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)(2)CuX4, where nBP(+) denotes the n-bromopyridinium cation and n = 2, 3, or 4 and X = Cl- or Br- and include (2BP)(2)CuBr4, (3BP)(2)CuBr4, (4BP)(2)CuBr4, (2BP)(2)CuCl4, (3BP)(2)CuCl4, and (4BP)(2)CuCl4. The structures all consist of isolated pyridinium cations and flattened tetrahedral CuX42- 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 Angstrom 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)(2)CuCl4, (4BP)(2)CuBr4, and the (4BP)(2)CuX4 salts. A double chain network exists in (3BP)(2)CuBr4, while the structure of (4BP)(2)CuCl4 contains a two-dimensional network. A common feature in all six networks is the existence of bibridged [CuX42- - (nBP(+))(2) - CuX42-] units, while the more complex double chain and layer networks also contain monobridged [CuX42- - (nBP(+)) - CuX42-] 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 pi-pi and pi-Br interactions. In general, the pi-pi 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.