The Electrostatic Nature of Aryl−Bromine−Halide Synthons: The Role of Aryl−Bromine−Halide Synthons in the Crystal Structures of the <i>trans</i>-Bis(2-bromopyridine)dihalocopper(II) and <i>trans</i>-Bis(3-bromopyridine)dihalocopper(II) Complexes

Abstract

The role of C−Br···X synthons in the structures of Cu(nbp)2X2 (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)2CuX4 and (nBP)X structures (nBP = n-bromopyridinium cations; X = Br- or 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)2CuX4 and (nBP)X salts compared with the Cu(nbp)2X2 compounds. The supramolecular assembly of these Cu(nbp)2X2 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 (rvdW) in Cu(3bp)2X2 but less than the sum of the rvdW by ∼0.24 Å in Cu(2bp)2X2. The C−Br···X angles are close to linear, ranging from 158.15(8)° to 167.90(8)° for Cu(3bp)2Cl2 and Cu(2bp)2Cl2 respectively. The Cu(2bp)2X2 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)2X2 compounds are based on the Cu···X semicoordinate bond. These chains interact via C−Br···X synthons to form the three-dimensional structure.