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–

A Ligand-Bridged Heterotetranuclear (Fe₂Cu₂) Redox System with Fc/Fc⁺ and Radical Ion Intermediates

The redox pair [(μ-abcp)­{Cu­(dppf)}₂]^2+/+ (abcp = 2,2′-azobis­(5-chloropyrimidine) and dppf =1,1′-bis­(diphenylphosphino)­ferrocene) has been structurally characterized to reveal the lengthening of the NN and shortening of the CN_azo bonds on reduction, each by about 0.04 Å. These and other charge forms, [(μ-abcp)­{Cu­(dppf)}₂]^<i>n</i>+ (n = 0, 3+, 4+), have been investigated spectroelectrochemically (UV–vis–near-IR, EPR) to reveal an abcp-based second reduction and a stepwise ferrocene-centered oxidation of the 2+ precursor. In contrast to the small but detectable comproportionation constant of <i>K</i>_c = 17 for the Fc/Fc⁺ mixed-valence (3+) charge state, the monocationic radical complex exhibits a very large <i>K</i>_c value of 10¹⁶

A Ligand-Bridged Heterotetranuclear (Fe₂Cu₂) Redox System with Fc/Fc⁺ and Radical Ion Intermediates

The redox pair [(μ-abcp)­{Cu­(dppf)}₂]^2+/+ (abcp = 2,2′-azobis­(5-chloropyrimidine) and dppf =1,1′-bis­(diphenylphosphino)­ferrocene) has been structurally characterized to reveal the lengthening of the NN and shortening of the CN_azo bonds on reduction, each by about 0.04 Å. These and other charge forms, [(μ-abcp)­{Cu­(dppf)}₂]^<i>n</i>+ (n = 0, 3+, 4+), have been investigated spectroelectrochemically (UV–vis–near-IR, EPR) to reveal an abcp-based second reduction and a stepwise ferrocene-centered oxidation of the 2+ precursor. In contrast to the small but detectable comproportionation constant of <i>K</i>_c = 17 for the Fc/Fc⁺ mixed-valence (3+) charge state, the monocationic radical complex exhibits a very large <i>K</i>_c value of 10¹⁶

Evidence for Bidirectional Noninnocent Behavior of a Formazanate Ligand in Ruthenium Complexes

Redox series of the complexes [Ru­(L)­(L′)₂]^<i>n</i>, L = 1,5-diphenyl-3-(4-tolyl)-formazanate and L′ = 2,4-pentanedionate (acac^–), 2,2′-bipyridine (bpy), or 2-phenylazopyridine (pap), were studied by cyclic and differential pulse voltammetry and by TD-DFT-supported spectroelectrochemistry (UV–vis–NIR, EPR). The precursors [Ru^III(L^–)­(acac^–)₂], [Ru^II(L^–)­(bpy)₂]­ClO₄, and [Ru^II(L^–)­(pap)₂]­ClO₄ were identified in their indicated oxidation states by X-ray crystal structure determination. The six-membered formazanato-ruthenium chelate rings have an envelope conformation with puckering of the metal. DFT calculations indicate a pronounced sensitivity of the N–N bond lengths toward the ligand oxidation state. Several electrochemically accessible charge states were analyzed, and the derived oxidation numbers Ru^II, Ru^III, or Ru^IV, L′ or (L′)^•–, and L^–, L^•2–, or the new formazanyl ligand L^• for the two-way noninnocent formazanate reflect the increasing acceptor effect of the ancillary ligands L′ in the series acac^– < bpy < pap

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

Evidence for Bidirectional Noninnocent Behavior of a Formazanate Ligand in Ruthenium Complexes

