Stibonium ions

For the stibonium ions the successive substitution of all four phenyl groups in Ph4Sb by methyl indicates a tremendous difference between As and Sb in sensitivity to... 

The mechanism of reduction at mercury apparently changes dramatically when As is replaced by Sb. This is seen in the shift from one two-electron process to two one-electron processes, where the potential of the first reduction is shifted at least 0.5 V in the positive direction relative to the single reduction process for the analogous arsonium salt. The potential of the second reduction process remains close to that for the arsonium compound cf Table 1. The main difference between the arsonium and the stibonium salts is their tendency to adsorb on the mercury electrode. From electrocapillary measurements, in which the drop time (t) of the mercury electrode is measured as a function of potential ( ) in the absence and in the presence of the stibonium ions, it appears that the t/ relationship is severely effected by the presence of the stibonium ions the effect is the greater, the larger... 

Electrophilic aromatic substitution is the mechanism suggested in both cases the initial step being attack of the cation radical on the aromatic compound (ArH) (reaction 101), followed by deprotonation of the intermediate (reaction 102) and further oxidation of the radical to the resulting arsonium or stibonium ion (reaction 103). 

Summary Bissilylated onium ions of the elements N -> Sb are formed by intramolecular addition of transient silylium ions to EPh2 groups (E = N, P, As, Sb). Solutions of the onium salts in aromatic hydrocarbons are stable at room temperature for days, with the exception of the stibonium ion, which decomposes slowly. The cations were isolated as their [B(C6F5)4] salts and were characterized by NMR spectroscopy supported by quantum mechanical calculations. 

The cations are prepared from 2,5-disilaheptanes, 3, by hydride transfer reactions. The transient silylium ion 4 undergoes an intramolecular reaction to the more stable cyclic onium ion 2. The precursor silanes, 3, are synthesized by salt metathesis reaction from compounds of the type Ph2ELi, where E is the relevant Group 15 element, and the silylchloride 5. The salts 2 [B(C6Fs)4]" are isolated after washing with pentane as white to yellow microcrystalline powders in nearly quantitative yield. Solutions of the cations 2a-c in aromatic solvents are stable at room temperature. The stibonium ion 2d decomposes in solution slowly, yielding unidentified products. 

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