Study of the effects of ion pairing and activity coefficients on the separation in standard potentials for two-step reduction of dinitroaromatics

The electrochemical reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, occurs with potential inversion. That is, the standard potential for formation of the anion radical is shifted in the negative direction from the standard potential for the anion radical/dianion couple. This behavior has been attributed to significant structural changes accompanying the reduction steps. In this work, an assessment was made of the magnitude of the effects of activity coefficients and ion pairing, two effects which contribute to potential inversion. 1,4-Dinitrobenzene, 3, and 2,5-dimethyl-1,4-dinitrobenzene, 4, were studied in acetonitrile and N,N-dimethylformamide with R(4)N(+) salts as electrolytes (R = CH(3)-, CH(3)CH(2)-, CH(3)(CH(2))(3)-, and CH(3)(CH(2))(7)-) at concentrations from 0.010 to 0.100 M. Significant ion pairing between the dianion, A(2-), and R(4)N(+) was found for (CH(3))(4)N(+) with both 3 and 4 while the effects of the other electrolytes were smaller. The data were successfully interpreted without recourse to other ion pairs, e.g., ion pairing between the anion radical and the electrolyte cation. Ion pair formation constants are reported along with the infinite-dilution values of the difference in the two standard potentials. The effects of activity coefficients and ion pairing at 0.10 M electrolyte do not exceed 100 mV for (CH(3)N(+)) and are only 20 to 60 mV for (CH(3))(4)N(+), a cation commonly used in studies of potential inversion. It is concluded that structural changes accompanying the reduction, rather than activity and ion pairing effects, are the dominant factors underlying potential inversion.     

Evaluation of the extent of conjugation in symmetrical and asymmetrical aryl-substituted acetophenone azines using electrochemical methods.

The electrochemical behavior of a series of symmetrical and unsymmetrical aryl-substituted acetophenone azines (1-X/Y, where X and Y are 4-NO2, 4-CN, H, 3-OCH3, 4-OCH3, 4-CH3, and 4-N(CH3)2) was studied in acetonitrile and N,N-dimethylformamide (DMF) solution using cyclic voltammetry (CV). Compounds 1-X/Y, where neither X or Y are nitro substituents, undergo successive reduction to their radical anion (1-X/Y.-) and then dianion (1-X/Y2-), respectively. In all cases, the formation of the radical anion is completely reversible and the standard reduction potentials, Eo1-X/Y/1-X/Y.- could be determined. The reversibility of the second electron transfer is substituent dependent with certain dianions sufficiently basic to be protonated under our conditions. Standard reduction potentials (Eo1-X/Y/1-X/Y.-) for the formation of radical anions exhibit a large substituent effect with values differing by more than 0.66 V throughout the series going from 1-4-CN/4-CN to 1-4-OCH3/4-OCH3; similar substituent effects were determined for the formation of the dianion. The nitro-containing azines deviate from the above-mentioned behavior. With the exception of 1-4-NO2/4-NO2, they exhibit single electron waves that have values of Eo1-X/Y/1-X/Y.- within 40 mV of each other and thus the reduction is not subject to the same substituent effect as the other azines. 1-4-NO2/4-NO2 exhibits an Eo at a similar potential, but is a two-electron reversible wave with features indicative of a reduction system containing two localized, nonconjugated redox centers. The reduction potentials of all the aryl azines were correlated with Hammett sigma parameters to look at variations in Eo1-X/Y/1-X/Y.- vs SCE as a function of substituent. The small rho values in combination with the other electrochemical data provide support for single bond character of the N-N bond and evidence for a lack of strong electronic communication between the two aryl centers through the azomethine bonds, especially for those systems with electron-withdrawing groups.     

