Structural studies of halogen adducts of some cyclic selenides and tellurides by 1H, 13C, 77Se and 125Te NMR. Evidence for the formation of molecular complexes of selenoxanthone and selenanthrene with bromine

Abstract
1H, 13C, 77Se and 125Te NMR spectra were recorded for selenoxanthone (1a), telluroxanthone (2a), selenanthrene (3a), related compounds and their halogen adducts to elucidate the structure of the adducts in solutions. Although selenides react with bromine to give trigonal bipyramidal (TB) hypervalent complexes in general, 1a reacted with bromine to yield a molecular complex (MC). Its chlorine adduct was found to be a TB. The effective electronegativity of selenium in 1a may be modulated by conjugation with the strongly electron‐withdrawing carbonyl group. In the case of 2a, its halogen adducts were TBs. The structure of the bromine adduct of 3a was also demonstrated to be an MC from NMR data.