Structures and photoluminescence of zinc(ii) coordination polymers based on in situ generated 1H-tetrazolate-5-propionic acid ligands

Abstract

Hydrothermal reaction of ZnCl₂, NCCH₂CH₂COONa and NaN₃, in the absence/presence of auxiliary ligand 2,2′-bipy/4,4′-bipy, gave birth to three novel coordination polymers, namely, [Zn(tzp)]_n (1), [Zn₂(tzp)(N₃)(OH)(2,2′-bipy)]_n (2) and [Zn₂(tzp)₂(4,4′-bipy)]_n (3) (H₂tzp = 1H-tetrazolate-5-propionic acid). The tzp²⁻ ligand was in situ generated through the [2 + 3] cycloaddition reaction of nitrile and azide, which was first untilized to synthesize coordination compounds. Polymer 1 behaves as a three-dimensional (3-D) network built up by the fusion of one-dimensional (1-D) pseudo-channels formed by the linkage of tzp²⁻ ligands and tetrahedral Zn nodes. When the chelating 2,2′-bipy was introduced into the reaction system, a two-dimensional (2-D) puckered network 2 was constructed, where 1-D pseudo-channels, similar to those in 1, are bridged by azide ions in an end-on (EO) mode. Employment of the linear bridging 4,4′-bipy ligand as the auxiliary ligand resulted in the formation of an unusual 3-D polymer 3, where the connection of Zn and tzp²⁻ produces a 2-D network, which are further joined by 4,4′-bipy. The tzp²⁻ ligands in 1–3 show the flexible conformations and different coordination modes, with the tetrazolate group being the 1,4-N mode and the carboxylate group being the syn–anti, syn–syn and monodentate mode, respectively. The results suggest that the flexible nature of the tzp²⁻ ligand as well as the introduction of auxiliary ligands is responsible for the formations of 1–3. Their photoluminescent properties and thermal stabilities have also been discussed.