Behavior of electrogenerated bases in room-temperature ionic liquids

The reductive electrochemistry of substituted benzophenones in the aprotic room-temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bistriflimide occurs via two consecutive one-electron processes leading to the radical anion and dianion, respectively. The radical anion exhibited electrochemical reversibility at all time-scales whereas the dianion exhibited reversibility at potential sweep rates of >or=10 V s(-1), collectively indicating the absence of strong ion-paring with the RTIL cation. In contrast, reduction in 1-butyl-3-methylimidazolium bistriflimide is complicated by proton-transfer from the [Bmim] cation. At low potential sweep rates, reduction involves a single two-electron process characteristic of either an electrochemical, chemical, electrochemical (ECE) or disproportion-type (DISP1) mechanism. The rate of radical anion protonation in [Bmim] is governed by basicity and conforms to the Hammett free-energy relation. At higher potential sweep rates in [Bmim][NTf2], reduction occurs via two consecutive one-electron processes, giving rise to the partially reversible generation of the radical anion and the irreversible generation of the dianion, respectively. Also, the redox potentials for the reversible parent/radical anion couples were found to be a linear function of Hammett substituent constants in both RTIL media and exhibited effectively equivalent solvent-dependent reaction constants, which are similar to those for reduction in polar molecular solvents such as acetonitrile or alcohols.     

Strukturen und Molekül-Eigenschaften ladungsgestörter Moleküle. 52. Mitteilung. 2, 3-Diphenylchinoxalin-Radikalanionen in Lösung und im Kristall

Structures and Molecular Properties of Charge-Pertubed Molecules. 2, 3-Diphenylquinoxaline Radical Anions in Solution and in Crystals The Na⊕ and K⊕ radical-ion salts of 2, 3-diphenylquinoxaline seem to be (according to a structural database search) among the first ones of N-heterocyclic radical anions in crystals. The one-electron reduction in aprotic 1, 2-dimethoxyethan (DME) solution at metal mirrors and the crystallization under Ar have been preceded by cyclovoltammetric (CV) and ESR/ENDOR measurement. The first electron insertion at ?1.63 V proves to be reversible, whereas the irreversible second step, which is accompanied by an overcrossing of the CV line, can be rationalized by an ‘ECE-DISP’ mechanism via a dianion redox disproportionation. The ENDOR spectrum resolves four ¹H couplings and allows to simulate the ESR spectrum including the ¹⁴N hyperfine splittings. Both dark-blue single crystals of the radical ion salts $[2,3{\rm - diphenylquinoxaline}^{{.} \ominus} {\rm Met}^ \oplus ({\rm DME})]^.$ show unexpected similarities for Met⊕ = Na⊕, K⊕ despite the 36-pm difference in their ionic radii. The largest structural changes inflicted by the one-electron reduction of the N-heterocyclic molecule are observed in the vicinity of the N-centers bearing the highest effective nuclear charge. The DME-chelated metal cations coordinate at the N electron pairs and form Met⊕(DME)-bridged polymer chains of the radical anion, which are differently ondulated in the Na⊕ and K⊕ radical anion salts. The take-home lesson suggests that many more N-heterocyclic molecules might be analogously reduced under optimized conditions and isolated as single crystals.     

取代苯醌在乙腈中的电化学和光谱电化学研究

研究了含有甲基、甲氧基、氟或氯等不同取代基的对苯醌衍生物在乙腈中的电化学和紫外-可见光谱电化学性质,并探讨了取代基对化合物电化学性质的影响.结果表明,每个化合物均可以发生两步可逆的单电子还原反应,分子中的供电子基能使还原反应电位发生负移,而吸电子基则可使电位发生正移.还原电位的变化值(ΔE)与取代基哈密特常数(∑σ)之间呈线性关系,其方程为:ΔE1=0.386 9∑σ-0.073 5(V),R2=0.996,ΔE2=0.280 3∑σ-0.114 5(V),R2=0.981.在控制电位还原时化合物的紫外可见光谱具有明显的变化,表明两步还原反应的产物分别为阴离子自由基(R)nQ.-和负二价阴离子(R)nQ2-(R=—CH3,—OCH3,—Cl,—F;n=0～4).     

Role of electron-transfer processes in reactions of diarylcarbenium ions and related quinone methides with nucleophiles

Second-harmonic alternating current voltammetry has been used to determine one-electron reduction potentials of 15 diarylcarbenium ions and 5 structurally analogous quinone methides, which have been employed as reference electrophiles for the development of nucleophilicity scales. A linear correlation (r(2) = 0.993) between the empirical electrophilicity parameters E and the reduction potentials in acetonitrile (E = 14.091E degrees (red) - 0.279) covering a range of 1.64 V (or 158 kJ mol(-)(1)) has been observed. For a large number of nucleophiles, it has been demonstrated that the observed activation free energies of the electrophile-nucleophile combinations are 61-195 kJ mol(-)(1) smaller than the free energy change of electron transfer from nucleophile to electrophile, which definitely excludes outer-sphere electron transfer occurring during these reactions.     

