Selenium homocycles

Sulfur or selenium homocycles containing one (or more) exocyclic halogen substituent are prepared by the oxidation of the element with arsenic or... 

Similar, but less stable, selenium homocycles, as well as mixed sulfur-selenium related heterocycles formed by heteroatom replacement, are also well docnmented. Cationic chalcogen homocycles are also known and have been reviewed. ... 

It has been shown recently that at 20 °C the unstable iodosulfanes (n = 1,2,...) decompose to give homocyclic sulfur molecules and iodine In a similar fashion SCjIj prepared from Se Clj and solid KI in carbon disulfide solution produces Seg and polymeric selenium Mixed sulfur-selenium ring molecules can be prepared by the reaction of mixtures of Se2Cl2 and chlorosulfanes (S Cl2 n = 1, 2, 4, 6-8) with potassium iodide in carbon disulfide solution. The subsequent decomposition of the unstable iodides results in the formation of mixed SeoS8 molecules. The general reaction scheme can be presented as follows ... 

Spectrophotometric and potentiometric studies of the reduction of SeCl4 by elemental selenium in eutectic NaCl-AlCl3 melts at 150°C indicate that Se can exist in up to five different oxidation states, which have been interpreted in terms of the dications Se 2+, with n = 2, 4, 8, 12, or 16, but the exact nature and the structures of these species are unknown, and no pure compounds have been isolated from these mixtures (84, 85). Oxidation of (C2F5)2Se2 by either SbF5 or 02(Sb2Fn) results in a wine-red cation of composition (SeC2Fs)2 "+, where n is probably equal to 2. A homocyclic structure has been discussed for this species (86). 

Waals distance of 400 pm as an upper limit. A stable homocyclic compound with a selenium-selenium double bond is not known. 

Many of the above-mentioned compounds have been first prepared or properly characterized after 1968. It therefore can be expected that new species of similar types will be synthesized in the near future. Their investigation by modern analytical techniques will finally help to elucidate the molecular structure of more complex systems such as amorphous, glassy, and liquid elemental selenium. In this context the already better understood homocyclic sulfur compounds and, in particular, the mixed species, the heterocyclic selenium sulfides (55), can serve as models to study the structural and chemical behavior of the selenium-selenium bond. 

Several types of homocyclic structures of selenium atoms are known. Neutral selenium rings Se with n = 5-10 have been studied in the vapor phase by mass spectrometry. In solution, Se species with = 6-8 have been observed by HPLC techniques. The crystal structure of See and three modifications of Seg have been determined by X-ray diffraction,... 

Several elements form homocyclic rings. Rhombic sulfur, the thermodynamically stable form at room temperature, consists of rings in the crown conformation. Unstable modihcalions, S , are known which include n — 6 through n = 36. In fact, sulfur has more allotnH> than any other elemeni. Selenium also forms hvc-, six-, seven-, and eight-membered rings, but they are unstable wilh respect to the chain form. 

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