Stable Radical Anions Research Articles

Electrochemical Reduction of 1,4-Disubstituted Phthalazines

Electrochemical reduction in dimethylformamide of unsymmetrical 1-Cl-4-X-phthalazines and symmetrical 1,4-X2-phthalazines (X = Cl, PhO, MeO, EtO, i-PrO) was studied by voltammetry and electrolysis-ESR. The electron trasfer on 1,4-dichlorophthalazine and unsymmetrical 1-Cl-4-X-phthalazines induces anionoid elimination of Cl and X, pyridazine ring cleavage, and phthalonitrile formation. In this previously unknown process the transfer of two electrons induces cleavage of three σ bonds and formation of two new π bonds. The reduction of 1,4-dialkoxy(diaryloxy)phthalazines involves formation of stable radical anions which undergo no heteroring cleavage. Possible mechanisms of the processes studied were discussed. It was assumed that the same mechanism with pyridazine ring cleavage may be realized with other phthalazine derivatives and heteroannelated pyridazines containing readily leaving nucleofuge groups α to nitrogens.

Fulleropyrrolidine-containing sterically hindered phenol. Synthesis, structure, and properties

Fulleropyrrolidine containing a sterically hindered phenolic fragment was synthesized by the reaction of fullerene C60 with N-methylglycine and 3,5-di-tert-butyl-4-hydroxybenzaldehyde. Electrochemical reduction of fulleropyrrolidine-containing phenol 1 and the corresponding phenoxide ion proceeded stepwise to form stable radical anions, dianions, and trianions. The radical anion (g = 2.0000) and the phenoxyl radical (g = 2.0045) obtained by chemical oxidation with lead dioxide were identified by ESR spectroscopy. The electron affinity of fulleropyrrolidine was estimated at 2.58 eV. For the phenoxide ion, the electrochemical gap was determined (ΔE = EIox – EIred = 0.47 V). The heats of formation and the energies of the frontier orbitals of fulleropyrrolidine and its transformation products were evaluated by the PM3 method.

The cathodic reduction of activated olefins. Experimental conditions allowing the specific hydrogenation of the enone derived from ergosterol

The reduction of the bulky enone 1 derived from ergosterol was performed in non-aqueous media without and with proton donor. A fairly stable anion radical was obtained and characterized. When 1 was reduced in the presence of an efficient proton donor (benzoic acid, trifluoroacetic acid) saturation of the double bond was achieved with reasonable yield. The overall process as a function of experimental conditions is discussed.

Observation of Highly Stable Radical Anions of Ladder Oligosilanes

Abstract The radical anions of ladder oligosilanes were generated by the reduction with alkali metals and observed by UV–visible–NIR and ESR spectroscopies. The radical anions of the longer ladder oligosilanes are highly stable at room temperature.

The electrochemical behaviour of electron deficient benzoheterocycle triosmium clusters

The electrochemical behaviour of the electron deficient clusters [Os3(CO)9(μ3-η2-(L-H))(μ-H)] (L=quinoline (1); 3-amino-quinoline (2); 5-amino quinoline (3); 6-methoxy quinoline (4); phenanthridine (5); 5,6-benzoquinoline (6); quinoxaline (7); 2-methyl benzimidazole (8); 2-methyl benzotriazole (9); 2-methyl benzothiazole (10); benzothiazole (11); 2-methyl benzoxazole (12)) is reported. The complexes all exhibit two one-electron electrochemically quasi-reversible reduction processes and in the case of 5 and 6 stable radical anions are formed following a one-electron reduction that is both electrochemically and chemically reversible. The electrochemical behaviour of the electron precise decacarbonyl analogues of 4–12 (4′–12′) was also studied in order to gain some insight into changes in the nature of the lowest unoccupied molecular orbital (LUMO) on going from the electron deficient to the electron precise cluster for a given ligand. The observed electrochemical potentials for 1–12 correlate with n to σ* electronic transitions (excluding 3 and 6) but not with the other absorptions for two series of complexes and the free ligands. The infrared spectroelectrochemistry, and ESR of 5− was also studied in an attempt to confirm the stability of these radical anions and to further elucidate their electronic and molecular structure.

