One-electron Reduction Waves Research Articles

A comparative kinetic investigation of ethylcinnamate electrohydrodimerization on mercury and liquid gallium

Electrohydrodimerization (EHD) of ethylcinnamate (EC) on mercury takes place both from hydrotropic solutions of tetraethylammonium- p-toluenesulphonate (TEA-PTS) and from dilute aqueous solutions of the strong surfactant Triton X-100. In both cases the one-electron reduction wave due to hydrodimer formation satisfies the requirements for a rate-determining homogeneous coupling of the electrochemically generated anion radicals. The kinetics of EC EHD on liquid gallium is identical with that on mercury at the same temperature of 31°C in hydrotropic solutions of TEA-PTS; conversely, in aqueous Triton X-100 it is faster than on mercury and satisfies the requirements for a rate-determining heterogeneous radical-radical coupling. This behaviour is explained by a stronger adsorptivity of EC and its intermediate reduction products on gallium than on mercury, so that Triton X-100 does not succeed in displacing these species completely from the electrode surface. The general trend consisting in a higher adsorptivity of organic compounds with conjugated double bonds and/or aromatic rings on gallium than on mercury is explained on the basis of the difference in hydrophilicity between these two metals.

New intermediate-spin chloroiron(III) complex with a mixed nitrogen– sulfur co-ordination sphere ‡

A new tetradentate proligand, 6,6′-bis(2,2-diphenyl-2-sulfanylethyl)-2,2′-bipyridine (H2L), involving two nitrogen bases and two thiols, has been synthesized. The crystal structure of its chloroiron(III) complex [FeL(Cl)] exhibits a five-co-ordinate iron in a distorted square-pyramidal structure including a long Fe–Cl axial bond. Its effective magnetic moment (µeff = 3.8 µB from 10 to 300 K) and its Mossbauer parameters [δ = 0.328(5) mm s–1 and ΔEQ = 2.543(9) mm s–1 at 293 K] are consistent with a pure intermediate S = spin state for the iron(III). This complex exhibits a sulfur-to-iron charge transfer absorption at 490 nm and a one-electron reduction wave at –415 mV (vs. SSCE). Upon reaction with tetrabutylammonium hydroxide it is converted into the µ-oxo complex [Fe2L2O], which has been characterized by mass spectral and elemental analyses. Its formation is reversible in dmf upon addition of trifluoroacetic acid in the presence of chloride anion, as shown by 1H NMR and UV/VIS spectroscopy and electrochemistry. The [FeL(Cl)] complex appears to be the first example of an iron(III) complex with a mixed nitrogen–sulfur co-ordination sphere in a pure S = ground spin state.

Complexes of nickel(II), palladium(II) and platinum(II) with 2,6-bis(mercaptomethyl)pyridine

2,6-Bis(mercaptomethyl)pyridine (H 2L) and its binuclear nickel(II), palladium(II) and platinum(II) complexes, containing the dianion as a bridging ligand, were prepared and characterized by physicochemical and spectroscopic data. Square-planar geometries are proposed for these divalent metal complexes. Cyclic voltammetry of Ni 2L 2 in DMSO shows a reversible one-electron reduction wave at −1.48 V vs a Ag/AgBF 4 reference electrode. Quantitative reduction of this complex by controlled potential electrolysis at −1.80 V yielded the corresponding Ni I product as shown by ESR spectroscopy.

Spectroscopic Properties of One-Electron-Reduced Species of Dichloro(phthalocyaninato)antimony(V) Cation

