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

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

Synthesis, Spectral Characterization, Structures, and Oxidation State Distributions in [(corrolato)Fe^III(NO)]^<i>n</i> (<i>n</i> = 0, +1, −1) Complexes

Two novel <i>trans</i>-A₂B-corroles and three [(corrolato)­{FeNO}⁶] complexes have been prepared and characterized by various spectroscopic techniques. In the native state, all these [(corrolato)­{FeNO}⁶] species are diamagnetic and display “normal” chemical shifts in the ¹H NMR spectra. For two of the structurally characterized [(corrolato)­{FeNO}⁶] derivatives, the Fe–N–O bond angles are 175.0(4)° and 171.70(3)° (DFT: 179.94°), respectively, and are designated as linear nitrosyls. The Fe–N (NO) bond distances are 1.656(4) Å and 1.650(3) Å (DFT: 1.597 Å), which point toward a significant Fe^III → NO back bonding. The NO bond lengths are 1.159(5) Å and 1.162(3) Å (DFT: 1.162 Å) and depict their elongated character. These structural data are typical for low-spin Fe­(III). Electrochemical measurements show the presence of a one-electron oxidation and a one-electron reduction process for all the complexes. The one-electron oxidized species of a representative [(corrolato)­{FeNO}⁶] complex exhibits ligand to ligand charge transfer (LLCT) transitions (cor­(π) → cor­(π*)) at 399 and 637 nm, and the one-electron reduced species shows metal to ligand charge transfer (MLCT) transition (Fe­(dπ) → cor­(π*)) in the UV region at 330 nm. The shift of the ν_NO stretching frequency of a representative [(corrolato)­{FeNO}⁶] complex on one-electron oxidation occurs from 1782 cm^–1 to 1820 cm^–1, which corresponds to 38 cm^–1, and on one-electron reduction occurs from 1782 cm^–1 to 1605 cm^–1, which corresponds to 177 cm^–1. The X-band electron paramagnetic resonance (EPR) spectrum of one-electron oxidation at 295 K in CH₂Cl₂/0.1 M Bu₄NPF₆ displays an isotropic signal centered at <i>g</i> = 2.005 with a peak-to-peak separation of about 15 G. The in situ generated one-electron reduced species in CH₂Cl₂/0.1 M Bu₄NPF₆ at 295 K shows an isotropic signal centered at <i>g</i> = 2.029. The 99% contribution of corrole to the HOMO of native species indicates that oxidation occurs from the corrole moiety. The results of the electrochemical and spectroelectrochemical measurements and density functional theory calculations clearly display a preference of the {FeNO}⁶ unit to get reduced during the reduction step and the corrolato unit to get oxidized during the anodic process. Comparisons are presented with the structural, electrochemical, and spectroelectrochemical data of related compounds reported in the literature, with a particular focus on the interpretation of the EPR spectrum of the one-electron oxidized form

Sensitivity of a Strained C–C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands

The new compounds [Ru­(acac)₂(BIAN)], BIAN = bis­(arylimino)­acenaphthene (aryl = Ph (<b>1a</b>), 4-MeC₆H₄ (<b>2a</b>), 4-OMeC₆H₄ (<b>3a</b>), 4-ClC₆H₄ (<b>4a</b>), 4-NO₂C₆H₄ (<b>5a</b>)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d­(CN) < 1.334 and 1.425 Å < d­(CC) < 1.449 Å, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru^III(BIAN^•–) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C–C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru­(acac)₂(BIAN)] produces electron paramagnetic resonance (EPR) and UV–vis–NIR (NIR = near infrared) spectroelectrochemically detectable Ru^III species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH₃ to NO₂ has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(μ-tppz)­[Ru­(Cl)­(BIAN)]₂}­(ClO₄)₂, tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine; aryl (BIAN) = Ph ([<b>1b</b>]­(ClO₄)₂), 4-MeC₆H₄ ([<b>2b</b>]­(ClO₄)₂), 4-OMeC₆H₄ ([<b>3b</b>]­(ClO₄)₂), 4-ClC₆H₄ ([<b>4b</b>]­(ClO₄)₂), were also obtained and investigated. The structure determination of [<b>2b</b>]­(ClO₄)₂ and [<b>3b</b>]­(ClO₄)₂ reveals <i>trans</i> configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV–vis–NIR, EPR) indicate oxidation to a weakly coupled Ru^IIIRu^II mixed-valent species but reduction to a tppz-centered radical state. The effect of the π electron-accepting BIAN ancillary ligands is to diminish the metal–metal interaction due to competition with the acceptor bridge tppz

Sensitivity of a Strained C–C Single Bond to Charge Transfer: Redox Activity in Mononuclear and Dinuclear Ruthenium Complexes of Bis(arylimino)acenaphthene (BIAN) Ligands

