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 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 Br 2 in MeCN are reported, together with the crystal structures of the 10‐E‐3, T‐shaped adducts 2⋅ Br 2 ( 12 ), 3⋅ Br 2 ( 13 ) and 4⋅ Br 2 ( 14 ). A conductometric and spectrophotometric investigation into the reaction between 1 – 4 and Br 2 , carried out in MeCN, allows the equilibria involved in the formation of the isolated 10‐E‐3 (E=S, 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 Br 2 and I 2 , DFT calculations have been carried out on 1 – 8, 19 and 20 , and 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 diffraction studies show 12 – 14 to have a T‐shaped (10‐E‐3; E=S, Se) hypervalent chalcogen nature. They contain an almost linear Br‐E‐Br (E=S, 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 Se−Br distances are asymmetric [2.529(1) vs. 2.608(1) Å], 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 (E=S, 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 Br 2 , 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 (E=S, Se) systems resemble those of the Br 3 − and IBr 2 − anions: the Raman spectrum of a symmetric Br‐E‐Br group shows only one peak near 160 cm −1 , as found for symmetric Br 3 − and IBr 2 − 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 IBr 2 − 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.