Two Zn-based MOFs constructed with biphenyl-2,2',5,5'-tetracarboxylic acid and flexible N-donor ligands: syntheses, structures and properties.

Two new Zn-based metal-organic frameworks (MOFs) based on biphenyl-2,2',5,5'-tetracarboxylic acid, i.e. H(o,m-bpta), and N-donor ligands, namely, poly[[(μ-biphenyl-2,2',5,5'-tetracarboxylato)bis{[1,3-phenylenebis(methylene)]bis(1H-imidazole)}dizinc(II)] dimethylformamide monosolvate dihydrate], {[Zn(CHO)(CHN)]·CHNO·2HO} or {[Zn(o,m-bpta)(1,3-bimb)]·CHNO·2HO} (1) {1,3-bimb = [1,3-phenylenebis(methylene)]bis(1H-imidazole)}, and poly[[(μ-biphenyl-2,2',5,5'-tetracarboxylato)bis{[1,4-phenylenebis(methylene)]bis(1H-imidazole)}dizinc(II)] monohydrate], {[Zn(CHO)(CHN)]·HO} or {[Zn(o,m-bpta)(1,4-bimb)]·HO} (2) {1,4-bimb = [1,4-phenylenebis(methylene)]bis(1H-imidazole)}, have been synthesized under solvothermal conditions. The complexes were characterized by IR spectroscopy, elemental analysis, single-crystal X-ray diffraction and powder X-ray diffraction analysis. Structurally, the (o,m-bpta) ligands are fully deprotonated and combine with Zn ions in μ-coordination modes. Complex 1 is a (3,4)-connected porous network with honeycomb-like [Zn(o,m-bpta)] sheets formed by 4-connected (o,m-bpta) ligands. Complex 2 exhibits a (2,4)-connected network formed by 4-connected (o,m-bpta) ligands linking Zn ions in left-handed helical chains. The cis-configured 1,3-bimb and 1,4-bimb ligands bridge Zn ions to form multi-membered [Zn(bimb)] loops. Optically, the complexes show strong fluorescence and display larger red shifts compared to free H(o,m-bpta). Complex 2 shows ferroelectric properties due to crystallizing in the C polar point group.