Unusual Zn5(μ3-OH)2(COO−)8 secondary building units for the construction of a metal–organic framework

A hydrostable and twofold interpenetrating three-dimensional zinc-organic framework with rob topology based on 4,4'-oxydibenzoate and 3,3'-dimethyl-4,4'-bipyridine ligands

Metal-organic frameworks (MOFs) are a new class of porous materials that have received widespread attention due to their potential applications in gas storage and/or separation, catalysis, luminescence, and so on. The title compound, poly[[(μ2-3,3'-dimethyl-4,4'-bipyridine-κ(2)N:N')bis(μ4-4,4'-oxydibenzoato-κ(4)O:O':O'':O''')dizinc] tetrahydrate], {[Zn2(C14H8O5)2(C12H12N2)]·4H2O}n, has been prepared by the solvothermal assembly of Zn(NO3)2·6H2O, 4,4'-oxydi(benzoic acid) and 3,3'-dimethyl-4,4'-bipyridine. The two Zn(II) atoms adopt the same five-coordinated distorted square-pyramidal geometry (i.e. ZnO4N), bonding to four O atoms from four different 4,4'-oxydibenzoate (oba) ligands and one N atom from a 3,3'-dimethyl-4,4'-bipyridine (dmbpy) ligand. The supramolecular secondary building unit (SBU) is a paddle-wheel [Zn2(COO)4] unit and these units are linked by oba ligands within the layer to form a two-dimensional net parallel to the b axis, with the dmbpy ligands pointing alternately up and down, which is further extended by dmbpy ligands to form a three-dimensional framework with rob topology. The single net leaves voids that are filled by mutual interpenetration of an independent equivalent framework in a twofold interpenetrating architecture. The title compound shows thermal stability up to 673 K and is stable in aqueous solutions in the pH range 5-9. Excitation and luminescence data observed at room temperature show that it emits a bright-blue fluorescence.