Structural diversity and fluorescent properties of CdII coordination polymers with 5-halonicotinates regulated by solvent and ligand halogen-substituting effect

Abstract

Seven Cd^II coordination polymers have been prepared from CdCl₂ and 5-halonicotinic acids (HL–X, X = F or Cl) in different solvent media using a similar layered diffusion method. A pair of polymorphic isomers [Cd(L–F)₂]_n (1 and 2) can be assembled from CH₃CN–H₂O and CH₃OH–H₂O systems, respectively, featuring the 3-D (3,5)-connected (4·6²)(4·6⁷·8²) network (5-connected node for each Cd^II ion and 2- or 3-connected linker for the two different types of L–F⁻ ligand) and the 3-D (3,6)-connected ant framework with the point symbol of (4·6²)₂(4⁴·6²·8⁸·10). While in DMF–H₂O solvent, a 2-D layered complex [Cd(L–F)Cl(DMF)]_n (3) is obtained with the (3,5)-connected net of (3·4·5)(3²·4·5·6²·7⁴) topology, where the chloride anion is involved in coordination. When the HL–Cl ligand is utilized instead of HL–F, the assembling reactions in CH₃CN–H₂O and DMF–H₂O media lead to the 3-D (3,5)-connected network of [Cd(L–Cl)₂]_n (4) and the 2-D (3,5)-connected net of [Cd(L–Cl)Cl(DMF)]_n (6), which are isostructural to complexes 1 and 3, respectively. [Cd(L–Cl)₂(H₂O)₂]_n (5) can be generated from the CH₃OH–H₂O medium, showing a 2-D (4,4) layered structure, and the similar reaction in the C₂H₅OH–H₂O solvent will produce a 3-D coordination framework {[Cd(L–Cl)₂(H₂O)_0.5](C₂H₅OH)_0.75(H₂O)_0.25}_n (7) with the (3,6)-connected (3·4·5)(3²·4⁴·5⁵·6²·7²) net. These structural divergences can be ascribed to the significant solvent modulation and halogen-substituting effect of 5-halonicotinates. Furthermore, their solid-state properties such as thermal stability and fluorescence have also been investigated.