Mechanistic aspects of the reaction between Br2 and chalcogenone donors (LE; E=S, Se): competitive formation of 10-E-3, T-shaped 1:1 molecular adducts, Charge-transfer adducts, and [(LE)2]2+ dications.

The synthesis and spectroscopic characterisation of the products obtained by treatment of N,N'-dimethylimidazolidine- 2-thione (1), N,N'-dimethylimidazolidine- 2-selone (2), N,N'-dimethylbenzoimidazole- 2-thione (3) and N,N'-dimethylbenzoimidazole-2-selone (4) with Br2 in MeCN are reported, together with the crystal structures of the 10-E-3, T-shaped adducts 2 ´ Br2 (12), 3 ´ Br2 (13) and 4 ´ Br2 (14). A conductometric and spectrophotometric investigation into the reaction between 1 ± 4 and Br2 , carried out in MeCN, allows the equilibria involved in the formation of the isolated 10-E-3 (EˆS, Se) hypervalent compounds to be hypothesised. In order to understand the reasons why S and Se donors can give different product types on treatment with Br2 and I2 , DFT calculations have been carried out on 1 ± 8, 19 and 20, and on their corresponding hypothetical [LEX]‡ cations (Lˆorganic framework; EˆS, Se; XˆBr, I), which are considered to be key intermediates in the formation of the different products. The results obtained in terms of NBO charge distribution on [LEX]‡ species explain the different behaviour of 1 ± 8, 19 and 20 in their reactions with Br2 and I2 fairly well. X-ray diffraction studies show 12 ± 14 to have a T-shaped (10-E-3; EˆS, Se) hypervalent chalcogen nature. They contain an almost linear Br-E-Br (EˆS, Se) system roughly perpendicular to the average plane of the organic molecules. In 12, the Se atom of each adduct molecule has a short interaction with the Br(1) atom of an adjacent unit, such that the Se atom displays a roughly square planar coordination. The SeÿBr distances are asymmetric [2.529(1) vs. 2.608(1) Š], the shorter distance being that with the Br(1) atom involved in the short intermolecular contact. In contrast, in the molecular adducts 13 and 14, which lie on a two-fold crystallographic axis, the Br-E-Br system is symmetric and no short intermolecular interactions involving chalcogen and bromine atoms are observed. The adducts are arranged in parallel planes; this gives rise to a graphite-like stacking. The new crystalline modification of 10, obtained from acetonitrile solution, confirms the importance of short intermolecular contacts in determining the asymmetry of Br-E-Br (EˆS, Se) and I-Se-I groups in hypervalent 10-E-3 compounds. The analogies in the conductometric and spectrophotometric titrations of 1 and 2 ± 4 with Br2 , together with the similarity of the vibrational spectra of 11 ± 14, also imply a T-shaped nature for 11. The vibrational properties of the Br-E-Br (EˆS, Se) systems resemble those of the Br3 ÿ and IBr2 ÿ anions: the Raman spectrum of a symmetric Br-E-Br group shows only one peak near 160 cmÿ1, as found for symmetric Br3 ÿ and IBr2 ÿ anions, while asymmetric Br-E-Br groups also show an antisymmetric Br-E-Br mode at around 190 cmÿ1, as observed for asymmetric Br3 ÿ and IBr2 ÿ ions. Therefore, simple IR and Raman measurements provide a useful tool for distinguishing between symmetric and asymmetric Br- E-Br groups, and hence allow predictions about the crystal packing of these hypervalent chalcogen compounds to be made when crystals of good quality are not available.