Electron-transfer reactions with significant changes in structure. Unsymmetrical crowded ethylenes

The electrochemical reduction mechanisms of xanthylideneanthrone, 6, thioxanthylideneanthrone, 7, 10-(diphenylmethylene)anthrone, 8, and 9-(diphenylmethylene)-9H-fluorene, 9, have been studied in dimethylformamide. The reduction of the first two compounds proceeds from folded forms of the neutral to twisted forms of the anion radical according to a square scheme. The data for reduction of 8 can be well accounted for by the same square scheme. However, one-step reduction with concerted electron transfer and structural change cannot be ruled out. Compound 9, whose fluorene ring system cannot fold, exists only in twisted forms in the neutral, anion radical, and dianion. Consequently, there are no major changes in structure upon reduction, and the compound is reduced in two reversible steps with the second complicated by rapid loss of the dianion that is probably due to protonation by components of the medium.     

Density functional theory study of redox pairs: 2. Influence of solvation and ion-pair formation on the redox behavior of cyclooctatetraene and nitrobenzene

A study of the electrochemical behavior of cyclooctatetraene (COT) and nitrobenzene with Density Functional Theory and the conductor like solvation model (COSMO) is reported. The two-electron reduction of the tub-shaped COT molecule is accompanied by a structural change to a planar structure of D(4)(h)() symmetry in the first electron addition step, and to a fully aromatic structure of D(8)(h)() symmetry in the second electron addition step. Theoretical models are examined that are aimed at understanding the electrolyte- and solvent-dependent redox behavior of COT, in which a single 2e(-) redox wave is observed with KI electrolyte in liquid ammonia solution (DeltaDeltaE(disp) = [E(-2) - E(-1)] - [E(-1) - E(0)] < 0, inverted potential), while two 1e(-) redox waves are observed (DeltaDeltaE(disp) > 0) with NR(4)(+)X(-) (R = butyl, propyl; X(-) = perchlorate) electrolyte in dimethylformamide solution. In all cases, the computed reaction energy profiles are in fair agreement with the experimental reduction potentials. A chemically intuitive theoretical square scheme method of energy partitioning is introduced to analyze in detail the effects of structural changes and ion-pair formation on the relative energies of the redox species. The structural relaxation energy for conversion of tub-COT to planar-COT is mainly apportioned to the first reduction step, and is therefore a positive contribution to DeltaDeltaE(disp). The effect of the structural change on the disproportionation energy for COT is counteracted by the substantially more positive reduction potential for planar-(COT)(-1) in comparison to tub-(COT)(-1). Ion pairing of alkali metal counterions with the anionic reduction products gives rise to a negative contribution to DeltaDeltaE(disp) because the second ion-pairing step is more exothermic than the first, and the reduction of [KA] (A = COT, NB) is more exothermic than the reduction of A(-1). For COT, this negative energy differential term as a result of ion pairing predicts the experimentally observed inversion in the two 1e(-) potentials (DeltaDeltaE(disp) < 0). Nitrobenzene is treated with the same computational protocol to provide a system for comparison that is not complicated by the major structural change that influences the COT energy profile.     

Analysis of the substituent effect on the reactivity modulation during self-protonation processes in 2-nitrophenols

