Assembly of Three 2D Metal-Organic Frameworks (MOFs) Derived from Flexible Ligands, 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (3-bpd) and/or 1,2-bis(4-pyridyl)ethane (dpe)

Three coordination polymers, {[Cd(3-bpd)(2)(NCS)(2)]center dot C2H5OH}(n) (1), {[Cd(3-bpd)(dpe)(NO3)(2)]center dot(3-bpd)}(n) (2), {[Cd(dpe)(2)(NCS)(2)]center dot 3-bpd center dot 2H(2)O}(n) (3) (3-bpd = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene; dpe = 1,2-bis(4-pyridyl)ethane), were prepared and structurally characterized by a single-crystal X-ray diffraction method. In compound 1, each Cd(II) ion is six-coordinate bonded to six nitrogen atoms from four 3-bpd and two NCS- ligands. The 3-bpd acts as a bridging ligand connecting the Cd(II) ion to generate a 2D layered metal-organic framework (MOF) by using a rhomboidal-grid as the basic building units with the 4(4) topology. In compound 2, the Cd(II) ion is also six-coordinate bonded to four nitrogen atoms of two 3-bpd, two dpe and two oxygen atoms of two NO3- ligands. The 3-bpd and dpe ligands both adopt bismonodentate coordination mode connecting the Cd(II) ions to generate a 2D layered MOF by using a rectangle-grid as the basic building units with the 4(4) topology. In compound 3, two crystallographically independent Cd(II) ions are both coordinated by four nitrogen atoms of dpe ligands in the basal plane and two nitrogen atom of NCS- in the axial sites. The dpe acts as a bridging ligand to connect the Cd(II) ions forming a 2D interpenetrating MOFs by using a square-grid as the basic unit with the 4(4) topology. All of their 2D layered MOFs in compounds 1-3 are then arranged in a parallel non-interpenetrating ABAB-packing manner in 1 and 2, and mutually interpenetrating manner in 3, respectively, to extend their 3D supramolecular architectures with their 1D pores intercalated with solvent (ethanol in 1 or H2O in 3) or free 3-bpd molecules in 2 and 3, respectively. The photoluminescence measurements of 1-3 reveal that the emission is tentatively assigned to originate from pi-pi* transition for 1 and 2 and probably due to ligand-center luminescence for compounds 3, respectively.