Intermolecular Bonding Interactions Within Acenaphthene Peri-Diselenide Radical Cations: Combining Chalcogen Bonding and Pancake Bonding

Cyclic 1,2-diselenides are known to exhibit three σ-holes around the Se atoms—two in the prolongation of the Se–Se bond and one merging the two electron-depleted areas in the prolongation of the C–Se bonds. Among them, naphthalene peri-diselenides were recently isolated in their radical cation form with halometallates, affording 1:1 salts, where charge activation upon oxidation leads to strong Se•••Cl/Br chalcogen bond (ChB) interactions. Acenaphthene peri-diselenide (1) is shown here to be readily oxidized to its cation radical, showing various association modes of the 1+• species in the solid state. When engaged in electrocrystallization experiments with (Bu4N)(FeCl4), acenaphthene peri-diselenide (1) afforded two salts of the radical cation, formulated as (1)2(Fe2Cl6O) and (1)(FeCl4), depending on the electrocrystallization conditions. In both salts, short and highly directional Se•••Cl chalcogen bond (ChB) interactions take place, simultaneously involving all three σ-holes surrounding the diselenide bridge, with the reduction ratio (relative to the van der Waals Se•••Cl contact distance of 1.90 + 1.75 = 3.65 Å) reduced to 0.89. In addition, while cation radicals in (1)2(Fe2Cl6O) adopt head-to-tail alternated stacks without any intermolecular Se•••Se contacts, they organize in (1)FeCl4 into twisted-cofacial dimers, with an intra-dimer Se•••Se contact as short as 3.197(2) Å, i.e., a reduction ratio of 0.84. Such structural organization is reminiscent of those observed in dithiadiazolyl/diselenadiazolyl radicals, which associate into dimers, most often in a cis-cofacial geometry. Here, the 1+• radicals adopt either a head-to-tail (trans-cofacial) geometry (in the Fe2Cl6O2− salt) or a distorted twisted-cofacial geometry (in the FeCl4− salt), further stabilized by the chelating FeCl4−.