Varying Electronic Structures of Diosmium Complexes from Noninnocently Behaving Anthraquinone-Derived Bis-chelate Ligands

The new compounds [(bpy)₂Os^II(μ-L₁^2–)­Os^II(bpy)₂]­(ClO₄)₂ ([<b>1</b>]­(ClO₄)₂) and [(pap)₂Os^II(μ-L₁^2–)­Os^II(pap)₂]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂) (H₂L₁ = 1,4-dihydroxy-9,10-anthraquinone, bpy = 2,2^/-bipyridine, and pap = 2-phenylazopyridine) and [(bpy)₂Os^II(μ-L₂^•–)­Os^II(bpy)₂]­(ClO₄)₃ ([<b>3</b>]­(ClO₄)₃) and [(pap)₂Os^II(μ-L₂^2–)­Os^II(pap)₂]­(ClO₄)₂ ([<b>4</b>]­(ClO₄)₂) (H₂L₂ = 1,4-diamino-9,10-anthraquinone) have been analytically identified as the <i>meso</i> and <i>rac</i> diastereoisomers, respectively. The paramagnetic [<b>3</b>]­(ClO₄)₃ was also characterized by crystal structure determination. In CD₃CN solution, [<b>3</b>]­(ClO₄)₃ displays rather narrow but widely split (13 > δ > −8 ppm) resonances in the ¹H NMR spectrum, yet no EPR signal was observed down to 120 K. Cyclic voltammetry and differential pulse voltammetry reveal several accessible redox states on oxidation and reduction, showing that the replacement of 1,4-oxido by imido donors causes cathodic shifts and that the substitution of bpy by the stronger π-accepting pap ligands leads to a strong increase of redox potentials. Accordingly, system <b>3</b>^<i>n</i> with the lowest (2+/3+) potential was synthetically obtained in the mono-oxidized (3+) form. The (3+) intermediates display small comproportionation constants <i>K</i>_c of about 10³ and long-wavelength near-infrared absorptions; an EPR signal with appreciable <i>g</i> splitting (1.84, 1.96, and 2.03) was only observed for <b>4</b>³⁺, which exhibits the smallest spin density on the osmium centers. An oxidation state formulation [Os^III(μ-L^•3–)­Os^III]³⁺ with some [Os^II(μ-L^2–)­Os^III]³⁺ contribution was found to best describe the electronic structures. UV–vis–NIR absorption spectra were recorded for all accessible states by OTTLE spectroelectrochemistry and assigned on the basis of TD-DFT calculations. These results and additional EPR measurements suggest rather variegated oxidation state situations, e.g., the pap ligands competing with the bridge L for electrons, while the oxidation produces mixed spin systems with variable metal/ligand contributions

Noninnocently Behaving Bridging Anions of the Widely Distributed Antioxidant Ellagic Acid in Diruthenium Complexes

Dinuclear compounds [L₂Ru­(μ-E)­RuL₂]^<i>n</i> where L is acetylacetonate (acac^–, 2,4-pentanedionate), 2,2′-bipyridine (bpy), or 2-phenylazopyridine (pap) and EH₄ is ellagic acid, an antioxidative bis-catechol natural product, were studied by voltammetric and spectroelectrochemical techniques (UV–vis–NIR and electron paramagnetic resonance (EPR)). The electronic structures of the isolated forms (NBu₄)₂[(acac)₂Ru­(μ-E)­Ru­(acac)₂] ((NBu₄)₂[<b>1</b>]), [(bpy)₂Ru­(μ-E)­Ru­(bpy)₂]­ClO₄ ([<b>2</b>]­ClO₄), and [(pap)₂Ru­(μ-E)­Ru­(pap)₂] ([<b>3</b>]) were characterized by density functional theory (DFT) in conjunction with EPR and UV–vis–NIR measurements. The crystal structure of (NBu₄)₂[<b>1</b>] revealed the <i>meso</i> form and a largely planar Ru­(μ-E)Ru center. Several additional charge states of the compounds were electrochemically accessible and were identified mostly as complexes with noninnocently behaving pap^0/•– or bridging ellagate (E^<i>n</i>–) anions (<i>n</i> = 2, 3, 4) but not as mixed-valence intermediates. The free anions E^<i>n</i>–, <i>n</i> = 1–4, were calculated by time-dependent DFT to reveal NIR transitions for the radical forms with <i>n</i> = 1 and 3 and a triplet ground state for the bis­(<i>o</i>-semiquinone) dianion E^2–

Varying Electronic Structures of Diosmium Complexes from Noninnocently Behaving Anthraquinone-Derived Bis-chelate Ligands