Radical anions from 5-fluorouracil derivatives. A theoretical study of their cleavage and orbital isomerism

Theoretical calculations were performed on the dissociation of radical anions of 5-fluorouracils substituted at N1. The reaction (1-CR1R2R3-5-fluorouracil)−→(5-fluorouracil)−+R1R2R3C was calculated for several substituents R, including the N1–C5 dimers 1. Two main types of radical anions were observed. Radical anions of π type have the unpaired spin density located at the carbonyl and C–C double bond conjugated system of the uracil ring while the σ species present unpaired spin density mainly at the N1–C(R1R2R3) bond. The electronic nature of the most stable radical anion was found to be determined by the electron affinity of the substituent at N1.The activation barrier for the cleavage of π radical anions arises from its avoided crossing with the σ potential surface. The σ radical anions, which have stabilised SOMOs, dissociate with lower activation barriers than the π intermediates. These observations correlate with the lengthening and increment of unpaired spin density at their N1–C bond. Gas phase calculations are compared with results in aqueous solution. Other fragmentation paths are proposed for substituents like RNO2 or halogen.

1,4-Bis(alkylsulfonyl)benzenes. A new and unexpected cathodic cleavage leading to the corresponding phenoxy ions

1,4-Bis(alkylsulfonyl)benzenes 2 exhibit the formation of a fairly stable anion radical under cathodic electron transfer. Its fragmentation leads both to 1-alkylsulfonyl-4-alkyl benzenes ( ipso substitution) and 4-alkylsulfonylphenoxy ions, the presence of the latter suggesting a new mode of cleavage for strongly activated sulfones.

Ab initio G2 and DFT calculations on electron affinity of cyclopentadiene, silole, germole and their 2,3,4,5-tetraphenyl substituted analogs: structure, stability and EPR parameters of the radical anions

Energies, geometries and electronic structures of cyclopentadiene (1), silole (2), germole (3) and their radical anions (RAs) 1a–3a have been studied by quantum-chemical methods at the G2(MP2, SVP) and DFT B3LYP/6-31G(d,p) levels. According to G2(MP2, SVP) calculations 1–3 have negative electron affinities (EAs) equal to −1.09, −0.06 and −0.15 eV, respectively, and hence their RAs are unstable in the gas phase. 1,2,3,4-Tetraphenylcyclopenta-1,3-diene (4) and 2,3,4,5-tetraphenylsubstituted-2 (5) and 3 (6) and their radical anions (4a–6a) were studied at the B3LYP/6-31G(d)//AM1 level. Substitution of cyclodienes results in significant stabilization of RAs 4–6 because of the effective delocalization of an extra electron within phenyl groups, especially those in positions 2 and 5, so 4–6 all have a positive EA. Our best estimates give for 4, 5 and 6 EA values 0.98, 1.30 and 1.13 eV respectively. The calculated hyperfine coupling (hfc) constants a(H) for radical anions 4a–6a are in a good agreement with experimental values. Variation of group 14 element (C, Si, Ge) in the series 1–3 and 4–6 do not produce a monotonous change of EA and hfc values.

Formation of Neutral C7H2 Isomers from Four Isomeric C7H2 Radical Anion Precursors in the Gas Phase

