Halogen Bonds in Square Planar 2,5-Dihalopyridine-Copper(II) Bromide Complexes

Abstract

Halogen bonding in self-complementary 1:2 metal–ligand complexes obtained from copper(II) bromide (CuBr2) and seven 2,5-dihalopyridines were analyzed using single-crystal X-ray diffraction. All presented discrete complexes form 1D polymeric chains connected with C–X···Br–Cu halogen bonds (XB). In (2-chloro-5-X-pyridine)2·CuBr2 (X = Cl, Br, and I) only the C5-halogen and in (2-bromo-5-X-pyridine)2·CuBr2 (X = Cl, Br, and I) both C2- and C5-halogens form C–X···Br–Cu halogen bonds with the X acting as the XB donor and copper-coordinated bromide as the XB acceptor. The electron-withdrawing C2-chloride in (2-chloro-5-X-pyridine)2·CuBr2 complexes has only a minor effect on the C5–X5···Br–Cu XBs, and the X5···Br distances follow the expected order I5 < Br5 < Cl5 with RXB values of 0.91, 0.94, and 0.99, respectively. In (2-bromo-5-X-pyridine)2·CuBr2 complexes, due to the polarization of both halogens, the C2–X2···Br–Cu and C5–X5···Br–Cu RXB values are very similar [0.92–0.99] as a result of the competition between C2- and C5-halogens for XB formation. In addition to the classical halogen bonds, the square-planar CuII complexes exhibit C2–X2···Cu (X = Cl and Br) contacts perpendicular to the Br–Cu–Br plane with shorter C2–Br2···Cu than C2–Cl2···Cu contacts. These interactions induce a pseudo-octahedral geometry for CuII ions. Notably, C2–X2···Br–Cu halogen bonds and the additional C2–X2···Cu contacts are slightly enhanced by the C5-halogen electronegativity.