A voltammetric and spectroelectrochemical ESR study of the reduction processes of five substituted 4-R-2-nitrophenols (R = -H, -OCH(3), -CH(3), -CN, -CF(3)) in acetonitrile was performed. In the potential range considered here (-0.2 to -2.5 V vs Fc+/Fc), two reduction signals (Ic and IIc) were detected; the first one was associated with the formation of the corresponding hydroxylamine via a self-protonation pathway. The voltammetric analysis at the first reduction signal showed that there are differences in the reduction pathway for each substituted 4-R-2-nitrophenol, being the E1/2 values determined by the inductive effect of the substituent in the meta position with respect to the nitro group, while the electron-transfer kinetics was determined by the protonation rate (k(1)+ ) of the anion radical electrogenerated. However, at potential values near the first reduction peak, no ESR signal was recorded from stable radical species, indicating the instability of the radical species in solution. Nevertheless, an intense ESR spectrum generated at the second reduction peak was detected for all compounds, indicating the monoelectronic reduction of the corresponding deprotonated 4-R-2-nitrophenols. The spin-coupling hyperfine structures revealed differences in the chemical nature of the electrogenerated radical; meanwhile, the -CF(3) and -CN substituents induced the formation of a dianion radical structure, and the -H, -CH(3), and -OCH(3) substituents provoked the formation of an anion radical structure due to protonation by acetonitrile molecules of the initially electrogenerated dianion radical. This behavior was confirmed by analyzing the ESR spectra in deuterated acetonitrile and by performing quantum chemical calculations of the spin densities at each site of the electrogenerated anionic radicals.     

On the kinetics and energetics of one-electron oxidation of 1,3,5-triazines

One-electron oxidation of 1,3,5-triazines is observed with both excited uranyl ion (*UO2(2+)) and sulfate radical anion (SO4.-) in aqueous solution, but not with Tl2+, indicating that the standard reduction potentials E degree of 1,3,5-triazine radical cations are = 2.3 +/- 0.1 V vs. NHE, consistent with theoretical calculations; this suggests that if triazines inhibit electron transfer during photosynthesis, they would need to act on the reductive part of the electron transport chain.     

Incorporation of stable organic radicals of the tris(2,4, 6-trichlorophenyl)methyl radical series to pyrrole units as models for semiconducting polymers with high spin multiplicity. 1

The condensation reactions between (4-amino-2,6-dichlorophenyl)bis(2, 4,6-trichlorophenyl)methyl radical and acetylacetone or 1, 4-bis(5-methyl-2-thienyl)-1,4-butanedione yield [2,6-dichloro-4-(2, 5-dimethyl-1-pyrrolyl)phenyl]bis(2,4,6-trichlorophenyl)methyl radical (3(*)()) and [2,6-dichloro-4-[2, 5-bis(5-methyl-2-thienyl)-1-pyrrolyl]phenyl]bis(2,4, 6-trichlorophenyl)methyl radical (4(*)()), respectively. EPR studies of both radicals 3(*)() and 4(*)() in CH(2)Cl(2) solution suggest a weak electron delocalization with coupling constant values of 1.25 and 1.30 G, respectively, with the six aromatic hydrogens. Their electrochemical behavior was analyzed by cyclic voltammetry. Both radicals show reversible reduction processes at E degrees = -0.69 V and -0.61 V versus SSCE, respectively, and anodic peak potentials at E(p)(a) = 1.10 and 0.72 V, respectively, versus SSCE at a scan rate (nu) of 200 mV s(-)(1), being reversible for radical 4(*)(). X-ray analysis of radical 3(*)() shows a high value (65 degrees ) of the dihedral angle between the 2,5-dimethylpyrrolidyl moiety and the phenyl ring. Smooth oxidation of radical 4(*)() in CH(2)Cl(2) containing trifluoroacetic acid gives an ionic diradical species with a weak electron interaction (|D/hc| = 0.0047 cm(-)(1)). A Curie plot of the Deltam(s)() = +/-2 signal intensity versus the inverse of the absolute temperature in the range between 4 and 70 K suggests a triplet or a nearly degenerate singlet-triplet ground state.     

Modeling [Fe-Fe] hydrogenase: evidence for bridging carbonyl and distal iron coordination vacancy in an electrocatalytically competent proton reduction by an iron thiolate assembly that operates through Fe(0)-Fe(II) levels

