Dinuclear Cu2X62- complexes (X=Cl, Br) (as well as higher nuclearity complexes) in the presence of planar pyridinium cations aggregate as stacks in which each copper(II) center assumes a quasi-square planar geometry with the closest coordinating halide ligands, and longer, axial, semicoordinate bond(s) to halide ligands in neighboring complexes. In the title compound, however, the dinuclear complexes are not stacked. Instead the copper centers are isolated with strictly four-coordinate, flattened tetrahedral geometries despite the presence of a planar pyridinium cation. Here, however, the vicinal arrangement of the methyl groups on the cation require the cations to form approximate inversion-related pairs that generate a bulkier supramolecular dication that prevents stacking of the complexes, as is generally found when bulky, nonplanar organic cations are present. At the same time, aggregation of complexes does occur through short Br...Br contacts (3.431 Å or 3.556 Å) that link dinuclear complexes into layers. In previous halidocuprate(II) structures with vicinal trimethylpyridinium cations, such as (1,2,3-trimethylpyridinium)CuCl3(H2O), (1,2,6-trimethylpyridinium)2CuX4, and (1,2,3-trimethylpyridinium)2Cu5Br10, a dominant structural motif is layers of zipper-like ribbons of organic cations separating inorganic complexes. Stacking of these layers, in turn, generates inversion-related organic cation pairs. At high temperature in the (1,2,6-trimethylpyridinium)2CuX4 salts, the structures undergo a transformation in which the layer structure is disrupted, but the inversion-related cation pairs persist. The title structure now underscores that the (quasi)-inversion-related cation pair, acting as a supramolecular dication, can act as the dominant organizing factor in the structure.

(1,2,6-Trimethylpyridinium)2Cu2Br6: The vicinal trimethylpyridinium cation pair as supramolecular dication

255th National Meeting (2018)

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