Successive Reduction Processes Research Articles

Electrochemical Study of Aromatic Ketones in 1-Ethyl-3-methylimidazolium tetrafluoroborate

The electrochemical reduction of 9-fluorenone and benzophenone has been studied in the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) ionic liquid. A comparison of potential separation for successive 9-fluorenone reduction processes in EMIBF4 at 100 mV/s with those in acetonitrile provides evidence for complexation of the 9-fluorenone dianion by the EMI+ cation. Complexation of the 9-fluorenone carbonyl oxygen by added BF3 etherate has also been studied, producing a positive potential shift for the reduction process. The reduction of benzophenone in EMIBF4 shows two successive electron transfers only at higher scan rates (1 V/s), suggesting that proton transfer and / or complexation of benzophenone reduction products by the EMI+ cation may be involved. The peak potential separation for the benzophenone reduction processes at 1 V/s is similar to that for 9- fluorenone at 100 mV/s, indicating that the EMI+ cation also complexes the benzophenone dianion.

Electrocatalytic activity of carbon-supported Pt–Au nanoparticles for methanol electro-oxidation

Pt-modified Au nanoparticles on carbon support were prepared and analyzed as electrocatalysts for methanol electro-oxidation. In this paper, a novel chemical strategy is described for the preparation and characterization of carbon-supported and Pt-modified Au nanoparticles, which were prepared by using a successive reduction process. After preparing Au colloid nanoparticles (∼3.5 nm diameter), Au nanoparticles were supported spontaneously on the surface of carbon black in the aqueous solution. Then a nanoscaled Pt layer was deposited on the surface of carbon-supported Au nanoparticles by the chemical reduction. The structural information and electrocatalytic activities of the Pt-modified Au nanoparticles were confirmed by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and cyclic voltammetry (CV). The results indicate that carbon-supported Au nanoparticles were modified with the reduced Pt atoms selectively. The Pt-modified Au nanoparticles showed the higher electrocatalytic activity for methanol electro-oxidation reaction than the commercial one (Johnson–Matthey). The increased electrocatalytic activity might be attributed to the effective surface structure of Pt-modified Au nanoparticles, which have a high utilization of Pt for surface reaction of methanol electro-oxidation.

2,2‘-Dipyridylketone (dpk) as Ancillary Acceptor and Reporter Ligand in Complexes [(dpk)(Cl)Ru(μ-tppz)Ru(Cl)(dpk)]n+where tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine

The tppz-bridged diruthenium(II) complex [(dpk)(Cl)Ru(II)(mu-tppz)Ru(II)(Cl)(dpk)](ClO4)2, [2](ClO4)2, and mononuclear [(dpk)(Cl)Ru(II)(tppz)](ClO4), [1](ClO4) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, dpk = 2,2'-dipyridylketone], have been synthesized. The 260 mV separation between successive one-electron oxidation couples in [2]2+ translates to a relatively small comproportionation constant, Kc, of 2.5 x 10(4) for the intermediate. It is shown how electrochemical data (E(ox), E(red), Kc) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(mu-tppz)RuCl(L)]n, particularly the competition between L and tppz for electron density from the metal. According to EPR (g1 = 2.470, g2 = 2.195, and g3 = 1.873 at 4 K) the intermediate [2]3+ is a mixed-valent Ru(II)Ru(III) species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (epsilon = 1500 M(-1) cm(-1)). The width at half-height (Deltanu(1/2)) of 700 cm(-1) of the IVCT band is much smaller than the calculated value of 3584 cm(-1), obtained by using the Hush formula Deltanu(1/2) = (2310E(op))(1/2) (E(op) = 5556 cm(-1), energy of the IVCT transition) which would be applicable to localized (Class II) mixed-valent Ru(II)Ru(III) systems. Valence delocalization in [2]3+ is supported by the uniform shift of the nu(C=O) band of the N,N'-coordinated dpk ligands from 1676 cm(-1) in the Ru(II)Ru(II) precursor to 1690 cm(-1) in the Ru(2.5)Ru(2.5) form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but pi-conjugated organic carbonyl group. The apparent contradiction between the moderate value of Kc and the narrow IVCT band is being discussed considering "borderline" or "hybrid" "Class II-III" concepts of mixed-valency, as well as coordination aspects, i.e., the bis-tridentate nature of the pi-acceptor bridging ligand. Altogether, the complex ions [1]+ and [2]2+ display four and five successive reduction processes, respectively, involving both tppz- and dpk-based unoccupied pi orbitals. The one-electron reduced form [2]+ has been assigned as a tppz*- radical-anion-containing species which exhibits a free-radical-type EPR signal at 4K (g(parallel) = 2.002, g(perpendicular) = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (epsilon = 1100 M(-1) cm(-1)).