The new compounds [Ru­(acac)₂(BIAN)], BIAN = bis­(arylimino)­acenaphthene (aryl = Ph (<b>1a</b>), 4-MeC₆H₄ (<b>2a</b>), 4-OMeC₆H₄ (<b>3a</b>), 4-ClC₆H₄ (<b>4a</b>), 4-NO₂C₆H₄ (<b>5a</b>)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d­(CN) < 1.334 and 1.425 Å < d­(CC) < 1.449 Å, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru^III(BIAN^•–) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C–C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru­(acac)₂(BIAN)] produces electron paramagnetic resonance (EPR) and UV–vis–NIR (NIR = near infrared) spectroelectrochemically detectable Ru^III species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH₃ to NO₂ has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(μ-tppz)­[Ru­(Cl)­(BIAN)]₂}­(ClO₄)₂, tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine; aryl (BIAN) = Ph ([<b>1b</b>]­(ClO₄)₂), 4-MeC₆H₄ ([<b>2b</b>]­(ClO₄)₂), 4-OMeC₆H₄ ([<b>3b</b>]­(ClO₄)₂), 4-ClC₆H₄ ([<b>4b</b>]­(ClO₄)₂), were also obtained and investigated. The structure determination of [<b>2b</b>]­(ClO₄)₂ and [<b>3b</b>]­(ClO₄)₂ reveals <i>trans</i> configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV–vis–NIR, EPR) indicate oxidation to a weakly coupled Ru^IIIRu^II mixed-valent species but reduction to a tppz-centered radical state. The effect of the π electron-accepting BIAN ancillary ligands is to diminish the metal–metal interaction due to competition with the acceptor bridge tppz

Sensitivity of the Valence Structure in Diruthenium Complexes As a Function of Terminal and Bridging Ligands

The compounds [(acac)₂Ru^III(μ-H₂L^2–)­Ru^III(acac)₂] (<i>rac</i>, <b>1</b>, and <i>meso</i>, <b>1</b>′) and [(bpy)₂Ru^II(μ-H₂L^•–)­Ru^II(bpy)₂]­(ClO₄)₃ (<i>meso</i>, [<b>2</b>]­(ClO₄)₃) have been structurally, magnetically, spectroelectrochemically, and computationally characterized (acac^– = acetylacetonate, bpy = 2,2′-bipyridine, and H₄L = 1,4-diamino-9,10-anthraquinone). The N,O;N′,O′-coordinated μ-H₂L^<i>n</i>– forms two β-ketiminato-type chelate rings, and <b>1</b> or <b>1</b>′ are connected via NH···O hydrogen bridges in the crystals. <b>1</b> exhibits a complex magnetic behavior, while [<b>2</b>]­(ClO₄)₃ is a radical species with mixed ligand/metal-based spin. The combination of redox noninnocent bridge (H₂L⁰ → → → →H₂L^4–) and {(acac)₂Ru^II} → →{(acac)₂Ru^IV} or {(bpy)₂Ru^II} → {(bpy)₂Ru^III} in <b>1</b>/<b>1</b>′ or <b>2</b> generates alternatives regarding the oxidation state formulations for the accessible redox states (<b>1</b>^<i>n</i> and <b>2</b>^<i>n</i>), which have been assessed by UV–vis–NIR, EPR, and DFT/TD-DFT calculations. The experimental and theoretical studies suggest variable mixing of the frontier orbitals of the metals and the bridge, leading to the following most appropriate oxidation state combinations: [(acac)₂Ru^III(μ-H₂L^•–)­Ru^III(acac)₂]⁺ (<b>1</b>⁺) → [(acac)₂Ru^III(μ-H₂L^2–)­Ru^III(acac)₂] (<b>1</b>) → [(acac)₂Ru^III(μ-H₂L^•3–)­Ru^III(acac)₂]⁻/[(acac)₂Ru^III(μ-H₂L^2–)­Ru^II(acac)₂]⁻ (<b>1</b>^–) → [(acac)₂Ru^III(μ-H₂L^4–)­Ru^III(acac)₂]^2–/[(acac)₂Ru^II(μ-H₂L^2–)­Ru^II(acac)₂]^2– (<b>1</b>^2–) and [(bpy)₂Ru^III(μ-H₂L^•–)­Ru^II(bpy)₂]⁴⁺ (<b>2</b>⁴⁺) → [(bpy)₂Ru^II(μ-H₂L^•–)­Ru^II(bpy)₂]³⁺/[(bpy)₂Ru^II(μ-H₂L^2–)­Ru^III(bpy)₂]³⁺ (<b>2</b>³⁺) → [(bpy)₂Ru^II(μ-H₂L^2–)­Ru^II(bpy)₂]²⁺ (<b>2</b>²⁺). The favoring of Ru^III by σ-donating acac^– and of Ru^II by the π-accepting bpy coligands shifts the conceivable valence alternatives accordingly. Similarly, the introduction of the NH donor function in H₂L^<i>n</i> as compared to O causes a cathodic shift of redox potentials with corresponding consequences for the valence structure