Divalent zinc and cadmium coordination polymers of a new flexible tetracarboxylate ligand: syntheses, crystal structures and properties

A series of seven metal-organic frameworks, namely, {[Zn-4(bptc)(2)(bpp)(2)]center dot 3.5H(2)O}(n) (1), {[Zn-2(bptc)(bix)]center dot H2O}(n) (2), {[Zn-2(bptc)(bpe)(1.5)(H2O)(3)]center dot H2O}(n) (3), [Zn-2(Hbptc)(mu(3)-OH)(4,4'-bpy)](n) (4), [Cd-2(bptc)(bpa)(2)(H2O)](n) (5), [Cd-2(bptc)(bpp)(2)(H2O)(2)](n) (6), {[Cd-4(bptc)(2)(bix)(2)(H2O)(2)]center dot 4.5H(2)O}(n) (7) (bptc = 2,2', 3,3'benzophenonetetracarboxylate), have been hydro(solvo) thermally synthesized through the reaction of 2,2', 3,3'-benzophenone tetracarboxylic dianhydride (2,2', 3,3'-bptda) with divalent zinc and cadmium salts in the presence of ancillary nitrogen ligands (bpa = 1,2-bis(4-pyridyl) ethane, bpe = 1,2-bis(4-pyridyl)ethene, bpp = 1,3-bi(4-pyridyl)propane, bix = 1,4-bis(imidazol-1-ylmethyl)benzene, and bpy = 4,4'bipyridine). Due to various coordination modes and conformations of the versatile 2,2',3,3'-bptc ligand and co-ligands, these complexes exhibit structural and dimensional diversity. In compounds 1-5, M-bptc (M = Zn or Cd) ribbons are connected together through bpp, bix, bpe, bpy and bpa, respectively, to form metal-organic sheets, which are then united together to generate 3D supramolecular structures through interlayer hydrogen bonding, C-H ... pi, or pi ... pi interactions. In the structure of 4, the ribbon is formed from interlaced coaxial meso-helical Zn-Hbptc chains, and the meso-helical chains are stabilized by the bpa co-ligand in 5. Compound 6 features a 3D metal-organic framework in which [Cd-2(bptc)](n) meso-helices are bridged by bpp ligands to exhibit a (2,6,2)-connected self-penetrating network with the Schlafli symbol (12)(4(2)12(9)16(4))(4)(2). Compound 7 also exhibits a 3D metal-organic framework in which [Cd-2(bptc)](n) layers are connected by pillared bix ligands to yield a (4,4,8)-connected network with the Schlafli symbol (4(16)6(12))(4(4)6(2))(4(4)6(2))(2). Compounds 1 and 7 display intense blue fluorescence emissions at 460 and 465 nm, respectively, and may be suitable as excellent candidates of blue fluorescent materials.