Magnetoelectrochemistry of 4,4′-bis(dimethylamino)biphenyl and 4,4′-dinitrobiphenyl azacrown macrocyclic lactams

The voltammetric behaviour at carbon fibre microelectrodes under the application of static magnetic fields of two series of macrolactams containing in their structure 4,4′-bis(dimethylamino)biphenyl or 4,4′-dinitrobiphenyl groups in MeCN solution is described. The response of 4,4′-dinitrobiphenyl receptors is dominated by two successive one-electron reduction processes at −0.9 and −1.6 V versus AgCl/Ag. 4,4′-bis(dimethylamino)biphenyl-containing receptors display two one-electron oxidations above +0.8 and +1.0 V. In both cases, a dihedral/planar interconversion precedes the second electron transfer step. Upon application of moderate (0.05–0.2 T) static magnetic fields to the electrochemical cell, the rate of such dihedral/planar interconversion is lowered for both the reduction of 4,4′-dinitrobiphenyl receptors and the oxidation of 4,4′-bis(dimethylamino)biphenyl lactams. The electrochemical response of N-methylated receptors, for which different cisoid–cisoid, cisoid–transoid, and transoid–transoid forms exist, exhibits a significant peak splitting that can be associated to the presence of such conformational isomers. Application of magnetic fields produces a relative enhancement of some peaks that can be interpreted in terms of differential magnetoconvection involving such conformational isomers.

Wells–Dawson-derived tetrameric complexes {K 28H 8[P 2W 15Ti 3O 60.5] 4} electrochemical behaviour and electrocatalytic reduction of nitrite and of nitric oxide

At low pH, trivacant polyoxometalate α-[P 2W 15O 56] 12- reacts with titanyl and forms tetrameric {K 28H 8[P 2W 15Ti 3O 60.5] 4} ( 1). The purity of [P 2W 15Ti 3O 60.5] 4 36− has been confirmed by IR spectroscopy, elemental analysis, and 31P NMR spectrometry. The electrochemical behaviour of the tetrameric complexes in aqueous solution has been investigated at various pHs. [P 2W 15Ti 3O 60.5] 4 36− exhibits successive reduction processes of the W atoms in a negative potential range. All waves are pH dependent at pH < 4. The anion showed an electrocatalytic effect on the reduction of HNO 2, but this effect is greatly influenced by pH. At low pH (pH < 3.5) the cathodic peak current of the first W VI/V couple is enhanced while its anodic couterpart disappears. When pH > 3.5 no catalytic effect is observed. On the contrary, upon saturation of the POM solution with NO, even at pH > 3.5, electrocatalytic reduction of NO was observed. The tetrameric tri-Ti IV-substituted Wells–Dawson polyoxometalate [P 2W 15Ti 3O 60.5] 4 36− was not degraded by repeated cycling between its oxidized and reduced states.

