A study of the halogen...halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C--X1...X2--C moieties (where theta1=C--X1...X2 and theta2=X1...X2--C; ri=X1...X2 distance). The distributions of the contacts within the sum of van der Waals radii (rvdW) versus thetai (theta1=theta2) show a maximum at theta approximately 150 degrees for X=Cl, Br, and I. This maximum is not seen in the distribution of F...F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I...I>Br...Br>Cl...Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp2>sp>sp3. Attaching an electronegative atom to the carbon atom strengthens the halogen...halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.

Two types of halogen...halide synthons are investigated on the basis of theoretical and crystallographic studies; the simple halogen...halide synthons and the charge assisted halogen...halide synthons. The former interactions were investigated theoretically (ab initio) by studying the energy of interaction of a halide anion with a halocarbon species as a function of Y...X- separation distance and the C-Y...X- angle in a series of complexes (R-Y...X-, R=methyl, phenyl, acetyl or pyridyl; Y=F, Cl, Br, or I; X-=F-, Cl-, Br-, or I-). The theoretical study of the latter interaction type was investigated in only one system, the [(4BP)Cl]2 dimer, (4BP=4-bromopyrdinium cation). Crystal structure determinations, to complement the latter theoretical calculations, were performed on 13 n-chloropyridinium and n-bromopyridinium halide salts (n=2-4). The theoretical and crystallographic studies indicate that these interactions are controlled by electrostatics and are characterized by linear C-Y...X- angles and separation distances less than the sum of van der Waals radius (rvdW) of the halogen atom and the ionic radii of the halide anion. The strength of these contacts from calculations varies from weak or absent, e.g., H3C-Cl...I-, to very strong, e.g., HCC-I...F- (energy of interaction ca. -153 kJ/mol). The strengths of these contacts are influenced by four factors: (a) the type of the halide anion; (b) the type of the halogen atom; (c) the hybridization of the ipso carbon; (d) the nature of the functional groups. The calculations also show that charge assisted halogen...halide synthons have a comparable strength to simple halogen...halide synthons. The nature of these contacts is explained on the basis of an electrostatic model.

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.

The role of C-Br‚‚‚X synthons in the structures of Cu(nbp) 2 X 2 (nbp ) n-bromopyridine; n ) 2 and 3) are investigated. A comparison of the role of these synthons in these and in the previously published (nBP) 2 CuX 4 and (nBP)X structures (nBP ) n-bromopyridinium cations; X ) Bror Cl -; n ) 2, 3, or 4) indicate that electrostatic effects, the positive charge on the bromopyridinium cation and the negative charge on the halide anion, play a major role in the strength and directionality of C-Br‚‚‚X synthons. The data indicates that the C-Br‚‚‚X synthons are stronger in the (nBP) 2 CuX 4 and (nBP)X salts compared with the Cu(nbp) 2 X 2 compounds. The supramolecular assembly of these Cu(nbp) 2 X 2 complexes is dominated by nontraditional C-Br‚‚‚X synthons and Cu‚‚‚X semicoordinate bonds. The bromine-halide distances are almost equal to the sum of their van der Waals radii (r vdW ) in Cu(3bp) 2 X 2 but less than the sum of the r vdW by ∼0.24 Å in Cu(2bp) 2 X 2 . The C-Br‚‚‚X angles are close to linear, ranging from 158.15(8)°to 167.90(8)°for Cu(3bp) 2 Cl 2 and Cu(2bp) 2 Cl 2 respectively. The Cu(2bp) 2 X 2 units form chain structures based on the C-Br‚‚‚X synthons. These chains interact via C-H‚‚‚X and π-π stacking to form the three-dimensional structure. In contrast, the chain structures in Cu(3bp) 2 X 2 compounds are based on the Cu‚‚‚X semicoordinate bond. These chains interact via C-Br‚‚‚X synthons to form the three-dimensional structure.