Synthesis and structural characterization of a new open-framework zinc terephthalate Zn3(OH)2(bdc)2·2DEF, with infinite Zn–(μ3-OH)–Zn chains

A new zinc carboxylate Zn3(OH)2(bdc)2·2DEF was synthesized under mild hydrothermal conditions (100 °C, 40 h) in the presence of 1,4-benzenedicarboxylic (bdc) acid, in N,N′-diethylformamide (DEF) solvent. Its structure, characterized by means of single-crystal XRD analysis, consists of connection of ZnO2(OH)2 tetrahedra corner-sharing with ZnO4(OH)2 octahedra. Two adjacent ZnO2(OH)2 tetrahedra have a common edge corresponding to hydroxy group, which is also linked to the octahedrally coordinated zinc atoms with a image configuration. This resulting connection mode generates infinite chains running along the c-axis, and connected to each other via the bdc ligand. Three-dimensional framework is therefore formed with channels running along the c-axis, parallel to the infinite Zn–OH–Zn chains. Within the tunnels are trapped the DEF species, which interacts via hydrogen-bond to the μ3-hydroxy of the zinc chains with the terminal oxygen atom of the formamide function. Analysis by solid state NMR (1H and 13C) has confirmed both the presence of occluded solvent molecules within the pores and the incorporation of carboxylate moieties into the framework. The structure of Zn3(OH)2(bdc)2·2DEF is closely related to those of MOF-69 series, constructed with longer organic linkers (4,4′-biphenyldicarboxylate and 2,6-naphthalenedicarboxylate). The structure is observed to loose crystallinity upon heating and removal of the occluded DEF moieties.