The Aryl Bromine−Halide Ion Synthon and Its Role in the Control of the Crystal Structures of Tetrahalocuprate(II) Ions

The role of the arylbromine-halide ion (C−Br···X^-) synthons in the development of the supramolecular frameworks is explored in a set of six bromopyridinium tetrahalocuprate(II) salts. The compounds belong to the series (nBP)₂CuX₄, where nBP⁺ denotes the n-bromopyridinium cation and n = 2, 3, or 4 and X = Cl^- or Br^- and include (2BP)₂CuBr₄, (3BP)₂CuBr₄, (4BP)₂CuBr₄, (2BP)₂CuCl₄, (3BP)₂CuCl₄, and (4BP)₂CuCl₄. The structures all consist of isolated pyridinium cations and flattened tetrahedral CuX₄^2- anions. The supramolecular assembly of these ionic species is dominated by the novel C−Br···X^- synthon and the more traditional N−H···X^- synthon. The C−Br···X^- synthon is invariably characterized by essentially linear C−Br···X^- angles with Br···X^- contacts 0.3−0.4 Å less than the sum of the van der Waals radii. In contrast, the N−H···X^- synthons show a variety of geometries:  linear, symmetric bifurcated, and asymmetric bifurcated. In all cases, low dimensional supramolecular networks are developed based on combinations of the C−Br···X^- and N−H···X^- synthons. These include chain networks in (3BP)₂CuCl₄, (4BP)₂CuBr₄, and the (4BP)₂CuX₄ salts. A double chain network exists in (3BP)₂CuBr₄, while the structure of (4BP)₂CuCl₄ contains a two-dimensional network. A common feature in all six networks is the existence of bibridged [CuX₄^2- − (nBP⁺)₂ − CuX₄^2-] units, while the more complex double chain and layer networks also contain monobridged [CuX₄^2- − (nBP⁺) − CuX₄^2-] units. These units then aggregate into the final crystal structures generally with coplanar stacking of the substituted pyridinium cations. The stacking interactions between cations include both π−π and π−Br interactions. In general, the π−π stacking is not optimal and, in some cases, it is nonexistent. Comparison with other previous studies show the competitive nature of the C−Br···X^- and N−H···X^- synthons in halocuprate(II) structures.