IR spectroelectrochemistry of Fe4{Me(CH2S)3}2(CO)8 (4Fe6S) in the nu(CO) region shows that the neutral and anion forms have all their CO groups terminally bound to the Fe atoms; however, for the dianion there is a switch of the coordination mode of at least one of the CO groups. The available structural and nu(CO) spectra are closely reproduced by density-functional theory calculations. The calculated structure of 4Fe6S2- closely mirrors that of the diiron subsite of the [Fe-Fe] hydrogenase H cluster with a bridging CO group and an open coordination site on the outer Fe atom of pairs of dithiolate-bridged Fe0FeII subunits connected by two bridging thiolates. Geometry optimization based on the all-terminal CO isomer of 4Fe6S2- does not give a stable structure but reveals a second-order saddle point ca. 11.53 kcal mol(-1) higher in energy than the CO-bridged form. Spectroelectrochemical studies of electrocatalytic proton reduction by 4Fe6S show that slow turnover from the primary reduction process (E1/2'=-0.71 V vs Ag/AgCl) involves rate-limiting protonation of 4Fe6S- followed by reduction to H:4Fe6S-. Rapid electrocatalytic proton reduction is obtained at potentials sufficient to access 4Fe6S2-, where the rate of dihydrogen elimination from the FeIIFeII core of 4Fe6S is ca. 500 times faster than that from the FeIFeI core of Fe2(mu-S(CH2)3S)(CO)6. The dramatically increased rate of electrocatalysis obtained from 4Fe6S over all previously identified model compounds appears to be related to the features uniquely common between it and the H-cluster, namely, that turnover involves the same formal redox states of the diiron unit (FeIFeII and Fe0FeII), the presence of an open site on the outer Fe atom of the Fe0FeII unit, and the thiolate-bridge to a second one-electron redox unit.     

1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazolidyl: a long-lived radical anion and its stable salts

[1,2,5]Thiadiazolo[3,4-c][1,2,5]thiadiazole (1) is synthesized in 62% yield by fluoride ion-induced condensation of 3,4-difluoro-1,2,5-thiadiazole with (Me(3)SiN=)(2)S. The reversible electrochemical reduction of 1 leads to the long-lived [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (2) and further to the dianion (3). The radical anion 2 is also obtained by the chemical reduction of the precursor 1 with t-BuOK in MeCN. The radical anion 2 is characterized by ESR spectroscopy in solution and in the crystalline state. The stable salts [K(18-crown-6)][2] and [K(18-crown-6)][2].MeCN (8 and 9, respectively) are isolated from the spontaneous decomposition of the [K(18-crown-6)][PhXNSN] (6, X = S; 7, X = Se) salts in MeCN solution followed by XRD characterization. The radical anion 2 acts as a bridging ligand in 8 and as chelating ligand in 9. The structural changes observed by XRD in going from 1 to 2 are explained by means of DFT/(U)B3LYP/6-311+G calculations.     

Redox potentials of iron—nitrosyl complexes: DFT calculations

Quantum chemical calculations of the structure and solvation energies of mono- and dianions of Roussin’s red salt esters [Fe₂(μ-RS)₂(NO)₄] (R = Me, Et, Prⁱ, Bu^t) in solutions in THF and acetonitrile were carried out. In monoanions, an additional electron is localized on one Fe(NO)₂ fragment, which leads to significant structural distortion of the anion compared to neutral molecule. The second electron is localized on the other Fe(NO)₂ fragment; this causes symmetrization of the dianion geometry. There are good linear correlations between the calculated and experimental redox potentials of these systems. A relationship was proposed for estimation of the redox potentials of related iron—nitrosyl complexes. The standard redox potentials of the complex with R = Ph in water, DMSO, and acetonitrile evaluated using this expression lie between −0.5 and −0.6 V.     

Electrochemically induced chemically reversible proton-coupled electron transfer reactions of riboflavin (vitamin B2)