Consideration of theoretical calculations \[E3LYP/aug-cc-pVDZ//B3LYP/6-31G(d)\] of the structures of ten C7H2 neutral isomers and the nine corresponding C7H2 radical anions have led us to synthesize four stable C7H2 radical anions in the ion source of our ZAB 2HF mass spectrometer, and to convert these to C7H2 neutrals. The four radical anion isomers prepared were (i) \[(HC equivalent to C)(2)C=C=C\](-.) \[from the reaction between (HC equivalent to C)(3)COCH3 and HO- \], (ii) \[HC=C=C=C=C=C=CH\](-.) \[from the reaction between HC equivalent to C-C equivalent to C-CD(OH)-C equivalent to CH and HO-\], (iii) \[C=C=C=C=C=C=CH2\](-.) \[from the reaction between DC equivalent to C-C equivalent to C-C equivalent to C-CH2OCH2CH3 and HO-\], and (iv) \[C equivalent to C-CH2-C equivalent to C-C equivalent to C\](-.) \[from the bis desilylation reaction of (CH3)(3)Si-C equivalent to C-CH2-C equivalent to C-C equivalent to C-Si (CH3)(3)With SF6-.\]. The four anions were further characterized by their collisional activation (negative ion) and charge reversal (CR, positive ion) mass spectra. The anions were converted into their corresponding neutrals by charge stripping, and the correspondence between the charge reversal (CR) and neutralization reionization (-NR+) mass spectra of each anion is taken as evidence that within the time frame of the -NR+ experiment (some 10(-6) s), each neutral is stable and undergoes no major rearrangement or interconversion to a more stable isomer. Theory and experiment are in accord for these systems.

Powerful Oxidizing Properties of a Planar [8]Annulene: Tetrakis(perfluorocyclobuta)cyclooctatetraene and Its Radical Anion

The central eight-membered ring of tetrakis(perfluorocyclobuta)cyclooctatetraene (1) is flat both in the crystal and in solution, but tetrakis(perfluorocyclopenta)cyclooctatetraene (2) assumes a tub shape similar to that of parent cyclooctatetraene. This sterically induced structural difference strongly affects the physical properties of compounds 1 and 2. The first and second reduction potentials of 1 in solution, +0.79 and +0.14 V vs. SCE, are considerably more positive than the corresponding values, +0.20 and −0.21 V, of 2. Accordingly, mere contact with Hg metal in N,N-dimethylformamide converts 1 to an unusually stable radical anion 1.− that is persistent in air. The temperature-independent coupling constant, aF=+0.320 mT, of the sixteen equivalent β-19F-nuclei in 1.− is much smaller than the expected value, and the g factor of 1.− is markedly lower than those generally found for structurally related radical anions. In contrast, the characteristic data for 2.−, generated from 2 with K metal in 1,2-dimethoxyethane, comply with the expected data. The coupling constants aF=+0.976 and +0.935 mT, each of eight β-19F-nuclei in 2.−, average +0.951 mT upon raising the temperature from 220 to 260 K. The gas-phase electron affinity of 1, A=3.4±0.2 eV, which was estimated from the reduction potential in solution and from charge-transfer absorption spectra obtained from a series of methylated benzenes and naphthalenes, is among the highest known for neutral organic molecules.

Thermodynamic Stabilities and Resonance Demand of Aromatic Radical Anions in the Gas Phase

Abstract The electron affinities (EAs) for a series of methyl benzoates, acetophenones, benzaldehydes, and benzophenones were determined by measuring the electron-transfer equilibria in the gas phase. The effect of substituents (X) in the benzene ring on the EAs of this series as well as the nitrobenzene and benzonitrile series were correlated in terms of the Yukawa–Tsuno equation, It has been shown that the r− value, representing the degree of π-delocalization of the negative charge into the substituent (X) at position 4, increases linearly with decreasing EA of the unsubstituted member (X = H) of the respective series, i.e., 0.65 for nitrobenzene vs. 1.05 for benzaldehyde and 1.30 for benzonitrile. This tendency of the r− value leads to the conclusion that the negative charge of a less-stable radical anion is more significantly distributed in the benzene ring than in a stable radical anion, like that of nitrobenzene. Ab initio MO calculations show that when the EA of a Y-substituted benzene decreases, the total negative charge on the substituent (Y) in the radical anion also decreases, and that the bond lengths (C2C3/C5C6) shorten. These features are consistent with the conclusion derived from the experimentally obtained r− value.

