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

We have synthesized and characterized a Ru II -OH 2 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 η 6 -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 Ce IV salt to produce the corresponding Ru III (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 Ru IV (O) complex, 6 , which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the Ru III (OH) complex, 5 , exhibits disproportionation to the corresponding Ru IV (O) complex, 6 , and a mixture of the Ru II 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 Ru IV (O) complex ( 6' ) lacking the HB moieties in the SCS.