Abstract Electrochemical properties of the titled complex in dichloromethane (DCM) solutions have been investigated by cyclic and rotating-disk-electrode voltammetry. Two successive reversible one-electron reduction waves (E1/2 = −0.23 and −0.65 V vs. ferrocene/ferricinium+, respectively) and one irreversible oxidation wave (Epa = 1.32 V) have been observed. These were assigned as phthalocyaninate-ring-centered. Ring-reduced species of the titled complex was generated by both chemical and electrochemical reduction of the complex in DCM solutions and was characterized by electronic absorption (UV; 250—1100 nm), magnetic circular dichroism (MCD; 250—800 nm), and ESR spectroscopy. A quite sharp ESR signal was observed at g = 2.0041 (with a width of 0.5 mT) for the electrochemically-generated species, confirming the formation of monomer but not of dimer. The reduced species showed some prominent bands in its UV spectra, at 328 (B1; π → π*), 382 (LMCT), 553 (n → π*), 599 (π* → π*), and 1059 nm (Q; π → π*). The MCD spectra showed sharp bands at 328 and 599 nm as Faraday C-terms, indicating that the ground state of the reduced species is orbitally degenerated. Thus the phthalocyanine-ring is not significantly Jahn–Teller-distorted and is approximately D4h in symmetry even upon reduction.

Reduction of Acetylated Tetraphenylethylenes: Electrochemical Behavior and Stability of the Related Reduced Anions

The synthesis and characterization of 1-(4-acetylphenyl)-1,2,2-triphenylethylene (2), 1,1-bis(4-acetylphenyl)-2,2-diphenylethylene (3a), trans-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3b), cis-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3c), 1,1,2-tris(4-acetylphenyl)-2-phenylethylene (4), and 1,1,2,2-tetrakis(4-acetylphenyl)ethylene (5) are described. The molecular orbital energies, charge densities, and electronic spectra of the neutral and reduced species of 2 and 3a−3c were calculated using INDO/1 parameters for structures that had been optimized geometrically with AM1 parameters. The calculated electronic spectra for the neutral compounds compare favorably with the experimentally observed absorption spectra. Cyclic voltammetry of these compounds in deoxygenated THF containing 0.3 M [n-Bu4N]PF6 showed that 2 and 3a have irreversible one-electron first reduction waves, whereas 3b, 3c, 4, and 5 display quasi-reversible two-electron first reduction waves. Chemical reduction of 2−4 with Na/Hg in THF results in highly colored solutions, the absorbance spectra of which have been compared to the calculated electronic spectra for the radical anions and dianions of these compounds. Reduction of 2 results in dimerization to a pinacol product, whereas those of 3b and 4 result only in formation of the tetraarylethane product. 3a gives both pinacol and tetraarylethane upon reduction.

Synthesis, Characterization, and Theoretical Study of Sulfur-Containing Donor-Acceptor DCNQI Derivatives with Photoinduced Intramolecular Electron Transfer.

The donor-acceptor compounds N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]dithiin-6,11-quinonediimine (9a) and N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]oxathiin-6,11-quinonediimine (10a) and their methyl-substituted derivatives (9b and 10b-d, respectively) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The (1)H-NMR spectra evidence the existence of a syn/anti isomerism in solution. Both experimental and theoretical data suggest that the preferred configuration of the N-CN groups corresponds to a syn isomer for 9 and to an anti isomer for 10. The X-ray analysis performed for 9b reveals that molecules are not planar and pack in vertical stacks showing an overlap between donor and acceptor moieties of adjacent molecules. In agreement with X-ray data, theoretical calculations predict that both for 9 and 10 the acceptor DCNQI moiety is folded and adopts a butterfly-type structure and the donor moiety is bent along the line passing through the heteroatoms. The energy difference between planar and butterfly structures is calculated to be < 3 kcal/mol at the ab initio 6-31G level. The UV-vis spectra present a broad absorption in the visible which corresponds to a photoinduced intramolecular electron transfer from the high-energy HOMO furnished by the donor moiety to the low-energy LUMO located on the DCNQI fragment. Cyclic voltammetry displays one oxidation peak to the cation and two one-electron reduction waves to the anion and dianion. Theoretical calculations show the planarization of the acceptor/donor moiety induced by reduction/oxidation. The formation of stable radical anions is corroborated by the intense EPR signals recorded for reduced 9. The assignment of the hyperfine coupling constants of the EPR spectra is consistent with the existence of a preferred syn configuration.

