Solar Cell Sensitizer Models [Ru(bpy-R)₂(NCS)₂] Probed by Spectroelectrochemistry

Complexes [Ru­(bpy-R)₂(NCS)₂], where R = H (<b>1</b>), 4,4′-(CO₂Et)₂ (<b>2</b>), 4,4′-(OMe)₂ (<b>3</b>), and 4,4′-Me₂ (<b>4</b>), were studied by spectroelectrochemistry in the UV–vis and IR regions and by in situ electron paramagnetic resonance (EPR). The experimental information obtained for the frontier orbitals as supported and ascertained by density functional theory (DFT) calculations for <b>1</b> is relevant for the productive excited state. In addition to the parent <b>1</b>, the ester complex <b>2</b> was chosen for its relationship to the carboxylate species involved for binding to TiO₂ in solar cells; the donor-substituted <b>3</b> and <b>4</b> allowed for better access to oxidized forms. Reflecting the metal-to-ligand (Ru → bpy) charge-transfer characteristics of the compounds, the electrochemical and EPR results for compounds <b>1</b>–<b>4</b> agree with previous notions of one metal-centered oxidation and several (bpy-R) ligand-centered reductions. The first one-electron reduction produces extensive IR absorption, including intraligand transitions and broad ligand-to-ligand intervalence charge-transfer transitions between the one-electron-reduced and unreduced bpy-R ligands. The electron addition to one remote bpy-R ligand does not significantly affect the N–C stretching frequency of the Ru^IINCS unit. Upon oxidation of Ru^II to Ru^III, however, the single N–C stretching band exhibits a splitting and a shift to lower energies. The DFT calculations serve to reproduce and understand these effects; they also suggest significant spin density on S for the oxidized form

Noninnocence of Indigo: Dehydroindigo Anions as Bridging Electron-Donor Ligands in Diruthenium Complexes

Complexes of singly or doubly deprotonated indigo (H₂Ind) with one or two [Ru­(pap)₂]²⁺ fragments (pap = 2-phenylazopyridine) have been characterized experimentally [molecular structure, voltammetry, electron paramagnetic resonance (EPR), and UV–vis–near-IR spectroelectrochemistry] and by time-dependent density functional theory calculations. The compound [Ru­(pap)₂(HInd^–)]­ClO₄ ([<b>1</b>]­ClO₄) was found to contain an intramolecular NH---O hydrogen bond, whereas [{Ru­(pap)₂}₂(μ-Ind^2–)]­(ClO₄)₂ ([<b>2</b>]­(ClO₄)₂), isolated as the meso diastereoisomer with near-IR absorptions at 1162 and 991 nm, contains two metals bridged at 6.354 Å distance by the bischelating indigo dianion. The spectroelectrochemical study of multiple reversible reduction and oxidation processes of <b>2</b>^<i>n</i> (<i>n</i> = 4+, 3+, 2+, 1+, 0, 1–, 2–, 3–, 4−) reveals the stepwise addition of electrons to the terminal π-accepting pap ligands, whereas the oxidations occur predominantly at the anionic indigo ligand, producing an EPR-identified indigo radical intermediate and revealing the suitability of deprotonated indigo as a σ- and π-donating bischelating bridge

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

Metal–Metal Bridging Using the DPPP Dye System: Electronic Configurations within Multiple Redox Series

Redox series [L_<i>n</i>Ru­(μ-DPPP)­RuL_<i>n</i>]^<i>k</i>, H₂DPPP = 2,5-dihydro-3,6-di-2-pyridylpyrrolo­(3,4-<i>c</i>)­pyrrole-1,4-dione and L = 2,4-pentanedionato (acac^–), 2,2′-bipyridine (bpy), and 2-phenylazopyridine (pap), have been studied by voltammetry (CV, DPV), EPR, and UV–vis–NIR spectroelectrochemistry, supported by TD-DFT calculations. Crystal structure analysis and ¹H NMR revealed oxidation states [(acac)₂Ru^III(μ-DPPP^2–)­Ru^III(acac)₂] and [(bpy)₂Ru^II(μ-DPPP^2–)­Ru^II(bpy)₂]²⁺ for the corresponding precursors, isolated as <i>rac</i> diastereomers. Oxidation was observed to occur mainly at the bridging ligand (DPPP^2– → DPPP^•–), whereas the site of reduction (DPPP, Ru, or L) depends on effects from the ancillary ligands L. The metal coordination of a derivative of the pigment forming 2,5-dihydro-pyrrolo­(3,4-<i>c</i>)­pyrrole-1,4-dione (DPP) dyes and the analysis of corresponding multistep redox series add to the previously recognized coordinative and electron transfer potential of dye molecules of the azo, indigo, anthraquinone, and formazanate type