The new compounds [(bpy)₂Os^II(μ-L₁^2–)­Os^II(bpy)₂]­(ClO₄)₂ ([<b>1</b>]­(ClO₄)₂) and [(pap)₂Os^II(μ-L₁^2–)­Os^II(pap)₂]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂) (H₂L₁ = 1,4-dihydroxy-9,10-anthraquinone, bpy = 2,2^/-bipyridine, and pap = 2-phenylazopyridine) and [(bpy)₂Os^II(μ-L₂^•–)­Os^II(bpy)₂]­(ClO₄)₃ ([<b>3</b>]­(ClO₄)₃) and [(pap)₂Os^II(μ-L₂^2–)­Os^II(pap)₂]­(ClO₄)₂ ([<b>4</b>]­(ClO₄)₂) (H₂L₂ = 1,4-diamino-9,10-anthraquinone) have been analytically identified as the <i>meso</i> and <i>rac</i> diastereoisomers, respectively. The paramagnetic [<b>3</b>]­(ClO₄)₃ was also characterized by crystal structure determination. In CD₃CN solution, [<b>3</b>]­(ClO₄)₃ displays rather narrow but widely split (13 > δ > −8 ppm) resonances in the ¹H NMR spectrum, yet no EPR signal was observed down to 120 K. Cyclic voltammetry and differential pulse voltammetry reveal several accessible redox states on oxidation and reduction, showing that the replacement of 1,4-oxido by imido donors causes cathodic shifts and that the substitution of bpy by the stronger π-accepting pap ligands leads to a strong increase of redox potentials. Accordingly, system <b>3</b>^<i>n</i> with the lowest (2+/3+) potential was synthetically obtained in the mono-oxidized (3+) form. The (3+) intermediates display small comproportionation constants <i>K</i>_c of about 10³ and long-wavelength near-infrared absorptions; an EPR signal with appreciable <i>g</i> splitting (1.84, 1.96, and 2.03) was only observed for <b>4</b>³⁺, which exhibits the smallest spin density on the osmium centers. An oxidation state formulation [Os^III(μ-L^•3–)­Os^III]³⁺ with some [Os^II(μ-L^2–)­Os^III]³⁺ contribution was found to best describe the electronic structures. UV–vis–NIR absorption spectra were recorded for all accessible states by OTTLE spectroelectrochemistry and assigned on the basis of TD-DFT calculations. These results and additional EPR measurements suggest rather variegated oxidation state situations, e.g., the pap ligands competing with the bridge L for electrons, while the oxidation produces mixed spin systems with variable metal/ligand contributions

Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10-anthraquinone-Bridged Diruthenium Compounds

The unsymmetrical diruthenium complexes [(bpy)₂Ru^II(μ-H₂L^2–)­Ru^III(acac)₂]­ClO₄ ([<b>3</b>]­ClO₄), [(pap)₂Ru^II(μ-H₂L^2–)­Ru^III(acac)₂]­ClO₄ ([<b>4</b>]­ClO₄), and [(bpy)₂Ru^II(μ-H₂L^2–)­Ru^II(pap)₂]­(ClO₄)₂ ([<b>5</b>]­(ClO₄)₂) have been obtained by way of the mononuclear precursors [(bpy)₂Ru^II(H₃L^–)]­ClO₄ ([<b>1</b>]­ClO₄) and [(pap)₂Ru^II(H₃L^–)]­ClO₄ ([<b>2</b>]­ClO₄) (where bpy = 2,2′-bipyridine, pap = 2-phenylazopyridine, acac^– = 2,4-pentanedionate, and H₄L = 1,4-diamino-9,10-anthraquinone). Structural characterization by single-crystal X-ray diffraction and magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR)) were used to establish the oxidation state situation in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet–visible–near-infrared (UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron transfer series of the potentially class I mixed-valent dinuclear compounds, considering the redox activities of differently coordinated metals, of the noninnocent bridge and of the terminal ligands. Comparison with symmetrical analogues [L₂^′Ru­(μ-H₂L)­RuL₂^′]^<i>n</i> (where L′ = bpy, pap, or acac^–) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged, with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands

Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10-anthraquinone-Bridged Diruthenium Compounds

The unsymmetrical diruthenium complexes [(bpy)₂Ru^II(μ-H₂L^2–)­Ru^III(acac)₂]­ClO₄ ([<b>3</b>]­ClO₄), [(pap)₂Ru^II(μ-H₂L^2–)­Ru^III(acac)₂]­ClO₄ ([<b>4</b>]­ClO₄), and [(bpy)₂Ru^II(μ-H₂L^2–)­Ru^II(pap)₂]­(ClO₄)₂ ([<b>5</b>]­(ClO₄)₂) have been obtained by way of the mononuclear precursors [(bpy)₂Ru^II(H₃L^–)]­ClO₄ ([<b>1</b>]­ClO₄) and [(pap)₂Ru^II(H₃L^–)]­ClO₄ ([<b>2</b>]­ClO₄) (where bpy = 2,2′-bipyridine, pap = 2-phenylazopyridine, acac^– = 2,4-pentanedionate, and H₄L = 1,4-diamino-9,10-anthraquinone). Structural characterization by single-crystal X-ray diffraction and magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR)) were used to establish the oxidation state situation in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet–visible–near-infrared (UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron transfer series of the potentially class I mixed-valent dinuclear compounds, considering the redox activities of differently coordinated metals, of the noninnocent bridge and of the terminal ligands. Comparison with symmetrical analogues [L₂^′Ru­(μ-H₂L)­RuL₂^′]^<i>n</i> (where L′ = bpy, pap, or acac^–) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged, with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands

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