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

Abstract

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