Isomeric Diruthenium Complexes of a Heterocyclic and Quinonoid Bridging Ligand: Valence and Spin Alternatives for the Metal/Ligand/Metal Arrangement

5,7,12,14-Tetraazapentacene-6,13-quinone (L) reacts with 2 equiv of [Ru­(acac)₂(CH₃CN)₂] to form two linkage isomeric bis­(chelate) compounds, [{Ru^II(acac)₂}₂(μ-L)], blue <b>1</b>, with 5,6;12,13 coordination and violet <b>2</b> with 5,6;13,14 coordination. The linkage isomers could be separated, structurally characterized in crystals as <i>rac</i> diastereomers (ΔΔ/ΛΛ), and studied by voltammetry (CV, DPV), EPR, and UV–vis–NIR spectroelectrochemistry (<i>meso</i>-<b>1</b>, <i>rac</i>-<b>2</b>). DFT and TD-DFT calculations support the structural and spectroscopic results and suggest a slight energy preference (Δ<i>E</i> = 263 cm^–1) for the <i>rac</i>-isomer <b>1</b> as compared to <b>2</b>. Starting from the Ru^II–(μ-L⁰)–Ru^II configurations of <b>1</b> and <b>2</b> with low-lying metal-to-ligand charge transfer (MLCT) absorptions, the compounds undergo two reversible one-electron oxidation steps with open-shell intermediates <b>1</b>^<b>+</b> (<i>K</i>_c = 4 × 10⁴) and <b>2</b>^<b>+</b> (<i>K</i>_c = 6 × 10⁵). Both monocations display metal-centered spin according to EPR, but the DFT-calculated spin densities suggest a Ru^III(μ-L^•–)­Ru^III three-spin situation with opposite spin density at the bridging ligand for the <i>meso</i> form of <b>1</b>⁺, estimated to lie 1887 cm^–1 lower in energy than <i>rac</i>-<b>1</b>^<b>+</b>, which is calculated with a Class II mixed-valent situation Ru^III–(μ-L⁰)–Ru^II. A three-spin arrangement Ru^III–(μ-L^•–)–Ru^III with negative spin density at one metal site is suggested by DFT for <i>rac</i>-<b>2</b>^<b>+</b> which is more stable by Δ<i>E</i> = 890 cm^–1 than <i>rac</i>-<b>1</b>^<b>+</b>. Reduction of <b>1</b> or <b>2</b> (<i>K</i>_c = 10⁷–10⁸) occurs mainly at the central bridging ligand with notable contributions (30%) from the metals in <b>1</b>^<b>–</b> and <b>2</b>^<b>–</b>. The mixed-valent Ru^III(μ-L)­Ru^II versus radical-bridged Ru^III(μ-L^•–)­Ru^III alternative is discussed comprehensively in comparison with related valence-ambiguous cases

1,5-Diamido-9,10-anthraquinone, a Centrosymmetric Redox-Active Bridge with Two Coupled β‑Ketiminato Chelate Functions: Symmetric and Asymmetric Diruthenium Complexes

The dinuclear complexes {(μ-H₂L)­[Ru­(bpy)₂]₂}­(ClO₄)₂ ([<b>3</b>]­(ClO₄)₂), {(μ-H₂L)­[Ru­(pap)₂]₂}­(ClO₄)₂ ([<b>4</b>]­(ClO₄)₂), and the asymmetric [(bpy)₂Ru­(μ-H₂L)­Ru­(pap)₂]­(ClO₄)₂ ([<b>5</b>]­(ClO₄)₂) were synthesized via the mononuclear species [Ru­(H₃L)­(bpy)₂]­ClO₄ ([<b>1</b>]­ClO₄) and [Ru­(H₃L)­(pap)₂]­ClO₄ ([<b>2</b>]­ClO₄), where H₄L is the centrosymmetric 1,5-diamino-9,10-anthraquinone, bpy is 2,2′-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [<b>1</b>]­ClO₄, [<b>2</b>]­ClO₄, [<b>3</b>]­(ClO₄)₂, [<b>4</b>]­(ClO₄)₂, and [<b>5</b>]­(ClO₄)₂ reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV–vis–NIR spectro-electrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in <b>3</b>³⁺ versus largely bridge-centered spin in <b>4</b>³⁺a result of the presence of Ru^II-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system <b>5</b>³⁺ with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H₂L^2– bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites

1,5-Diamido-9,10-anthraquinone, a Centrosymmetric Redox-Active Bridge with Two Coupled β‑Ketiminato Chelate Functions: Symmetric and Asymmetric Diruthenium Complexes

