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

Abstract

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 (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 Br2 and I2, 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 Br2 and I2 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) A], 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 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 (E = S, Se) systems resemble those of the Br3- and IBr2- anions: the Raman spectrum of a symmetric Br-E-Br group shows only one peak near 160 cm(-1), as found for symmetric Br3- 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 Br3- and IBr2- 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 th...

One‐Pot Synthesis and Structure of Organochalcogen Halides with Mixed Valence States

“…However, complexes 1 and 2 cannot be classified as charge‐transfer (CT) complexes. Indeed, based on their structures and literature precedents4,6 they should be considered CT‐type compounds where the donor–acceptor interaction occurs from the μ‐Br2 and μ‐I4 atoms of the octahedral [PhTeX 4 ] fragment, which acts as donor, to the empty σ* molecular orbital located on the S–Te axis of the [PhTeS(R)] + moiety.…”

Reactions Between Chalcogen Donors and Dihalogens/Interalogens: Typology of Products and Their Characterization by FT‐Raman Spectroscopy

“…Indeed, no hypervalent sulfur compounds containing the I−S−I moiety are known, and only three hypervalent selenium compounds containing the I−Se−I framework have recently been structurally characterized [71, 115]. Only three examples of a Br−S−Br type hypervalent sulfur compound with dibromine have been structurally characterized [59, 88, 116], while analogous compounds from selenium containing substrates are numerous [65, 69, 85, 88, 117]. As expected, hypervalent sulfur and selenium compounds containing the linear Cl−E−Cl ( = , ) group are very well known [65, 69, 86].…”

“…In fact, the vibrational properties of a Br−E−Br group resemble those of (Br−X−Br) − anions ( = , ) [65, 66, 88, 117]. The FT-Raman spectrum of a symmetrical Br−E−Br group only shows one Raman peak near 160 cm −1 (see Figure 10), as found in symmetric 3 − and 2 − anions, which can be assigned to the symmetric stretching vibration of the three-body system.…”

“…This variety of products, besides being very puzzling from a kinetic and thermodynamic point of view [66, 77, 88, 89, 93], represents a serious challenge when it comes to characterize the outcome of the reactions between chalcogen-donor ligands and dihalogens and interhalogens, especially when an X-ray crystal structure determination is not possible. The FT-Raman spectroscopy was proved to be of particular help in giving qualitative structural information particularly in the case of compounds from reactions with diiodine [65].…”

Bond Analysis in Dihalogen–Halide and Dihalogen–Dimethylchalcogenide Systems

“…The reaction of molecules containing chalcogen donor atoms, E (E = S, Se), with dihalogens, such as Br 2 and I 2 , and interhalogens, such as ICl and IBr, can afford a great variety of products, depending on the experimental conditions used, including the polarity of the solvent, the reaction molar ratio, the acid–base nature of the starting materials, and the chemical environment of the donor atoms …”