Halogen bond and polymorphism in trans-bis(2-iodo-5-halopyridine)dihalocopper(ii) complexes: crystallographic, theoretical and magnetic studies

The effect of halogen bonding interactions on polymorphism in Cu(2I5YP)(2)X-2 has been investigated, where 2I5YP = 2-iodo-5-halopyridine, Y = Br or Cl and X = Cl or Br; these complexes are abbreviated as 2I5Y-X. Two conformers are possible for 2I5Y-X complexes: the syn-conformer where the two iodine atoms are located on the same side of the coordination sphere and the anti-conformer where the iodine atoms are located on the opposite side of the coordination sphere. The anti-conformers are more stable than the corresponding syn-conformer by 4.0 kJ mol(-1) in 2I5Y-Br, whereas, in 2I5Y-Cl the syn- and anti- have almost the same stability (energy difference = 0.45 kJ mol(-1)). In 2I5Br-Cl, crystals of the syn- and anti-conformers were characterized using single crystal X-ray diffraction. However, only the anti-conformer of 2I5Br-Br was produced. The syn-conformer crystallizes in the monoclinic space group C2/c. Two packing polymorphs of anti-2I5Cl-Cl were characterized; they crystallized in the P1 and P2(1)/n space groups. On the molecular level, the syn-conformer is stabilized by C6-HMIDLINE HORIZONTAL ELLIPSISX hydrogen bonding interactions and the anti-conformer is stabilized by I2MIDLINE HORIZONTAL ELLIPSISCu interactions. On the supramolecular level, the C-IMIDLINE HORIZONTAL ELLIPSISCl and C-YMIDLINE HORIZONTAL ELLIPSISCl interactions are stronger in the C2/c phase than the corresponding interactions in the P2(1)/n phase as indicated by YMIDLINE HORIZONTAL ELLIPSISCl and IMIDLINE HORIZONTAL ELLIPSISCl distances. The possibility of crystallizing the syn-conformer depends on the nature of the halogens atoms in the complex; the lighter the halogen on position two of the pyridine ring and the heavier the halogen on position five of the pyridine ring, the higher the possibility of crystallizing the syn-conformer. syn-Conformers are not observed in 2I5Y-Br complexes. These conclusions were supported by density functional theory calculations and analyzing the electron density using quantum theory of atoms in molecules (QTAIM). Both conformers of 2I5Br-Cl and 2I5I-Cl complexes show antiferromagnetic interactions; the data were fit to a 1D-uniform AFM chain model. The interactions are stronger in the syn-conformer than the corresponding anti-conformer as indicated by J/k(B) values, which are -3.77(8) K, -1.88(6) K, -1.75(6) K and -1.5(4) K for syn-2I5Br-Cl, anti-2I5Br-Cl, syn-2I5I-Cl and anti-2I5I-Cl, respectively.