Redox series of the complexes [Ru­(L)­(L′)₂]^<i>n</i>, L = 1,5-diphenyl-3-(4-tolyl)-formazanate and L′ = 2,4-pentanedionate (acac^–), 2,2′-bipyridine (bpy), or 2-phenylazopyridine (pap), were studied by cyclic and differential pulse voltammetry and by TD-DFT-supported spectroelectrochemistry (UV–vis–NIR, EPR). The precursors [Ru^III(L^–)­(acac^–)₂], [Ru^II(L^–)­(bpy)₂]­ClO₄, and [Ru^II(L^–)­(pap)₂]­ClO₄ were identified in their indicated oxidation states by X-ray crystal structure determination. The six-membered formazanato-ruthenium chelate rings have an envelope conformation with puckering of the metal. DFT calculations indicate a pronounced sensitivity of the N–N bond lengths toward the ligand oxidation state. Several electrochemically accessible charge states were analyzed, and the derived oxidation numbers Ru^II, Ru^III, or Ru^IV, L′ or (L′)^•–, and L^–, L^•2–, or the new formazanyl ligand L^• for the two-way noninnocent formazanate reflect the increasing acceptor effect of the ancillary ligands L′ in the series acac^– < bpy < pap

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

At the Borderline between Metal–Metal Mixed Valency and a Radical Bridge Situation: Four Charge States of a Diruthenium Complex with a Redox-Active Bis(<i>mer</i>-tridentate) Ligand

The complex ions [L³Ru­(μ,η³:η³-BL)­RuL³]^<i>n</i>+ (<b>1</b>^{<b><i>n</i>+</b>}, L³ = 4,4′,4″-tri-<i>tert</i>-butyl-2,6,2′,6″-terpyridine and H₂BL^2– = 1,2-bis­(salicyloyl)­hydrazide(2−)) were isolated with PF₆^– or ClO₄^– counterions (<i>n</i> = 1) and as bis­(hexafluorophosphate) (<i>n</i> = 2). Structural, electrochemical, and spectroscopic characterization reveals the monocation as intermediate (<i>K</i>_c = 10^8.2) in the three-step reversible redox system <b>1</b>^{<b>0/+/2+/3+</b>}. The <b>1</b>^<b>+</b> ion has the molecule-bridged (Ru- - -Ru 4.727 Å) ruthenium centers involved in five- and six-membered chelate rings, and it exhibits long-wavelength absorptions at λ_max 2240, 1660, and 1530 nm (ε_max = 1000, 3000, and 8000 M^–1 cm^–1, respectively), which would be compatible with a Ru^IIIRu^II mixed-valent situation or with a coordinated radical ion bridge. In fact, EPR and DFT analysis of <b>1</b>^<b>+</b> reveals that the spin is equally distributed over the ligand bridge and over both metals. The oxidized paramagnetic ions <b>1</b>^<b>2+</b> and <b>1</b>^<b>3+</b> have been studied by ¹H NMR and EPR and by TD-DFT supported UV–vis–NIR and MIR (mid-IR) spectroelectrochemistry. The capacity of various kinds of bis­(<i>mer</i>-tridentate) bridging ligands (π donors or π acceptors, cyclometalated or noncyclometalated) for mediating metal–metal interactions is discussed

Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly <i>π-</i>accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system <b>1</b>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –)

Ancillary Ligand Control of Electronic Structure in o-Benzoquinonediimine-Ruthenium Complex Redox Series: Structures, Electron Paramagnetic Resonance (EPR), and Ultraviolet−Visible−Near-Infrared (UV-vis-NIR) Spectroelectrochemistry

The compounds Ru­(acac)₂(Q) (<b>1</b>), [Ru­(bpy)₂(Q)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), and [Ru­(pap)₂(Q)]­PF₆ ([<b>3</b>]­PF₆), containing Q = <i>N,N</i>′-diphenyl-<i>o</i>-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac^–), π-accepting 2,2^/-bipyridine (bpy), or strongly <i>π-</i>accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system <b>1</b>⁺–<b>1</b>^– prefers trivalent ruthenium with corresponding oxidation states Q⁰–Q^2– of the noninnocent ligand, the series <b>2</b>²⁺–<b>2</b>⁰ and <b>3</b>²⁺–<b>3</b>^– retain Ru^II. The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in <i>o</i>-quinonediimine ligands [Kalinina et al., <i>Inorg. Chem</i>. <b>2008</b>, <i>47</i>, 10110] and to the discussion of the most appropriate oxidation state formulation Ru^II(Q⁰) or Ru^III(Q^• –)