Bridging the gap: attractive 3c-4e interactions in peri-substituted acenaphthylenes

A series of peri-substituted acenaphthylenes that contain mixed halogen-chalcogen functionalities at the 5,6-positions in 1-6 [Acenapyl[X](EPh) (Acenapyl = acenaphthylene-5,6-diyl; X = Br, I; E = S, Se, Te)] and chalcogen-chalcogen moieties in 7-11 [Acenap(EPh)(EPh) (Acenap = acenaphthene-5,6-diyl; E/E = S, Se, Te)] have been prepared from their corresponding acenaphthene analogues A1-A11 by utilising 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) for the dehydrogenation of the ethane backbone. The related dihalide compounds 13 and 14 Acenapyl[XX] (XX = BrBr, II) have also been prepared by following a similar procedure, and 1,2,5,6-tetrabromo-1,2-dihydroacenaphthylene A0 was prepared as an intermediate by following an alternative route to 13. The series of acenaphthylene compounds have remarkably similar molecular structures to their acenaphthene counterparts; they exhibit an expected increase in peri separation as heavier congeners occupy the close peri positions. The presence of the ethene bridge, however, naturally compresses the nearest bay angle as it increases the splay of the exocyclic peri atoms, thereby resulting in a minor increase in separation relative to equivalent acenaphthenes. The structures of 1-11, 13 and 14 are discussed and compared with previously reported analogous naphthalene and acenaphthene compounds. Similar to acenaphthene derivatives, aromatic ring conformations and the location of p-type lone pairs influences the geometry of the peri region. Under appropriate geometric conditions, quasi-linear three-body C-Ph-EZ (E = Te, Se, S; Z = Br/E) fragments provide an attractive component for the EZ interaction. DFT studies confirm the onset of formation of three-center, four-electron bonding, similar in extent to that observed in analogous acenaphthenes, despite an increase in the peri distances.