Polycyclooctatetraeneoxy alkane polyanionic polyradicals

The room temperature potassium reduction of 1,2,3-triscyclooctatetraeneoxypropane, in hexamethylphosphoramide (HMPA), yields an anion radical, which disproportionates so strongly to the dianion diradical that the anion radical cannot be observed via EPR. The dianion diradical has one unpaired electron in a primary and one in a secondary ring system, and it can be readily reduced to the corresponding trianion triradical. An analogous reduction of 1,2,3,4-tetrakiscyclooctatetraeneoxybutane does produce an observable anion radical, but it also is readily reduced to the system corresponding to one electron per eight-membered ring (the tetraanion tetraradical). These results and those obtained from systems containing two cyclooctatetraene (COT) moieties are explained in terms of the geometry changes COT undergoes upon one-electron reduction, the interactions between reduced and adjacent unreduced ring systems, and the electron- electron repulsion present in the polyanion polyradicals.     

A new approach to the spectral study of unstable radicals and ions in solution by the use of an inert gas glovebox system: observation and analysis of the infrared spectra of the radical anion and dianion of p-terphenyl

By using a Fourier-transform infrared spectrometer contained in an inert gas glovebox system (oxygen and water concentrations: <0.1 ppm), high-quality infrared absorption spectra have been observed for the radical anion and dianion of p-terphenyl in tetrahydrofuran solutions. Density functional theory with the B3LYP nonlocal exchange-correlation functional and the 6-311+G** basis set has been used for the calculations of the structures and infrared spectra of the neutral species, radical anion, and dianion of p-terphenyl. The observed infrared spectra of the radical anion and dianion are in good agreement with those calculated by density functional theory. The origin of the strong infrared absorption intensities characteristic of the radical anion and dianion are discussed in terms of changes in electronic structures induced by specific normal vibrations (electron-molecular vibration interaction).     

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.     

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.     

Anion radicals of di-trans-[12]annulene and heptalene in a one-pot synthesis from a common fire retardant

[reaction: see text] Low temperature (-100 degrees C) dehydrohalogenation of 1,2,5,6,9,10-hexabromocyclododecane (a common fire retardant) with potassium tert-butoxide in THF followed by one-electron reduction yields the anion radical of the di-trans form of [12]annulene. This system yields a well-resolved EPR signal that reveals that most of the spin density resides on one side (the planar side) of the anion radical. Five of the carbons in this [12]annulene system are twisted from the plane of the remaining seven carbons, and the rate of rearrangement between the degenerate conformations is on the EPR time scale (k = 10(6)-10(7) s(-1)). Warming of the solution results in the formation of a sigma-bond between the two internal carbons, loss of molecular hydrogen, and consequent generation of the anion radical of heptalene. Tractable quantities of neutral heptalene can be obtained via the reoxidation of this anion radical with iodine.     

Studies of potential inversion in the electrochemical reduction of 11,11,12,12-tetracyano-9,10-anthraquinodimethane and 2,3,5,6-tetramethyl-7,7,8,8-tetracyano-1,4-benzoquinodimethane

The electrochemical reduction of the title compounds, 2a and 3a, and 7,7,8,8-tetracyanoquinodimethane, 1a, was studied in acetonitrile. The reduction of 1a shows normal ordering of potentials, i.e., the potential for insertion of the first electron, E degrees1, is more positive than the potential for the second step of reduction, E degrees2. Thus, E degrees1 - E degrees2 > 0. By contrast, 2a and 3a show inversion of potentials where introduction of the second electron occurs with greater ease than the first (E degrees1 - E degrees2 < 0). The extent of inversion has been determined by simulation of the cyclic voltammograms obtained at 298 and 257 K. Electron paramagnetic resonance measurements at room temperature of solutions containing equimolar mixtures of the neutral and dianion allow determination of the concentration of anion radicals from which the disproportionation equilibrium constant and E degrees1 - E degrees2 can be calculated. The results were in good agreement with the voltammetric determinations. Calculations were conducted to characterize the structural changes accompanying reduction to the anion radical and dianion forms. Fast scan experiments at low temperatures (up to 10 000 V/s at 257 K; 500 V/s at 233 K) were conducted in an attempt to detect intermediates in the reduction, but none was found. Thus, it is not possible to state whether structural change and electron transfer are concerted or occur in discrete steps.     

Mono- and Dianion of a Bis(benzobuta)tetraazapentacene Derivative

A bis(benzobuta)tetraazapentacene derivative was reduced to its radical anion and its dianion, using potassium [18]crown-6 anthracenide in THF. Both reduced species were characterized by UV/Vis spectroscopy of the isolated species and by spectroelectrochemistry. Two distinct single-crystal structures of the dianion and an EPR spectrum of the radical anion were obtained. Contrary to other azaacenes, the lowest energy absorption in the UV/Vis spectrum of the dianion is redshifted in comparison to that of the neutral compound.     

Quinonoid oligothiophenes as electron-donor and electron-acceptor materials. A spectroelectrochemical and theoretical study

Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.     

