Roles of halide anions in the structural diversity of Zn(II) complexes containing the flexible N,N′-di(4-pyridyl)adipoamide ligand

A series of Zn(II) complexes with N,N'-di(4-pyridyl)adipoamide ligands, {[ZnX2(L)]center dot H2O}(infinity) (X = Cl, 1, Br, 2 and I, 3; L = N,N'-di(4-pyridyl)adipoamide), [Zn2X4(L)(2)]center dot 2DMF (X = Cl, 4 and Br, 5) and [Zn2I4(L)(2)]center dot 4DMF center dot C4H10O, 6, were prepared and their structures determined by X-ray crystallography. Complexes 1 and 2 form double-stranded helical chains which are supported by the N-H center dot center dot center dot O and O-H center dot center dot center dot X hydrogen bonds involving the guest water molecules and the halide anions, whereas 3 shows sinusoidal chains which are interlinked by the N-H center dot center dot center dot O hydrogen bonds and pi-pi interactions. Complexes 1-3 represent a unique example that the halide anions show significant effect on folding and unfolding of the Zn(II) double-stranded helical coordination polymers. Complexes 1-3 can be transformed to the corresponding complexes 4-6, which are dinuclear with 34-membered metallocycles. In these complexes, the chloride and bromide anions play the same role in the crystal structures, while the iodide anion is distinct.