Determination of heterogeneous rate constants for the electrochemical reduction of fullerene C 60 by the method of cyclic voltammetry on a hemispherical ultramicroelectrode

A sealed cell for the study of low temperature electrochemistry is described. From room temperature to 223 K the kinetic properties of the reduction of fullerene C 60 in CH 3CN + toluene (volume ratio 1:5) containing 0.1 M TBAPF 6 were studied. Six one-electron reduction waves of C 60 have been obtained under our experimental conditions. The results show that the diffusion activation energy of C 60 is 18.0 kJ mol −1. The heterogeneous rate constants of the first five redox couples of C 60 have been measured at different temperatures. The activation energies for the first five electroreductions were evaluated.

Synthesis, electrochemistry and ab initio molecular orbital calculations on some norbornane- and bicyclo[2.2.2]octane-fused 5,12-bis(dicyanomethylidene)naphthalene systems

A series of norbornane- and bicyclo[2.2.2]octane-fused tetracyanonaphthoquinodimethane (benzoTCNQ) derivatives, 4,6,8,10 and 12 has been synthesized. Cyclic voltammetry measurements revealed two one-electron reduction waves for the benzo-TCNQ systems 4, 8 and 10 which possess either a cyclobutane or a norbornane moiety fused to the quinonoid ring, but a single (presumably two-electron) reduction wave for 6 and 12 which possess a bicyclo[2.2.2]octane group fused to the gmnonoid ring. The benzo-TCNQ systems were found to be potent electron acceptors. The results of (U)HF/3-21G calculations carried out on representative benzo-TCNQ derivatives suggest that steric interactions between the quinonoid ring and its adjacent fused alicyclic group are largely responsible for the observed decrease in the electron affinities along the series: 8 > 4 > 6. The MO calculations further suggest that LUMO energies of neutral TCNQ derivatives are qualitatively reliable predictors of electron affinities. The calculated (gas phase) internal reorganization energies, λi, for the benzo-TCNQ systems are ca. 54 kJ mol–1 and are comparable to that calculated for the dicyanovinyl (DCV) system, 27. The similarity between the λi values for the benzo-TCNQ systems and 27, together with the experimental finding that benzo-TCNQ systems are reduced at ca. 1.7 V less negative potentials than 27, should make the norbornane- and bicyclo [2.2.2] octane-benzo-TCNQ systems superior to the DCV moiety as acceptor chromophores in giant multichromophoric systems.

Electrochemistry of polyoxometalates immobilized in ion exchange polymer films

The polyoxometalate ions PMo12O403−, PW12O403−, and SiW12O404−are incorporated in polymeric ruthenium(II)(vinyl)bipyridine (poly-Ru(vbpy)32+) films from aqueous and dioxane–water electrolytes. Despite their large mass the ions exist as freely diffusing species that compensate for up to 30% of the charge in poly-Ru(vbpy)32+. An investigation of the effect of environmental conditions on electrochemical behavior reveals that the first two one-electron reduction waves of SiW12O404−coalesce into a single two-electron reaction and those of PW12O403−shift significantly in potential upon a change from pure aqueous to 50(v/v)% dioxane/water solvent. The observation is attributed to destabilization of the one-electron reaction products as the solvent is enriched is dioxane. Incorporation of polyoxometalates in protonated poly(vinyl)pyridine and poly-Ru(vbpy)32+films from dioxane–water solvent results in differences in electrochemical behavior. Polyoxometalate anions incorporated in poly-Ru(vbpy)32+films catalyze the electrochemical reduction of hydrogen ion. Keywords: polyoxometalate, electrochemistry, poly-Ru(vbpy)32+, electrocatalysis, immobilization.

