Inter-element linkage in 1,2- and 1,4-bis(arylselanyl)benzenes with halogens

The criteria to distinguish the structure of halogen adducts of aryl chalcogenides in solutions based on the NMR chemical shifts are confirmed by ab initio molecular orbitals (MO) calculations based on the gauge-including atomic orbitals (GIAO) theory. The criteria are applied to determine the structure of halogen adducts of 1,2-bis(phenylselanyl)benzene (1), 1,4-bis(phenylselanyl)benzene (2), and 1,4-bis(p-tert-butylphenylselanyl)benzene (3) (1·nX2, 2·nX2, and 3·nX2, respectively: n=1 and 2 and X=Cl, Br, and I) in CDCl3. The structure of 1·Br2 is demonstrated to be trigonal bipyramidal (TB) not only in the solution but also in crystals. The TB formation of 1·Br2 is just the opposite of the MC (molecular complexes) formation of selenanthrene with bromine in the solution. The driving force for the TB and MC formation is discussed based on the structure of the parent selenides. The structure of 2·2X2 and 3·2X2 is (TB, TB) for X=Cl and Br and (MC, MC) for X=I. On the other hand, the structure of 1·2Br2 is revealed to be TB at one SeBr2 moiety but MC for the other SeBr2 group, which is described as (TB, MC). The bromine exchange is observed in 1·2Br2 in the conditions of NMR measurements. The rate of bromine exchange becomes sharp as excess bromine is added to the 1·2Br2 solution, which shows that the structural (TB, MC)⇄(MC, TB) site exchange in 1·2Br2 is accelerated by the excess bromine and/or its derivatives. Ab initio MO calculations are performed on the adducts to understand their structural features and on the proposed intermediate to confirm the mechanism.