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.

Crystal structures of six iodopyridinium tetrahalocuprate(II) salts are reported, (nIP) 2 CuX 4 , where X = Cl or Br, nIP is the n-iodopyridinium cation, and n = 2, 3, or 4. The supramolecular structure of these salts is developed based on N−H•••X hydrogen bonding and C−I•••X halogen bonding interactions. Comparing these structures with the previously published structures of the general formulas (nCP) 2 CuX 4 and (nBP) 2 CuX 4 , where nCP + and nBP + are the n-chloropyridinium and n-bromopyridinium cations, respectively, allows us to investigate the competition between the halogen and hydrogen bonding interactions. Henceforth, the general formula (nYP) 2 CuX 4 will be used to represent the 18 structures where nYP + is the n-halopyridinium cation. Isomorphism has been observed in these structures. Isomorphic structures are divided into four sets. Analysis of the isomorphic structures allows us to apply the separation of variables principle; upon comparison of isomorphic structures, complications arise from geometrical factors due to the isomeric nature of the nYP + cation and effects of intermolecular forces other than N−H•••X hydrogen bonding, and C−I•••X halogen bonding interactions are minimized and hence can be ignored. Comparing halogen and hydrogen bonding interaction parameters within each isomorphous set allows us to investigate the competition between these interactions. As the organic halogen becomes heavier and the halide ligand is unvaried, the N•••X distance is either unvaried or becomes longer. In contrast, the Y•••X distance becomes shorter even though heavier halogens have a larger radius. For example, for the isomorphous structures (2BP) 2 CuCl 4 and (2IP) 2 CuCl 4 , the N•••Cl distances are 2.926 Å and 3.070 Å, respectively, whereas the corresponding Y•••Cl distances are 3.322 Å and 3.316 Å. Theoretical calculations have shown that bifurcated hydrogen bonding interactions are stronger than the corresponding linear ones. Also, calculations have shown that as the organic halogen becomes heavier, the halogen bonding interactions become stronger. This agrees with crystal structure data; as the organic halogen gets heavier and the halide ligand is unvaried, the difference between the two legs of the bifurcated hydrogen bond becomes larger (weaker hydrogen bonding interactions). For example, the three (4YP) 2 CuBr 4 structures are isomorphous; the difference between the two legs of the hydrogen bond are 0.117 Å, 0.191 Å, and 0.246 Å for (4CP) 2 CuBr 4 , (4BP) 2 CuBr 4 , (4IP) 2 CuBr 4 , respectively. Surprisingly, the above two trends are valid in all isomorphous sets without exception, which is rare in solid state chemistry. Analysis of the Cu−X bond distances indicates that the Cu−X bond distance of the halogen acceptor is always shorter than that of the corresponding proton acceptor; which agrees with the theoretical calculations; hydrogen bonding interactions are stronger than the corresponding halogen bonding interactions.

The title compound crystallizes in both a yellow and a green phase. The pyridinium cations form bifurcated hydrogen bonds to the CuCl4 2- anions to form molecular species. The packing of these molecular species force the CuCl4 2- chromophore to be distorted more toward a square planar geometry in the green phase. The packing in both phases appears to be strongly influenced by C−H···Cl electrostatic interactions.

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The crystal structure of 2(€6 204)3•8€2 50 (1), a complex obtained by slow hydrolysis of o-chloranil (C6C1402) in acidic ethanol solution, has been determined by single-crystal X-ray diffraction techniques with data collected by counter methods. The structure of Na3¡ 6 20(0 )( 803)2]• 20 (2), the trianion of tirón, has also been determined by X-ray crystallography. Crystals of 1 form in triclinic space group PI with a = 8.990 (1) A, b = 10.503 (2) Á, c = 13.598 (1) A, a = 99.02 (1)°, 8 = 91.50 (1)°, y = 94.44 (1)°. The observed density of 1.643 g cm"3 is in agreement with the calculated value of 1.671 g cm"3 for one unit of Pr2(C6Cl204)3-8C2H5OH per unit cell. The crystal structure is a three-dimensional network of alternating (C6C1204)2" and Pr3+ ions in which the chloranilate rings lie about crystallographic inversion centers. The resulting coordination sphere about Pr3+ consists of six chloranilate oxygen atoms from three symmetry-independent chloranilate ions and three ethanol oxygen atoms (six of the eight ethanol molecules in the full formula-the remaining two are simply in the crystal lattice), arranged in an approximately tricapped trigonal-prismatic fashion. Full-matrix least-squares refinement of the structure has converged with R and 7?w indices (on |P|) of 0.025 and 0.040 with use of 3057 symmetry-independent reflections with F2 > 3 ( 02). Crystals of 2 form from aqueous solution in orthorhombic space group Pnma with a = 16.018 (2) A, b = 6.972 (2) A, and c = 9.700 (1) A. The [C6H20(OH)(S03)2]3" species exhibits rigorous Cs symmetry, with only the two symmetry-related oxygen atoms of each S03 moiety lying out of the plane of the anion. There are no unusual aspects of the molecular geometry of 2. Full-matrix least-squares refinement of the structure has converged with R and Rw indices (on |F|) of 0.034 and 0.045 with use of the 805 observations with F2 > 3 ( 02).