Radical Anion Complexes of Tris(1,3-diphenyltriazenido)aluminum

Electrochemical studies of Al(dpt)3 (Hdpt = 1,3-diphenyltriazene) by cyclic voltammetry in THF solution reveal three successive pseudo reversible one-electron reduction waves (E112 = -1.50, -1.84, and -2.16 V). The chemical reduction of Al(dpt)3 by sodium metal in THF allows for the isolation of the radical anion complexes [Na(THF)x]n[Al(dpt)3], n = 1 ( l ) , 1 (2), and 3 (3). Characterization by EPR, NMR, UV-visible, and X-ray photoelectron (XP) spectroscopy, in addition to the X-ray structural determination of [PPN] [Al(dpt)3] (4), supports the formation of the f i s t homologous series of ligand-centered aluminum(II1) radical anion complexes. Analogous electrochemical reduction series are observed for the p-methyland p-methoxy-substituted triazenides. The dependence of the complex reduction potentials is discussed with respect to the UV-visible spectra of the unreduced complex and the ligand's Hammett substituent constant (a). In contrast, irreversible electrochemical reduction (-1.5 to -2.2 V) occurs for the pentafluoroand p-fluoro-, p-chloro-, and p-bromo-substituted triazenido complexes. Irreversible reduction also occurs for the alkyl and aryloxide compounds Al(R)z(dpt) (R = 'Bu, 'Bu), Al('Bu)(dpt)z, Al(BHT)z(dpt), and Al(BHT)(dpt)z (BHT-H = 2,6-di-tert-butyl-4-methylphenol). Ab initio molecular orbital calculations have been carried out on the model compounds Al(HNNNH)3 and [Al(HNNNH)3I3-. The identity of the frontier molecular orbitals and calculated structures are considered in relation to experimental data.

Voltammetric Studies of the Redox Polymer Poly[4,4′-bipyridinium-1,1′-diylcarbonyl-2,5-thiophenediyl Dichloride] in Dimethyl Sulfoxide

Abstract The electrochemical behavior of a redox polyelectrolyte, poly[4,4′-bipyridinium-1,1′-diylcarbonyl-2,5-thiophenediyl dichloride] at mercury electrode has been examined in dimethyl sulfoxide containing different concentrations of tetrabutylammonium iodide. The techniques of sampled DC polarography, pulse polarography (normal and differential), and cyclic voltammetry have been used. All polarograms revealed three one-electron reduction waves which are found either quasi-reversible or irreversible and usually adsorption-controlled by cyclic voltammetric experiments. The results were discussed by taking into account the implications of different concentrations of supporting electrolytes and polycation conformations on the adsorption processes consisting of preceding and following chemical reactions coupled with charge transfer reactions occurring at the electrode.

The cyclic voltammetry of a new copper(II) phthalocyaninate substituted with four 15-membered tetraazamacrocycles and its pentanuclear complexes

A new copper(II) phthalocyaninate substituted with four 15-membered tetraazamacrocycles (15M-TAMCuPc) and its pentanuclear complexes (Ni,Cu) have been studied in dimethyl sulphoxide solution by cyclic voltammetric and coulometric methods. The cyclic voltammogram of 15M-TAMCuPc is characterized by one cathodic wave which has a totally irreversible nature. However, four one-electron reduction waves which show quasi-reversible behaviour at all sweep rates were observed for the pentanuclear Cu(II) complex of 15M-TAMCuPc. On the other hand two one-electron reduction waves were observed for the pentanuclear Ni(II) complex of 15M-TAMCuPc. One of them shows reversible behaviour and the other shows quasi-reversible behaviour at all sweep rates. The reaction mechanisms and diffusion coefficients have been investigated.

Synthesis of novel chloro-substituted N, N′-dicyanoquinonediimines. Formation of charge transfer complexes and copper radical-anion salts

Novel chloro-substituted N, N′-dicyanoquinonediimines (DCNQIs) ( 5) have been synthesized from the corresponding quinones ( 4) by reaction with bis(trimethylsilyl)carbodiimide (BTC) and titanium tetrachloride. Cyclic voltammetry (CV) measurements of the prepared DCNQIs ( 5) exhibited the presence of two one-electron reversible reduction waves to the corresponding radical anion and dianion. The first reduction potential revealed a better acceptor ability of compounds 5 than the parent DCNQI, these values being strongly dependent on the presence of chlorine atoms. The 1H NMR high resolution spectral data showed, when possible, the presence of the syn-anti isomers in these compounds ( 5). Charge transfer complexes ( 6) of the novel DCNQIs ( 5) with the donor tetrathiafulvalene (TTF) and also the copper radical-anion salts ( 7) have been obtained. Room temperature conductivities of the charge transfer complexes ( 6) and charge transfer salts ( 7) were measured and the temperature dependence exhibited a semiconducting behaviour.

