Arrangement of σ-holes at the halogen atom in halonium cations

Abstract

Halonium cations are key entities that can stabilize transition states in organocatalysis. The distribution of the electrostatic potential on the crystal structures and theoretical models was investigated in the current study. The Cambridge Structural Database (CSD) survey revealed 478 structures of the [L⋯X⋯L]⁺ (X – halogen atom, L – ligands attached to the halogen) structural motif, which were divided by the values of the L⋯X⋯L angles. The value of this angle determined the number of σ-holes at the halonium cation and hence its ability to accommodate nucleophilic attack. Complexation with HCN as a Lewis base showed that one or two such ligands can be attracted depending on the number of σ-holes. Further investigation into the electrostatic potential distribution on the surface of the model halonium [HF₂C₂–X–C₂F₂H]⁺ cations revealed that altering the values of the C⋯X⋯C angles leads to the following consequences: two σ-holes can merge into a single belt-like region, or they may both disappear entirely. The unusual fluctuations in the distribution of σ-holes caused by such geometry maneuvering were observed.