Synthesis and mixed valence aspects of [{(L)ClRu}2(μ-tppz)]n+incorporating 2,2′-dipyridylamine (L) as ancillary and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) as bridging ligand

The tppz-bridged diruthenium complex [{(L)ClRuII}2(μ-tppz)](ClO4)2, [1](ClO4)2 {tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, L = 2,2′-dipyridylamine} and its mononuclear counterpart [(L)ClRuII(tppz)]ClO4, [2](ClO4) have been synthesized. The 380 mV separation between successive RuII/RuIII couples in [1]2+ leads to a comproportionation constant (Kc) of 2.7 × 106. Consequently, the RuIIRuIII species [1]3+ exhibits a rather narrow intervalence charge transfer band at 1700 nm, suggesting a class III mixed-valence state, the electronic coupling constant (Vab) is calculated at 2940 cm−1. Complex [1]3+ displays a rhombic EPR spectrum at 4 K (g1 = 3.390, g2 = 2.278, g3 = 1.697), characteristic of ruthenium(III) in a distorted octahedral environment. Both complexes show two successive tppz-based reduction processes [(tppz)0/−1 and (tppz)−1/−2]. The one-electron reduced species [1]+ is a tppz radical anion species with an intense low-energy band at 1105 nm and an axial EPR signal at 4 K (g1 = 2.008, g2 = g3 = 1.994). [1]2+ and [2]+ exhibit moderately strong emissions at 740 nm and 668 nm, respectively, in EtOH–MeOH glass at 77 K.

Study of the kinetics of processes during electrochemical deposition of zirconia from nonaqueous electrolytes

The processes of electrochemical deposition of zirconia from nonaqueous electrolytes on different substrates have been investigated. Cyclic voltammetry and linear sweep voltammetry have been used to investigate the oxygen and hydrogen reduction processes preceding/accompanying the discharge of the zirconium complex being identified. On the basis of simultaneous XPS and SEM analyses, the potential region of zirconia layer formation has been defined. The kinetic parameters of successive and parallel reduction processes have been determined. The factors hindering the formation of thick zirconia layers have been discussed.

Synthesis, structure and spectroelectrochemical properties of a dinuclear ruthenium complex exhibiting a strong electronic interaction across a 1,2,4,5-tetrazine bridging ligand

The reaction of [Ru(bpy)2(EtOH)2]2+ (bpy = 2,2′-bipyridine) with the potentially bridging ligand 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,4-dihydro-1,2,4,5-tetrazine) (H2L) results in formation of the diruthenium complex [(bpy)2RuII(L)RuII(bpy)2]4+ ([1]4+) in which the dihydrotetrazine unit of H2L has undergone oxidative dehydrogenation to give an aromatic tetrazine unit connecting the two metal centres [L = 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine]. The crystal structures of H2L and [1(ClO4)4]·2H2O have been determined. In MeCN solution, complex [1]4+ undergoes two successive one-electron oxidation processes at 1.25 and 1.70 V versus SCE, corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)/Ru(III) state (Kc = 4 × 107) at intermediate potentials. Four successive ligand-based reduction processes at −0.13, −0.99, −1.58 and −1.94 V versus SCE are also observed, of which the first two are associated with the bridging tetrazine unit and the other two are reductions of the coordinated bipyridine molecules. A UV/VIS/NIR spectroelectrochemical study was carried out on [1]n+ for n = 3–6; the mixed-valence species [1]5+ displays a narrow π–π* transition at 1534 nm, indicative of a class III mixed-valence state. The chemically generated one-electron-reduced species [1]3+ shows an EPR signal (g = 2.0085) characteristic of a ligand-centred radical. [1]4+ is weakly luminescent at 77 K.

Electrochemistry of vanadium-doped tetragonal and monoclinic ZrO2 attached to graphite/polyester composite electrodes

The electrochemistry of monoclinic and tetragonal vanadium-doped zirconias (VZrO2), prepared from gel precursors with vanadium loadings ranging from 0.5 to 15 mol%, has been studied using abrasive-conditioned graphite/polyester composite electrodes immersed in aqueous HCl and HClO4 solutions. Isolated vanadium centers form a solid solution in the zirconia lattice with a solubility limit close to 5 mol%. Above 5 mol%, finely dispersed V2O5 is formed. Vanadium centers located at the boundary sites of the zirconia lattice display successive one-electron transfer processes near to +0.25 and +0.10 V vs. SCE, whereas finely dispersed V2O5 yields three successive reduction processes at +0.46, +0.30, and +0.16 V vs. SCE. Electrochemical data indicate the presence of both V5+ and V4+ centers in the lattice of monoclinic and tetragonal zirconias, the V5+/V4+ ratio decreasing as the vanadium loading increases.

