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 Ru-II-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 eta 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.

Automated summary

We synthesized and characterized a Ru-II-OH2 complex and studied its coordination equilibrium and oxidation reaction. The complex exhibits η6 coordination and intramolecular hydrogen bonding, which are important for catalytic oxidation reactions.