A voltammetric study of Keggin-type heteropolymolybdate anions

The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo 12O 40] n− (XS, P, As, V, Si, Ge; n = 2–4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.

New insights into the electrochemistry of molybdenum(VI) dioxo complexes containing a tetradentate S2(NH)2-type ligand

The electrochemistry of dioxomolybdenum(VI) with the tetradentate S[sub 2](NH)[sub 2]-type ligand N,N[prime]-bis(2-mercaptophenyl)-2,3-diaminobutane (mPhabH[sub 4]), at neutral, acidic, and basic conditions, has been investigated. The molecular structure of (mPhabH[sub 2])Mo[sup VI]O[sub 2] (1) has been determined: space group P2[sub 1]2[sub 1]2[sub 1] (No. 19), a = 8.667(<1), b = 12.884(<1), c = 19.877(3) [Angstrom], V = 2220 [Angstrom][sup 3]; Z = 4. The cis-dioxo complex 1 displays a highly distorted octahedral structure. Electrochemical, one-electron reduction of the Mo(VI) compound 1 is followed by a fast dehydration reaction leading to [(mPhab)MoO][sup [minus]] (4), which can be further reduced in a subsequent one-electron reduction step to the Mo(IV) dianion. At neutral conditions this process appears as a single two-electron reduction. Upon addition of N-methylimidazole (NMI) the dehydration rate decreases and two separate reduction steps can be observed. Acidic conditions also lead to a decrease of the dehydration rate and to two separate one-electron reductions. In basic solution the Mo(VI) complex 1 is reduced by methanol to [(mPhab)Mo[sub V]O][sup [minus]] (4), which shows both a quasi-reversible one-electron reduction wave to form the Mo(IV) dianion and a quasi-reversible oxidation wave. Synthetically, [(mPhab)Mo[sup V]O][sup [minus]] (4) can also be obtained via deoxygenation of 1 by Ph[submore » 2]PMe in the presence of N-methylimidazole, presumably via formation of a Mo(IV)-NMI adduct that undergoes a comproportionation reaction with 1 and subsequent dehydration to the mononuclear Mo(V) complex 4. Neither [mu]-oxo-bridged Mo(V) dimers nor mononuclear Mo(IV) species can be detected. 30 refs., 5 figs., 4 tabs.« less

Synthesis, electrochemical properties and effect of substituents on π-extended TCNQ and DCNQI systems

The synthesis of novel π-extended TCNQ and DCNQI derivatives from the corresponding 1,4-anthraquinones is described. The cyclic voltammetric data reveal that both TCNQ-type and DCNQI-type acceptors present two one-electron reduction waves to the corresponding anion-radical and dianion. These novel 2,3-fused TCNQ and DCNQI π-extended systems are, according to the electrochemical values, better acceptors than the previously reported 2,3–5,6-fused TCNQ and DCNQI molecules. A good linear correlation between the first reduction potential of the substituted DCNQI-type derivatives versus the Hammett's σ3 constant has been found.

Novel molecules for the design of organic conductors. Synthesis of 7,7,8,8-tetracyano-2,5-bis(3-phenylpropyl)-p-quinodimethane and N,N′-dicyano-2,5-bis(3-phenylpropyl)-p-quinodiimine and X-ray structure of the TCNQ derivative

The novel 7,7,8,8-tetracyano-2,5-bis(3-phenylpropyl)-p-quinodimethane 2 and N,N′-dicyano-2,5-bis(3-phenylpropyl)-p-quinodiimine 3 in which the acceptor TCNQ and DCNQI moieties are covalently linked to weak donors, phenyl rings, by three methylene units have been synthesized and studied as intended donor–acceptor–donor (D–A–D) systems. The cyclic voltammetry data reveal two one-electron reversible reduction waves to the corresponding anion-radical and dianion. The novel molecules 2 and 3 behave, in solution, as simple TCNQ and DCNQI derivatives, respectively. A structural study by X-ray diffraction on the TCNQ-type molecule 2 shows a very planar, nondistorted TCNQ ring with isolated D ⋯ A ⋯ D triplets between the central TCNQ moiety of a molecule and two phenyl rings of two other different molecules.

