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

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posted on 2003-04-19, 00:00 authored by Roger D. Willett, Firas Awwadi, Robert Butcher, Salim Haddad, Brendan Twamley
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 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 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 [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 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.