Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere.

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6') lacking the HB moieties in the SCS.

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

A RuII-NH3 complex, 2, was oxidized through a proton-coupled electron transfer (PCET) mechanism with a CeIV complex in water at pH 2.5 to generate a RuV═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 RuIII-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 RuIV-NH2 complex (4) formed from 5 by 1e-/1H+ reduction is unstable and quickly disproportionates into 3 and 5.