Radical Cations of Aromatic Selenium Compounds: Role of Se•••X Nonbonding Interactions

Selenium centered radical cations in aliphatic selenium compounds are stabilized by formation of two-center-three electron (2c-3e) hemi bonds either with nearby heteroatoms forming monomer radicals or with selenium atoms of the parent molecules forming diner radicals. Such radicals in aromatic selenium compounds would generally be stabilized as monomers by the delocalization of the spin density along the aromatic ring. To test the assumption if aromatic selenides having Se center dot center dot center dot X nonbonding interactions can show different types of radical cations, we have performed pulse radiolysis studies of three structurally related aromatic selenium compounds and the results have been substantiated with cyclic voltammetry and quantum chemical calculations. The three aromatic selenium compounds have functional groups like -CH2N(CH3)(2) (1), -CH2OH (2), and -CH3 (3) at ortho position to the -SeCH3 moiety. The energy of Se center dot center dot center dot X nonbonding interactions (E-nb) for these compounds is in the order 1 (Se center dot center dot center dot N) > 2 (Se center dot center dot center dot O) > 3 (Se center dot center dot center dot H) Radical cations, 1(center dot+), 2(center dot+) and 3(center dot+) were produced by the one electron oxidation of 1, 2 and 3 by radiolytically generated (OH)-O-center dot and Br-2(center dot-) radicals. Results on transient spectra, lifetime, and secondary reactions of 1(center dot+), 2(center dot+), and 3(center dot+) indicated that 1(center dot+) shows a significantly different absorption spectrum, longer lifetime, and less oxidizing power compared to those of 2(center dot+) or 3(center dot+). Quantum chemical calculations suggested that 1(center dot+) is stabilized by the formation of a 2c-3e bond between Se and N atoms, whereas 2(center dot+) and 3(center dot+) acquire stability through the delocalization of the spin density on the aromatic ring. These results provide evidence for the first time that stronger nonbonding interactions between Se center dot center dot center dot N in the ground state, facilitate the formation of stabilized radical cations, which can significantly influence the redox chemistry and the biological activity of aromatic selenium compounds.