EPR and voltammetric evidence for the reversible dimerization of anion radicals of aromatic meta-substituted diesters and dithioic S,S ′-diesters

The electrochemical behaviour of some alkyl meta-substituted (R = Me or Pr) diesters and dithioic S,S′-diesters of pyridine and benzene (trivial names dipicolinate and isophthalate esters) have been studied by cyclic voltammetry (scan rates = 0.1–50 V s–1) in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The cyclic voltammetry data can be interpreted as suggesting that the compounds undergo a one electron reduction to form the corresponding radical anions, which rapidly decompose via a chemical step, i.e. an EC mechanism, with the chemical step occurring in the order of several milliseconds. However, in situ electrochemical-EPR experiments performed during the one electron reduction of the diesters have confirmed the existence of relatively stable radical anions with a much greater lifetime (t1/2 > 2–5 s). Furthermore, the very simple nature of the EPR spectra, combined with product analysis data from bulk controlled potential electrolysis experiments, provide very good evidence that the anion radicals are the primary radicals formed by one electron reduction of the starting materials. Therefore, the apparent discrepancy in the lifetimes of the radical anions obtained by voltammetric and spectroscopic methods have been rationalized by considering a reversible dimerization mechanism. Rate constants have been fitted to the cyclic voltammetry data by digital simulation techniques and were estimated to be kf ≈ 103–104 L mol–1 s–1 (for a radical–radical coupling step) and kb ≈ 10–1–100 s–1 (for the dianion dimer to dissociate to form two anion radicals). Only approximate rate constants could be derived from the experimental curves because a large number of variables needed to be included in the simulations, meaning that no unique combination of variables would give a reasonable data fit. The dimerization reaction is also complicated by a competing reaction where the radical anions or dianions irreversibly decay to form stable products. Thus, the competing decay reaction has also been included in the electrochemical simulations and gives values of kd ≈ 10–1–100 s–1 (assuming a first-order decay).

Cyclovoltammetric and EPR study of arylazophosphonates

Monofunctional arylazophosphonates p-X–C6H4N12N2PO(OCH3)21a–f with X=H (1a, 1b), Cl (1c), CH3O 2a–c with X=–(2a), O (2b) and CO (2c) were prepared. They showed relatively high photochemical and thermal stability. Monofunctional compounds 1a–f were reversibly reduced at the cathode in the region -0.85 to -1.35 V (vs. SCE) and the corresponding peak potentials correlated well with Hammett constants of the substituents. Bifunctional compounds 2a–c were reversibly reduced at -0.68 to -1.34 V. The first peak was reversible, and the second peak, following closely, was quasireversible. Generally, the anodic oxidation was irreversible. Using EPR spectroscopy, stable anion radicals were generated from the monomeric arylazophosphonates 1a–f. The unpaired spin density was centred on the azo and phosphonate group, with average splitting constants aN10.80, aN20.37 and aP0.88 mT, and nonequivalent ortho and meta protons aHo,o0.31 and 0.39 mT, aHm,m0.11 and 0.12 mT. Splitting constants aN1, aN2 and aP correlated well with the Hammett constants of the substituents X with a positive slope for aN2 and aP and a negative one for aN1. The EPR spectra of anion radicals from the bifunctional compounds 2a–c are more complex and indicate a consecutive rearrangement of primary anion radicals.

The Cathodic Cleavage of the Nitrobenzoyl Group from Protected Aliphatic Amines in N,N-Dimethylformamide

The electrochemical reduction of aliphatic amines protected by the 4- and 3- nitrobenzoyl group in N,N-dimethylformamide was reported. The compounds are reduced in two cathodic steps. The first one at about -1 V vs. SCE occurs with the formation of the rather stable anion radicals, involving one electron transfer . The reduction to dianion occurs at potentials between -1.5 and -2.0 V vs. SCE by an ECE process and leads to cleavage of the CN bond in yields above 50%.

Remarkably stable radical anions derived from clusters [HOs 3(CO) 9(L)], L= ortho-metallated α-diimine: a spectro-electrochemical study and theoretical rationalization

