Synthesis, X-ray structures, and spectroscopic and electrochemical properties of (mu-oxo)bis(mu-carboxylato)diruthenium complexes having six imidazole bases as terminal ligands

Complexes [Ru2O(O2CR)(2)(1-MeIm)(6)](ClO4)(2) (la-c), [Ru2O(O2CR)(2)(ImH)(6)](ClO4)(2) (2a,b), and [Ru2O(O2CR)(2)(4-MeImH)(6)](ClO4)(2) (3a,b) with a (mu-oxo)bis(mu-carboxylato)diruthenium(III) core have been prepared by reacting Ru2Cl(O2CR)(4) with the corresponding imidazole base, viz. 1-methylimidazole (1-MeIm), imidazole (ImH), and 4-methylimidazole (4-MeImH) in methanol, followed by treatment with NaClO4 in water (R: Me, a; C6H4-p-OMe, b; C6H4-p-Me, c). Diruthenium(III,IV) complexes [Ru2O(O2CR)(2)(1-MeIm)(6)](ClO4)(3) (R: Me, 4a; C6H4-p-OMe, 4b; C6H4-p-Me, 4c) have been prepared by one-electron oxidation of 1 in MeCN with K2S2O8 in water. Complexes la, 2a . 3H(2)O, and 4a . 1.5H(2)O have been structurally characterized. Crystal data for the complexes are as follows: la, orthorhombic, P2(1)2(1)2(1), a = 7.659(3) Angstrom, b = 22.366(3) Angstrom, c = 23.688(2) Angstrom, V = 4058(2) Angstrom(3), Z = 4, R = 0.0475, and R-w = 0.0467 for 2669 reflections with F-o > 2 sigma(F-o); 2a . 3H(2)O, triclinic, , a = 13.735(3) Angstrom, b = 14.428(4) Angstrom, c = 20.515(8) Angstrom, alpha = 87.13(3)degrees, beta = 87.61(3)degrees, gamma = 63.92(2)degrees, V = 3646(2) Angstrom(3), Z = 4, R = 0.0485 and R-w = 0.0583 for 10 594 reflections with F-o > 6 sigma(F-o); 4a . 1.5H(2)O triclinic, , a = 11.969(3) Angstrom, b = 12.090(6) Angstrom, c = 17.421(3) Angstrom, alpha = 108.93(2)degrees, beta = 84.42(2)degrees, gamma = 105.97(2)degrees, V = 2292(1) Angstrom(3), Z = 2, R = 0.0567, and R-w = 0.0705 for 6775 reflections with F-o > 6 sigma(F-o). The complexes have a diruthenium unit held by an oxo and two carboxylate ligands, and the imidazole ligands occupy the terminal sites of the core. The Ru-Ru distance and the Ru-O-oxo-Ru angle in la and 2a . 3H(2)O are 3.266(1), 3.272(1) Angstrom and 122.4(4), 120.5(2)degrees, while in 4a . 1.5H(2)O these values are 3.327(1) Angstrom and 133.6(2)degrees. The diruthenium(III) complexes 1-3 are blue in color and they exhibit an intense visible band in the range 560-575 nm. The absorption is charge transfer in nature involving the Ru(III)-d pi and O-oxo-p pi orbitals. The diruthenium(III,IV) complexes are red in color and show an intense band near 500 nm. The diruthenium(III) core readily gets oxidized with K2S2O8 forming quantitatively the diruthenium(III,IV) complex. The visible spectral record of the conversion shows an isosbestic point at 545 nm for 1 and at 535 nm for 2 and 3. Protonation of the oxide bridge by HClO4 in methanol yields the [Ru-2(mu-OH)(mu-O2CR)(2)](3+) core. The hydroxo species shows a visible band al 550 nm. The pK(a) value for la is 2.45. The protonated species are unstable. The 1-MeIm species converts to the diruthenium(III,IV) core, while the imidazole complex converts to [Ru(ImH)(6)](3+) and some uncharacterized products. Complex [Ru(ImH)(6)](ClO4)(3) has been structurally characterized. The diruthenium(III) complexes are essentially diamagnetic and show characteristic H-1 NMR spectra indicating the presence of the dimeric structure in solution. The diruthenium(III,IV) complexes are paramagnetic and display rhombic EPR spectral features. Complexes 1-3 are redox active. Complex 1 shows the one-electron reversible Ru-2(III)/(RuRuIV)-Ru-III, one-electron quasireversible (RuRuIV)-Ru-III/Ru-2(IV), and two-electron quasireversible Ru-2(III)/Ru-2(II) couples near 0.4, 1.5, and -1.0 V vs SCE In MeCN-0.1 M TBAP, respectively, in the cyclic and differential pulse voltammetric studies. Complexes 2 and 3 exhibit only reversible Ru-2(III)/(RuRuIV)-Ru-III and the quasireversible (RuRuIV)-Ru-III/Ru-2(IV) couples near 0.4 and 1.6 V vs SCE, respectively, The observation of a quasireversible one-step two-electron transfer reduction process in 1 is significant considering its relevance to the rapid and reversible Fe-2(III)/Fe-2(II) redox process known for the tribridged diiron core in the oxy and deoxy forms of hemerythrin.