The electrochemical behavior of the naturally occurring vitamin B(2), riboflavin (Fl(ox)), was examined in detail in dimethyl sulfoxide solutions using variable scan rate cyclic voltammetry (ν = 0.1 - 20 V s(-1)) and has been found to undergo a series of proton-coupled electron transfer reactions. At a scan rate of 0.1 V s(-1), riboflavin is initially reduced by one electron to form the radical anion (Fl(rad)(?-)) at E(0)(f) = -1.22 V versus Fc/Fc(+) (E(0)(f) = formal reduction potential and Fc = ferrocene). Fl(rad)(?-) undergoes a homogeneous proton transfer reaction with the starting material (Fl(ox)) to produce Fl(rad)H(?) and Fl(ox)(-), which are both able to undergo further reduction at the electrode surface to form Fl(red)H(-) (E(0)(f) = -1.05 V vs Fc/Fc(+)) and Fl(rad)(?2-) (E(0)(f) = -1.62 V vs Fc/Fc(+)), respectively. At faster voltammetric scan rates, the homogeneous reaction between Fl(rad)(?-) and Fl(ox) begins to be outrun, which leads to the detection of a voltammetric peak at more negative potentials associated with the one-electron reduction of Fl(rad)(?-) to form Fl(red)(2-) (E(0)(f) = -1.98 V vs Fc/Fc(+)). The variable scan rate voltammetric data were modeled quantitatively using digital simulation techniques based on an interconnecting "scheme of squares" mechanism, which enabled the four formal potentials as well as the equilibrium and rate constants associated with four homogeneous reactions to be determined. Extended time-scale controlled potential electrolysis (t > hours) and spectroscopic (EPR and in situ UV-vis) experiments confirmed that the chemical reactions were completely chemically reversible.     

A radical-anion chain mechanism initiated by dissociative electron transfer to a bicyclic endoperoxide: insight into the fragmentation chemistry of neutral biradicals and distonic radical anions

The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N-dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the O-O bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be -0.55+/-0.15 V and 20+/-3 kcal mol(-1), respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the O-O bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene-cis-1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane-cis-1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a beta-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals.     

Interpretation of solute retention of some monovalent inorganic anions in anion-exchange chromatography using a dicarboxylic Acid as an eluent.

In single-column anion-exchange chromatography, the retention volume of some monovalent inorganic anions (Cl(-), Br(-), NO(3)(-), NCS(-) and NO(2)(-)) were observed as a function of the pH of a mobile phase at a fixed concentration of 2-phenylmalonic acid or 1,4-benzenediacetic acid used as an eluent. The experimental retention volume of such an anion was decreased with an increase in the pH of a mobile phase, and was able to be described by the following equation taking account of anion-exchange equilibria of a sample anion with a hydrogen dicarboxylate ion (HE(-)) and with a dicarboxylate ion (E(2-)): alpha(1s)/V(R)'[HE(-)] = 1/m(T)wK(ex1) + (2K(a2)/m(T)w(2)K(ex2))(V(R)'/alpha(1s)[H(+)]), where V(R)', m(T), w, K(a2), K(ex1) and K(ex2) are the adjusted retention volume of a given sample anion, the capacity for the anion-exchange of column packings and the weight of column packings packed into a separating column, the second acid-dissociation constant of the dicarboxylic acid used as an eluent, and equilibrium constants for the anion exchange of a sample anion with a monovalent hydrogen dicarboxylate ion and with a divalent dicarboxylate ion, respectively. The term alpha(1s), defined as K(as)/([H(+)] + K(as)), where K(as) is the acid-dissociation constant of HX, is the mole fraction of a sample anion, X(-), and is equal to 1 when using a strong acid anion as a sample anion.     

Electrochemical Reduction of Cyclohex-2-enones

The electrochemical reduction of the cyclohex-2-enones 1a–1e (mercury cathode, CH₃CN, Bu₄NBF₄) was studied by means of cyclic voltammetry, d.c. polarography, coulometry and chemical product analysis. Compounds 1a–1c give a mixture of the hydrodimers 4 and 5 via formation of the radical anion 2 by an irreversible one electron transfer, followed by protonation and dimerization of the allylic radical 3 . The 6-halocyclohex-2-enones 1d and 1e exhibit two distinct reduction waves. The first corresponds to an irreversible two electron transfer with formation of the halide anion and the enolate anion 6 which gives 1b by protonation. The second wave corresponds to a quasi-reversible one electron transfer to 6 to afford the radical dianion 7 (Scheme 2).     

