Synthesis and crystal structure of poly[μ-2,6-di­methyl­pyrazine-μ3-iodido-μ2-iodido-dicopper(I)] with an unusual CuI substructure

The asymmetric unit of the title compound, [Cu₂I₂(C₆H₈N₂)]_n, consists of six crystallographically independent Cu^I cations, six unique iodide anions and three independent 2,6-di­methyl­pyrazine ligands, all of them located in general positions. Three of the six copper cations display trigonal planar coordinations (one 2,6-di­methyl­pyrazine ligand and two iodide ions), whereas the remaining cations are tetra­hedrally coordinated (one 2,6-di­methyl­pyrazine ligand and three iodide ions). In the extended structure, two different CuI substructures are observed. In one of them, discrete (CuI)₄ units are observed, which show a ladder-like arrangement. Two of the cations are threefold, the other two cations are fourfold coordinated and are linked by two μ-1,1 and two μ-1,1,1 bridging iodide anions. In the other substructure (CuI)₂ rings built up of one threefold and one fourfold coordinated copper cation as well as one μ-1,1 and one μ-1,1,1 bridging iodide anion are connected into chains. These CuI substructures are linked by bridging 2,6-di­methyl­pyrazine ligands into layers that lie parallel to the ab plane. The layers are connected by a number of C—H⋯I inter­actions. Analyzing the CuI substructures in related copper(I) iodide coordination compounds with pyrazine deriv­atives as neutral ligands revealed that the same ladder-like CuI substructure is observed in all of them, which is completely different from that observed in the title compound.