Tunable Electrochemical and Catalytic Features of BIAN- and BIAO-Derived Ruthenium Complexes

This article deals with a class of ruthenium–BIAN-derived complexes, [Ru^II(tpm)­(R-BIAN)­Cl]­ClO₄ (tpm = tris­(1-pyrazolyl)­methane, R-BIAN = bis­(arylimino)­acenaphthene, R = 4-OMe ([<b>1a</b>]­ClO₄), 4-F ([<b>1b</b>]­ClO₄), 4-Cl ([<b>1c</b>]­ClO₄), 4-NO₂ ([<b>1d</b>]­ClO₄)) and [Ru^II(tpm)­(OMe-BIAN)­H₂O]²⁺ ([<b>3a</b>]­(ClO₄)₂). The R-BIAN framework with R = H, however, leads to the selective formation of partially hydrolyzed BIAO ([<i>N</i>-(phenyl)­imino]­acenapthenone)-derived complex [Ru^II(tpm)­(BIAO)­Cl]­ClO₄ ([<b>2</b>]­ClO₄). The redox-sensitive bond parameters involving NC–CN or NC–CO of BIAN or BIAO in the crystals of representative [<b>1a</b>]­ClO₄, [<b>3a</b>]­(PF₆)₂, or [<b>2</b>]­ClO₄ establish its unreduced form. The chloro derivatives <b>1a</b>⁺–<b>1d</b>⁺ and <b>2</b>⁺ exhibit one oxidation and successive reduction processes in CH₃CN within the potential limit of ±2.0 V versus SCE, and the redox potentials follow the order <b>1a</b>⁺ < <b>1b</b>⁺ < <b>1c</b>⁺ < <b>1d</b>⁺ ≈ <b>2</b>⁺. The electronic structural aspects of <b>1a</b>^<i>n</i>–<b>1d</b>^<i>n</i> and <b>2</b>^<i>n</i> (<i>n</i> = +2, +1, 0, −1, −2, −3) have been assessed by UV–vis and EPR spectroelectrochemistry, DFT-calculated MO compositions, and Mulliken spin density distributions in paramagnetic intermediate states which reveal metal-based (Ru^II → Ru^III) oxidation and primarily BIAN- or BIAO-based successive reduction processes. The aqua complex <b>3a</b>²⁺ undergoes two proton-coupled redox processes at 0.56 and 0.85 V versus SCE in phosphate buffer (pH 7) corresponding to {Ru^II–H₂O}/{Ru^III–OH} and {Ru^III–OH}/{Ru^IVO}, respectively. The chloro (<b>1a</b>⁺–<b>1d</b>⁺) and aqua (<b>3a</b>²⁺) derivatives are found to be equally active in functioning as efficient precatalysts toward the epoxidation of a wide variety of alkenes in the presence of PhI­(OAc)₂ as oxidant in CH₂Cl₂ at 298 K, though the analogous <b>2</b>⁺ remains virtually inactive. The detailed experimental analysis with the representative precatalyst <b>1a</b>⁺ suggests the involvement of the active {Ru^IVO} species in the catalytic cycle, and the reaction proceeds through the radical mechanism, as also supported by the DFT calculations