The dinuclear complexes {(μ-H₂L)­[Ru­(bpy)₂]₂}­(ClO₄)₂ ([<b>3</b>]­(ClO₄)₂), {(μ-H₂L)­[Ru­(pap)₂]₂}­(ClO₄)₂ ([<b>4</b>]­(ClO₄)₂), and the asymmetric [(bpy)₂Ru­(μ-H₂L)­Ru­(pap)₂]­(ClO₄)₂ ([<b>5</b>]­(ClO₄)₂) were synthesized via the mononuclear species [Ru­(H₃L)­(bpy)₂]­ClO₄ ([<b>1</b>]­ClO₄) and [Ru­(H₃L)­(pap)₂]­ClO₄ ([<b>2</b>]­ClO₄), where H₄L is the centrosymmetric 1,5-diamino-9,10-anthraquinone, bpy is 2,2′-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [<b>1</b>]­ClO₄, [<b>2</b>]­ClO₄, [<b>3</b>]­(ClO₄)₂, [<b>4</b>]­(ClO₄)₂, and [<b>5</b>]­(ClO₄)₂ reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV–vis–NIR spectro-electrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in <b>3</b>³⁺ versus largely bridge-centered spin in <b>4</b>³⁺a result of the presence of Ru^II-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system <b>5</b>³⁺ with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H₂L^2– bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites

1,5-Diamido-9,10-anthraquinone, a Centrosymmetric Redox-Active Bridge with Two Coupled β‑Ketiminato Chelate Functions: Symmetric and Asymmetric Diruthenium Complexes

The dinuclear complexes {(μ-H₂L)­[Ru­(bpy)₂]₂}­(ClO₄)₂ ([<b>3</b>]­(ClO₄)₂), {(μ-H₂L)­[Ru­(pap)₂]₂}­(ClO₄)₂ ([<b>4</b>]­(ClO₄)₂), and the asymmetric [(bpy)₂Ru­(μ-H₂L)­Ru­(pap)₂]­(ClO₄)₂ ([<b>5</b>]­(ClO₄)₂) were synthesized via the mononuclear species [Ru­(H₃L)­(bpy)₂]­ClO₄ ([<b>1</b>]­ClO₄) and [Ru­(H₃L)­(pap)₂]­ClO₄ ([<b>2</b>]­ClO₄), where H₄L is the centrosymmetric 1,5-diamino-9,10-anthraquinone, bpy is 2,2′-bipyridine, and pap is 2-phenylazopyridine. Electrochemistry of the structurally characterized [<b>1</b>]­ClO₄, [<b>2</b>]­ClO₄, [<b>3</b>]­(ClO₄)₂, [<b>4</b>]­(ClO₄)₂, and [<b>5</b>]­(ClO₄)₂ reveals multistep oxidation and reduction processes, which were analyzed by electron paramagnetic resonance (EPR) of paramagnetic intermediates and by UV–vis–NIR spectro-electrochemistry. With support by time-dependent density functional theory (DFT) calculations the redox processes could be assigned. Significant results include the dimetal/bridging ligand mixed spin distribution in <b>3</b>³⁺ versus largely bridge-centered spin in <b>4</b>³⁺a result of the presence of Ru^II-stabilizig pap coligands. In addition to the metal/ligand alternative for electron transfer and spin location, the dinuclear systems allow for the observation of ligand/ligand and metal/metal site differentiation within the multistep redox series. DFT-supported EPR and NIR absorption spectroscopy of the latter case revealed class II mixed-valence behavior of the oxidized asymmetric system <b>5</b>³⁺ with about equal contributions from a radical bridge formulation. In comparison to the analogues with the deprotonated 1,4-diaminoanthraquinone isomer the centrosymmetric H₂L^2– bridge shows anodically shifted redox potentials and weaker electronic coupling between the chelate sites

Organosilicon Radicals with Si–H and Si–Me Bonds from Commodity Precursors

The cyclic alkyl­(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)₂SiH₂ (<b>1</b>), (cAAC)­SiMe₂-SiMe₂(cAAC) (<b>2</b>), and (cAAC)­SiMeCl-SiMeCl­(cAAC) (<b>3</b>) have been isolated as molecular species. All the compounds are stable at room temperature for more than 6 months under inert conditions in the solid state. All radical species were fully characterized by single-crystal X-ray structure analysis and EPR spectroscopy. Furthermore, the structure and bonding of compounds <b>1</b>–<b>3</b> have been investigated by theoretical methods. Compound <b>1</b> contains the SiH₂ moiety and this is the first instance, where we have isolated <b>1</b> without an acceptor molecule