Synthesis and structures of helical and meso-helical coordination polymers directed by the conformation restriction of flexible/angular pyridine-containing ligands

Abstract

Six coordination polymers, {[Ni(L1)₂(H₂O)₂]·(H₂O)₄}_n (1), {[Co(L1)₂(H₂O)₂]·(H₂O)₄}_n (2), [Co(L2)₂]_n (3), {[Hg₃(L3)₂Cl₆]·(CH₃OH)₄·(H₂O)₂}_n (4), {[Hg₃(L3)₂I₆]·(H₂O)₃}_n (5) and {[Zn₃(L3)₂Cl₆]·(CH₃OH)₂·(H₂O)₄}_n (6) (L1 = 3,5-bis(4-pyridylmethoxy)benzoate, L2 = 3,5-bis(3-pyridylmethoxy)benzoate, L3 = pyridin-4-ylmethyl-3,5-bis(pyridin-4-ylmethoxy)benzoate were synthesized under hydrothermal (solvothermal) conditions and structurally determined by single-crystal X-ray diffraction. 1 and 2 are isomorphous, exhibiting a double helical chain, in which L1 acts as a bidentate ligand to bridge two metal ions using the monodentate carboxylate group and one pyridyl arm. In complex 3, each L2 performs as a tridentate ligand using the chelating carboxylate group and one pyridyl arm to link two metal ions, forming a 2-D homochiral layer in which neighbouring helical chains with the same chirality are interlinked through metal ions. Complexes 4, 5 and 6 are constructed from ligand L3, which acts as a tridentate ligand to bridge three metal ions via three pyridyl arms. 4 and 5 are 1-D meso-helical chains but 6 is a zigzag chain. In 4, each HgCl₂ connects with two L3 to form a helical subunit; neighbouring subunits are in opposite helicity and mediated by two other HgCl₂ to form a chair-shaped conformation and this is extended into a double-chain network. 5 contains a helical half-a-turn reversal, which is constructed with two L3 linking two HgI₂, showing a shell-like conformation. Complex 6 also contains shell-like conformations similar to those in 5, but it is a zigzag chain. The results indicate that the flexible/angular ligands can adopt different conformations, while the conformation restriction of flexible/angular ligands, together with the coordination preferences of the metal centers, plays a critical role in the construction of these novel coordination polymers.