Structure, electron-accepting properties, and reactivity of nitrostilbenes

Nitrostilbenes, nitrostyrenes, nitrotoluenes, and nitrodibenzyls in neutral, radical onion, and dianion forms are studied by quantum chemical, electron spectroscopy, ESR spectroscopy, and cyclic voltammetry methods. The calculated electron affinities of nitroarenes are compared with electrochemical reduction potentials. The dianion form of stilbene is stable provided that it has two nitro groups in the paraposition of the benzene ring. Nitrodibenzyls are converted to the corresponding nitrostilbenes by electrochemical reduction; this is explained by dissociative cleavage of C–H bonds in the trianion radical form of dibenzyl. In reactions of nitroarenes with bases, regioselectivity is determined by spin density distribution and by the conformation composition of the radical onions. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 640–654, July–August, 1998.     

Quantum-chemical study of tautomers of reduced forms of anthraquinone

The quantum-chemical method B3LYP/6-311G(d,p) was used to calculate structural parameters of four conformers of 9,10-dihydroxyanthracene, two conformers of 9-hydroxyanthrone, and the corresponding anions, dianion, and radical anion. The energy of 9,10-dihydroxyanthracene in a gas phase is higher and in aqueous solution lower than the energy of 9-hydroxyanthrone. The dianion can exist exclusively in a polarizable medium.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—VI. IDENTIFICATION OF THE FREE RADICALS GENERATED DURING THE PHOTOLYSIS OF MUSK AMBRETTE, MUSK XYLENE AND MUSK KETONE

Musk ambrette (4-tert-butyl-3-methoxy-2,5-dinitrotoluene), a common component of perfumes, soaps, and some food flavorings, can cause cutaneous photosensitization reactions including photoallergy. These may be mediated through free radicals formed during photolysis. When musk ambrette was photolyzed under nitrogen in basic methanol, two distinct nitro anion radicals were identified by electron spin resonance. One radical was centered on a nitro group in the plane of the aromatic ring, while the other was centered on a nitro group twisted out of the plane of the ring due to steric hindrance by bulky substituents on either side of the group: the two radicals appeared to interconvert and maintain an equilibrium concentration ratio. Two closely related compounds which are also used in perfumes, but have not been reported to cause photosensitizing reactions, also produced free radicals during photolysis. Musk xylene (2,4,6-trinitro-1,3-dimethyl-5-tert-butylbenzene) generated two nitro anion radicals, both of which were centered on twisted nitro groups, while musk ketone (3,5-dinitro-2,6-dimethyl-4-tert-butylacetophenone) produced only one nitro anion radical, which is also twisted. Athough these nitro anion radicals are probably the first step in the photolysis of these nitroarornatic molecules, it seems likely that in vivo they will undergo further reduction to produce more reactive species including the corresponding nitroso and hydroxylamine derivatives. In addition, autoxidation of the nitro anion radical intermediate forms superoxide.     

A Mixed-Anion System Consisting of a Germyl Anion and Anions Delocalized on Conjugated Carbon Ring Skeletons

A trianion consisting of one germyl anion and two anions delocalized on conjugated carbon ring skeletons was synthesized by trimerization of the germanium analogue of the anthryl anion, which exhibits high germylene character. In addition, the generation of the putative intermediate of this reaction, the radical anion of 9-germaanthracene, was confirmed by the formation of the corresponding dimeric dianion. The isolated trianion and dianion were characterized by X-ray crystallographic analysis together with NMR spectroscopy and theoretical calculations.     

Quantitative evaluation of the mechanism of electroreduction of benzoyl cyanides

The mechanism of reduction of benzoyl cyanide, 6, p-methoxybenzoyl cyanide, 7, and p-chlorobenzoyl cyanide, 8, has been studied in acetonitrile (6 and 7), N,N-dimethylformamide (6), and acetonitrile containing water (all three compounds). The reaction proceeds by initial reduction to form the anion radical followed by dimerization to produce an intermediate dianion, the dianion of the dicyanohydrin of benzil. The latter loses cyanide to give the anion of the monocyanohydrin of benzil, which undergoes two parallel reactions: expulsion of cyanide to give the corresponding benzil and rearrangement to the monoanion of mandelonitrile benzoate. The addition of water brings about an increase in the dimerization rate constant and an associated increase in the amount of benzil that is produced. The standard potentials for the initial reduction step have been evaluated, and their dependence on the substituent is discussed. The dimerization rate constants have also been evaluated.     

Compounds of potassium and tin(II) with diiminopyridine ligands: EPR spectroscopy and theoretical study

Potassium and tin(II) derivatives of the paramagnetic forms of the redox-active 2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine were synthesized and studied by means of EPR spectroscopy. It was shown that the reaction with metallic potassium involves the three-electron step reduction of the organic ligand to form a radical anion, dianion, and trianion radical, respectively. The reactions of the tin(II) diiminopyridine derivatives with metallic magnesium give complexes comprising the radical anion form of the ligand and a low-valence metal center. The geometries of the paramagnetic products and their spin density distributions were explored in terms of the density functional theory.     