Ferrocene in conjugation with a Fischer carbene: Synthesis, NLO, and electrochemical behavior of a novel organometallic push-pull system

Using an iterative sequence of Wittig olefination, reduction, oxidation, and condensation of an active methylene group to carbonyl, it was possible to prepare a series of organometallic push-pull molecules [(CO)(5)M=C(OCH3)(-CH=CH-)(n)(C5H4)Fe(C5H5), M = W, Cr, n = 1-4] in which ferrocene is the donor element and a Fisher carbene moeity is the acceptor group. The molecular first hyperpolarizability beta was determined by hyper-Rayleigh scattering experiments. The beta values ranged from 110 x 10(-30) to 2420 x 10(-30) esu in acetonitrile, and they are among the highest reported for organometallic molecules so far. Electrochemical measurements are consistent with the push-pull nature of these compounds.

Hydrogel-polymer electrolytes for electrochemical capacitors: an overview

Electrochemical capacitors are electrochemical devices with fast and highly reversible charge-storage and discharge capabilities. The devices are attractive for energy storage particularly in applications involving high-power requirements. Electrochemical capacitors employ two electrodes and an aqueous or a non-aqueous electrolyte, either in liquid or solid form; the latter provides the advantages of compactness, reliability, freedom from leakage of any liquid component and a large operating potential-window. One of the classes of solid electrolytes used in capacitors is polymer-based and they generally consist of dry solid-polymer electrolytes or gel-polymer electrolyte or composite-polymer electrolytes. Dry solid-polymer electrolytes suffer from poor ionic-conductivity values, between 10(-8) and 10(-7) S cm(-1) under ambient conditions, but are safer than gel-polymer electrolytes that exhibit high conductivity of ca. 10(-3) S cm(-1) under ambient conditions. The aforesaid polymer-based electrolytes have the advantages of a wide potential window of ca. 4 V and hence can provide high energy-density. Gel-polymer electrolytes are generally prepared using organic solvents that are environmentally malignant. Hence, replacement of organic solvents with water in gel-polymer electrolytes is desirable which also minimizes the device cost substantially. The water containing gel-polymer electrolytes, called hydrogel-polymer electrolytes, are, however, limited by a low operating potential-window of only about 1.23 V. This article reviews salient features of electrochemical capacitors employing hydrogel-polymer electrolytes.

Pt metal-CeO2 interaction: Direct observation of redox coupling between Pt-0/Pt2+/Pt4+ and Ce4+/Ce3+ states in Ce0.98Pt0.02O2-delta catalyst by a combined electrochemical and x-ray photoelectron spectroscopy study

