Other Compounds of Selenium

Other Compounds of Selenium.—8everal methods for the synthesis of tri-selenadiborolans (56) and the structure and formation of the cyclic five-membered ring have been discussed. The reaction of RgBX (X = I or H) 

The synthesis and properties of neutral chelates of the new ligand iViV-dimethylthioselenocarbonate with the tripositive ions of Ga, In, and T1 have been reported. Trischelates were obtained for the 3+ ions and a 1 1 chelate (58) for T1+. X-ray photoelectron spectra have been determined  

Other Compounds of Selenium.—The crystal structure of K8e(8CN)3,2H20 has been determined.The compound was prepared by the reaction  

In the crystals, the selenium trithiocyanate ions are dimerized (45), with the two selenium atoms and the six sulphur atoms approximately coplanar. The six cyano-groups are located on the same side of the plane and each of the two selenium atoms has an approximately square-planar co-ordination. An almost identical arrangement was found for the crystal structure of K8e(8eCN)3,5H20 in which eight selenium atoms participate in the dimerized unit (46). The unit can 

382 (I j xhiebault, L. Guen, R. Eholie, and J. Flahaut, Bull. Soc. chim. France, 1975, 967. 

Opedal, and J. Askog, Acta Chem. Scand. (A), 1975, 29, 225. 

Dimethyltin selenidc has been shown to react with diorganyliodoboranes to give R2B—Se—BR2. The compounds with R = C6H,i and QH, are stable, but with R = CHj and C4H9 decomposition to polymeric (RBSe) was observed. The reaction of 3,4-xylenyl-l,l-di-iodoborane with polymeric iodoboron selenide at 180°C has been shown to yield xyleno-l,2,5-selenadiborolen (48). 

Other Compounds of Selenium.—A study of the crystal and molecular structures of bis(NN-diethyldiselenocarbamato)selenium(ii) has shown that, except for the terminal methyl groups, the molecules are planar, with non-crystallographic C2 (mm) symmetry, and they contain two types of Se—Se bond, of lengths 2.45 and 2.80 

Breitinger and W. Morell, Inorg. Nuclear Chem. Letters, 1974, 10, 409. 

Inorganic Chemistry of the Main-group Elements over the individual Se—N—Se units, but the slight flattening of the chair configuration may indicate more extensive delocalization. 

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Some epidemiological studies report data from populations exposed to selenium in the food chain in areas with high selenium levels in soil. It is likely that selenite, selenate, and the selenium found in food and in dietary supplements comprise the majority of selenium compounds to which oral, off-site selenium exposures will occur at or near hazardous waste sites. Aside from the variation in effective dose, the health effects from exposure to selenate, selenite, and dietary selenium are not expected to differ greatly. However, oral exposures to many other compounds of selenium could occur (primarily through soil or edible plant ingestion) if those compounds were deposited at the site, or if local environmental conditions greatly favor transformation to those forms. Heavy metal selenides, aluminum selenide, tungsten diselenides, and cadmium selenide are used in industry and may end up in waste sites. 

Selenides. Selenium forms compounds with most elements. Biaary compounds of selenium with 58 metals and 8 nonmetals, and alloys with three other elements have been described (55). Most of the selenides can be prepared by a direct reaction. This reaction varies from very vigorous with alkah metals to sluggish and requiring high temperature with hydrogen. 

Extensive structural, optical, and electronic studies on the chalcopyrite semiconductors have been stimulated by the promising photovoltaic and photoelectrochem-ical properties of the copper-indium diselenide, CuInSe2, having a direct gap of about 1.0 eV, viz. close to optimal for terrestrial photovoltaics, and a high absorption coefficient which exceeds 10 cm . The physical properties of this and the other compounds of the family can be modulated to some extent by a slight deviation from stoichiometry. Thus, both anion and cation deficiencies may be tolerated, inducing, respectively, n- and p-type conductivities a p-type behavior would associate to either selenium excess or copper deficiency. 

The ease with which selenium combines with many other elements to form binary compounds has already been mentioned (p. 299). Only one compound with hydrogen is known, but a series of polyselenides of the alkali metals corresponding to the polysulphides has been obtained. Selenium forms a hexafluoride, SeFG, but no other hexalialide has been isolated. The tetrahalides arc the most stable, and mixed chloro-bromides of quadrivalent selenium, SeCl Br, are known. Selenium monochloride, SeaCla, and selenium monobromide, Se2Br2, also exist as comparatively stable liquids at ordinary temperatures. No compound of selenium and iodine is known. Selenium oxychloride, SeOCl2, and the oxybromide, SeOBr2, are extremely reactive and useful compounds. 

