From loop-like chain to helix: a result of symmetry breaking triggered by the replacement of coordination water

Abstract

Four novel Cd(II) coordination polymers derived from different ligands, [(Cd·NIP·Him·H₂O)·H₂O]_n (1), [Cd₂·(NIP)₂·(py)₂·H₂O)·1.5H₂O]_n (2), [Cd₂·(Q)₂·H₂O]_n (3) and [Cd·Q·(H₂O)₂]_n (4), where NIP = 5-nitroisophthalate, Him = imidazole, py = pyridine and Q = quinolinonate, have been synthesized and structurally determined by X-ray diffraction. Complexes 1 and 4 have 1D loop-like chain-shaped structures with two centrosymmetric subunits fusing alternately. Complex 2 shows a 2D folding sheet structure made up of double-stranded helices. Complex 3 adopts a 3D framework structure formed by the 1D loop-like chain linking the left- and right-handed single strand helices. The 1D chain in complex 3 is similar to the chains in complexes 1 and 4. The structural investigation of these coordination polymers shows that the variations of coordination modes have an important influence on the complex structures, with a slight change in coordination modes inducing a great change in structures. This may provide a better opportunity to gain insight into the helices formation mechanisms in coordination polymers. A geometric symmetry breaking model has been proposed to explain the formation of the helical structures.