Synthesis, crystal structures and fluorescence properties of coordination polymers based on tetrahedral ligands and secondary bidentate linkers

Two coordination polymers, namely {[Co2L(dpe)2]·0.42H2O} (1), and {[Zn4L2(dpe)4]·4H2O} (2), were solvothermally prepared from a flexible tetrapodal ligand tetrakis[(3-carboxyphenyl)oxamethyl]methane acid (H4L) and a secondary bidentate linker (E)-1,2-di(pyridin-4-yl)ethane (dpe). Single-crystal X-ray diffraction analysis revealed that these compounds possess similar three-dimensional structures, in which the frameworks are constructed from dinuclear SBUs (secondary building units) [M2(COOR)4N4] (M = Co or Zn) and tetrahedral L4− linkers. The overall structures of both compounds can be described as (4, 6)-connected binodal networks, with a Schläfli symbol of {42537}{4255647282} according to the topological analysis. Compound 2 exhibits strong photoluminescence at room temperature, with a peak at 422 nm (excitation at 370 nm), most probably from an intraligand and/or ligand-to-ligand charge transfer (LLCT). The emission of compound 2 was quenched efficiently by Fe3+ ion, suggesting that 2 could be used as a fluorescence sensor for Fe3+.