Twisting and Tilting 1,1′-Bis(dialkylphosphino)ferrocene Bound to Low Valent Tricarbonylmaganese(I to −I)

Recently we had reported the <i>noninnocent</i> behavior of 1,1′-bis­(diphenylphosphino)­ferrocene (dppf) in Fe­(CO)₃dppf [Ringenberg et al.<i>, Inorg. Chem.</i>, <b>2017</b>, <i>56</i>, 7501]. Moving to the left in the periodic table, HMn­(CO)₃(dRpf) where dRpf = dppf (<b>1H</b>) and 1,1′-bis­(diisopropylphosphino)­ferrocene (dippf) (<b>2H</b>) were synthesized. The hydride ligand was removed by protonation with [(Et₂O)₂H]­[B­(Ar^F)₄] ([B­(Ar^F)₄]⁻ = tetrakis­[3,5-bis­(trifluoromethyl)­phenyl]­borate), resulting in the rapid evolution of H₂ followed by the formation of an Fe→Mn interaction. The reaction mechanism was determined by <i>in situ</i> IR experiments which show that directly following protonation both [<b>1</b>]⁺ and [<b>2</b>]⁺ offer an open manganese coordination site that allows for the formation of an intramolecular Fe→Mn dative bond. This process is significantly faster for [<b>2</b>]⁺ than for [<b>1</b>]⁺. The reduction chemistry as studied by cyclic voltammetry (CV) reveals that both complexes change from a distorted octahedral coordination with an Fe→Mn interaction to an open square-pyramidal configuration which is more stable for [<b>1</b>]⁰ than [<b>2</b>]⁰. Reoxidation of this square-pyramidal species proceeds more reversibly for <b>2</b> versus <b>1</b> due to the faster ferrocene ligand reorganization. The electrochemical mechanism was studied by <i>in situ</i> spectroscopic techniques, e.g., IR, UV–vis–NIR (near IR), and EPR spectroelectrochemistry (SEC) as well as by CV simulation. The new complexes described offer an exciting platform for the development of electrocatalysts for the reduction of CO₂ to CO, or for proton reduction (2H⁺ + 2e^– → H₂)

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

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

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

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

Oxidation of Natural Bioactive Flavonolignan 2,3-Dehydrosilybin: An Electrochemical and Spectral Study

The electrochemical oxidation of the natural antioxidant 2,3-dehydrosilybin (<b>DHS</b>) was investigated in acetonitrile. The spectral changes during two electron and two proton oxidation registered by in situ IR spectroelectrochemistry show that the electron transfer is followed by a subsequent chemical reaction with traces of water. A benzofuranone derivative (<b>BF</b>) is formed by ECEC (electron transfer–chemical reaction–electron transfer–chemical reaction) process at the potential of the first oxidation wave. A minor difference in the chemical structures of flavonolignans <b>DHS</b> and silybin, the presence of a double bond between atoms C-2 and C-3 in the <b>DHS</b> molecule, causes the formation of completely different oxidation products. <b>BF</b> was for the first time identified as the product of the oxidation of flavonolignan <b>DHS</b>. Its formation was proved by electroanalytical, chromatographic, and spectroelectrochemical techniques. Molecular orbital calculations support the experimental findings

Electrochemical Evidence for Hemilabile Coordination of 1,3-Dimethyllumazine to [1,1′-Bis(diorganophosphino)ferrocene]copper(I)

The complex cations [Cu­(dippf)­(DML)]⁺ ([<b>1</b>]⁺) and [Cu­(dppf)­(DML)]⁺ ([<b>2</b>]⁺), where dippf = 1,1′-bis­(diisopropylphosphino)­ferrocene, dppf = 1,1′-bis­(diphenylphosphino)­ferrocene, and DML = 1,3-dimethyllumazine, were prepared and crystallized as BF₄^– or PF₆^– salts. Structure determinations of the tetrafluoroborates revealed asymmetric O⁴,N⁵ chelation of DML to copper­(I) with longer Cu–O bonds of about 2.25 Å. Reversible oxidation to [<b>1</b>]²⁺ and [<b>2</b>]²⁺ proceeds at the ferrocene units, while reduction leads to the neutral radical complexes [<b>1</b>] and [<b>2</b>] with the unpaired electron localized on the DML ligand. The occurrence of two voltammetric steps for the one-electron-reduction process is attributed to a two-species equilibrium caused by the hemilabile coordination of DML. Electrochemical and spectroelectrochemical measurements (UV–vis, IR) reveal increased coordination lability of the reduced complexes and their slow fragmentation

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