Coupling reactions of ynamines. Syntheses, structural characterization and electrochemistry of some nido and closo ferracyclopentadiene complexes

The reaction of [Fe 2(CO) 7C(Ph)C(NEt 2)] ( 1) with ynamines RCCNEt 2 (R  Me, C 3H 5, or CH 2Ph) at 20°C yielded dinuclear ferracyclopentadiene complexes [Fe 2(CO) 6C(Ph)C(NEt 2)C-(NEt 2)C(R)] (R  Me ( 2), R  CH 2Ph ( 3), R  C 3H 5 ( 4)). Complex 5, [Fe 2(CO) 6C(C 3H 5)C(NEt 2)C-(NEt 2)C(C 3H 5)], was obtained directly from the reaction of C 3H 5CCNEt 2 with [Fe 2(CO) 9]. All complexes were characterized by a combination of microanalytical and spectroscopic techniques. The molecular structure of 4 was determined X-ray crystallographically: triclinic, P 1 , a = 8.820(2) Å, b = 9.261(2) Å, c = 18.470(2) Å, α = 89.40(1)°, β = 81.81(1)°, γ = 70.23(2)°, R w = 0.0406 for 2580 reflections. The nido complexes 4 and 5 transformed to the closo tripledecker compounds [Fe 3(CO) 8 C(Ph)C(NEt 2)C(NEt 2)C(R)] (R  Ph ( 6); R  C 3H 5 (7)) by an excess of [Fe 2(CO) 9] in refluxing hexane. Both complexes were characterized by IR, 1H NMR, mass spectra and chemical analysis. The X-ray structure of 6 was carried out: monoclinic, P2 1 / c, a = 14.900(3), b = 11.149(2), c = 18.453(4) Å, β = 91.14(2)°, R w = 0.0414 for 2304 reflections. Only one isomer could be isolated for the nido compounds. The nido-closo transformation did not modify the coupling within the butadiene moiety. Electrochemistry of the nido and closo complexes is reported. The closo compounds showed two reversible one-electron reduction waves whereas the nido species were completely irreversibly reduced.

Synthesis, electrochemistry, and crystal structure of N,N'-dicyanoquinonediimine (DCNQI) derivatives containing fused benzene rings

A series of novel fused aromatic N,N-dicyanoquinonediimines (DCNQI) derivatives (1, 5, 7, and 9) and N-cyanoquinonemonoimines (4, 6, 8, and 10) has been synthesized by reaction of the corresponding quinones with bis(trimethylsilyl)carbodiimide (BTC). The acceptor ability is discussed in term of both steric and electronic effects. Cyclic voltammetric data reveal two one-electron reduction waves of the corresponding radical-anion and dianion of each compound. The first reduction potentials are more negative than those of the more simple DCNQI (2a) and TMDCNQI (2b) acceptors

Polarography of Uranyl Complex with Chromotropic Acid

The polarography characteristic of uranyl ion in chromotropic acid solution was investigated systematically over the pH range 2.0 to 10.0 with ligand concentrations varying from 0.010 M to 0.200 M. At pH 5.5, the one-electron reversible reduction waves were obtained. The temperature coefficient of the half-wave potential was obtained to be −0.32 mV per degree and the mean value of id/h1/2 is 0.340 ± 0.003. The electrode reaction is UO2(HA)24− + c = UO2(HA)2− + HA3− Where pH 5.5, an irreversible and diffusion-controlled wave was obtained. The diffusion coefficients and kinetic parameters of complex species were determined by deducing instantaneous equations.