Sulfur radical anion

Nowadays, ultramarine-type pigments are produced synthetically. Inside the zeolite structure the highly reactive sulfur radical anions are well protected which explains the stability of the blue color over thousands of years in air. However, the species responsible for the blue color should not be confused with the sulfur radical cations responsible for the blue color of sulfur solutions in fuming sulfuric acid (oleum) and similar oxidizing mixtures... 

By mass spectrometry sulfur radical anions with up to 25 atoms have been detected and there is photoelectron spectroscopic evidence for chainlike as well as cychc isomers of 8g and 87 [141]. 

A remarkable example of a persistent radical anion is the semiprecious stone, lapis lazuli, known and appreciated as a pigment since ancient times. The species imparting the blue hue is trisulfur radical anion, S , accompanied by variable fractions of S, which introduces a green tint the sulfur radical anions are incarcerated... 

The rare 1,2,3-dithiazine ring system was prepared by the relatively unusual method of electrochemical synthesis <89BSF433,90BSF427). The electrolysis is performed using a carbon-sulfur cathode, which acts as a rich source of highly nucleophilic sulfur radical anions. These attack the substrate a,)S-unsaturated nitriles (166) or (168) to give radical intermediates which lead to the 1,2,3-dithiazine products by attack on the solvent (167) or by dimerization (169) (Scheme 17). 

Basic copper(ll) carbonate Cobalt(ll) stannate Cobalt(ll)-doped alumina glass Calcium copper(ll) silicate Sulfur radical anions in a sodium aluminosilicate matrix Barium manganate(VII) sulfete Copper(ll) phthalocyanine Basic oopper(ll) sulfete Iron(lll) hexacyanoferrate(ll) Cobalt(ll) silicate Basic copper(ll)... 

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

Hydroxyl radicals, generated from hydrogen peroxide and titanium trichloride, add to the sulfur atom of 2-methylthiirane 1-oxide leading to the formation of propene and the radical anion of sulfur dioxide (Scheme 102) (75JCS(P2)308). 

Another way to prepare fluorinated sulfides is the photochemical alkylation of sulfides or disulfides by perfluoroalkyl iodides [69, 70, 71] (equations 62-64). Reaction of trifluoromethyl bromide with alkyl or aryl disulfides in the presence of a sulfur dioxide radical anion precursor, such as sodium hydroxymethanesulfi-nate, affords trifluoromethyl sulfides [72] (equation 65). 

Fluorinated sulflnates are prepared from sodium dithionite and liquid per-fluoroalkyl halides [74] (equation 67). For the transformation of the gaseous and poorly reactive trifluoromethyl bromide, it is necessary to use moderate pressure [75] (equation 68) These reactions are interpreted by a SET between the intermediate sulfur dioxide radical anion and the halide The sodium trifluorometh-anesulfinate thus obtained is an intermediate for a chemical synthesis of triflic acid. 

Perfluoroalkylatwn ofpyridines by perfluoroalkyl bromides or iodides occurs m the presence of sulfur dioxide radical anion precursors, such as sodium hydroxy-methanesulfinate [755, 757, 158] (equation 135)... 

Sulfur diimides may be reduced chemically (by alkali metals) or electrochemically to the corresponding radical anions [S(NR)2] , which... 

Scheme 10.5 Skeletal scrambling of sulfur diimide radical anions...

Sulfur is known to be easily reducible in nonaqueous solvents and its reduction products exist at various levels of reduction of polysulfide radical anions (S . ) and dianions (Sm2 ) 173], Recently Be-senhard and co-workers [74] have examined the effect of the addition of polysulfide to LiC104-PC. Lithium is cycled on an Ni substrate with Qc=2.7 C cm 2 and cycling currents of 1 mA cm. The cycling efficiency in PC with polysulfide is higher than that without an additive. The lithium deposition morphology is compact and smooth in PC with added polysulfide, whereas it is dendritic in PC alone. 

