such as sodium ethylbenzene-sulfonate (EBS), which gave the corresponding 1-phenylethanol derivative as the product. In the substrate oxidation, complex 5 was converted to the corresponding Ru^IIINH₃ complex, 3. The formation of 1-phenylethanol derivative from EBS and that of 3 indicate that complex 5 as the oxidant does not perform nitrogen-atom transfer, in sharp contrast to other high-valent metal–imido complexes reported so far. Oxidation of cyclobutanol by 5 afforded only cyclobutanone as the product, indicating that the substrate oxidation by 5 proceeds through a hydride-transfer mechanism. In the kinetic analysis on the C–H oxidation, we observed kinetic isotope effects (KIEs) on the C–H oxidation with use of deuterated substrates and remarkably large solvent KIE (sKIE) in D₂O. These positive KIEs indicate that the rate-determining step involves not only cleavage of the C–H bond of the substrate but also proton transfer from water molecules to 5. The unique hydride-transfer mechanism in the substrate oxidation by 5 is probably derived from the fact that the Ru^IVNH₂ complex (4) formed from 5 by 1e^–/1H⁺ reduction is unstable and quickly disproportionates into 3 and 5.