Optical and electrochemical properties of heteroditopic ion receptors derived from crown ether-based calix[4]arene with amido-anthraquinone pendants

Two heteroditopic receptors based on a calix[4]arene crown ether containing amidoanthraquinone pendants in cone and 1,3-alternate conformations (1 and 2, respectively) were synthesized. Photophysical properties of 1 and 2 were studied by UV-vis and fluorescence spectrophotometry in dried CH(3)CN. Both 1 and 2 showed the highest sensitivity towards F(-) through the appearance of a new charge transfer band at 500 nm and the enhancement of the emission spectra at λ(em) = 542 nm and 528 nm respectively. Interestingly, in the presence of K(+), the fluorescence intensity of 1 at 542 nm increased around 2 fold compared to that in the absence of K(+) upon addition of F(-), while this phenomenon was not observed in the case of receptor 2. Cyclic voltammograms of receptors 1 and 2 showed two consecutive one-electron reversible waves in 40% v/v CH(3)CN in CH(2)Cl(2), corresponding to two single-electron reductions to give mono- and dianions species at E(1/2)I = -1.21 V and E(1/2)II = -1.66 V as well as E(1/2)I = -1.25 V and E(1/2)II = -1.71 V, respectively. H(2)PO(4)(-) gave remarkable potential shifts (ca. 200 mV) of the second reduction waves (E(1/2)II) of both free 1 and 2. In the presence of K(+), only receptor 1 gave remarkable potential shifts in its redox wave II upon adding F(-) and AcO(-). Therefore, receptors 1 and 2 exhibited dual sensing modes by fluorescence spectrophotometry and cyclic voltammetry. The topology of ligands also played an important role in cooperative binding properties of heteroditopic receptor 1 possessing a closer distance between a cation and an anion binding. On the other hand, the two ion binding sites of receptor 2 were separated by a longer distance and did not support the cooperative binding. This resulted in the abstraction of K(+) from receptor 2 upon addition of anions.     

Relationship between molecular structure and electron targets in the electroreduction of benzocarbazolediones and anilinenaphthoquinones. Experimental and theoretical study

We report the synthesis and voltamperometric reduction of 5H-benzo[b]carbazole-6,11-dione (BCD) and its 2-R-substituted derivatives (R = -OMe, -Me, -COMe, -CF(3)). The electrochemical behavior of BCDs was compared to that of the 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) previously studied. Like PANs, BCDs exhibit two reduction waves in acetonitrile. The first reduction step for the BCDs represents formation of the radical anion, and the half-wave potential (E(1/2)) values for this step are less negative than for that of the PANs. The second reduction wave, corresponding to the formation of dianion hydroquinone, has E(1/2) values that shift to more negative potentials. A good linear Hammett-Zuman (E(1/2) vs sigma(p)) relationship, similar to that for the PAN series, was also obtained for the BCDs. However, unlike the PANs, in the BCDs, the first reduction wave was more susceptible to the effect of the substituent groups than was the second wave, suggesting that the ordering of the two successive one-electron reductions in BCDs is opposite that in PANs. This is explained by the fact that the electron delocalizations in the two systems are different; in the case of BCDs there is an extra aromatic indole ring, which resists loss of its aromatic character. The electronic structures of BCD compounds were, therefore, investigated within the framework of the density functional theory, using the B3LYP hybrid functional with a double zeta split valence basis set. Our theoretical calculations show that the O(1).H-N hydrogen bond, analogous to that previously described for the PAN series, is not observed in the BCDs. Laplacians of the critical points (nabla(2)rho) and the natural charges for the C-O bonds indicate that the first reduction wave for the BCDs corresponds to the C(4)-O(2) carbonyl, while in the PAN series the first one-electron transfer occurred at the C(1)-O(1) carbonyl. Natural bond orbital analysis showed that, in all the BCDs, the lowest unoccupied molecular orbital (LUMO) is located at C(4), whereas for the PANs, the LUMO is found at C(1). The good correlation between the LUMO energy values and the E(1/2) potentials (wave I) established that the first one-electron addition takes place at the LUMO. Analysis of the molecular geometry confirmed that, in both series of compounds, the effect of the substituent groups is mainly on the C(4)-O(2) carbonyl. These results explain the fact that reduction of the C(4)-O(2) carbonyl (voltammetric wave II in the PANs and voltammetric wave I in the BCDs) is more susceptible to the effect of the substituent groups than is reduction of the C(1)-O(1) carbonyl (wave I in the PANs and wave II in the BCDs).