Sulfur homocycles cations

All known solid state structures shown in Fig. 1 contain sulfur homocycles. While the 84 cation is a regular square, the 8g cation adopts an approximately Cs symmetric exo-endo conformation with three transannular contacts and 819 consists of two seven membered rings linked by a penta-sulfur chain and incorporates two tricoordinate sulfur atoms. An analysis of the soHd state anion contacts of the 84 and 8g cations showed that the positive charge is delocalized almost equally over the entire cations [11, 23]. 

In the section dealing with electrophilic attack at carbon some results on indazole homocyclic reactivity were presented nitration at position 5 (Section 4.04.2.1.4(ii)), sulfon-ation at position 7 (Section 4.04.2.1.4(iii)) and bromination at positions 5 and 7 (Section 4.04.2.1.4(v)). The orientation depends on the nature (cationic, neutral or anionic) of the indazole. Protonation, for instance, deactivates the heterocycle and directs the attack towards the fused benzene ring. A careful study of the nitration of indazoles at positions 2, 3, 5 or 7 has been published by Habraken (7UOC3084) who described the synthesis of several dinitroindazoles (5,7 5,6 3,5 3,6 3,4 3,7). The kinetics of the nitration of indazole to form the 5-nitro derivative have been determined (72JCS(P2)632). The rate profile at acidities below 90% sulfuric acid shows that the reaction involves the conjugate acid of indazole. 

Only molecular S7 and Se7 are firmly known, yet only the former has been characterized by X-ray analysis. However, Re complexes of Se7 are known55,56 and the neutral Se7 homocycle cocrystallized with salts of polyselenides.57,58 S7+ is thought to be present in S02 solutions of sulfur cations however, this is an unproven but likely hypothesis. Other than that, only the structures of Sig2+ and Se172+ contain E7 homocycles (see there). 

In solution, however, lattice-stabilisation effects no longer apply and the dissociation of homocyclic sulfur dications occurs readily. The formation of the cation radical 85" in solutions of sulfur in 65% oleum has been firmly established by EPR studies of samples that were 92% enriched with 8 (/= 3/2). The 16-line spectrum shown in Figure 12.13 is consistent with a cyclic structure in which the unpaired electron interacts with five equivalent 8 nuclei [(2x5x3/2)-Hl = 16]. 

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. ... 

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