Introducing (inter-)halogen polysulfates - a study on the influence of halogen bonding on weakly coordinating anions

We present the isolation and structural elucidation of the first (inter-)halogen polysulfates [I₃]₄[S₄O₁₃]₂(SO₃), [IBr₂]₂[S₄O₁₃], [ICl₂][HS₂O₇] and [ICl₂]₂[S₂O₇]. All compounds are accessible by the reaction of I₂, Br₂ and ICl₃ with neat SO₃ in sealed glass ampoules. The novel compounds show strong donor-acceptor interactions between the oxoanionic polysulfates and the electrophilic central iodine atom of the cations. The strength of the halogen bond increases with electron withdrawal from the iodine. The electron density donation from the polysulfates into the antibonding σ*-molecular orbitals leads to a significant weakening of the X-I bond. The influence of the HaB on the bonding situation within the anions is analyzed and compared to that of Li₂[S₄O₁₃], which we synthesized as a reference alkaline metal polysulfate, showing purely electrostatic cation-anion interactions. Density functional theory (DFT) investigations and resulting MEP surface plots and QTAIM and NBO analyses show the physical nature of the HaB and reveal their energetic driving force. These insights extend our view on the influence of non-covalent interactions on anions we usually call 'weakly coordinating'.