DETERMINATION OF MENADIONE BASED ON ITS POLAROGRAPHIC CATALYTIC WAVE IN THE PRESENCE OF POTASSIUM IODINATE

A method for the determination of menadione was proposed. In 0.2 mol/L HOAc–NaOAc (pH 4.7) buffer solution containing 4.0 × 10−3 mol/L KIO3, menadione yields a well-defined polarographic catalytic wave with a peak potential of −0.95 V (vs. SCE). The second order derivative peak current is linearly proportional to the menadione concentration in the range of 4.0 × 10−8–2.0 × 10−6mol/L with the correlation coefficient of 0.999 (n = 8). The detection limit is 2.0 × 10−8mol/L. The sensitivity of the catalytic wave is 10 times higher than that of the corresponding reduction wave in the absence of KIO3. The proposed method was used to directly determine the menadione contents in clinical injections in good agreement with the labeled amount without preseparation. The mechanism of polarographic catalytic wave of menadione was discussed. The polarographic reduction wave of menadione itself in the acetate buffer solution is ascribed to that the carbonyl group of menadione is reduced in consecutive two electrons and two protons addition to hydroquinone via semiquinone (quinone-hydroquinone) free radical intermediates. In the presence of KIO3, the carbonyl group of menadione is chemically regenerated due to oxidation of semiquinone free radical intermediates by KIO3 and a series of its derived species formed during six-electron successive reduction process. The maximum current of the first charge transfer is enhanced, and the second electroreduction step of menadione is inhibited.

Electrochemical, EPR, and Magnetic Studies on Microcrystals of the [C60⊂(p-Benzyl-calix[5]arene)2]•8Toluene and Its One-Electron-Reduced Encapsulation Complex

The electrochemical behavior of microcrystals of the [C60⊂(p-benzyl-calix[5]arene)2]•8toluene (referred to as C60⊂L2) encapsulation complex adhered to electrode surfaces in contact with CH3CN (electrolyte) has been studied using cyclic voltammetric and microgravimetric techniques. Six successive one-electron reduction processes, of which the first two have chemically reversible characteristics, are observed at a glassy carbon electrode when large Bu4N+ cations are used as the electrolyte, although dissolution of the reduced solid into the bulk solution also accompanies the reduction processes. Under the same conditions but with an electrolyte containing much smaller Li+, Na+, or Ba2+ cations, all reduction processes were found to be chemically irreversible. These chemically reversible/irreversible reduction processes are attributed to the reversible/irreversible intercalation processes of large/small electrolyte cations into the lattice of C60⊂L2 crystals. In EPR studies, the g values of 2.0020 ± 0.0002 a...

Revisiting the synthesis of [Mo6(η5-C5Me5)O18]−. X-Ray structural analysis, UV-visible, electrochemical and multinuclear NMR characterization

In methanol, [n-Bu4N][MoCp*O3] reacted with [n-Bu4N]2[Mo4O10(OMe)4Cl2] to yield [n-Bu4N][Mo6Cp*O18] in reasonable yield, thus opening a new route in the synthesis of organometallic derivatives of polyoxometalates. The crystal and molecular structure of [n-Bu4N][Mo6Cp*O18]·Me2CO has been determined by single-crystal X-ray diffraction. The ion [Mo6Cp*O18]– has been thoroughly characterized by multinuclear NMR in solution. The 95Mo and 17O NMR data support the conclusion that the Cp* ligand is a better σ + π donor than the oxo ligand. The complex is electrochemically active and displays three successive one-electron reduction processes.