Pt ions-CeO2 interaction in Ce1-xPtxO2-delta (x=0.02) has been studied for the first time by electrochemical method combined with x-ray diffraction and x-ray photoelectron spectroscopy. Working electrodes made of CeO2 and Ce0.98Pt0.02O2-delta mixed with 30% carbon are treated electrochemically between 0.0-1.2 V in potentiostatic (chronoamperometry) and potentiodynamic (cyclic voltametry) mode with reference to saturated calomel electrode. Reversible oxidation of Pt-0 to Pt2+ and Pt4+ state due to the applied positive potential is coupled to simultaneous reversible reduction of Ce4+ to Ce3+ state. CeO2 reduces to CeO2-y (y=0.35) after applying 1.2 V, which is not reversible; Ce0.98Pt0.02O2-delta reaches a steady state with Pt2+:Pt4+ in the ratio of 0.60:0.40 and Ce4+:Ce3+ in the ratio of 0.55:0.45 giving a composition Ce0.98Pt0.02O1.74 at 1.2 V, which is reversible. Composition of Pt ion substituted compound is reversible between Ce0.98Pt0.02O1.95 to Ce0.98Pt0.02O1.74 within the potential range of 0.0-1.2 V. Thus, Ce0.98Pt0.02O2-delta forms a stable electrode for oxidation of H2O to O-2 unlike CeO2. A linear relation between oxidation of Pt2+ to Pt4+ with simultaneous reduction in Ce4+ to Ce3+ is observed demonstrating Pt-CeO2 metal support interaction is due to reversible Pt-0/Pt2+/Pt4+ interaction with Ce4+/Ce3+ redox couple.

Electrochemical phase formation (ECPF): Nucleation growth vis-a-vis adsorption models

The nucleataon growth model of electrochemical phase formation is analysed for the hnear potential sweep input Apart from deducing diagnostic criteria and method~ of estimating model parameters, the predictions of the nucleation growth model are compared and contrasted with those of a sample adsorption model A dastlnCtlOn is made possible between adsorption and phase transition, which seems useful for understanding the nature of ECPF phenomena, especially underpotentlal deposition (UPD).

Two-dimensional phase transitions at electrochemical interfaces

A two-state Ising model has been applied to the two-dimensional condensation of tymine at the mercury-water interface. The model predicts a quadratic dependence of the transition potential on temperature and on the logarithm of the adsorbate concentration. Both predictions have been confirmed experimentally.

Electrochemical Reduction of Oriented Graphene Oxide Films: An in Situ Raman Spectroelectrochemical Study

Graphene oxide (GO) is assembled on a gold substrate by a layer-by-layer technique using a self-assembled cystamine monolayer. The negatively charged GO platelets are attached to the positively charged cystamine monolayer through electrostatic interactions. Subsequently, it is shown that the GO can be reduced electrochemically using applied DC bias by scanning the potential from 0 to -1 V vs a saturated calomel electrode in an aqueous electrolyte. The GO and reduced graphene oxide (RGO) are characterized by Raman spectroscopy and atomic force microscopy (AFM). A clear shift of the G band from 1610 cm-1 of GO to 1585 cm-1 of RGO is observed. The electrochemical reduction is followed in situ by micro Raman spectroscopy by carrying out Raman spectroscopic studies during the application of DC bias. The GO and RGO films have been characterized by conductive AFM that shows an increase in the current flow by at least 3 orders of magnitude after reduction. The electrochemical method of reducing GO may open up another way of controlling the reduction of GO and the extent of reduction to obtain highly conducting graphene on electrode materials.

Electrochemical phase formation (ECPF) and macrogrowth Part I. Hemispherical models

The phenomenological theory of hemispherical growth is generalised to time-dependent nucleation and growth-rates. Special cases, which include models with diffusion-controlled rates, are analysed. Expressions are obtained for small and large time behaviour and peak characteristics of potentiostatic transients, and their use in model parameter estimation is discussed. Two earlier equations are corrected. Numerically calculated transients which are presented exhibit some interesting features such as a maximum preceding the steady state, oscillations and shoulder.

Electrochemical phase formation (ECPF) and macrogrowth Part II. Two-rate models

A general theory is evolved for a class of macrogrowth models which possess two independent growth-rates. Relations connecting growth-rates to growth geometry are established and some new growth forms are shown to result for models with passivation or diffusion-controlled rates. The corresponding potentiostatic responses, their small and large time behaviours and peak characteristics are obtained. Numerical transients are also presented. An empirical equation is derived as a special case and an earlier equation is corrected. An interesting stochastic result pertaining to nucleation events in the successive layers is proved.

