Structural characterization of protonated benzene-seleninic acid, the dihydroxyselenonium ion

The structure of the dihydroxyphenylselenonium ion (C6H7O2Se+) has been det ermined in its benzenesulfonate (C6H5O3Se-) and p-toluenesulfonate (C7H7O3S -) salts. Whereas the former salt is disordered, the latter less dense salt is well defined. This difference in crystallization behaviour is attribute d to a C-H . . .O hydrogen bond involving the methyl group of the p-toluene sulfonate ion. The two salts display very similar hydrogen-bond arrangement s and differ only with respect to the stacking of the phenyl groups. The di hydroxyselenonium ion is a strong acid with a pK value of -0.9 determined f rom the variation of the Se-77 chemical shift. A comparison with the two de protonated species reveals a systematic increase in the Se-O bond lengths a nd the pyramidal configuration around Se with the number of protons attache d.