Competition between coordination and hydrogen bonding in networks constructed using dipyridyl-1H-pyrazole ligands

Abstract

The reaction between zinc(II) acetate and 3,5-di(4-pyridyl)-1H-pyrazole (HL¹1) is crucially dependent on the solvent, with products based on different architectures formed in DMF, methanol and water. When the reaction is undertaken in DMF, the product is [Zn₃(OAc)₆(HL¹1)₄] 1, in which Zn(OAc)₂ centres link Zn₂(OAc)₄(HL¹1)₄ units into double-stranded zig-zag chains. Each chain is ‘broken’ after every fourth HL¹1 ligand, though the links between the chains ensure a coordination polymer is formed. The reaction in methanol produces [Zn(OAc)₂(HL¹1)]·0.5MeOH 2 which also contains double-stranded chains, though in this case both chains are continuous. The hydrothermal reaction gives [Zn(OAc)₂(HL¹1)₂]·H₂O 3, which contains discrete zinc complex molecules that are linked into double-stranded chains through hydrogen bonds. Zinc(II) chloride reacts with HL¹1 to form [ZnCl₂(L¹1)] 4. The structure of 4 consists of chains that are cross-linked into sheets by N–H⋯Cl hydrogen bonds. The nitrates of zinc(II), cobalt(II) and nickel(II) react with 3,5-di(3-pyridyl)-1H-pyrazole (HL²2) to form [M(MeOH)₄(HL²2)₂](NO₃)₂·2H₂O (5, M = Zn; 6, M = Co; 7, M = Ni). These isostructural compounds contain discrete cations that are linked into double-stranded chains by O–H⋯N hydrogen bonds, with further hydrogen bonds connecting these into sheets. The structures of 1–3 and 5–7 all contain double-stranded chains, with both bridging ligands and hydrogen bonds linking individual chains into pairs.