Reduction of the cluster [Os 3(CO) 10( i Pr-PyCa)], i Pr–PyCa= σ- N, σ- N′-pyridine-2-carbaldehyde- N-isopropyl-imine, ( 1) follows an ECEC sequence. The initial one-electron reduction step produces the radical anion 1 − which was found stable on a subsecond time scale at low temperatures. At room temperature an open-structure radical anionic transient 1a − is formed which instantaneously reduces to 1a 2−. Subsequent attack of the latter species at yet nonreduced 1 yields an Os–Os bonded cluster dimer [ 1a– 1a] 2−. In contrast to the reactivity of 1 − , the radical anions of the clusters [HOs 3(CO) 9(C 5H 3N-2-C(H)N– i Pr)] ( 2), [HOs 3(CO) 9(6-CH 2–C 5H 3N-2-C(H)N– i Pr)] ( 3), containing the ortho-metallated ligands o- i Pr–PyCa{6-X} (X=H, Me}, are substantially more stable. At room temperature in THF, 2 − and 3 − slowly disproportionate to moderately stable dianionic species 2b and 3c. IR and 1H-NMR spectra of 2b have revealed the presence of bridging CO and terminal hydride ligands, and the dominant localization of the negative charge on the metallic core. The structure of 3c is probably similar. The latter dianions were also obtained by direct one-electron reduction of 2 − and 3 − , respectively. Both radical anions are inherently stable at T=253 K. According to extended Hückel molecular orbital calculations on models of the parent clusters 1– 3, the higher stability of 2 − and 3 − relative to that of 1 − has its origin in larger localization of the odd electron at the lowest π*-orbital of the ortho-metallated ligands compared to the σN, σN′-chelated i Pr–PyCa ligand, and in higher cluster-diimine binding energies of 2 and 3 which drop due the one-electron reduction less than calculated for the couple 1/ 1 − . IR and 1H-NMR spectra have revealed that 2,3-dipyrid-2′-ylbenzoquinoxaline (dpb) coordinates to the Os 3 core as an ortho-metallated ligand, forming [HOs 3(CO) 9(dpb-14-yl)] ( 4), in contrast to shorter 2,3-dipyrid-2′-ylpyrazine (dpp). The corresponding radical anion 4 − is inherently stable already at room temperature, mainly due to the large π-acceptor capacity of the ortho-metallated dpb ligand.

Toward a Square Grid Polymer: Electrochemistry of Tentacled Tetragonal Star Connectors, C4R4−Co−C5(HgY)5, on Mercury

Electrochemical reduction of cyclobutadienecyclopentadienylcobalt derivatives, C4R4−Co−C5(HgY)5 [R = C6H5−, Y = −SMe (3), −SCOMe (4), −SCH2CH2SCH3 (5), −SCH2CH2SCOCH3 (6); R = p-EtOCOC6H4−, Y = −SEt (7), −SCOMe (8), −SCH2CH2SCH3 (9), −SCH2CH2SCOCH3 (10)], and the parent C4R4−Co−C5H5 [R = C6H5− (11) and R = p-EtOCOC6H4− (12)] has been examined. Reversible one-electron “metal-centered” reduction of 11 (−2.24 V vs Ag/AgCl) yields a stable radical anion. In 12, it takes place at −1.70 V. In 3−10, similar metal-centered reduction occurs irreversibly, accompanied by an adsorption pre-wave at −0.7 to −1.15 V. We propose that the radical anion loses a −HgSX substituent soon after it is formed. Electrochemical cleavage of the substituents occurs before the metal-centered reduction in 4 and 8 and afterward in the others. The cleaved thiolate fragments yield an anodic peak. In all compounds with ester groups, ester reduction is observed at −2.1 to −2.5 V.

Synthesis and Properties of Dihexylbithienoquinonoid Derivatives with Head-to-head, Head-to-tail, and Tail-to-tail Orientations

Abstract 2,2′-(Dihexyl-2,2′-bithienylidene-5,5′-diylidene)bis(propanedinitrile)s (dihexylbithienoquinonoid derivatives), in which two 3-hexylthiophene moieties are connected with head-to-tail (HT) and tail-to-tail (TT) orientations, were synthesized. HT bithienoquinonoid was isolated in a single form together with a small amount of its bromo derivative, similarly to the case for the head-to-head (HH) orientational isomer. On the other hand, TT isomer exhibited a peculiar feature in solution, affording an equilibrium mixture of two geometrical isomers. Electronic and electrochemical properties of these bithienoquinonoids were studied and compared with those of HH isomer. The results proved that all these bithienoquinonoid compounds possess highly polarizable and electron-acceptable properties. HH bithienoquinonoid was reduced through one two-electron transfer process, while HT and TT isomers were reduced through two one-electron transfer processes via stable radical anions to the corresponding dianions.