Electrogenerated radical anions in room-temperature ionic liquids

The sequential two-electron reduction of benzaldehyde to the radical anion and dianion species in 1-butyl-3-methylimidazolium triflimide and 1-butyl-1-methylpyrrolidinium triflimide is reported. In 1-butyl-1-methylpyrrolidinium triflimide, the heterogeneous electrochemistry and follow-up chemical reactivity are essentially equivalent to that in conventional molecular-solvent-based electrolytes where no interaction with the media is observed. In 1-butyl-3-methylimmidazolium triflimide, reduction occurs via the same two heterogeneous processes; however, the apparent heterogeneous rate constants are smaller by ca. 1 order of magnitude which leads to quasi-reversible electrochemical behavior. Since the bulk viscosities of the liquids are similar, the slower heterogeneous kinetics are attributed to local interfacial viscosity due to local ordering in the imidazolium-based medium. Also, a dramatic anodic shift in the reduction potentials is observed in 1-butyl-3-methylimidazolium triflimide media that is attributed to a stabilizing interaction of the radical anion and dianion species with the imidazolium cation.     

Dissolving metal reduction of acetylenes: a computational study

The two-electron, two-proton reduction of alkynes to trans-alkenes has been studied computationally using the polarizable continuum regime to model liquid ammonia, the solvent in which such reductions are generally carried out. Two computational approaches have been used. In one, the energies of species (alkyne, radical anion, vinyl radical, vinyl anion, dianion, and alkene) that are implicated as possible participants in the reduction are obtained using high level ab initio single-point computations under the polarizable continuum model (PCM) conditions. In the other approach, the same species are surrounded by ten explicit ammonia molecules before undergoing the same single-point PCM analysis. It has been shown that the two methods provide nearly identical results in terms of relative energies. Other findings include the probable bent nature of the radical anion species in ammonia, the likelihood that the trans stereochemistry of the reduction is determined at the vinyl anion stage, and the elimination of a dianion as a possible species that determines the stereochemical result. Various observations relating the solvent effects of ammonia are made relative to known gas-phase properties of the species studied.     

Formation of a phosphorus-phosphorus bond by successive one-electron reductions of a two-phosphinines-containing macrocycle: crystal structures, EPR, and DFT investigations.

Chemical and electrochemical reductions of the macrocycle 1 lead to the formation of a radical monoanion anion [1](*)(-) whose structure has been studied by EPR in liquid and frozen solutions. In accord with experimental (31)P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding sigma P-P orbital. Clearly, a one-electron bond (2.763 A) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 A apart (crystal structure). A subsequent reduction of this radical anion gives rise to the dianion [1](2)(-) which could be crystallized by using, in the presence of cryptand, Na naphthalenide as a reductant agent. As shown by the crystal structure, in [1](2)(-), the two phosphinine moieties adopt a phosphacyclohexadienyl structure and are linked by a P-P bond whose length (2.305(2) A) is only slightly longer than a usual P-P bond. When the phosphinine moieties are not incorporated in a macrocycle, no formation of any one-electron P-P bond is observed: thus, one-electron reduction of 3 with Na naphthalenide leads to the EPR spectrum of the ion pair [3](*)(-) Na(+); however, at high concentration, these ion pairs dimerize, and, as shown by the crystal structure of [(3)(2)](2)(-)[(Na(THF)(2))(2)](2+) a P-P bond is formed (2.286(2) A) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon-sodium-phosphorus bridges.     

Interannular communication in the radical anions of bis-cyclooctatetraene systems

[reaction: see text] The room-temperature potassium reduction of 1,2-bis-cyclooctatetraeneoxypropane yields two different regio-spin isomer anion radicals in equilibrium (COT-O-C(Me)HCH(2)CH(2)-O-COT(*)(-) = (*)(-)COT-O-C(Me)HCH(2)CH(2)-O-COT) that is shifted far to the right. The presence of the unreduced ring perturbs the spin density on the reduced ring. Addition of more electrons generates the diradical dianion ((*)(-)COT-O-C(Me)HCH(2)CH(2)-O-COT(*)(-)), and the anion radical on the secondary side splits the degeneracy of the psuedo-ortho protons of the anion radical on the primary side.     

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.     

Synthesis of 4-(ω-X-alkyl)benzonitriles (X = 1,3-dioxan-2-yl,CN, CO<Subscript>2</Subscript>Et) by the reaction of terephthalonitrile dianion with ω-X-alkyl bromides in liquid ammonia

The main products of the reaction of terephthalonitrile dianion disodium salt with ω-X-alkyl bromides (2-(2-bromoethyl)-1,3-dioxane, 5-bromovaleronitrile, ethyl 6-bromohexanoate) in liquid ammonia are the corresponding 4-(ω-X-alkyl)benzonitriles. Similar reactions of benzonitrile radical anion sodium salt lead to ω-X-alkylbenzenes. In both cases the formation of products is due to selective ipso-alkylation of anionic forms that indicates the nucleophilic activity of terephthalonitrile dianion and benzonitrile radical anion in these reactions and the realization of alkylation via S _N 2 mechanism.