Superalkali Li3M (M = Cl, Br, I) as a Lewis base in halogen bonding: A heavier halogen is a stronger Lewis base than a lighter halogen

The Li3M–XY (M = Cl, Br, and I; XY = ClCl, BrBr, ClF, BrCl, and BrF) complexes have been studied with theoretical calculations at the MP2/aug-cc-pVTZ level. The results showed that the Br atom in Li3Br is a stronger halogen acceptor than the Cl atom in Li3Cl when it interacts with dihalogen molecules although their strength difference is small and the Li3I is the strongest Lewis base. To the best of our knowledge, this is the first report on halogen bonds involved with superatoms. The formation of halogen bonding has been analyzed with natural bond orbital (NBO), atoms in molecules (AIM), and energy decomposition. The results showed that the orbital interaction plays an important role for the stabilization of the complexes, which is different from conventional halogen bonds.