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

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

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

Separation of Metal Binding and Electron Transfer Sites as a Strategy To Stabilize the Ligand-Reduced and Metal-Oxidized Form of [Mo(CO)₄L]

The zerovalent metal in [Mo­(CO)4(bmiq)] binds the two imidazole-N-imine donors of 2,3-bis­(1-methylimidazol-2-yl)­quinoxaline (bmiq), resulting in a seven-membered chelate ring coordinated in cis configuration. DFT calculations confirm the preference for a seven-membered vs five-membered ring chelation alternative as well as the experimental structural parameters. The complex is reversibly reduced in CH2Cl2 at −2.08 V and reversibly oxidized at −0.14 V vs ferrocenium/ferrrocene. The facilitated oxidation to a stable cation is attributed to the donor effect from the imidazole rings. In agreement with the DFT-calculated characteristics of the HOMO and LUMO, the in situ EPR studies at a Pt electrode reveal a MoI signature for the cation (g1 = 1.967, g2 = 1.944, g3 = 1.906; Aiso(95,97 Mo) = 50 G) and a quinoxaline radical-type EPR spectrum with dominant 14N coupling (2 N) of 6.0 G for the anion. IR spectroelectrochemistry confirms these assignments, showing small (Δν ≤ 20 cm–1) low-energy shifts of carbonyl stretching bands on reduction but significantly larger high-energy shifts (Δν = 77–142 cm–1) after oxidation. The neutral compound with a weak, broad MLCT absorption band at 500 nm is photolabile in solution. The unusual stability of both the anion and the cation is attributed to the spatial and electronic separation of the sites for electron loss (at the metal) and for electron uptake (at the uncoordinated quinoxaline ring)

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

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

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

Separation of Metal Binding and Electron Transfer Sites as a Strategy To Stabilize the Ligand-Reduced and Metal-Oxidized Form of [Mo(CO)₄L]

The zerovalent metal in [Mo­(CO)4(bmiq)] binds the two imidazole-N-imine donors of 2,3-bis­(1-methylimidazol-2-yl)­quinoxaline (bmiq), resulting in a seven-membered chelate ring coordinated in cis configuration. DFT calculations confirm the preference for a seven-membered vs five-membered ring chelation alternative as well as the experimental structural parameters. The complex is reversibly reduced in CH2Cl2 at −2.08 V and reversibly oxidized at −0.14 V vs ferrocenium/ferrrocene. The facilitated oxidation to a stable cation is attributed to the donor effect from the imidazole rings. In agreement with the DFT-calculated characteristics of the HOMO and LUMO, the in situ EPR studies at a Pt electrode reveal a MoI signature for the cation (g1 = 1.967, g2 = 1.944, g3 = 1.906; Aiso(95,97 Mo) = 50 G) and a quinoxaline radical-type EPR spectrum with dominant 14N coupling (2 N) of 6.0 G for the anion. IR spectroelectrochemistry confirms these assignments, showing small (Δν ≤ 20 cm–1) low-energy shifts of carbonyl stretching bands on reduction but significantly larger high-energy shifts (Δν = 77–142 cm–1) after oxidation. The neutral compound with a weak, broad MLCT absorption band at 500 nm is photolabile in solution. The unusual stability of both the anion and the cation is attributed to the spatial and electronic separation of the sites for electron loss (at the metal) and for electron uptake (at the uncoordinated quinoxaline ring)