The electrostatic nature of aryl-bromine-halide synthons:: The role of aryl-bromine-halide synthons in the crystal structures of the trans-bis(2-bromopyridine)dihalocopper(II) and trans-bis(3-bromopyridine)dihalocopper(II) complexes

The role of C-Br center dot center dot center dot 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)CuX4 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 center dot center dot center dot X synthons. The data indicates that the C- Br center dot center dot center dot X synthons are stronger in the (nBP)(2)CuX4 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 center dot center dot center dot X synthons and Cu center dot center dot center dot 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 similar to 0.24 angstrom in Cu(2bp)(2)X-2. The C- Br center dot center dot center dot X angles are close to linear, ranging from 158.15(8)degrees to 167.90(8)degrees 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 center dot center dot center dot X synthons. These chains interact via C-H center dot center dot center dot X and pi-pi 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 center dot center dot center dot X synthons to form the three-dimensional structure.