Exploring hypervalency and three-centre, four-electron bonding interactions: Reactions of acenaphthenechalcogen donors and dihalogen acceptors

Abstract

Sterically crowded peri-substituted selenium and tellurium acenaphthene donors D1–D7 [Acenap(EPh)(Br) E = Se, Te; Acenap(SePh)(EPh) E = Se, S; Acenap(TePh)(EPh) E = S, Se, Te] react with dibromine and diiodine acceptors to afford a group of structurally diverse addition products 1–12, comparable in some cases to previously reported naphthalene analogues. Tellurium donors D4–D6 react conventionally with the dihalogens to afford insertion adducts 6–11 (X-R₂Te-X) exhibiting molecular see-saw geometries, characterised by hypervalentX-Te-Xquasi-linear fragments. The reactions of selenium donors D1–D3 with diiodine afford expected neutral charge-transfer (CT) spoke adducts 1, 4 and 5 (R₂Se-I-I) containing quasi-linear Se-I-I alignments. Conversely, treatment of D2 and D3 with dibromine results in the formation of two tribromide salts 2 and 3 containing bromoselanyl cations [R₂Se–Br]⁺⋯[Br–Br₂]⁻, each exhibiting a quasi-linear three-body Br–Se⋯E (E = Se, S) fragment. The peri-bonding in these species can be thought of as a weak hypervalent G⋯Se-X three-centre, four-electron (3c-4e) type interaction, closely related to the T-shaped 3c-4e interaction. Density-functional calculations performed on 2 and 3 and their bare cations (2a and 3a) reveal Wiberg bond indices of 0.25–0.37, suggesting substantial 3c-4e character in these systems. The presence of such an interaction operating in 2 and 3 alleviates steric strain within the peri-region and minimises the degree of molecular distortion required to achieve a relaxed geometry. Ditellurium donor D7 reacts with dibromine to afford an unorthodox insertion adduct 12 containing a Te–O–Te bridge and two quasi-linear Br–Te–O fragments, with the central tellurium atoms assuming a molecular see-saw geometry. Whilst DFT calculations indicate 12 is thermodynamically unfavourable, its formation is viable under experimental conditions.