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

A discrete pi-hole center dot center dot center dot sigma-hole dimer is synthesized and X-ray characterized. It presents a perfect thumbtack geometry where the sigma-hole of the linear [AuI2]- anion points to the pi-hole located above the central Au-atom of the [AuI4]- anion. Such discrete pi-hole center dot center dot center dot sigma-hole dimers are unprecedented in literature, since all mixed-valence gold(I/III) iodide compounds reported to date form infinite center dot center dot center dot([AuI4]-center dot center dot center dot[AuI2]-)n center dot center dot 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 center dot center dot center dot anion interaction has been studied considering two possibilities: (i) a pi-hole coinage bond or (ii) sigma-hole halogen bond using high-level density functional theory calculations, the quantum theory of atoms in molecules, and the noncovalent interaction plot index.