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.
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