Transients and stable radical from the deamination of α-alanine

The objects of investigation were single crystals of L-α-alanine, in which radical anion CH3 C˙H CO 2 − has been formed by radiation induced deamination of alanine. Previously, this stable radical has been spectrally identified (λmax, e=1100 M−1·cm−1), and its characteristics have found to be identical with characteristics of the same radical obtained by pulse radiolysis in aqueous solution. The mechanism of radical formation in the solid state is not known. Time resolved pulse radiolysis of single crystal alanine has shown more complicated way of the formation of the same radical in solid state than in aqueous solution. The electrons abstracted from the solid alanine molecule neutralise positive of zwitter-ion alanine. Ammonia is leaving the reaction-complex in time of milliseconds, leaving the stable radical anion.

Generation of Oxygen Radicals from Iron Complex of Orellanine, A Mushroom Nephrotoxin; Preliminary ESR and Spin-Trapping Stukes

Orellanine, [2,2′-bipyridine]-3,3′,4,4′-tetrol-l,I'-dioxide, is the toxin responsible for the lethal nephrotoxicity of some Cortinarius mushrooms. Our present ESR and spin-trapping studies of the redox properties of the system of non-illuminated orellanine, ferrous iron and dioxygen contribute to understanding the molecular mechanism of its toxicity. UV-visible spectrophoto-metry, cyclic voltammetry and ESR in frozen medium showed the formation of a wine-red tris complex, Fe(III)Or3. This ferric complex is easily reducible (EP =-565 mV vs Ag/AgCl/3M KCl at pH7), involving a one-electron reversible process. Spin-trapping using DMPO is employed to detect the generation of super-oxide anion and hydroxyl radicals. The instantaneous one-electron oxidation of ferrous ions in the presence of the toxin under air is concomitant with dioxygen consumption as supported by dioxygen consumption. GSH involves the toxin and ferrous ions under air in a redox cycling process resulting in the production of glutathionyl and oxygen free radicals, observed for the first time with an iron complex of a mushroom toxin. In most cases, EDTA is not able to prevent the Fe(III)Or3 and radical formation. The ortho-dihydroxylated groups borne by the di-N-oxidized bipyridine structure and not the bipyridine structure itself, are responsible for the formation of a stable ferric complex at pH 7, as they are for the generation of an apparently stable ortho-semiquinone anion radical. These one-electron mechanisms may play a major role in some of the known toxic effects of orellanine.

UV-Visible Spectrooelectrochemistry of the Reduction Products of Anthraquinone in Dimethylformamide Solutions: An Advanced Undergraduate Experiment

The redox properties of anthraquinone (AQ) may be used to model the behaviour of quinones in biological systems. AQ undergoes two successive one-electron reductions in aprotic solvents to form a stable radical anion (AQ.-) and a stable dianion (AQ2-) but this behaviour is altered in the presence of a proton donor. This advanced undergraduate experiment shows how cyclic voltammetry, digital simulations of cyclic voltammograms, and UV-visible spectroelectrochemistry may be used to examine the reduction behaviour of AQ in dimethylformamide (DMF), both in the absence and presence of benzoic acid.The cyclic voltammetry of AQ in DMF shows two reversible one-electron reductions. This allows the UV-visible spectra of AQ.- and of AQ2- to be determined using an optically transparent thin layer electrode (OTTLE) cell. AQH- may also be detected in the spectra if there are proton impurities. When benzoic acid is added to the DMF, the cyclic voltammograms are markedly altered with almost all the reduction occurring near the AQ/AQ.- potential and the corresponding oxidation at rather more positive potentials. The UV-visible spectroelectrochemistry shows AQH2 as the stable reduction product under these conditions while digital simulations of the cyclic voltammograms support a mechanism involving protonation of AQ.- followed by AQH. disproportionation.