Supramolecular isomerism in zinc hydroxide coordination polymers with pyridine-2,4-dicarboxylic acid: Two polymorphs with centrosymmetric two-dimensional and acentric three-dimensional coordination networks

Abstract

Reactions of Zn(BF₄)₂ and pyridine-2,4-dicarboxylic acid (2,4-pydcH₂) in the presence of 1,2-bis(4-pyridyl)ethylene or 1,3-bis(4-pyridyl)propane under hydro(solvo)thermal conditions yielded two polymorphic metal–organic coordination polymers formulated as Zn₂(OH)₂(2,4-pydc) (1 and 2). Polymorph 1 features a two-dimensional (2-D) layer-like structure that is constructed by 2,4-pydc ligands bridging between the Zn–OH–Zn double-chain units. Each single Zn–OH–Zn chain is composed of μ₂-OH groups connecting trigonal bipyramidal and tetrahedral Zn centers. Polymorph 2 is a 3-D coordination polymer containing 2-D Zn–OH–Zn sheets that consist of μ₂- and μ₃-OH groups and trigonal bipyramidal Zn centers. The sheets are pillared by 2,4-pydc ligands to form an acentric structural architecture. 1 and 2 are rare examples that the two polymorphs exhibit a centrosymmetric 2-D coordination network and an acentric 3-D coordination network, respectively. The different structures lead to differences in photoluminescent properties and thermal stabilities for 1 and 2.