Between sulfur dioxide radical anions, dithionite, and sulfoxylate/sulfite there exists a pH-dependent equilibrium465 (equation 86). Therefore, dithionite has been used as a source of sulfoxylate in order to prepare sulfinate and hence sulfones. Alkylation with triethyl oxonium fluoroborate leads to ethyl ethanesulfinate, alkyl iodides lead to symmetrical sulfones466 (equation 87). 

The absorption at 375 nm is assigned to the radical anion. OH radicals were found to give an optically active transient which absorbs at 360 nm and which is assigned by the authors to the radical anion formed by the reaction of OH with the sulfur atom of the thiomethyl bond of MTMSO,... 

A number of radical anions of sulfur-containing aromatic compounds have been studied essentially by means of ESR spectroscopy and sometimes by electronic spectroscopy. The studied compounds include aromatic rings separated by the oxidized sulfur functionality. The effects caused by the latter depend on the geometry and topology of the aromatic systems as well as on the electron-withdrawing ability of the other substituents. 

Table 3 presents the hyperfme splitting constants of some sulfur-containing aromatic radical anions. The series studied included the monoxides and dioxides of dibenzothio-phene 1, thioxanthene 2, thioxanthone 3, dibenzo[b,/] thiepin 4 and dithienothiophene dioxide 5. 

TABLE 3. Hyperfine splitting constants (gauss) of some sulfur-cpntaining aromatic radical anions obtained in 1,2-dimethoxyethane with K metal as the reductant at — 80 °C... 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

Keywords Sulfur chains Molecular structures Radical anions Spectra Redox reactions... 

To check the identity and purity of the products obtained in the above reactions it is not sufficient to analyze for the sulfur content since a mixture may incidentally have the same S content. Either X-ray diffraction on single crystals or Raman spectra of powder-like or crystalline samples will help to identify the anion(s) present in the product. However, the most convincing information comes from laser desorption Fourier transform ion cyclotron resonance (FTICR) mass spectra in the negative ion mode (LD mass spectra). It has been demonstrated that pure samples of K2S3 and K2S5 show peaks originating from S radical anions which are of the same size as the dianions in the particular sample no fragment ions of this type were observed [28]. 

Ionic polysulfides dissolve in DMF, DMSO, and HMPA to give air-sensitive colored solutions. Chivers and Drummond [88] were the first to identify the blue 83 radical anion as the species responsible for the characteristic absorption at 620 nm of solutions of alkali polysulfides in HMPA and similar systems while numerous previous authors had proposed other anions or even neutral sulfur molecules (for a survey of these publications, see [88]). The blue radical anion is evidently formed by reactions according to Eqs. (5)-(8) since the composition of the dissolved sodium polysulfide could be varied between Na2S3 and NaaS with little impact on the visible absorption spectrum. On cooling the color of these solutions changes via green to yellow due to dimerization of the radicals which have been detected by magnetic measurements, ESR, UV-Vis, infrared and resonance Raman spectra [84, 86, 88, 89] see later. 

The red tetrasulfide radical anion 84 has been proposed as a constituent of sulfur-doped alkali hahdes, of alkah polysulfide solutions in DMF [84, 86], HMPA [89] and acetone [136] and as a product of the electrochemical reduction of 8s in DM80 or DMF [12]. However, in all these cases no convincing proof for the molecular composition of the species observed by either E8R, Raman, infrared or UV-Vis spectroscopy has been provided. The problem is that the red species is formed only in sulfur-rich solutions where long-chain polysulfide dianions are present also and these are of orange to red color, too (for a description of this dilemma, see [89]). Furthermore, the presence of the orange radical anion 8e (see below) cannot be excluded in such systems. 

More recently, 84 may have been identified by ESR spectroscopy of solutions of Li2S ( >6) in DMF at 303 K. The lithium polysulfide was prepared from the elements in liquid ammonia. These polysulfide solutions also contain the trisulfide radical anion ( 2.0290) but at high sulfur contents a second radical at g=2.031 (Lorentzian lineshape) was formed which was assumed to be 84 generated by dissociation of octasulfide dianions see Eq. (32) [137],... 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

---