Mechanistic aspects of the reaction between Br-2 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 b y treatment of N,N'-dimethyl-imidazolidine-2-thione (1), N,N'-dimethylimida zolidine-2-selone (2), N,N'-dimethylbenzoimidazole-2-thione (3) and N,N'-di methylbenzoimidazole-2-selone (4) with Br-2 in MeCN are reported, together with the crystal structures of the 10-E-3, T-shaped adducts 2 . Br-2 (12), 3Br(2) (13) and 4 . Br-2 (14). A conductometric and spectrophotometric inve stigation into the reaction between 1-4 and Br-2, carried out in MeCN, allo ws the equilibria involved in the formation of the isolated 10-E-3 (E=S, Se ) hyper valent compounds to be hypothesised. In order to understand the rea sons why S and Se donors can give different product types on treatment with Br-2 and I-2, DFT calculations have been carried out on 1-8, 19 and 20, an d on their corresponding hypothetical [LEX](+) cations (L=organic framework ; E= S, Se; X = Br, 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 Br-2 and I-2 fairly well. X-ray diff raction studies show 12-14 to have a T-shaped (10-E-3; E = S, Se) hypervale nt chalcogen nature. They contain an almost linear Br-E-Br (E = S, Se) syst em 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 squa re planar coordination. The Se-Br distances are asymmetric [2.529(1) vs. 2. 605(1) Angstrom]. the shorter distance being that with the Br(1) atom invol ved in the short intermolecular contact. In contrast, in the molecular addu cts 13 and 14, which lie on a two-fold crystallographic axis, the Br-E-Br s ystem is symmetric and no short intermolecular interactions involving chalc ogen and bromine atoms are observed. The adducts are arranged in parallel p lanes; this gives rise to a graphite-like stacking. The new crystalline mod ification of 10, obtained from acetonitrile solution, confirms the importan ce of short intermolecular contacts in determining the asymmetry of Br-E-Br (E = S, Se) and I-Se-I groups in hypervalent 10-E-3 compounds. The analogi es in the conductometric and spectrophotometric titrations of 1 and 2-4 wit h Br-2, together with the similarity of the vibrational spectra of 11-14, a lso imply a T-shaped nature for 11. The vibrational properties of the Br-E- Br (E=S, Se) systems resemble those of the Br-3(-) and IBr2- anions: the Ra man spectrum of a symmetric Br-E-Br group shows only one peak near 160 cm(- 1), as found for symmetric Br-3(-) and IBr2- anions, while asymmetric Br-E- Br groups also show an antisymmetric Br-E-Br mode at around 190 cm(-1), as observed for asymmetric Br-3(-) and IBr2- ions. Therefore, simple IR and Ra man measurements provide a useful tool for distinguishing between symmetric and asymmetric Br-E-Br groups, and hence allow predictions about the cryst al packing of these hypervalent chalcogen compounds to be made when crystal s of good quality are not available.