Other areas of selenium chemistry, such as selenium rings, selenides, " organoselenium compounds, " and the focus of older, more comprehensive accounts of Se halide chemistry will not be covered here. Additionally, the vast environmental chemistry of selenium, for example, its aqueous and sediment speciation, plant uptake, nutritional and human health aspects, and appearance in coafl and coal-derived ash, are beyond the scope of this review. The previous reviews on Se-containing ligands, " biochemistry of selenium,as well as other relevant aspects, remain some of the most valuable sources for information on these subjects. 

The selenium compounds that are most likely to be encountered in air in occupational settings are dusts of elemental selenium, hydrogen selenide, and selenium dioxide. Other volatile selenium compounds (e.g., dimethyl selenide, dimethyl diselenide) might be encountered in some naturally occurring situations. Because selenium is converted from one form to another, as in plant biosynthesis of selenoamino acids, it is not clear which specific forms may be encountered at hazardous waste sites. If a hazardous waste site specifically contains deposits of compounds of selenium, those compounds could be released off-site in dust or air. Toxicity data for exposures via inhalation are available for elemental selenium, selenium dioxide, selenium oxychloride, hydrogen selenide, and dimethyl selenide. Because there are few studies of inhalation of selenium of any single form, all available studies of inhalation exposures to selenium compounds will be included in this discussion. 

Hydrogen selenide, a highly poisonous selenium compound, is a gas at room temperature, with a density much higher than air. Selenium oxychloride, also highly toxic, is more irritating and corrosive to the human respiratory tract than are other forms of selenium because the compound hydrolyzes to hydrogen chloride (HC1), which can then form hydrochloric acid in humid air and in the respiratory tract (Dudley 1938). Hydrogen selenide and selenium oxychloride are occupational exposure hazards that are not expected to be much of a concern at hazardous waste sites. 

Selenocysteine was identified in 1976 (57) in a protein produced by Clostridium stricklandii, and it is thought to be the form in which selenium is incorporated, stoichiometricaHy, into proteins. Studies with rats show that over 80% of the dietary selenium given them is incorporated into proteins, thus selenocysteine takes on metaboHc importance. Selenoproteins having known enzymatic activities contain selenocysteine at the active sites. Two other forms of metabohc selenium are recognized methylated selenium compounds are synthesized for excretion, and selenium is incorporated into some transfer ribonucleic acids (tRNAs) in cultured cells (58). Some of the more important seleno-compounds are Hsted in Table 4. Examples of simple ring compounds are shown in Eigure 4. 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Chemical Properties. The most significant chemical property of L-ascorbic acid is its reversible oxidation to dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid has been prepared by uv irradiation and by oxidation with air and charcoal, halogens, ferric chloride, hydrogen peroxide, 2,6-dichlorophenolindophenol, neutral potassium permanganate, selenium oxide, and many other compounds. Dehydro-L-ascorbic acid has been reduced to L-ascorbic acid by hydrogen iodide, hydrogen sulfide, 1,4-dithiothreitol (l,4-dimercapto-2,3-butanediol), and the like (33). 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

The only sulfur isotope with a nuclear spin is which is quadrupolar (/ = 3/2) and of low natural abundance (0.76%). In view of these inherent difficulties and the low symmetry around the sulfur nuclei in most S-N compounds, S NMR spectroscopy has found very limited application in S-N chemistry. However, it is likely that reasonably narrow resonances could be obtained for sulfur in a tetrahedral environment, e.g. [S(N Bu)4], cf. [S04] . On the other hand both selenium and tellurium have isotopes with I = Vi with significant natural abundances ( Se, 7.6% and Te, 7.0%). Consequently, NMR studies using these nuclei can provide useful information for Se-N and Te-N systems. 

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te N interaction. Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te N interaction include 8-(dimethylamino)-l-(naphthyl)tellurium bromide, 2-(bromotelluro)-A-(p-tolyl)benzylamine, and 2-[(dimethylammo)methyl]phenyltellunum iodide. Intramolecular donation from a nitrogen donor can also be used to stabilize the Se-I functionality in related compounds." ... 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

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