Synthesis of Dichloro(phthalocyaninato)antimony(V) Perchlorate, Tetrafluoroborate, and Hexafluorophosphate and Electrochemical Reinvestigation on the New Complex Salts

Abstract The titled new salts were synthesized by exchanging the unknown counter anion of [Sb(pc)Cl2]+ salt (pc2− = phthalocyaninate, C32H16N82−), which was the first product of the reaction of phthalonitrile with antimony(III) trichloride, with perchlorate, tetrafluoroborate, and hexafluorophosphate. Elemental analytical data and infrared spectra of the new salts confirmed that these complexes are composed of one [Sb(pc)Cl2]+ cation and one mononegative counter ion. The introduction of electrochemically inactive counter-anions exerted little effect on the electronic spectrum and the first and second pc-ring reduction potentials of the complex in dichloromethane, but has made it possible to observe the third and fourth reduction processes, which were not observed for the initial salt. Up to four successive reduction processes of a pc-ring were observed for the first time in a noncoordinating solvent. Although the 1Δ2E1/2 (half-wave potential interval between the first and second reduction processes; 0.45 V), and 3Δ4E1/2 (0.58 V) values for [Sb(pc)Cl2]+ were typical of those for the known phthalocyanines, the 2Δ3E1/2 value (0.31 V) was much smaller. The anomaly in the spacing of the reduction processes for [Sb(pc)Cl2]+ is discussed in terms of the lack of a significant Jahn–Teller distortion of the pc-ring upon reduction.

Electrochemical and spectral properties of octakis(hexylthio)-substituted phthalocyanines

The redox properties of octakis(hexylthio)-substituted phthalocyanines with 2H, Zn II and Cu II in the phthalocyanine core have been investigated by cyclic voltammetry in dichloromethane and the energies of photo-excitation and oxido-reduction processes have been compared. Two successive one-electron reduction and one-electron oxidation processes were observed for the complexes used in this work. The displacement of two protons in the inner core with metal ion was found to shift the first reduction potential of phthalocyanine ring to more negative values as a result of the π-back donation of the filld d π of metal ion into the empty pc-π ∗ orbitals. The evaluation of cyclic voltammetry data together with the absorption spectra showed that photoexciatation arises from the orbitals below the HOMO for all the complexes used in this work.

Spectral and Electrochemical Properties of the Diastereoisomeric Forms of Azobis(2-pyridine)-Bridged Diruthenium Species

A series of dinuclear complexes of ruthenium(II) have been synthesized in which α-azodiimines {such as azobis(2-pyridine), apy, and azobis(4-methyl-2-pyridine), mapy} act as the bridge and 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (Me2bpy) as the terminal ligands. The diastereoisomeric forms of each species {ΔΛ (meso) and ΔΔ/ΛΛ (rac)} have been separated by cation-exchange chromatography and characterized by 1H-NMR spectroscopy. Electronic spectral and electrochemical studies show there to be differences in inter-metal communication between the diastereoisomeric forms in each case. Comparison of the spectroelectrochemical behavior of the range of complexes has allowed unequivocal assignment of the site of the successive reduction processes observed in dinuclear complexes of this type.

The electrochemical reduction mechanism of [N,N′-1,2-phenylenebis(salicylideneiminato)]cobalt(II)

The electrochemical reduction mechanism of [CoL][L =N,N′-1,2-phenylenebis(salicylideneiminate)] has been investigated in dimethylformamide by means of cyclic voltammetry and controlled-potential electrolysis. The cobalt(II) complex shows three successive one-electron reduction processes. The first electron uptake is metal-centred yielding [CoIL]–, whereas the second and third electron transfers are both ligand based, corresponding to the reduction of one imine double bond. This leads to the formation of a new cobalt complex [CoI(HL–)]2– which has the CoII–CoI redox couple located at a potential ca. 600 mV negative of the potential of the same redox couple of the original [CoL] complex. Decay reactions of some reduction intermediates have been kinetically characterised and the corresponding rate constants have been determined.

Probabilistic Communication Net as a Nonoriented Graph

This paper proves a necessary and sufficient realizability condition of the s-expected terminal capacity matrix of a probabilistic communication net. The proof is based on the successive reduction processes of a matrix, which provide a technique for designing a tree topology and determing edge capacities and reliabilities that satisfy the matrix.