A Comparative Study of Electrochemical Kinetic Parameters by the Potential Step Method. Electrode Reactions of CyDTA and EDTA Complexes of Cu(II) at DME

By using the same current-time (I-t) curves, electrochemical kinetic parameters are determined by two methods, (a) using the ratio of current at a given potential to the diffusion-controlled limiting current and (b) curve fitting method, for the reduction of Cu(II)–CyDTA complex. The analysis by the method (a) shows that the rate determining step involves only one electron although the overall reduction of the complex involves two electrons suggesting thereby the stepwise reduction of the complex. The nature of I-t curves suggests the adsorption of intermediate species at the electrode surface. Under these circumstances more reliable kinetic parameters can be obtained by the method (a) compared to that of (b). Similar observations are found in the case of reduction of Cu(II)–EDTA complex.

Physicochemical, Electrochemical, and Spectroscopic Characterization of Zinc-Based Room-Temperature Molten Electrolytes and Their Application in Rechargeable Batteries

Room-temperature zinc ion-conducting molten electrolytes based on acetamide, urea, and zinc perchlorate or zinc triflate have been prepared and characterized by various physicochemical, spectroscopic, and electrochemical techniques. The ternary molten electrolytes are easy to prepare and can be handled under ambient conditions. They show excellent stability, high ionic conductivity, relatively low viscosity, and other favorable physicochemical and electrochemical properties that make them good electrolytes for rechargeable zinc batteries. Specific conductivities of 3.4 and 0.5 mS cm(-1) at 25 degrees C are obtained for zinc-perchlorate-and zinc-triflate-containing melts, respectively. Vibrational spectroscopic data reveal that the free ion concentration is high in the optimized composition. Rechargeable Zn batteries have been assembled using the molten electrolytes, with gamma-MnO2 as the positive electrode and Zn as the negative electrode. They show excellent electrochemical characteristics with high discharge capacities. This study opens up the possibility of using acetamide-based molten electrolytes as alternate electrolytes in rechargeable zinc batteries. (C) 2009 The Electrochemical Society.

Synthesis, spectral and electrochemical properties of donor/acceptor substituted fluoroarylporphyrins

Spectroscopic and electrochemical redox properties of a series of fluorinated porphyrins bearing donor-acceptor groups and their Zn(II) and Cu(II) derivatives are presented. The magnitude of the ring reduction potentials and charge transfer properties derived from spectral data depend on the nature and position of the substituent(s), (nitro/dimethylamino) and the central metal ions.

Electrochemical reduction of hydrogen peroxide on stainless steel

Electrochemical reduction of hydrogen peroxide is studied on a sand-blasted stainless steel (SSS)electrode in an aqueous solution of NaClO4.The cyclic voltammetric reduction of H2O2 at low concentrations is characterized by a cathodic peak at -0 center dot 40 V versus standard calomel electrode(SCE).Cyclic voltammetry is studied by varying the concentration of H2O2 in the range from 0 center dot 2 mM to 20 mM and the sweep rate in the range from 2 to 100 mV s(-1)Voltammograms at concentrations of H2O2 higher than 2 mM or at high sweep rates consist of an additional current peak, which may be due to the reduction of adsorbed species formed during the reduction of H2O2. Amperometric determination of H2O2 at -0 center dot 50 V vs SCEprovides the detection limit of 5 A mu M H2O2. A plot of current density versus concentration has two segments suggesting a change in the mechanism of H2O2 reduction at concentrations of H2O2 a parts per thousand yen 2 mM. From the rotating disc electrode study, diffusion co-efficient of H2O2 and rate constant for reduction of H2O2 are evaluated.

Hydrogel membrane electrolyte for electrochemical capacitors

Polymer electrolytes are known to possess excellent physicochemical properties that are very useful for electrochemical energy systems. The mobility in polymer electrolytes is understood to be mainly due to the segmental motion of polymer chains and the ion transport is generally restricted to the amorphous phase of the polymer. Gel polymer electrolytes (GPE) that are formed using plastizicers and polymers along with ionic salts are known to exhibit liquid-like ionic conductivity while maintaining the dimensional stability of a solid matrix. In the present study, the preparation and characterization of poly(vinyl alcohol)-based hydrogel membranes (PHMEs) as electrolyte for electrochemical capacitors have been reported. VaryingHClO4 dopant concentration leads to different characteristics of the capacitors. The EC comprising PHME doped with 2 M HClO4 and black pearl carbon (BPC) electrodes has been found to exhibit a maximum specific capacitance value of 97 F g(-1), a phase angle value of 78A degrees, and a maximum charge-discharge coulombic efficiency of 88%.