Structure and Spectroelectrochemistry (UV/Vis, IR, EPR) of the Acceptor-Bridged Heterodinuclear Complex [(η⁵-C₅Me₅)ClRh(μ-bptz)Re(CO)₃Cl](PF₆), bptz = 3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine

The title complex contains two organometallic reaction centers which are known to engage in hydride transfer catalysis (Rh) or in CO2 activation (Re), each after reductive elimination of the respective chloride ligand. The bridged heterodinuclear compound has been structurally characterized in the form where the chloride ligands are in cis-configuration relative to the bptz plane. The complex was subjected to cyclic voltammetry and spectroelectrochemical reduction to reveal an electrochemically reversible one-electron uptake by the bptz bridge, a rhodium chloride-dissociative second reduction to yield neutral [(η5-C5Me5)Rh(μ-bptz)Re(CO)3Cl], and a quasi-reversible third one-electron reduction

Structure and Spectroelectrochemistry (UV/Vis, IR, EPR) of the Acceptor-Bridged Heterodinuclear Complex [(η⁵-C₅Me₅)ClRh(μ-bptz)Re(CO)₃Cl](PF₆), bptz = 3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine

The title complex contains two organometallic reaction centers which are known to engage in hydride transfer catalysis (Rh) or in CO2 activation (Re), each after reductive elimination of the respective chloride ligand. The bridged heterodinuclear compound has been structurally characterized in the form where the chloride ligands are in cis-configuration relative to the bptz plane. The complex was subjected to cyclic voltammetry and spectroelectrochemical reduction to reveal an electrochemically reversible one-electron uptake by the bptz bridge, a rhodium chloride-dissociative second reduction to yield neutral [(η5-C5Me5)Rh(μ-bptz)Re(CO)3Cl], and a quasi-reversible third one-electron reduction

Valence Delocalization despite Weak Metal−Metal Coupling in a Bis(organoosmium(III,II)) Complex with a Pyrazine Bridge

Carbonyl vibrational spectroelectrochemistry of {(μ-pz)[Os(PiPr3)2(CO)(H)Cl]2}0/+ in dichloromethane reveals valence delocalization of the mixed-valent state despite relatively weak metal−metal coupling, as evident from the comproportionation constant Kc = 104.3 and the intervalence charge-transfer band at 1705 nm (ε = 1250 M-1 cm-1, Δν1/2 = 3700 cm-1). The rather low charge and nonpolar medium, i.e., the absence of valence trapping by counterions or solvent molecules, favor this particular situation

(Spectro)electrochemical and Electrocatalytic Investigation of 1,1′-Dithiolatoferrocene–Hexacarbonyldiiron

Hexacarbonyldiiron bridged by a 1,1′-dithiolatoferrocene, [Fe­(C5H4S)2{Fe­(CO)3}2] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe­(C5H4S)2 site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction

Molecule-Bridged Mixed-Valent Intermediates Involving the Ru^I Oxidation State

The diruthenium(II) complexes {(μ-L)[RuCl(Cym)]2}(PF6)n, Cym = p-cymene = 4-isopropyltoluene, L = 2,2‘-azobispyridine = abpy and n = 1, or L = 2,5-bis(1-phenyliminoethyl)pyrazine = bpip and n = 2, were synthesized and characterized by NMR (n = 2) or EPR spectroscopy (n = 1). Whereas the (n = 1) species are ligand radical-bridged RuIIRuII complexes, the three-electron reduction under loss of both chloride ions produces the ions {(μ-L)[Ru(Cym)]2}+, which could be identified as RuI(4d7)-containing mixed-valent species (Ru0RuI or RuIRuII) through UV−vis−NIR spectroelectrochemistry (intervalence charge-transfer bands around 1500 nm) and EPR (rhombic g tensor anisotropy). The weak metal−metal interaction of the dσ electrons from the eg set is responsible for the small electrochemical coupling with comproportionation constants Kc ≈ 102

On the Question of Mixed-Valent States in Ligand-Bridged Dinuclear Organoplatinum Compounds [R<i>_k</i>Pt(μ-L)PtR<i>_k</i>]<i>ⁿ</i>, <i>k</i> = 2 or 4^†

Symmetrically dinuclear complexes between the bis-bidentate bridging ligands μ-L (μ-L = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis(1-phenyliminoethyl)pyrazine (bpip)) and the organoplatinum fragments PtMes2 (Mes = mesityl), PtMe2, or PtMe4 were synthesized as deeply colored compounds. Low-energy charge-transfer transitions from metal d orbitals (PtII) or metal−carbon σ bond combinations (PtIV) to low-lying π* orbitals of the π acceptor ligands are responsible for long-wavelength absorption maxima λmax(CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes indicate the formation of [PtIV]2(μ-L•-) and [PtII]2(μ-L•-) species, however, the latter exhibit a significant metal contribution according to a PtII/PtI formulation. Cyclic voltammetry reveals that the remarkable system [Mes2Pt(μ-bptz)PtMes2]n forms an enormously stabilized radical anion (n = 1−) with ΔE1/2 = 1250 V and Kc = 1021.2 and a PtIII/PtII mixed-valent state (n = 1+) with ΔE1/2 = 80 mV and Kc = 23. This small Kc value is attributed to the predominantly dσ orbital character of the redox orbitals on the Pt(II) centers

Reversible and Site-Specific Reduction of the Ligand Sides in a Molecular Rectangle with up to Eight Electrons

Reversible and Site-Specific Reduction of the Ligand Sides in a Molecular Rectangle with up to Eight Electron

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