Selenium rings

On the other hand, the situation is more complex for the 3-substituted derivatives. In such compounds the substituent can be considered electronically to be located either meta or para to the selenium ring atom (cf. resonance formulas 8 and 9 for a -I -M substituted derivative). However, the substituent-caused shifts in the 3-substituted derivatives indicate that the heteroatom and the substituents are para and not meta related. 77Se chemical shifts are more sensitive than the 13C shifts to changes in electron... 

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... 

The area under a chromatographic peak is proportional to the concentration of the corresponding.species in the sample analysed. Using empirical calibration functions, a rapid simultaneous qualitative and quantitative analysis of sulfur mixtures by HPLC is now possible. In a similar manner the selenium rings Seg, Se, and SCg have recently been separated by HPLC... 

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 ... 

PNICTOGEN-CHALCOGEN RINGS 11.7.1 Binary Phosphorus-Sulfur and -Selenium Rings... 

Scheme 11.13 Formation and structures of organophosphorus-selenium rings.

Several mechanisms have been proposed for the intriguing interconversions of sulfur (or selenium) rings. These include the formation of (i) radicals by homolytic S-S bond cleavage, (ii) thiosulfoxides of the type S =S via ring contraction (an intramolecular process) or (iii) spirocyclic sulfuranes (or sele-nanes) via an intermolecular process. A fourth alternative (iv) invokes nucleophilic displacement reactions. Generic examples of mechanisms (ii)-(iv) for homoatomic sulfur or selenium rings are depicted in Scheme 12.1. 

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. 

Neutral selenium rings Se have been studied in the vapor phase by mass spectroscopy, in solution by high-pressure liquid chromatography, and in the solid state by Raman spectroscopy and X-ray crystallography. 

The enthalpies of formation and atomization, the heat capacities, and the entropies of the selenium rings Se (n = 5-12), as derived from mass spectrometric measurements and statistical thermodynamics (8-10,12, 13, 57), are given in Table VI. 

The first derivative of a selenium ring was prepared by Passmore et al. (87) according to the following equations ... 

One of the major differences between two-coordinated sulfur and selenium is the tendency of the latter to increase its coordination number, as can be seen from the fairly strong intermolecular interactions between selenium rings in the various crystal structures. The shortest intermolecular distance observed (335 pm in y-Se8) is 65 pm shorter than the van der Waals distance (400 pm) but still 101 pm longer than the single bond distance defined above. In the case of sulfur rings the shortest observed intermolecular contact (323 pm in Si0 (88) is 37 pm shorter than the van der Waals distance (360 pm) but 101 pm longer than the single bond in S8. 

Heteroaromatic cations containing oxygen, sulfur, or selenium ring heteroatoms are far more susceptible to pseudobase formation than the corresponding AT-methyl cations. A comparison of data for structurally related cations is given in Table IV. It is clear that in most cases pKR + values fall in the order O < Se < S NMe, and a linear relationship of unit slope exists between pKk+ values for at least two series (20 and 21) of cations (Fig. 2). The pKR + values reported81 for the linear tricyclic system 22 are unusual in that they fall in the order Se < O < S and furthermore suggest that in these cases 22 are less susceptible to pseudobase formation than their isomers 21, which is unexpected in view of the relative pKR,... 

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. 

Eight-membered, and larger, sulfur-selenium rings may be obtained from melts of the elements, by reaction of chlorosulfanes with hydrogen selenide (or vice versa), from selenous acid, or by decomposition of mixtures of 80212 and 8212, although many of these reactions give rise to mixtures... 

The structures of mixed sulfur selenium rings are similar to their sulfur analogs. Structures in which the number of... 

Scheme 6 Some mixed sulfur-selenium rings preserving bonds between identical atoms...

The chemical shift of the arsine selenide 89 with a formal As=Se double bond (THF = tetrahydrofurane) is slightly positive (Scheme 15). Several arsenic-selenium ring anions have been described by Se NMR and X-ray crystallography 90 (as PPh4 salt) but in the enlN salt... 

Some interesting antimony selenium ring compounds have been published recently (Scheme 15), among them 97 with a spiro-antimony atom, a selenadistibirane 98 stabilized by bulky groups (Bbt = 2,6-bis[bis(trimethyl-silyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl) and the dianion 99, ° a congener of 91, with similar Se chemical shifts. 

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