Unexpected formation of highly stabilized tetrakis-(2-alkoxyphenyl)bismuthonium salts in the oxidation of tris-(2-alkoxyphenyl)bismuthanes with iodosylbenzene

Treatment of tris-(2-alkoxyphenyl)bismuthanes 1 with iodosylbenzene in methylene dichloride at 40 °C led to none of the expected bismuthane oxides 2 but, quite unexpectedly, gave tetrakis-(2-alkoxyphenyl)bismuthonium chlorides 3 in moderate to good yields. In some cases, bismuthonium formates 4 accompanied the main reaction products. Similar treatment in benzene in the presence of benzyl bromide, ethyl bromide, or 2,2,2-trifluoroethyl iodide led to the corresponding bismuthonium bromides 7 and iodides 8. Through anion exchange, a variety of bismuthonium salts including formate 4, tetrafluoroborate 11, toluene-p-sulfonate 12, bromide 7, iodide 8 and perchlorate 13 were prepared from the salt 3 in good yields. In contrast to the known tetraphenylbismuthonium salts, all of these new bismuthonium salts exhibited high thermal stability. The molecular structure of compound 7a was elucidated by X-ray analysis, where the four neighbouring oxygen atoms are found to surround the bismuth atom tetrahedrally via a weak through-space interaction with the metal, making the bismuth centre less susceptible to nucleophilic attack of the halide anion.