Selenium dications

The interaction of polycations with free chalcogens leads, in some cases, to new cluster compounds.185 Reaction of the aromatic tetraatomic selenium dication with diphenyldiselenide affords Ph2Se62+ - compound 159 with a six-membered selenium ring (Scheme 66).186... 

Gillespie and coworkers obtained the compound Ph2Sefi 1 Asl fi SO .293 The cation 107 consists of a six-membered selenium ring in a boat conformation with the phenyl rings at opposite corners. The average Se—Se bond length of 2.416 A is typical for selenium dications. The distance across the top of the boat is 3.550 A, which is smaller than the sum of the van der Waals radii (4.00 A) indicating a weak intracationic interaction. 

A number of NMR studies on unusual six-membered ring molecules containing two or more heteroatoms with at least one selenium or tellurium atom have appeared since GHEC-II(1996) was published. 1,4-Diselenoniabicyclo[2.2.0] hexane bis-(tetrafluoroborate) 26 was characterized spectroscopically, particularly by H, C, and Se NMR. The Se NMR shifts are remarkably downfield, S 1202, relative to other selenium dications 27 and 28 (S 439, 807, respectively). 

Polyatomic dications of sulfur, selenium and tellurium were discovered at the end of the 1960s during the investigation of intensively colored solutions known for almost a century. These solutions are formed when chalcogens are dissolved in concentrated sulfuric acid or oleum.The first dications studied by X-ray crystallography were a hydrosulfate of tetraatomic selenium dication (Se4 ) and tetrachloroaluminates of octaatomic selenium dication (Seg " ) and tetraatomic tellurium dication ( 04 " ). Up to now, a large volume of reliable... 

The larger atomic radius and larger polarizability of selenium and tellurium together with the lesser electronegativity of these elements results in increased stability of the corresponding dications. Similarities in chemical properties and electronic structure allow for formation of mixed dichalcogen dications as well. 

Synthetic approaches to selenium, tellurium and mixed dications are similar to the methods used for disulfonium dications. Oxidation of bis-selenides to diselenonium dications 104 with two equivalents of nitrosonium salts -NOBF4, NOPfV,113 occurs more readily than the oxidation of sulfides and affords the corresponding dications in better yields (Scheme 41).96,114... 

The possibility of preparation of selenium and tellurium containing dications by direct oxidation with triflic anhydride was investigated only recently.119 An example is provided by the synthesis of dication 110 from mixed bis-chalco-genide 109 (Scheme 42). 

The most versatile approach to disulfonium dications - reaction of triflic anhydride with monosulfoxides of bis-sulfides - has certain limitations in the case of selenium. Most importantly, selenoxides that contain (3-hydrogen atoms are labile.120 122 Trimethylsilyl triflate was used instead of triflic anhydride for synthesis of dication 112 from a selenoxide 111 (Scheme 43).123... 

The lesser electrophilicity of the selenium and tellurium derivatives is also displayed in their hydrolytic stability. In general, all diselenonium dications are more stable toward hydrolysis than the corresponding disulfonium dications. 1,5-Diselenoniabicyclo[3.3.0]octane 113, which is expected to be hydrolyzed to an unstable selenoxide, is stable in water.124 On the other hand, dication 112 is... 

Common synthesis of disulfonium dications involves formation of a S-S bond by oxidative coupling of two sulfide moieties. Involving for oxidative generation of S-S bond, thiocarbonyl compounds can lead to new-type dications containing disulfide moieties between two positively charged fragments. There are several disulfide dications of this type derived from thioureas, thiocarbon-ates and thioketones as well as selenium derivatives.137 148... 

It is interesting to compare S.. S and Se.. Se cation radicals in terms of oxidation. In general, oxidation of selenium requires considerably less energy than oxidation of sulfur. This means that Se.. Se species are more stable thermodynamically than their S.. S analogs. Nevertheless, Se.. Se species are readily oxidized to the corresponding dications. The reaction in Scheme 3-20 is an illustrative example (Asmus 2000) ... 

Similar chemistiy has been described for the vicinal -dications of selenium, tellurium, and mixed chalcogen systems, both for the preparation and reactions of these vicinal -dications.91... 

Cyclic selenium cations are known as monocations Se + observed in selenium vapor by mass spectrometry, and as dications Se 2+ of which... 

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). 

Reaction of the selenoxide 120 with triflic anhydride gave the selenurane dication 11 (Equation 27) <199600311, 1998PS565>. In a similar way, 2 was prepared as well as a derivative with two outer oxygens and one central selenium <1998PS565, 1997001767>. 

Reaction of diaminocyclopropeneselones with diaminochlorocyclopropenylium salts in acetonitrile afforded selenium bridged dications 2. ... 

In the course of the studies on transannular interaction [76] between heteroatoms, Furukawa and co-workers have recently reported some interesting new stable azasulfuranes [77], sulfurane dications bearing two sulfur atoms [78], and selenurane dications bearing two sulfur and selenium atoms as ligands which are annelated in two five-membered diazocine skeletons. As shown in Scheme 35, the central sulfur atom of the tris-sulfide 76 can be converted... 

Transannular interactions between the two selenium atoms of cyclic bis-selenides were only demonstrated in 1989 as a result of the formation of the diselenide dication (51) from 1,5-diselenacyclooctane (52) <89CC1789>. This is the first example of an isolable dication salt containing a selenium-selenium cr-bond. Similarly, dications (53) and (55) are prepared from precursors (54) and (56). 

Diselenide dication (51) is formed via a two-electron oxidation of 1,5-diselenacyclooctane (52) with two equivalents of a one-electron oxidizing agent, nitrosyl hexafluorophosphate or nitrosyl tetrafluoroborate as shown in Equation (16). Six- and ten-membered ring bis-selenides (56) and (54) are oxidized similarly to the dications (55) and (53) respectively <93T1605>. The ease with which l,4-diselenoniabicyclo[2.2.0] salt (55) is obtained from 1,4-diselenacyclohexane (56) is somewhat surprising since the transannular interaction between the two selenium atoms at 1,4-positions is not expected to be extensive. 

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