Anion···Anion [AuI₄]^-···[AuI₂]^- Complex Trapped in the Solid State by Tetramethylammonium Cations

A discrete π-hole···σ-hole dimer is synthesized and X-ray characterized. It presents a perfect thumbtack geometry where the σ-hole of the linear [AuI₂]^- anion points to the π-hole located above the central Au-atom of the [AuI₄]^- anion. Such discrete π-hole···σ-hole dimers are unprecedented in literature, since all mixed-valence gold(I/III) iodide compounds reported to date form infinite ···([AuI₄]^-···[AuI₂]^-) ·· chains in the solid state. If an excess of iodine is used for the synthesis, triiodide [I₃]^- ions are partially incorporated into the [AuI₂]^- sites, forming infinite chains. The nature of the anion···anion interaction has been studied considering two possibilities: (i) a π-hole coinage bond or (ii) σ-hole halogen bond using high-level density functional theory calculations, the quantum theory of atoms in molecules, and the noncovalent interaction plot index.