Chalcogen (Y) atoms in bivalent coordination are known to contain two separate σ-holes and so are thought to be capable of two chalcogen bonds (ChBs) to a pair of nucleophiles. However, the two σ-holes of the bivalent Y atoms within an aromatic ring coalesce so as to form an extended positive region. These molecules are thus able to engage in trifurcated ChBs to three separate nucleophiles, one directly between the two formal σ-holes. This sort of bonding is aided by the character of the low-lying vacant orbitals that are well situated to accept charge from three nucleophilic atoms in these positions. This principle is applied to design a series of receptors where three nucleophilic N atoms are located on a single molecule. The optimal receptor contains three amine groups separated by flexible ethyl groups. The interaction energy of this receptor with S, Se, and Te chalcogenadiazoles is computed to be 9.7, 13.9, and 20.5 kcal/mol, respectively. Consideration of binding, rather than pure interaction energy, would shift the designation of the optimal receptor to those with a more rigid pyridine core. Immersion of these complexes in water reduces these quantities somewhat but still leaves them in the 5-16 kcal/mol range.
Amonov et al. (Mon,) studied this question.